Faraday Enclosures

© 2009-2024 Kevan Hashemi Open Source Instruments Inc.


Home-Made Enclosures


[19-APR-24] What we call a Faraday enclosure is an enclosure made of electrically-conducting walls that is designed to stop microwaves entering the enclosed space. Enclosures made with conducting walls are usually called Faraday cages, but we call them Faraday enclosures to avoid confusing them with animal cages. We operate our Subcutaneous Transmitter (SCT) system inside Faraday enclosures so it does not have to compete with ambient microwave interference.

Figure: The FE3BS Faraday Enclosure with Shelf. Outer dimensions 91 cm wide, 60 cm high, 65 cm deep. The back wall accommodates six coaxial and six Ethernet feedthroughs.

For conducting walls we use stainless steel mesh and aluminum sheet metal. Conducting walls alone are insufficient to make an effective Faraday enclosure. We need an absorber of microwaves inside the enclosure as well. All our enclosures are equipped with slabs of microwave absorbing foam. We must also provide some means for bringing signals in and out of the enclosure without compromising its isolation. We provide coaxial and ethernet feedthrough connectors in the back wall of our rigid enclosures. We provide coaxial and ethernet feedthrough assemblies for our canopy enclosures.

Video: The FE5A Faraday Canopy, 208 cm high, 163 cm wide, 76 cm deep. Stainless steel mesh walls, aluminum floor, aluminum support frame, suspended absorbers, magnetically-sealed door seams. Coaxial and ethernet feedthroughs taped to floor. Antennas perpendicular to MA3 absorbers. Video of absorbers arranged on side walls here.

Our Faraday enclosures provide connected, conducting walls with one or more microwave-absorbing surfaces on the interior. Steel mesh allows light and air to enter and leave. Coaxial feedthroughs allow us to insert one or more antennas in the interior. Ethernet feedthroughs allow us to place Animal Cage Cameras (ACC) and Animal Location Trackers (ALT) in the enclosure. The enclosure not only reduces interference at the receiving antenna, but also stops our telemetry signals from interfering with any other system.

Figure: Coaxial Feedthrough (A3039E). Allows us to bring eight antenna signals out of a Faraday Canopy without compromising its isolation.

When our animal cages are stored on racks, we place the rack on a floor of aluminum sheet and cover the entire rack with a single canopy of stainless steel mesh. The canopy is supported by a frame made of aluminum tubes. We hang four 60-cm square absorbers from the frame inside the enclosure. We enter the canopy through one of two magnetically-sealed seams on either side of the front wall (see video). We bring coaxial antenna cables out of the enclosure by means of one or more Coaxial Feedthroughs (A3039E), which we tape to a convenient location on the edge of the floor. The bottom of the mesh wall fits between the interior and exterior cables. For Power over Ethernet (PoE) cables, we use an Ethernet Feedthrough (A3039D).

Figure: Four FE3A Faraday Enclosures and Recording System. Long coaxial cables run from the Octal Data Receiver (A3027E) to feedthroughs in the back of the enclosures. Short coaxial cables run from the feedthroughs inside the enclosure to Loop Antennas (A3015C).

Our SCT signals are broad-band and low-power. The Subcutaneous Transmitter (A3028) transmits 300 μW in the 902−928 MHz Industrial Scientific Medical (ISM) band. These signals are easily swamped by interference. Without a Faraday enclosure, a mobile phone base station can disrupt reception from implanted SCTs at a range of 1500 m. With a Faraday enclosure, the same disruption will not occur unless the base station is 50 m away. A secondary benefit of our Faraday enclosures is to guarantee that no detectable signal from our transmitters emerges from your telemetry recording room. In North America, South America, and the UK, any device may use the 902-928 MHz band provided the device's peak power output is less than 1 W. In other countries, the 902-928 MHz band is licensed for amateur radio and radiolocation systems, and regulations prohibit our system from radiating intentionally within the band. The average 902-928 MHz power escaping a Faraday enclosures containing ten implanted SCTs is <10 nW. This power generates an electric field strength <60μV/m at range 10 m, which satisfies the ≤210 μV/m electromagnetic emission limit for unintentional radiators under regulations common to all nations.

Figure: Microwave Absorbing Foam (MA3), Vinyl-Coated, 61 cm × 61 cm. We provide four such absorbers with each Faraday Canopy (FE5A). Foam is ML-10052-25. Hangs by grommets.

The most common way to detect SCT signals with a Loop Antenna (A3015C/D) plugged into an Octal Data Receiver (ODR) or Telemetry Control Box (TCB). Our Faraday enclosures provide a factor of one thousand attenuation of external microwave interference. In London, our enclosures provide robust reception on the top floor of a building, within a hundred meters of a mobile phone base station. But if we open an enclosure, reception is poor. In Edinburgh, on the top floor of another building, our enclosures were unable to provide robust reception within fifty meters of a set of three base stations. Meanwhile, in another location in London, on the ground floor at the center of a brick building, we obtain perfect reception with no Faraday enclosure at all.

Figure: An FE3AS Packed Up In a Crate for Shipping. In the past, we shipped enclosures partially-assembled in cardboard boxes. Now we ship them fully-assembled in crates.

Before you select a location for your telemetry system, we recommend you the exact latitude and longitude of your location, and altitude, so that we can check the location of local mobile phone base stations. If there are any within one hundred meters, facing directly towards you, and you are high enough off the ground to be directly in their line of transmission, we will advise you on measurements you can make of the interference power so as to determine whether your preferred space can support our SCT system. Rural locations have far less interference. In one rural setting we were able to receive SCT signals at a range of four meters with no Faraday enclosure. Aquatic settings also permit us to operate the SCT system without a metallic Faraday enclosure. The water itself reflects and attenuates external interference, creating an aquatic enclosure, so that we are able to operate in an urban location with four antennas immersed in the aquarium.


[14-MAR-24] We have two Faraday enclosures in active production: the FE3B and the FE5A. The table below lists these and earlier versions that we no longer manufacture. Starting in March 2024, we ship all our enclosures fully-assembled in a wooden crate, like this.

Part Height
of Cages
of Antennas
Description Status
FE3B 60 65 91 6 4 front-loading, no shelf, easy-replace mesh drawings, assembly, guide. Active
FE3BS 60 65 91 6 4 front-loading, clear shelf, easy-replace mesh drawings, assembly, guide. Active
FE5A 208 76 163 40 8 flexible mesh canopy for IVC rack, two vertical zippers, two microwave absorbers, metal floor, assembly. Active
FE2A 47 71 71 2 2 aluminum frame, top-loading, all walls mesh, assembly Discontinued
FE2B 33 71 71 2 1 aluminum frame, top-loading, rear wall solid Discontinued
FE2C 33 71 71 2 2 aluminum frame, front-loading, rear wall solid, customer-made Discontinued
FE2D 50 71 71 2 2 aluminum frame, front-loading, rear wall solid, customer-made Discontinued
FE2E 50 71 142 4 4 aluminum frame, front-loading, rear wall solid, double-wide, customer-made Discontinued
FE2F 33 64 62 2 2 aluminum frame, front-loading, rear wall solid Discontinued
FE3A 60 65 91 3 2 aluminum frame, front-loading, double-height, drawings, assembly, BOM. Discontinued
FE3AS 60 65 91 6 4 aluminum frame, front-loading, double-height, clear shelf, drawings, assembly, BOM. Discontinued
FE4A 61 69 137 NA 4 folding absorber walls, reflecting floor, mesh cover, for demonstrations Discontinued
Table: Exterior Dimensions of Faraday Enclosures.

The FE3B is a rigid-frame enclosure that will house two large rat cages with space for cameras mounted above. Its microwave absorber is glued to its ceiling. When equipped with a shelf, the FE3B becomes the FE3BS, and will house six mouse cages: three on its floor and three on the shelf. The front wall is a door that swings down, giving access to the entire interior space. The FE3BS comes with top-side handles for carrying it about, but if we remove these handles, we can stack several FE3BS on top of one another. We could, for example, assemble a mobile recording unit for twenty-four mouse cages by stacking four FE3BS on top of a four-wheeled platform.

The FE5A is a soft-walled canopy held up by a rigid frame that encloses an individually-ventilated cage (IVC) rack. Its microwave absorbers hang on the back or side walls, wrapped in vinyl. Magnetically-closed seams on the left and right allow us to open the front wall and service the cages, or roll the IVC rack out of the enclosure for cleaning. Both types of Faraday enclosure require that we bring coaxial and ethernet cables into the enclosed space via grounded feedthrough connectors. In the FE3BS, these feedthroughs are bolted into the back wall of the enclosure. In the FE5A, these feedthroughs are separate assemblies that we tape to the aluminum floor of the enclosure.

The interior width and depth of the aluminum frame enclosures are 1 cm smaller on account of the thickness of the aluminum brackets. The interior height is 6 cm shorter because of the foam absorber on the underside of the lid. The opening at the top of the enclosure is roughly 90 mm smaller on account of the width of the aluminum brackets. The 6-mm thick, clear, polycarbonate shelf provided by the FE3AS enclosure will sag by <2 mm when loaded by three 1-kg animal cages unless we insert the shelf support beneath its center. Without the shelf support, three 5.7-kg paint cans cause a sag of 6 mm, see here.

To enclose shelves or racks of cages, we recommend the Faraday Canopy (FE5A). Within the canopy, we can distribute sixteen antennas on the shelves or racks and connect them to two Eight-Four Coaxial Feedthroughs (A3039C) at the boundary of the enclosure. We connect the eight combined antenna signals to an Octal Data Receiver. So long as the antennas that are paired together are more than 60 cm apart, they provide increased cover within the rack.


[18-MAR-24] We ship our bench-top Faraday enclosures fully-assembled in wooden crates, like this. We ship our Faraday canopies disassembled in wooden crates. The crates arrive on a truck and are transferred to your receiving dock. At that point, you can either have the crate moved up to your laboratory, or you can open the crate on the dock. To open the crate, you must release the metal latches that hold it together, as we demonstrate in this movie. With the crate open, remove the fully-assembled enclosure or the disassembled canopy. The enclosures are equipped with handles, making it easy for two people to carry the enclosure to your laboroatory.

The FE5A arrives as a set of struts, joints, absorbers, cable ties, floor plates, aluminum tape, and the stainless steel mesh canopy. To assemble, we need a hammer and an automatic screw driver with a phillips head bit. See our pictorial FE5A Assembly guide. The square-section, hollow aluminum struts of the fram fits together with plastic press-fit joints. There are four simple connection joints to make the long vertical struts. There are four three-way corner joints to make the roof frame. There are four feet to put into the bottom of the vertical struts. We use the hammer to help the joints into the struts. When we drape the canopy over the frame, we get it square and settled, then we fasten the outer magnetic strip of both seams to its vertical strut with several self-drilling screws, as shown below.

Video: Faraday Canopy Assembly: Driving the Door Seam Screws into the Vertical Struts.

When we enclose an IVC rack in a Faraday Canopy, we must always accommodate the inlet and outlet air ventilation pipes. We must not place any electrical machinery inside the Faraday enclosure, other than antennas and Power over Ethernet cameras that we have tested to make sure they do not disrupt telemetry reception or biopotential integrity. So we must leave the ventilation pumps outside and run the air in and out of the enclosure with pipes that pass through the enclosure walls. We make two cross-cuts in the sides of the canopy to allow each ventilation pipes to enter. The cross-cuts are slightly bigger than the pipes, so that each will provide four triangles of steel mesh fabric that you can tape around the pipe outside the enclosure using aluminum tape. These two pipes will compromise the isolation of the enclosure, but the enclosure will still be effective enough to ensure reliable telemetry reception from the IVC rack.

Home-Made Enclosures

[27-OCT-23] Some customers prefer to make their own Faraday enclosures, which we call home-made enclosures. Home-made enclosures usually work well enough, but sometimes they don't. Our customers save money making their own enclosures, and they are able to direct work to their own machine shops. The most common problem with home-made enclosures is omission of the microwave absorbing foam. Another problem is failure to provide an adequate microwave-frequency seal around the door. An effective microwave seal consists a 1-mm gap, 10-mm deep with a right-angle bend at depth 5 mm. We are happy to support customers in their efforts to design and manufacture their own enclosures. We can provide microwave absorbing foam and steel mesh fabric to facilitate local construction.

Figure: Stack of Locally-Made Faraday Enclosures at Chang Gung Memorial Hospital in Taiwan. Enclosures made by Advance Biotech Inc., New Taipei City.

The simplest enclosure to make is one with a lid we can remove. We constructed our first enclosures out of aluminum angle-brackets, epoxy, and stainless steel mesh. Instructions for making these inexpensive, top-loading enclosures are in our FE2 Assembly manual. Enclosures with hinged front doors are harder to make, but they have the great advantage that they can be stacked one upon the other, as shown below.

Figure: The FE2C Stack, Built at ION. Five front-loading Faraday enclosures on top of one another with rolling wheels underneath. The stack can be moved around the laboratory. Each enclosure has its own microwave absorbing foam slab glued to its ceiling.

An easier door to assemble is a sliding door, such as in the FE2D designed and build by customers in Germany. A sliding door gives good isolation provided it is equipped with conducting foam gaskets around the edges, as shown in this close-up of the FE2D door corner.


[08-APR-09] We construct two Faraday enclosures with the help of cardboard boxes. The first uses aluminum foil as the conducting wall material and the second uses aluminum mosquito screen with 1.5-mm square holes. To test the effectiveness of the enclosures, we set up our 910-MHz SAW Oscillator (A3014SO) 5 m from our RF Spectrometer (A3008) and measured the power received by the spectrometer as we placed it in the enclosures.

ShieldingAntenna Power
Spectrometer Antenna Disconnected−80 dBm
Spectrometer Loop Antenna Unshielded−48 dBm
Foil-Lined Enclosure (poorly-sealed)−73 dBm
Foil-Lined Enclosure (well-sealed)−76 dBm
Aluminum Screen (poorly-sealed)−56 dBm
Aluminum Screen (well-sealed)−63 dBm
Table: Power Received for Various Shielding.

The power we received depended upon the exact position of the spectrometer, regardless of the shielding. We see a ±1 dB variation as we move the spectrometer by ±10 cm. But it appears that the foil enclosure gives us 28 dB attenuation and the screen enclosure gives us 16 dB. Sealing the foil and screen around the corners and lid of the enclosure is important: poor sealing can case a jump of 10 dB in the power inside the enclosure.

Figure: Foil Faraday Enclosure. The foil is taped onto a cardboard box 25 cm × 20 cm ×14 cm.

Whatever Faraday enclosure we use around our animal cages, it must be transparant and rugged. A foil shield is neither transparant nor rugged. The screen enclosure is both. It gives us 16 dB attenuation, which would drop the estimated −56 dBm ambient interference in the ION laboratory down to −72 dBm, and so increase our operating range there from 25 cm to around 200 cm.

Our aluminum mosquito screen was $1 per square foot. There are better meshes available, such as these. The vendor claims that their VeilShield gives 50 dB attenuation at 1 GHz. They sell it for $3 per square foot and it comes in 58-inch wide rolls.

[09-APR-09] We want to know if reflections inside a Faraday enclosure will cause more severe multi-path interference than outside a Faraday enclosure. We placed our spectrometer inside the foil enclosure. We attached an encapsulated A3013A to a stick. With the tip of the stick protruding from the enclosure, we rotated and moved the transmitter. The power measured by the spectrometer at the center of the transmitter's spectrum varied from −43 dBm down to −72 dBm. When we perform the same experiment outside the enclosure, the power varies from −33 dBm down to −78 dBm. It appears that our problems with multi-path interference and poor orientation are alleviated in the enclosure. Certainly, they are not exacerbated.

We placed the Data Receiver antenna in the foil enclosure. We placed a transmitter outside the enclosure at a range 30 cm from the antenna. We moved and rotated the transmitter. We obtained robust reception. There were precise locations where reception became intermittent. We placed the transmitter in the enclosure and the antenna outside the enclosure at a range of 30 cm from the transmitter. We obtained no reception.

This simple experiment proves that interference is limiting our operating range. Let us consider both stages of the experiment in turn.

When the antenna is in the enclosure, both the signal and the interference are attenuated by the Faraday enclosure. Let us assume the Faraday enclosure attenuates both signal and interference by 28 dB. The interference power drops to −88 dBm, on a par with the receiver's noise power (which is −90 dBm, see Noise and Interference). Our signal can drop to −4 dBm at the transmitter, −22 dB for 30 cm of air, −17 dB for unfavorable orientation, −10 dB for multi-path interference, −28 dB for the Faraday enclosure, which is −80 dBm. Our receiver works down to −76 dBm (see Demodulator). And so reception fails in a few precise orientations of the transmitter outside the Faraday enclosure, and is no worse than reception without the Faraday enclosure.

When the transmitter is in the enclosure, the signal is attenuated by 28 dB, but not the interference. Even with a precise, favorable orientation of the transmitter, we get no more than −54 dBm at the receiver. Our interference power remains at −68 dBm, so the minimum power required for reception is −56 dBm. When we place the transmitter in a random orientation, we will get far too little power, as little as −81 dBm, for reception. And so we obtained no reception at all the five or six times we placed the transmitter in the foil enclosure and closed the lid.

[We put a transmitter in a plastic bag and wrapped it in one layer of mosquito screen. We placed it in the foil enclosure with the receiving antenna. We obtained intermittent reception. We opened the enclosure and moved the transmitter, and repeated a few more times. We received anything from a 2% to 50% reception. This suggests that the mosquito screen is attenuating the signal by more than the foil enclosure attenuates the interference. A well-sealed Faraday enclosure made of mosquito screen should give us attenuation of 30 dB or more. At ION, such a enclosure would drop the −56 dBm interference down to −86 dBm, and our operating range would jump up to several meters. If we build a Faraday enclosure large enough to enclose four animal cages, it would be roughly 100 cm along its longest side. We can hope for prefect reception in all orientations and positions of the transmitter while implanted in an animal.

[13-APR-09] We apply an oscilloscope probe on points in the amplifier of our Demodulating Receiver (A3017). We know the gain of each stage the amplifier so we can convert peak-to-peak amplitude on the oscilloscope into power at the antenna input. We use 29 dB for the gain from RF to IF (see schematic) and 71 dB from RF to the output of U204. Our source of artificial interference was our 910-MHz SAW Oscillator (A3014SO). We put its 13-dBm output through a 20-dB attenuator and into quarter-wave wire antenna.

Receiver Transmitter Amplitude
and Point
Input Power Comments
none none 80 mV p-p @ U204 −86 dBm input noise
terminator and 1-m cable none 100 mV p-p @ U204 −84 dBm cable pick-up
antenna and 1-m cable none 600 mV p-p @ U204 −68 dBm ambient interference
bad message rate 3/s
antenna in foil enclosure
enclosure not grounded
none 130 mV p-p @ U204 −82 dBm attenuation 14 dB
antenna in foil enclosure
enclosure grounded and squeezed
none 80 mV p-p @ U204 −86 dBm attenuation >18 dB
antenna and 1-m cable −7 dBm, 910 MHz, 100 cm 130 mV p-p @ IF −40 dBm artificial interference
antenna in foil enclosure
enclosure not grounded
−7 dBm, 910 MHz, 100 cm 24 mV p-p @ IF −54 dBm attenuation 16 dB
antenna in foil enclosure
enclosure grounded
−7 dBm, 910 MHz, 100 cm 12 mV p-p @ IF −60 dBm attenuation 20 dB
antenna in foil enclosure
enclosure grounded and squeezed
−7 dBm, 910 MHz, 100 cm 220 mV p-p @ U204 −77 dBm attenuation 37 dB
terminator and 1-m cable −7 dBm, 910 MHz, 100 cm 200 mV p-p @ U204 −78 dBm cable pick-up
antenna in foil enclosure A3013A at 10 cm in enclosure 350 mV p-p @ IF −31 dBm 100% message reception
antenna in foil enclosure
enclosure grounded
A3013A at 10 cm in enclosure
−7 dBm, 910 MHz, 100 cm
350 mV p-p @ IF from A3013A
12 mV p-p @ IF from 910 MHz
−31 dBm
−60 dBm
100% message reception
antenna and 1-m cable A3013A at 10 cm
−7 dBm, 910 MHz, 100 cm
350 mV p-p @ IF from A3013A
130 mV p-p @ IF from 910 MHz
−31 dBm
−40 dBm
0% message reception
Table: Input Power and Reception for Various Antennas and Enclosures.

Our power measurements are accurate to ±2 dB. The amplifier gain is temperature-dependent and we must measure peak-to-peak amplitude on the oscilloscope screen. During the course of our experiments, we discovered that grounding the Faraday enclosure increased its performance by at least 6 dB. We used a BNC cable union at the point where the antenna cable enters the enclosure, and made sure that contact with copper tape connected the union to the enclosure. This gave us 6 dB more isolation from our 910-MHz interference. When we squeezed the enclosure with our hands, pressing together loose pieces of foil, we obtain up to 18 dB more isolation, bringing the total isolation provided by the enclosure to 37 dB. In the presence of −40 dB of 910-MHz interference, reception from an A3013A at range 10 cm goes from 0% with no isolation to 100% inside our foil enclosure.

[17-APR-09] We repeat some previous experiments with our foil enclosure, and increasing levels of ambient interference. We set up our 910 MHz, +13 dBm source with a 30 dB attenuator and an 80-mm antenna. At range 100 cm we receive −53 dBm with a Loop Antenna (A3015A). We place our antenna in the foil enclosure and receive −77 dBm. The foil enclosure is providing 24 dB isolation. Now we begin our experiments. At each step, we select a smaller attenuator for our 910 MHz source. We assume the ambient 910 MHz power around our enclosure increases in proportion to the decrease in our attenuator. We have the antenna in the foil enclosure connected to our Data Receiver (A3018). We hold a transmitter in the enclosure on a stick. We rotate and move the transmitter for one minute while monitoring with the Receiver Instrument. We calculate average, minimum, and maximum message reception over 1-s periods and record it in the table below.

910 MHz Attenuator
910 MHz Outside
Enclosure (dBm)
910 MHz In
Enclosure (dBm)
Average Reception
(% in 1 s)
Maximum Reception
(% in 1 s)
Minimum Reception
(% in 1 s)
33−56no enclosure621000
Table: Message Reception in Foil Enclosure with Ambient Interference Power. We are moving the transmitter from range 0 cm to 20 cm, rotating it, and pressing it up against the enclosure walls at random.

The foil enclosure allows a 20 cm operating range with up to −32 dBm ambient interference. The enclosure attenuates this −32 dBm to −56 dBm. Inside our foil enclosure, our operating range is 20 cm for −56 dBm of interference at the receiving antenna. We set up our 910 MHz source one meter from a receiving antenna with no Faraday enclosure. We added attenuators until the antenna received −56 dBm. We moved our transmitter around between 0 cm and 20 cm of the antenna, and monitored reception. The result is the final line in the table above.

Given the same interference power at the receiving antenna, reception from a subcutaneous transmitter appears to be slightly better inside a Faraday enclosure than outside. On average, reflections off the enclosure walls appears to help rather than hinder reception. The dominant effect of a Faraday enclosure is to attenuate ambient interference. In today's experiments, we obtained operating range 20 cm with −32 dBm interference outside the enclosure. We can obtain the same operating range without a Faraday enclosure only if ambient interference is lower than −56 dBm, or 0.4% of the power we can tolerate with a Faraday enclosure.

[29-APR-09] We have our prototype screen-walled Faraday enclosure. It measures 40 cm × 30 cm × 30 cm. The frame is made of 1.6-mm thick aluminum angle brackets. The walls are made of stainless steel VeilShield fabric. The fabric is taped to the frame with a variety of metal tapes.

Figure: Prototype Faraday Enclosure, with Steel Fabric Walls. The stick with a transmitter on the end shows how we manipulate a transmitter to test reception.

We perform some of the same experiments we did for the foil enclosure, to measure the attenuation of artificial interference by the enclosure. We turn on our 910-MHz transmitter and place our receiving antenna 100 cm away. We place the prototype enclosure around the antenna and make adjustments to its lid and walls.

None−35 dBm
No Antenna−65 dBm
Prototype Enclosure−59 dBm
Prototype Enclosure, double Al screen over lid−59 dBm
Prototype Enclosure, lid off−41 dBm
Prototype Enclosure, double Al screen under lid−59 dBm
Prototype Enclosure, single Al screen in front−60 dBm
Prototype Enclosure, double Al screen in front−60 dBm
Prototype Enclosure, double Al screen on left−61 dBm
Prototype Enclosure, single Al screen front and left−64 dBm
Prototype Enclosure, single Al screen front, right, and left−66 dBm
Prototype Enclosure, single Al screen front, right, top, and left−65 dBm
Prototype Enclosure, single Al screen in front−59 dBm
Prototype Enclosure−59 dBm
Table: Interference Power Received. The receiving loop antenna is 100 cm away from the vertical quarter-wave 910-MHz transmitting antenna. The front side of the enclosure faces the antenna.

The prototype enclosure on its own provides roughly 24 dB attenuation of interference. With extra layers of shielding on the sides, we can improve attenuation to 30 dB. Our experiment is not sensitive to further improvements.

910 MHz Outside
Enclosure (dBm)
(% in 1 s)
(% in 1 s)
(% in 1 s)
−68 76 100 12 RA outside enclosure, 10-mm TA in hand, range ≤40 cm
−68 99 100 93 RA 10 cm from corner, 10-mm TA in hand, enclosure lid off
−68 88 100 41 RA 10 cm from corner, 10-mm TA on stick, enclosure lid off
−68 100 100 96 RA 10 cm from corner, 50-mm TA in hand, enclosure lid off
−68 96 100 74 RA 10 cm from corner, 10-mm TA on stick, enclosure closed
−68 89 100 39 RA 10 cm from corner, 50-mm TA on stick, enclosure closed
−68 79 100 36 RA 10 cm from corner, 30-mm TA on stick, enclosure closed
−68 99 100 82 RA at floor center, 30-mm TA on stick, enclosure closed
−68 96 100 71 RA at floor center, 50-mm TA on stick, enclosure closed
−35 77 100 14 RA at floor center, 50-mm TA on stick, enclosure closed
−35 48 100 0 RA at floor center, 10-mm TA on stick, enclosure closed
−68 97 100 40 RA at floor center, 50-mm TA in fake rat, enclosure closed
−35 81 100 5 RA at floor center, 50-mm TA in fake rat, enclosure closed
−49 98 100 78 RA at floor center, 50-mm TA in fake rat, enclosure closed
−35 40 100 0 RA at floor center, 20-mm TA in fake rat, enclosure closed
−49 77 100 0 RA at floor center, 20-mm TA in fake rat, enclosure closed
Table: Message Reception in Prototype Enclosure. We use abbreviations: RA for receiving antenna, TA for transmitting antenna, and enclosure for prototype enclosure. In each experiment, we move the transmitter around at random in the full volume of the enclosure, even touching the sides and floor.

We simulated implanting the transmitter in a rat body by tying a latex glove filled with water to our stick, as shown below.

Figure: Fake Rat.

One thing that stands out from our observations is that the receiving antenna performs better at the center of the enclosure than in the corner.

[05-MAY-09] Pishan at ION (Institute of Neurology) made her own Faraday enclosure out of foil and a cardboard box. She placed an animal cage within the foil enclosure. The animal cage contained a rat with an A3013A implanted transmitter. She put an A3015A loop antenna inside the enclosure with the animal and the cage. The antenna was close to one wall, so as to make space for the cage. The cable shield was not connected to the foil around the box.

The foil enclosure blocked out all signals from transmitters outside the enclosure. Reception from the transmitter inside the enclosure was 85% in one test and 91% in another. Reception outside the enclosure with these same transmitters was 95% and 80% respectively.

Figure: Pishan's Foil Faraday Enclosure

Because the antenna is close to a wall of the box, we expect reception to drop from 100% to 90%, as it did when we moved the antenna into the corner of our prototype enclosure. But we expected reception outside the enclosure to be poorer, so as to give a contrast with the reception inside the enclosure. On this day, however, reception from both tested transmitters was good outside the enclosure. We will see what happens on other days.

[07-MAY-09] Pishan put the antenna in the center of the enclosure and released each of four animals into the enclosure in turn, like this. We received 0% from transmitters outside the enclosure, except when No12 was in the enclosure and No1 was outside, when we received 14% from No1 through the foil walls. Reception from No12 inside the enclosure was 93%. We saw 100% reception most of the time, but occasional seconds would go by with 0% reception. This loss contrasts with the loss we observe outside the enclosure, where reception is 30% to 90% all of the time. Reception from No10 was 94% in the enclosure, from No9 was 84%, and from No1 was 96%.

[21-MAY-09] We measure the mains hum on free-floating transmitter leads inside and outside our prototype Faraday enclosure. We see 2 mV p-p hum outside and less than 20 μV of hum inside the enclosure. When we put the enclosure's lid on, we see the following signal on the transmitter's X input.

Figure: Static Field as Lid Closes

We conclude that the Faraday enclosure gives us a factor of 100 drop in mains hum amplitude, or 40 dB attenuation.

[10-JUN-09] Today we completed our first large Faraday enclosure, which we call our Big Enclosure. The big enclosure is 70 cm × 70 cm × 40 cm, and is large enough to fit two animal cages and their water bottles with room to spare. As in our prototype enclosure, the walls of the big enclosure are made of VeilShield fabric and its edges are made of 1.6-mm aluminum L-brackets. The lid has been simplified, so that it takes up less height. We describe the construction of the big enclosure in our Assembly Manual.

Figure: The Big Enclosure

We set up a 910-MHz interference source and obtained the following power at our receiving antenna as we moved the antenna in and out of the enclosure, put on the lid, and sealed up gaps beneath the lid with aluminum slats.

None−44 dBm
No Antenna−82 dBm
Big Enclosure, original lid−57 dBm
Big Enclosure, lid gaps sealed−68 dBm
Big Enclosure, adjust lid− 82 dBm
Big Enclosure, adjust lid− 73 dBm
Big Enclosure, adjust lid− 63 dBm
Table: Interference Power Received. The receiving loop antenna is 6 m away from the vertical quarter-wave 910-MHz transmitting antenna.

With the gaps in the lid reduced with aluminum slats, the enclosure gives a minimum of 20 dB isolation, and with perfect lid adjustment, up to 40 dB isolation. The 20 dB variation in isolation with lid position is affected also by the location of the experimenter's bodies, and contact with the fabric walls.

We place a transmitter in the enclosure. We get good reception with the lid off, but when we put the lid on, we get no signal. We move the transmitter. We get a week signal. We continue with these movements and observations, and find that reception inside the enclosure is poor, although it is unaffected by turning on our external 910-MHz transmitter. We supposed that internal reflections in the big enclosure would be less severe than in the prototype enclosure, which was half the length and one tenth the volume. But we appear to have been mistaken.

We inserted a second loop antenna into the big enclosure and powered it with our Modulating Transmitter (A3014MT). We sweep the frequency of the transmitter through the 900−930 MHz band. We observe the IF amplitude in our Demodulating Receiver (A3017). This amplitude is proportional to the product of the power received by our antenna and the loss in our SAW band-pass filter. We obtained the following measurements with our oscilloscope.

Figure: Reception with Frequency. In each shot, top trace is IF in the receiver and bottom trace is TUNE in the modulating transmitter. Top-Left: no antennas, connection by 30 dB attenuator. Top-Right: two loop antennas close together in enclosure, lid off. Bottom-Left: two loop antennas, lid on. Bottom-Right: quater-wave transmits to loop, lid on.

With the 30dB attenuator, we see the response of the SAW filter in our receiver. With the two loop antennas and the lid off, we see a similar frequency response, with no notches. As soon as we put the lid on, the notches appear. As we move the antennas around, adjust the lid, and press upon the walls, the notches move around rapidly, and form complex patterns. Often the notches join up and we get holes in the frequency response. With a quarter-wave transmitting antenna, the holes become more severe.

With its highly reflective walls, and its dimensions greater than one wavelength of our signal, the big enclosure is a resonant chamber in which standing waves form as a matter of course, and destructive interference almost always occurs at one frequency or more frequencies in the 900−930 MHz band. We estimate that the narrow reception holes are as deep as 30 dB, so that we are suffering a 30 dB loss of power at one particular frequency because of multi-path interference.

We placed baffles made of mosquito screen in the corners of the enclosure. We observed fewer holes in the frequency response, but still enough to compromise reception from our transmitters. All we can do with our reflecting screens is re-direct the waves. But we observe that we get good performance inside the enclosure when we take the lid off. Radio waves leaving the top of the enclosure don't come back, or hardly any of them do. Removing the lid is equivalent to placing a microwave absorber in the lid. We removed the lid and moved a transmitter into all parts of the enclosure, including the corners, and rotated it rapidly and at random for sixty seconds. We obtained 98% message reception. With an absorber in the lid, we would obtain the same performance, so we are confident that such an absorber will solve our reflection problems.

[12-JUN-09] We repeated our interference measurements for the Prototype Enclosure. You will find our observations in the Prototype Enclosure section. We do not see sharp holes in reception with frequency. Our observation is consistent with our claim that it is the sharp holes that destroy reception of our transmitter signals. We conclude that internal reflections, leading to standing waves and other interference phenomena, take place far more easily in the Big Enclosure than the Prototype Enclosure. The Big Enclosure has sides that are a little over two wavelengths long. The Prototype Enclosure has sides that are one wavelength long.

[19-JUN-09] We received two 61-cm × 61-cm pieces of Eccosorb microwave absorbant foam. The AN-75 is 3 cm thick and the AN-77 is 6 cm thick. The black surface of the AN-75 reflects only 4% at 900 MHz, and the AN-77 reflects only 2%. What is not reflected is absorbed. Each foam pad consists of three layers of carbon-impregnated conducting layers. We placed the foam in the enclosure, either on the bottom or attached to the lid, and measured reception with a transmitter on a stick for each arrangement. We are delighted with our results.

Foam Arrangement Average
No foam33
AN-77 on bottom, black side up, antenna 25 cm from wall94
AN-77 on bottom, black side up95
AN-77 on bottom, black side up97
AN-75 on bottom, black side up93
AN-75 on bottom, black side down91
AN-75 under lid, black side down97
AN-75 under lid, black side down96
No foam, lid off90
No foam, lid off, −40dBm 910 MHz outside35
AN-75 under lid, black side down, −40dBm 910 MHz outside93
AN-75 10 cm below lid, black side down, −40dBm 910 MHz outside83
No enclosure, −40dBm 910 MHz outside0.3
No enclosure88
AN-77 beneath antenna, black side up, no enclosure89
AN-77 on top of lid, black side down18
AN-75 under lid, black side up95
AN-77 on bottom, black side down97
AN-77 on bottom, black side up, AN-75 under lid, black side down96
AN-75 on bottom, black side up85
AN-75 glued under lid, black side up, AN-77 on bottom, black side up97
AN-75 glued under lid, black side up87
AN-75 glued under lid, black side up, antenna 10 cm above bottom87
No foam52
AN-77 on bottom, black side down88
AN-77 on bottom, black side up98
AN-77 on bottom, black side down89
AN-77 on bottom, black side up98
AN-75 on bottom, black side up91
AN-75 on bottom, black side down81
AN-75 on bottom, black side up92
AN-77 glued under lid, black side down98
AN-75 on bottom, black side up96
No foam64
Carbon Sheet, 1 layer on bottom81
Carbon Sheet, 2 layers on bottom81
Carbon Sheet, 10 bottom82
Carbon Sheet, 8 bottom, 1 left and right94
Carbon Sheet, 2 bottom, 1 left, right, front, back98
AN-75 on bottom, black side up97
Table: Message Reception in Big Enclosure. In each experiment, we move the transmitter around at random in the full volume of the enclosure, even touching the sides and floor. Some rows are repeat experiments. By default, the antenna is in the center of the enclosure floor.

We looked at the reception versus frequency traces on the oscilloscope, and saw no sign of sharp notches or dead spots whenever we had one of the pieces of absorbant foam in the enclosure. We can find orientations when reception across the 902−908 MHz spectrum is greatly reduced, but these reductions can be explained by the relative orientation of the transmitting and receiving antennas alone.

We turned on 910 MHz interference power and compared the power we receive inside and outside the Big Enclosure when we have the AN-77 foam sitting on the bottom, with the antenna on the foam at the enclosure center. Received power dropped from −44 dBm to anything between −74 dBm and −80 dBm as we moved the lid around and changed the position of our own body. We conclude that the Big Enclosure, when equipped with absorbant foam, gives us 30-dB isolation. If keeps out 99.9% of all RF power in our spectrum.

Reception inside the Big Enclosure is better than reception outside, even when we don't generate our own interference.

[12-JUN-09] Reception is poor inside our Big Enclosure. We observed sharp holes in reception with frequency, which we believe are caused by internal reflections within the enclosure. Today we looked at the reception with frequency for the prototype enclosure. The result is the oscilloscope shots below.

Figure: Reception with Frequency. In each shot, top trace is IF in the receiver and bottom trace is TUNE in the modulating transmitter. Top-Left: two loop antennas close together in enclosure, lid off. Top-Right: lid on. Bottom-Left: lid on, antennas arranged to give biggest holes in reception. Bottom-Right: quater-wave transmits to loop, lid on.

Although we see holes in the reception with frequency, they are not nearly as sharp nor as deep as those we see in the same traces for the Big Enclosure.

[25-JUN-09] We glued the AN-75 (3 cm) black side up on the underside of a new lid. The lid does not perform well. We get only 87% reception. We repeat our experiments with the AN-77 (6 cm), flipping it from black side up to black side down. The black side is the conducting side, which is suppose to be the last layer of foam reached by an absorbed wave. Our understanding, and that of the manufacturer of the foam, suggests we will get best performance with the black side facing the enclosure boundary. In previous experiments, we noticed little difference between the orientations. At that time, our test transmitter battery voltage was above 2.5V. But today the voltage is down to 2.3V. With the weaker transmission power, the difference between the foam orientations is clear and repeatable. The enclosure performs better with the black side facing away from the enclosure boundary.

We removed the AN-75 from the lid, where it had been glued securely, and placed it in the bottom of the enclosure. We obtained 91% reception with the black side up, and 81% with the black side down. We resolve to glue the AN-77 black side down to the lid, and try again.

[29-JUN-09] With the AN-77 (6 cm) glued to the lid, black side down, we get 98% reception from our test transmitter, whose battery voltage is now 2.1V. We will ship this Faraday enclosure to ION.

[08-JUL-09] Our hope is that we can connect multiple Faraday enclosures to a single receiver, by combining the signals received from the antenna in each enclosure. We connected our Big Enclosure and our Prototype Enclosure to a single Data Receiver (A3018). In the Big Enclosure we had an AN-75 foam absorber, black side up in the bottom. In the Prototype Enclosure we placed a single thickness of microwave absorbant sheet (but note that this material does not actually absorb microwaves, because it is too thin to do so). We moved a transmitter around in the Big Enclosure and kept a transmitter in the Prototype Enclosure in a jar of water. We tried various ways of combining the signals.

Antenna Junction Reception in
Prototype Enclosure (%)
Reception in
Big Enclosure (%)
T-Junction Only10097
Two 3dB Attenuators10098
Two 6dB Attenuators9996
Two 12dB Attenuators10093
Two 18dB Attenuators10079
T-Junction Only, −40 dBm Outside10040
Two 3dB Attenuators, −40 dBm Outside10067
T-Junction Only, −40 dBm Outside10037
Two 3dB Attenuators, −40 dBm Outside9983
T-Junction Only, −40 dBm Outside10064
Two 6dB Attenuators, −40 dBm Outside10091
Two 12dB Attenuators, −40 dBm Outside10094
T-Junction Only, −40 dBm Outside10081
T-Junction Only, −40 dBm Outside9979
ZB4PD1-2000+ Combiner9899
ZB4PD1-2000+ Combiner, −40 dBm Outside93100
Table: Message Reception with Two Enclosures. We move a transmitter around inside the Big Enclosure and leave another transmitter stationary in a jar or water in the Prototype Enclosure.

The results with no attenuators, just a T-junction joining the two antenna cables, and outside interference at 910 MHz, are varied. We obtained between 37% and 81% reception. But the improvement in reception with attenuators in the presence of interference is significant. We obtained 94% reception from the moving transmitter with 12 dB attenuators and −40 dBm interference.

We have some microwave absorbent sheet made of carbon fibers. This material is only half a millimeter thick. It costs $30 per square meter. We placed layers of the sheet in the bottom of the enclosure and tested reception. One layer in the bottom improves reception from 64% to 81%. Multiple layers do nothing to improve reception. With one layer of sheet on the bottom and four side walls, we obtain 98% reception.

Figure: Faraday Enclosure with Animal Inside

[15-JUL-09] Pishan at ION has implanted transmitter No11 in a rat, put the rat in a plastic cage, and put the plastic cage within a big enclosure we sent to ION by air freight. Over the course of five minutes, with the rat moving around, she obtained an average of 99% reception, with reception of at least 80% of messages during 98% of the three hundred seconds, and a minimum reception during one particular second of 53%. This is the best reception we have ever obtained from a transmitter, implanted or not, in Boston or London. We conclude that the Big Enclosure with the 6-cm absorber is effective.

[01-SEP-09] We return from a visit to ION. Reception from two animals in a single Faraday enclosure remains perfect. You can see the two animals here. Pishan tells us she would like the enclosure to be 10 cm shorter so that she can lean into it more easily. The back side of the lid has broken off and remains held in place by aluminum tape. The glue joints broke on both sides and the small brackets fell off. We fixed a hole in the mesh walls with tape.

[09-SEP-09] We combine the antenna signals with the help of a Four-Way Antenna Combiner (AC4A), which consists of a ZB4PD1-2000 four-way power combiner and two BTRM-50 fifty-ohm terminators, all from Minicircuits. The transmitter in the jar in the prototype enclosure has been running continuously since 08-JUL-09, and it's battery voltage has dropped to 2.2 V. We are not surprised that reception drops to 93% in the presence of interference. But the transmitter in the big enclosure has a brand new battery. Reception is 100% even with interference.

The advantage of the combiner is that signals from one input do not travel out along the other inputs. Each antenna is isolated from the other, even as their signals are added in the combiner. There is a power loss of −6 dB in a 4-way combiner, but our earlier experiments revealed that attenuation of up to 12 dB does little to degrade reception.

We package the combiner, two terminators, and a 30-cm BNC cable in our Four-Way Antenna Combiner (AC4A). Connect two or more antennas to the input sockets on the power combiner. Cover any unused sockets with terminators. Connect the output socket on the combiner to the receiver input.

[05-NOV-09] We assist with the completion of an FE2B enclosure at ION and set up two cages with an antenna combiner. There are two animals in one cage and one in another (No6, No14, and No5 respectively). We place two transmitters in bags upon the floor of one cage (No1 and No2), to assure a weak signal, and a third on top of a cage (No3). Transmitter No6 has been showing problems with reception for some time. We obtain the following graph.

Figure: Reception from Two Enclosures

We see that the combination of two cages is effective.

[07-DEC-09] At ION, reception from two enclosures with the AC4A has been poor, as you can see in the second table of the Trials section in Flexible Wires. Some transmitters show average reception of 50%. Today, Pishan connected one enclosure at a time to a Data Receiver (A3018) and reception rose for all transmitters to 98% or higher. We resolve to repeat our experiments with antenna combiners in our own lab.

Antenna Junction Reception (%)
Single Enclosure, Direct, Q0126, 3 cm Foam98% (entire volume)
Single Enclosure, Direct, Q0126, 3 cm Foam90% (corners and walls)
Single Enclosure, Direct, Q0126, 6 cm Foam99% (entire volume)
Single Enclosure, Direct, Q0126, 6 cm Foam99% (corners and walls)
Single Enclosure, Combiner, Q0126, 6 cm Foam99% (entire volume)
Single Enclosure, Combiner, Q0126, 6 cm Foam97% (corners and walls)
Two Enclosures, Combiner, Q0126, 6 cm Foam99% (entire volume)
Two Enclosures, Combiner, Q0126, 6 cm Foam98% (corners and walls)
Two Enclosures, Combiner, P0193, 6 cm Foam100% (entire volume)
Two Enclosures, Combiner, P0193, 6 cm Foam99% (corners and walls)
Two Enclosures, Combiner, P0193, 6 cm Foam96% (entire volume), 98%, 98%, 99% (stationary)
Two Enclosures, Combiner, P0193, 6 cm Foam98% (entire volume), 96%, 97%, 100% (stationary)
Two Enclosures, Combiner, P0193, 6 cm Foam, −40dBm outside90% (corners and walls), 99%, 99%, 100% (stationary)
Two Enclosures, Lids Off, Combiner, P0193, 6 cm Foam, −40dBm outside0% (corners and walls), 0%, 0%, 0% (stationary)
Two Enclosures, Lids Off, Combiner, P0193, 6 cm Foam, −40dBm outside0% (entire volume), 0%, 0%, 0% (stationary)
Two Enclosures, Lids Off, Combiner, P0193, 6 cm Foam99% (entire volume), 98%, 99%, 101% (stationary)
Table: Message Reception with One or Two Enclosures. We move a single active transmitter around inside the Big Enclosure. If other transmitters are active, they are stationary. Cables are 240 cm from enclosure to combiner or receiver. Others are 30 cm.

In these tests, we introduce the exploration of "corners and walls" of an enclosure as compared to the "entire volume". In the past, we have explored the entire volume, spending only a quarter of the time close to the corners and walls. In our new "corners and walls" tests, the transmitter spends its entire time touching or almost-touching the corners and walls. Thus we obtain a more conservative measure of signal loss.

Figure: Top-Loading Faraday Enclosures Assembled from Kits of Parts. Each contains two animal cages with water bottles. Marked are (1) a prototype version of the FE2B with mesh side walls, (2) an FE2A, (3) an FE2B. Photograph taken at ION 04-NOV-10.

The RF amplifier in the Data Receiver (A3018A) has a tendency to oscillate. We try two receivers: Q0126 and P0193, to see if one of them will show instability with the antenna combiner. But both receivers perform well.

We see the Faraday enclosures isolating the transmitters from external −40dBm interference. When we remove the lids, reception drops to zero. With the −40dBm removed, reception is good with the lids off, although we see evidence of bad messages from ambient −68dBm interference. When we have multiple transmitters, two are in the second enclosure. Thus We cannot reproduce the poor reception observed at ION.

[16-OCT-12] We are presented with a new reception problem: we want the ISL to receive 146-MHz commands from an antenna outside the animal, while at the same time transmitting 910-MHz message to additional antennas outside the animal. The 146-MHz signal has wavelength 2 m. An effective, omnidirectional antenna is roughly 30 cm high, so that it will almost touch ceiling and floor of the enclosure. Under such circumstances, we expect severe multi-path interference.

We propose to use a single 146-MHz command antenna on a laboratory table. Our hope is that the single 146-MHz transmission will be powerful enough to be received reliably by all ISLs on the table. At the same time, we will distribute about this table eight independent 910-MHz data antennas. Our hope is that the transmission from any one ISL or SCT will be received correctly by at least one of the eight antennas.

This single-table transmitter system must at the very least be isolated from other ISL or SCT systems on nearby tables. At the same time, an isolating Faraday enclosure will provide immunity to unknown quantities of external interference. We propose to try out the Faraday enclosure shown below. The canopy is rather like a mosquito net, made out of RF reflecting fabric. Beneath the canopy we propose to install an RF absorbing mat that will serve both to block incoming RF and to dampen the reflection of internal RF.

Figure: A Canopy Enclosure. The canopy is made out of conducting, transparent fabric. The base mat is made out of microwave absorbing sheet, if such can be found.

[30-NOV-12] We purchased a variety conducting fabric from LessEMF.com. We set up our laptop to measure power received from our home wireless router. We wrapped it in various fabrics. We placed a base underneath of microwave absorber and covered it with various fabrics.

ArrangementReceived Power
No Fabric −32
Resistive Sheet wrap-around −62
Resistive Sheet base only −32
Resistive Sheet base SwissShield Daylight single-layer cover −47
Resistive Sheet base SwissShield Daylight double-layer cover −57
Resistive Sheet base Canopy Mesh Fabric single-layer cover −52
Canopy Mesh Fabric wrap-around with Resistive Sheet inside −52
Wall Shield wrap-around −55
Aluminum foil wrap-around −55
Table: WiFi 2.4-GHz Power Loss Due to Shielding. The attenuation of 2.4 GHz by the shielding may be far greater than the power loss because the router can increase its output power to compensate for a poor connection with the monitoring laptop.

The wrap-around with folded sides made of the Wall-Shield foil gave us a 20-dB drop in received power. This observation is inconsistent with our past experience with foil as a reflector of RF power. We included some resistive sheet within the wrap-around and this did nothing to improve isolation. We suspect that the router is increasing its output power when we wrap the lap-top, thus obscuring the isolation provided by the fabric. Nevertheless, we conclude that the SwissShield Daylight fabric will be sufficient to give us a 15-dB isolation at 2.4 GHz, which is what is suggested by its data sheet. At 1 GHz, we can hope for 20-dB isolation when combined with the same resistive sheet as a base mat.

The canopy must provide some degree of isolation from mains hum. Our existing steel and aluminum Faraday enclosures, with their connection to the antenna cable shield, provide 40 dB isolation from mains hum. As a result, we see no observable mains hum in our EEG recordings. This greatly facilitates EEG analysis, because we don't have to worry about mistaking mains hum for neurological activity. Fabric that provides isolation against RF may not provide isolation at mains hum frequencies (50 Hz, 60 Hz, and their first few harmonics). We placed a transmitter and antenna in a box on a table, with an AC power cord running nearby. We experimented with various fabrics and enclosures. We placed fabric over the power cord but under the box. We wrapped the box in fabric. We measured the amplitude of mains hum in each case.

ArrangementMains Hum
(mV rms)
No Fabric 9.6
SwissShield Daylight base only 0.52
SwissShield Daylight wrap-around 1.1
Canopy Mesh Fabric base only 0.22
Canopy Mesh Fabric wrap-around 1.0
VeilShield and Aluminum FE2B Enclosure <0.015
Table: Mains Hum Isolation by Fabrics.

We see that wrapping our transmitter in fabric that is not connected electrically to the coaxial antenna cable's shield causes an increase in mains hum. The SwissShield Daylight fabric uses strands of copper coated in cotton. Its surface is non-conducting. We will not be able to connect it to a cable shield. The Canopy Mesh Fabric, on the other hand, has a metallic surface and is a fine conductor. The resistive sheet is not conducting upon its surface, but if we peel back the thin plastic coating, we get to the carbon fibers that constitute the absorber. The resistance of the carbon layer is roughly 3 Ω per square. We plan to make a conducting enclosure out of mesh fabric and resistive sheet.

[24-JAN-13] We erect the first Faraday Canopy, shown below. The walls and ceiling are made of canopy mesh fabric. The base is made of a layer of resistive sheet, called microwave absorbing sheet by its manufacturer, on top of a layer of kitchen foil. The conducting walls, ceiling, and base reflect incoming microwaves. The resistive sheet we hope will absorb microwaves that manage to penetrate the conducting enclosure.

Figure: The Prototype Canopy Enclosure. Within the enclosure is its designer and creator, holding the first Implantable Lamp (A3024Y). The lamp is flashing in response to 146-MHz RF power emitted by the half-wave antenna in front of her.

To get a better idea of the size and shape of the enclosure, we invite you to view the following short video.

Figure: Movie Showing the Canopy in the OSI Office. The movie is an mpg file, which Microsoft Windows won't play for you unless you install an MPG player application.

We tucked the base of the canopy under the black resistive sheet so as to make a fragile but effective contact between the walls and the aluminum foil of the base mat. We placed a few extra pieces of black resistive sheet within the enclosure. We ran the cable of our RF spectrometer into the enclosure and tried to get an idea of how effective the enclosure was at blocking external interference in the 900-930 MHz band.

Figure: Peak Power Spectra Within and Outside the Enclosure. We use the RF Spectrometer (A3008C) to make our measurements. The vertical scale is logarithmic received power, −100 dBm to −20 dBm (0.1 nW to 10 μW) with 10-dB marks. The horizontal scale is linear frequency 880-950 MHz with marks every 5 MHz from 900-930 MHz. With no antenna we get a trace along the −80 dB line. Green: antenna in enclosure. Blue: antenna outside enclosure. Pink: outside at 3-m range with 910-MHz interference source turned on. Yellow: antenna inside the enclosure at 2-m range. Red: antenna inside the enclosure at 2-m range and double-layer of black resistive sheet.

We see cell-phone interference in the 860-900 MHz band. The enclosure appears to give us around 12 dB of isolation to these signals. There is a peak at 927 MHz that is hardly attenuated at all by the enclosure. We generate 910 MHz with a SAW Oscillator (A3014SO). With the antenna at 3 m outside the enclosure we receive 7.0 dB more power than with the antenna at 2 m inside. The difference suggests the enclosure offers 10 dB of isolation at 910 MHz. Our FE2B enclosure gives us 30 dB isolation.

According to the manufacturer's data sheet, the canopy mesh fabric should provide over 40 dB of reflection loss at 900 MHz. Our foil enclosure gave us 28 dB isolation, so we know that our foil base layer must give us at least 28 dB reflection isolation. Our base has in addition a layer of absorbant sheet which we know from our own measurements above will give us 30 dB isolation at 2.4 GHz, and from the data sheet should give us 27 dB isolation loss at 1 GHz. We note that this sheet does not isolate by absorbing, despite the manufacturer's claims, but rather by reflection.

Let us suppose that 99% of external 910-MHz power reflects from the sides of our canopy. The remaining 1% enters and reflects from the walls. Upon each reflection, it loses only 1% of its power, so ten reflections will hardly alter its strength. If ten reflections are likely, then the same RF photon will have ten chances to interact with our antenna. Only 1% of RF power enters, but it is ten times more likely to be detected. Thus our isolation is 90% rather than 99%.

[30-JAN-13] We apply 32 dBm of 146 MHz to a vertical half-wave antenna on the floor of the canopy. We check reception with a tuned A3024A Implantable Lamp. We obtain 100% reception throughout the enclosure. We drop the antenna power to 10 dBm and obtain 100% reception except when we bring the A3024 loop antenna within a few centimeters of the absorber on the floor, with the coil oriented in a vertical plane facing the transmitting antenna. We restore 32 dBm to our antenna and move the A3024A receiver outside the enclosure. At range 10 cm from the wall, reception is roughly 50%, which is far inferior to reception we obtain on the other side of the wall at almost the same range from the antenna, but with 22 dB less power transmitted. This observation suggests that the walls of the enclosure are providing at least 22 dB isolation at 146 MHz.

[13-MAY-13] We have added a floor of copper taffeta fabric and a door made with zippers. The Canopy Enclosure is now a Tent Enclosure. The tent is a complete, conducting enclosure when the zip door is closed. We add two socks to bring in cables. Each sock is 50 cm long and 10 cm in diameter. Its outer surface is canopy mesh fabric and its inner surface is absorbant sheet (which does not in fact absorb, but merely reflects, although we had not come to this realisation at the time we performed this experiment). Inside, we hang two panels of the same black sheet from the back and side walls, each panel 1 m wide and 1.5 m high. We use our spectrometer to measure the power picked up by an antenna in the tent.

Figure: Peak Power Spectra Within and Outside the Faraday Tent. Blue: antenna disconnected. Red: Antenna connected. The bump at 925 MHz is −70 dBm. Green: Transmitter near antenna.

The interference at 925 MHz has power −60 dB outside the tent and −70 dBm inside. If we turn on our 910 MHz reference source, we get −30 dBm at range 3 m with the tent door unzipped and held open. With the door zipped shut we get −40 dBm. Both measurements are consistent with interference attenuation of 10 dB. We take out our Prototype Enclosure and place it around the antenna inside the Faraday Tent, with the tent door open, and one layer of so-called absorbant sheet on the floor of the Prototype Enclosure. We get −48 dBm. We add a bundle of folded so-called absorbant sheet to the Prototype Enclosure and get −58 dBm.

We take a A3013P transmitter with a bent-wire antenna and an external battery and test reception inside the Faraday Tent over one-minute periods. With the transmitter next to the antenna we get 98.5% reception. We move at random throughout the tent, at ranges up to 2 m, and obtain 75%. At ranges up to 35 cm we get 89%. We repeat at 35 cm and get 83%. We add an A3021B active antenna combiner to the signal path and get 85% for ranges up to 2 m. We turn the WiFi off and get 87% up to 2 m. We turn phones off as well and get 84% up to 2 m and 91% up to 35 cm. In this last test, robustness is 88%.

[14-MAY-13] We set up the 910 MHz source 5 cm outside the wall of our Prototype Enclosure and measure power at 910 MHz with the Spectrometer (A3008B). The antenna inside the enclosure is 10 cm from the enclosure wall. We are using veil shield walls, and a resistive sheet described by the supplier as microwave absorbant sheet, even though it is too thin to absorb our 910-MHz radio waves. It does, however, act as a fine reflector.

ArrangementReceived Power
No enclosure −29
Enclosure with no absorber −37
Enclosure with absorber in lid −42
Enclosure with no absorber −37
Enclosure with absorber in floor −43
Enclosure with absorber in floor and lid −43
Enclosure with absorber in floor and lid,
cable shield isolated from enclosure walls
Enclosure with absorber in floor and lid,
re-connect cable shield to enclosure
Move antenna inside −42
Absorber top, bottom, side, back −43
Disconnect antenna cable from enclosure −62
Disconnect antenna cable from spectrometer −70
Re-connect antenna cable to spectrometer −61
Re-connect antenna to enclosure,
four absorbers in enclosure
Move antenna −52
Move antenna −54
Move antenna −45
Move antenna −51
Disconnect antenna from enclosure −66
Re-connect antenna, wrap entire enclosure in absorber. −68
Unwrap. −48
Remove interior absorbers. −47
Table: Attenuation by Prototype Enclosure, Revisited. In all cases the antennas are separated by 15 cm. The power source is 910 MHz, +13 dBm into a quarter-wave wire.

The most effective shielding in the above experiment is the double-wall of absorber wrap on the outside and veil shield on the inside, which gives the same 910 MHz power as when we disconnect the antenna from the spectrometer altogether.

[21-MAY-13] We set up the 910 MHz source 10 cm outside the wall of our Prototype Enclosure and measure power at 910 MHz with the Spectrometer (A3008B). The antenna inside the enclosure is 10 cm from the wall. The floor of the enclosure is filled with AN-75 absorber, black side up and cut to size. We try a selection of materials for the wall between the two antennas.

Figure: Attenuation by Various Wall Materials. We give the power received by the antenna, as measured by the spectrometer, for various fabrics placed between them. The rightmost column is the attenuation by the fabric wall, as compared to the power we received with no wall in place.

The "Bopp 11523" fabric is made by G. Bopp & Co. AG. It is a 304 stainless steel mesh made of 30-μm diameter wire on a 250-μm pitch, so that 78% of the are is open. This fabric is as light as the veil shield fabric, but stiffer and tougher. It is available in 1.2-m wide (48") rolls and costs $2.72 per square foot. Its performance appears to be better than all other fabrics except the copper taffeta, which is not transparent.

The manufacturer of the absorbant sheet specifies "shielding performance" of 28 dB at 1 GHz, and suggests that the absorbing power of the material will "scavenge any RF which might leak in". Our observations (see above and below), however, are consistent with 28 dB shielding through reflection and 0 dB through absorption, which is consistent with the theory of electromagnetic wave propagation and absorption. The thickness of the sheet is too small compared to the radio wavelength for there to be any significant probability of absorption.

[24-MAY-13] The Design and Engineering facility at the Institute of Neurology (ION) has designed and built a front-loading version of our big enclosure, which we will call the FE2C. The photograph below shows a single FE2C at ION.

Figure: The FE2C Enclosure, Built at ION.

The walls are veil shield mesh, as in the FE2B. Glued to the lid is an AN-77 absorber (see Absorbers) with the gray face exposed, in accordance with the manufacturer's instructions. Jon Henton of ION tells us that the absorbers, which he ordered directly from the manufacturer, came with an adhesive backing on the black face. The front face is hinged along the bottom edge to make a door. The door is held firmly in place by two locking nuts on either side. When open, the door is held horizontal by a wire. This design has been in use at ION for almost a year, and provides reliable reception. The great advantage of the front-loading enclosure is that they may be stacked upon one another, as shown here. The five-enclosure stack is on wheels so that it may be rolled around the animal laboratories. All parts were purchased, machined, and assembled at ION without assistance from OSI.

[21-JUN-13] We set up the canopy with two anti-static mats covering the floor. With the weight of the mats holding down and extending the floor, the canopy hangs well from the ceiling and is spacious inside. We run a coaxial cable through the floor-level cable sock and tape the cable shield to the conducting floor.

Figure: Interior of the Canopy. Rubber anti-static mats on the floor. Three AN-77 absorbers, gray side out, lean against the Canopy Mesh Fabric back wall. The floor under the mats is Copper Taffeta Fabric.

We have six AN-77 Eccosorb absorbers to use in our experiments. We begin by placing three against the back wall, black side facing the interior. We place the pick-up antenna of a Data Receiver (A3018C) on a stool in the center of the enclosure, as shown above, and Subcutaneous Transmitter (A3018A) No23.3 on the stool while we move around the interior of the enclosure. Reception is over 99.%.

We hold 23.3 in one hand and move it back and forth along the wall of absorbers, rotating and translating at 0-50 cm from the wall. The transmitter is 50-150 cm from the antenna. Reception is 76%. We repeat with un-encapsulated transmitter 24.5 and obtain 64%. Meanwhile, 23.3 on the stool gives 99.6%.

We place the stool up against the absorber wall. The antenna is 30 cm from absorber surface. We hold 23.3 and move it 0-30 cm on either side of the antenna, as if there were animal cages on either side. Reception is 95%. We add three more absorbers along the side wall, also black side facing the interior and repeat. We get 93%.

Rotate all six absorbers so gray side faces interior and repeat experiment 0-30 cm from antenna. Get 97.3%. Remove the three absorbers on the side wall and get 96%. Return antenna to center and move 0-50 cm from wall, 50-150 cm from antenna, holding 23.3. Get 80%. Restore the three absorbers to the side wall, gray side facing interior, and get 87%.

We place our 910 MHz +13 dBm source outside the zip-door of the canopy. The spectrometer antenna is on the stool in the center of the room. The antenna cable runs into the enclosure through a sock and is grounded by aluminum tape. We measure the attenuation of the 910 MHz power as we open and close the zip door. We arrange the absorbers in various ways along the back and side walls, and behind the antenna. We unplug the antenna and the antenna cable from the spectrometer to obtain a background. The canopy appears to provide no more than 6 dB of isolation.

[24-JUN-13] We return and bring the entire spectrometer and antenna cable into the canopy, running a single shielded LWDAQ root cable out to the LWDAQ Driver. We move the 910 MHz source to a place 50 cm outside one of the side walls, away from the zip door. We place the antenna on a stool 50 cm inside the wall. The difference in power with and without the zip door open is only 5 dB. We make a five-sided cube around the antenna, with the single open face against the enclosure wall. Isolation is now 23 dB. With only a single absorber behind the antenna we get 7 dB. Adding top, bottom, and two side walls to the absorber cube attenuates by 16 dB, which suggests that 97.5% of the power reaching the antenna is reflected off the canopy walls.

[15-AUG-13] Catherine Hills, veterinary doctor at ION, has tried passing the air flow in and out of an IVC rack through fine stainless steel mesh, a sample from BOPP. She reports, "Just to update you on the material I tried on the IVC rack. Unfortunately the material held the dust created from the cages, therefore with this building up it will make the motor unit work harder to achieve the correct air changes and removing the CO2 and ammonia. It will also increase the temperature and humidity within the cages, therefore we would have to cut the material around the pipes to ensure the environment of the cage is kept within the correct parameters." So we must allow pipes to enter and leave the enclosure, or we can opt to include the pumps and pipes within the enclosure, in which case we will have to determine how much mains hum these pumps will add to the EEG we record.

[19-SEP-13] We set up our Faraday Canopy and place within it three towers of microwave absorbers. Each absorber tower is 60 cm wide and 180 cm high, made of three 24-inch square AN77 foam absorbers. We arrange these along the back wall of the enclosure and turn on our 910 MHz +13 dBm interference power source. We run an antenna cable into the Faraday Canopy through a sock to measure penetrating 910 MHz. We have the 910 MHz source just outside the zip door of the canopy, on a box, and the spectrometer antenna 1 m away inside the enclosure. The following paragraph is transcribed from our notes, and shows that our canopy with rear wall of absorbers appears to give us at least 10 dB, and most often 20 dB of isolation form external interference.

Spectrometer outside enclosure, no antenna cable, 910 MHz source powered up but no antenna wire inserted, -74 dBm in spectrometer. Cable 92" x 2 into enclosure through sock -59 dBm. Ground cable union to floor with tape -72 dBm. Open zipper, insert 910 MHz antenna, 1 m between antennas, -32 dBm. Close zipper, have three absorber towers on back wall, get -47 dBm. Patch two 10-mm holes next to zipper, -52 dBm. Arrange towers as triangle (see here), -54 dBm. Open zipper, -35 dBm. Remove absorbers from enclosure, zipper open -31 dBm. Close zipper -42 dBm. Make box of absorbers around spectrometer antenna (see here) and zipper open -32 dBm, close zipper -49 dBm. Repeat triangle behind, zip open -36 dBm, zip closed -44 dBm. Make absorber wall at rear zip closed -47 dBm, zip open -30 dBm. Wall towers touching one another at rear wall, zip open -37 dBm, zip closed -48 dBm. Now Kirsten moves around the enclosure with the zip closed, and power varies from -57 dBm to -44 dBm depending upon where she stands. Swap right side absorber tower position with Kirsten's position, -42 dBm. Kirsten leaves enclosure -43 dBm. Wall at back again, zip closed K out -48, K in -46, zip open Kout -34 dBm. Absorber triangle again, Kin zip open -39 dBm, Kout zip open -37 dBm, Kout zip closed -43 dBm. Pushing on canopy walls and pulling, -42 dBm to -46 dBm. Spaced absorber towers at back, K in zip open -48 dBm, K out zip open -41 dBm. K out zip closed -46 dBm. Kin zip closed -43 dBm. Taped more holes K out zip open -37 dBm, zip closed -45 dBm. Close-up absorber wall at back, Kin zip closed -61 dBm, Kout zip closed -47 dBm. Tape over more holes -51 dBm. Towers around spectrometer antenna. K in zip closed -57 dBm, Kin zip open -44 dBm. Kin zip closed -61 dBm. K in zip open -45 dBm. K in zip closed -52 dBm.

Now we move the antenna off to one side, and our observations become confusing.

910 MHz 2 m from receive antenna, outside canopy corner, absorbers walls boxing in behind and either side of receive antenna. K in zip closed -47. K out zip open -53. (This is the first surprise.) K in zip open -45 dBm. Absorber wall at the back, K in zipper open -40 dBm, K out zipper open -42 dBm, K out zipper closed -45 dBm. (This contradicts our observations when the transmitter was outside the door, and opposite the absorber wall.) Move absorber wall so it is behind receive antenna along line from 910 MHz transmitter. K in zip open -40 dBm. K out zip open -39 dBm. K out zip closed -45. (Only 6 dB isolation.)

The isolation of the enclosure depends upon the direction of the incoming interference. Isolation could be as low as 6 dB. That's useful, but does not solve our interference problems.

Instead of a Faraday Enclosure, which works by reflecting incoming radio waves with conducting walls, we now consider making a different type of enclosure, one that provides isolation through interior absorbers and some exterior reflectors. The idea is to reduce the number of radio-wave photons entering the enclosure by a factor of ten or a hundred, and then either absorb or release these photons before they have crossed the interior of the chamber more than a couple of times.

We place our spectrometer on a stool 1 m from our 910 MHz source. We have no antenna in the spectrometer, but we do have an antenna in the transmitter. We see -65 dBm of 910 MHz. We add the antenna to the spectrometer and see -41 dBm. We place a 60-cm square absorber between the two, we get -48 dBm. Remove absorber, back to -41 dBm. We repeat this experiment in various ways. We find that the gray face of the absorber, when facing the transmitter, provides around 7 dB of isolation, while the black face provides only 5 dB.

[20-SEP-13] We arrange three antennas in a row, at 30-cm intervals, with their loops coaxial. We connect each antenna to its own independent amplifier and demodulator in our prototype Data Receiver (A3027). Ambient interference is −48 dBm in the 900-930 MHz band. We hold an A3019D transmitter in our fist. We rotate and translate the transmitter in the 30-cm cube between the left and center antennas. We obtain 96.5% reception. We disconnect the left and right antennas. We obtain 38% reception from the single, center antenna. We now lay down a mat of resistive sheet, and create walls of AN77 microwave absorbing foam, as shown below.

Figure: Absorbing Coffin. There are three antennas in the base. The floor is microwave resistive sheet. The walls are AN77 foam absorbers.

Within this open-topped coffin, we obtain 99.3% reception with all three antennas connected, moving the transmitter between the left and center antennas.

[24-SEP-13] We propose to try the following arrangement at ION at the end of October. Behind the rack stands wall of 3×3 AN-77 absorbers backed with aluminum. Two smaller walls of 1×3, also backed with aluminum, stand on the sides. A reflecting curtain of steel mesh hangs in front of the cages. A rubber mat hides a sheet of reflecting fabric beneath. The top of the rack is open, other than the metal that forms its frame and ventilation system.

Figure: Absorbing Enclosure for IVC Rack. This enclosure is designed to work with eight independent antennas arrange on the shelves.

The absorber walls are backed with aluminum, with the gray side of the absorber facing away from the aluminum and towards the animal cages in the rack. Of the900-MHz power incident upon the gray surface, roughly 90% will be absorbed on its way to the aluminum reflector, and 80% of what remains will be absorbed on its way out again, so the total absorption of the aluminum-backed will be 98.2% (the AN-77 data sheet specifies −17.5 dB reflection at 900 MHz). The conducting wall below and in front of the rack serves to reflect interference away from the rack, and also to keep SCT signals inside the rack, where they can be picked up by more distant antennas. The ceiling is open so that multiple reflections of waves between the ceiling and floor are unlikely. The figure below shows an arrangement of antennas in the rack.

Figure: Antennas Arranged on IVC Shelves. Horizontal spacing a and vertical spacing b. Absorber walls behind, left, and right. Reflecting curtain in front and reflecting mat below. Ceiling is open.

Using the dimensions in the figure above, let us suppose a = b. A transmitter at the center of one of the four central cages will have two antenna within 0.5 a and another two within 1.1 a, as well as others even farther away. A transmitter at the outer edge of the top-right or bottom-left cage will have two antenna at range 1.0 a and one at range 1.4 a. We expect the top-right and bottom-left cages to have the poorest reception.

Suppose a = 30 cm. In our office, with ambient interference of −48 dBm and no radio-frequency enclosure of any kind, reception from a transmitter buried in one hand is roughly 50% and at 50 cm it is roughly 25%. At ranges up to 120 cm, it is at least 10%. Assuming reception is independent for eight antennas, and considering the arrangement above, we would expect with these values a combined reception to be 89%. Now suppose that within our IVC rack, with our absorbers and reflectors, we reduce ambient interference by 6 dB, thus doubling the range at which a transmitter can dominate interference. We also convey our transmitter signal more effectively to the more distant antennas. Suppose reception we get 75%, 50%, and 20% at ranges 30 cm, 50 cm, and greater respectively. Combined reception should be 99%.

[31-OCT-13] In Pishan's recording room in the Anatomy Building of UCL, we obtain robust reception from a transmitter held in one fist at range 50 cm from a damped loop antenna on a table with no Faraday enclosure and no foam absorbers. On the same day on the tenth floor of the ION building of UCL, we cannot obtain robust reception without a Faraday enclosure, even when we place the transmitter on the floor next to an antenna. The Anatomy Building is made of brick. Pishan's recording room has 20-cm of brick on all four wall, has the ground below, because it is on the ground floor, and what we guess must be a plaster-coated wood ceiling. Brick provides roughly 1 dB/cm attenuation of 900 MHz, so interference power in this room is one hundred times weaker than in the street. Their wireless networks don't work well because of the brick. The ION building is made of cinderblocks and steel girders. There might be as little as five centimeters of light-weight concrete between the outside and the inside.

At ION, we set up an Octal Data Receiver and plug in four antennas. We arrange them as if they were on the shelves of an IVC rack, with no enclosure or absorbers around about. We measure reception as if in a cage between them by holding the entire transmitter in our fist and rotating at random for ten seconds, while watching reception in the Receiver Instrument. With one antenna we get 30±10% reception, two give us 60±10%, three gives us 90±5%, and four gives is 97±3%.

[31-DEC-13] We set up a small isolation chamber, as shown below. It consists of six AN-77 absorbers, gray sides inwards, standing on the concrete floor of our office at 5 Pratt Ave. We wrap an outer layer of resistive sheet around the absorbers. We put a cover of the same resistive sheet on top. We assume the concrete floor is a perfect absorber, because we are in a basement.

Figure: Small Isolation Chamber on Concrete Floor.

The 900-930 MHz interference in our 5 Pratt Ave location varies from one second to the next. It peaks at around −45 dBm, but can drop as low as −84 dBm. Almost all the power is concentrated in the 925-928 MHz band. Reception on an open work bench is reliable only if we set our transmitter right next to the antenna. We set up our A3008C spectrometer and measured the 925-928 MHz power for one minute with the pick-up antenna on the corner of our work bench, one the floor out in the open, in the center of our small isolation chamber, and in the corner of our isolation chamber.

Figure: Interference Power in Separate Minutes and in Various Locations.

The highest interference power we observe outside the enclosure is −45 dBm, but inside the enclosure the power never rises above −68 dBm. It appears that this enclosure is providing 23 dB of isolation. We place a Subcutaneous Transmitter (A3019D) inside the enclosure 50 cm from each of two antennas connected to an Octal Data Receiver (A3027C). Over the course of one minute we obtain 100.0% reception. We remove the enclosure lid and reception drops to 29%.

[01-JAN-14] For a discussion of the precautions necessary to provide better shielding with a conducting canopy, see Practical Shielding. In order to obtain >20 dB isolation with such a canopy, we would have to make sure that all our seams and doors were conducting surfaces interrupted by gaps of no more than a few millimeters wide. Instead of socks to allow pipes and wires into the canopy, we would have to insist upon bulkhead feedthroughs of shielded cables. Our box enclosures work well because their seems are taped-over and inflexible, and we use BNC feedthroughs. These precautions would make our proposed canopy impractical for animal experiments. As it is, we have seams and zips that may be acting as slits that resonate at our operating frequency in combination with our pick-up antenna, and cable shields entering the canopy that may be acting as dipole antennas in combination with the canopy mesh. These may be the reasons why our canopy performs so poorly.

[28-FEB-14] We set up the Faraday canopy in our new office. Ambient 902-930 MHz interference peaks at −43 dBm on our work bench. It enters via the roof and windows only, because we are in a basement. We place three antennas inside the cage, one on the floor and two on a box. We tape the shields of all three cables to the copper fabric floor of the canopy. Interference peaks at −61 dB, −52 dB, and −64 dB on the floor, box, and box.

[28-FEB-14] We make a curtain 2.0 m wide and 2.2 m high with curtain rings for the chamber test at ION. We hang the curtain in our office. We lay down a mat of copper taffeta fabric. We make a 1.2 m × 1.2 m absorber back wall and two 1.2 m × 0.6 m side walls. We suspend a piece of resistive sheet above the isolation chamber thus created. The result is shown below.

Figure: Isolation Chamber with Front Curtain.

We place three antennas on a table inside the chamber and three transmitters, which we move around to different places on the table. Reception without the chamber is 0-100% because of intermittent −58 dB ambient interference at 926 MHz. With the chamber as shown, we get reception 95-100%. When we move aside the curtain, reception drops to 70-100%. Peak interference power in the closed chamber is around −64 dB.

[07-MAR-14] In the new animal room on the tenth floor at ION in London, we test reception in an IVC rack with and without the radio-frequency isolation chamber shown below. Each cage is for one or more mice, and has a metal grating to hold food pellets under the lid.

Figure: IVC Rack with Isolation Chamber. We have a sheet of copper taffeta fabric on top and bottom. Behind and to the sides are walls of AN-77 absorbers backed by aluminum sheet. In front is a curtain of steel mesh fabric. Antennas are at locations marked "X". Cage labelling scheme as marked on the rack.

We place the eight antennas of an Octal Data Receiver (A3027B) between the floors of cages B7-F7 and B6-F6. We measure reception in the rack by removing one cage and rotating and translating an subcutaneous transmitter (A3019D) on the end of a stick for one minute within the volume the cage would have occupied. We record the fraction of messages received. When the curtain is in place, we cut a hole in the curtain and pass the stick through the hole. Before we install the isolation chamber, we measure the reception in cage spaces as follows: E7 = 92.5%, E6 = 99.2%, E5 = 96.7%, G7 = 85.1%, B7 = 91.6%, E7 = 91.8%, D5 = 99.6%. After we install the isolation chamber we get: E7 = 100.00%, F7 = 100.00%, H10 = 100.00%.

Without the chamber, reception is not quite good enough for reliable recording, even within 30 cm of two antennas. With the chamber, reception is 100.00% reliable everywhere.

[08-OCT-14] We have a self-propelled ball to which we can attach a transmitter and have it wander around on the floor of an isolation chamber. We set up a chamber in our office and measure reception with varying numbers of antennas, as we report here. With four antennas and the roof on, reception is 99.9%. With the roof off, it's only 86.8%.

Figure: The FE4A Isolation Chamber, Without Mesh Cover. This chamber, set up at the Society for Neuroscience Show in 2014, provided superb reception for randomly-moving transmitters on its carpeted floor, even with its reflecting mesh lid removed.

[15-JAN-15] We receiver recordings from four rats in separate cages in an FE2E enclosure at Philipps University, Marburg. They recorded with two, three, and four antennas connected to a Data Receiver (A3027C). Reception was as follows.

Table: Reception in the FE2E, Average During Five Minutes.

There was no single one-second interval in the entire 5-minute recording with 2, 3 or 4 antennas during which reception dropped below 80% for any one of the four channels. Thus robustness of reception was 100.0% for 2-4 antennas.

[05-FEB-15] At ION, we try three types of transmitter in their IVC rack isolation chamber. There is a large gap in the wall at the rear left, where the lower pipe exits, there being no hole in the wall for the pipe. With the front curtain sealed, we measure reception with three types of transmitter. With a single-channel, rat-sized A3028R (number R69.2) in various locations, we obtain 100% reception. With two-channel mouse-sized A3028A (number A55.13) in various locations we obtain 100% reception also. With the single-channel mouse-sized A3028B (number 55.10) in various locations we obtain 75% reception. With two implanted A3028Bs we also obtain 75% reception. When we open a 30-cm long 3-cm wide gap at the end of the front curtain, reception deteriorates for all transmitters.

[27-MAR-15] We receive 10 of ML-10052-25 microwave absorbing foam from Arc Technologies, specialists in microwave absorbers. We place a 24" square piece of the absorber on the floor of a Big Enclosure (FE2A) and measure reception from a moving A3028E transmitter on a stick. Our Data Receiver is an A3027D.

Foam Arrangement Average
ML-10052-25 on bottom, darker side up99.5
ML-10052-25 on bottom, darker side down98.8
AN-77 on bottom, black side up97.3
AN-75 on bottom, black side down96.7
ML-10052-25 on bottom, darker side up98.0
ML-10052-25 on bottom, darker side down97.1
No absorber78.0
ML-10052-25 on bottom, darker side up96.7
ML-10052-25 on bottom, darker side down97.8
AN-77 on bottom, black side up94.7
AN-75 on bottom, black side down97.5
Table: Message Reception in Big Enclosure with Antenna Centered. In each experiment, we move the transmitter around at random in the full volume of the enclosure, but avoiding touching the walls. By default, the antenna is in the center of the floor.

Interference in our office is powerful and intermittent, so we repeat our measurements. It looks like the ML-10052-25 performs better with the darker side facing the inside of the enclosure, which is the side with the identification letters on it. The AN-77 performs better with the darker side facing the outside of the enclosure. The AN-77 identification letters are on the lighter side. In both cases, the identification letters are visible in the optimal orientation. Average reception with the identification letters visible is 98.0% for ML-10052-25 and 97.3% for the AN-77.

[15-APR-15] We have our first FE2F front-loading enclosure, OSI designed and made. It has a front door with no straight path into the interior when closed. The mesh walls are glued in place with a frame, so they are taught. The corners are bolted together. It provides three antenna feedthroughs. We put a 25 cm × 41 cm box in the enclosure. Inside the box we place an A3028E attached to a self-propelled ball, and let the ball roll around in the box to simulate the movement of an animal.

Figure: FE2F Reception Test.

We measure reception with various numbers of antennas, door open and closed, and with our own 13 dBm, 910 MHz interference source turned on 4 m away. The table below presents average reception over one minute. During all these experiments, we have intermittent ambient 926-MHz interference of −55 dBm. Our 910 MHz source produces −50 dBm in the neighborhood of the enclosure.

AntennasDoor910 MHz SourceReceptionOther
1ClosedOn80.9%Taped all gaps in rear walls.
1ClosedOn80.1%Taped all gaps in rear wall and ceiling.
1Lid ClosedOff46.8%Big Enclosure for comparison, AN-77 on Floor, One 20-mm Hole
1Lid ClosedOff49.3%Big Enclosure
1Lid ClosedOff64.5%Big Enclosure, Lid Adjust
1Lid ClosedOn35.4%Big Enclosure
1Lid OpenOn1.1%Big Enclosure
Table: Reception in the First FE2F Enclosure.

[24-APR-15] We hear from ION that they have assembled a second IVC rack isolation chamber using our two-piece front curtain, mesh ceiling, copper taffeta floor mat, and fifteen absorbers on the aluminum back and side walls. With careful antenna arrangement, reception within the enclosure is excellent. They have to take care to clip the front curtains together to keep out interference.

[01-JUN-15] We have another FE2F enclosure. We set up the same reception test described above, with a moving transmitter in a dry box.

AntennasDoor910 MHz SourceReceptionOther
1ClosedOff99.5%A3028B with 30-mm Antenna
1ClosedOff99.5%A3028B with 50-mm Antenna
1ClosedOff99.4%A3028E with 50-mm Antenna
1ClosedOn99.5%A3028B with 30-mm Antenna
1OpenOn64.8%A3028B with 30-mm Antenna
Table: Reception in the First FE2F Enclosure.

We measure the 910 MHz power picked up by an antenna inside the enclosure with the door open, and get −45 dBm. We close the door and get −80 dBm. We disconnect the antenna and get −83 dBm. We open the door in stages, and measure power received inside the enclosure with the width of the opening between the lid and the frame.

Figure: Isolation versus Door Opening for FE2F. We define zero isolation as that provided when the door is fully open.

Compared to the door open, the door closed gives 35 dB isolation. With a 1-mm gap isolation drops to 30 dB. These measurements allow us to estimate the effect of a loose door, or of forcing the door open a few millimeters in order to allow an additional cable to enter the enclosure, like this.

[24-JUN-15] ION measures reception in an IVC rack isolation chamber. They arrange four antennas around four mouse cages, each with a mouse and A3028B transmitter. Two other cages are on the far edges of the rack. They measure reception during four hours with an Antenna Combiner (A3021B) and Data Receiver (A3018D) and then for four hours with an Octal Data Receiver (A3027D).

Table: Reception in New IVC Rack.

Reception with the Octal Data Receiver is robust for all six transmitters. With eight antennas, reception should be robust from fourteen transmitters.

[15-SEP-15] We close the door of our FE2F, sealing the antennas inside. We move an A3028E transmitter around outside. We receive 0.0% of its messages. When we place the transmitter inside the enclosure, we receive 100%.

[15-JAN-16] The two IVC Isolation Chambers are working well at ION, but only so long as the edges of the curtain are clamped on sides, top, and bottom to the absorber walls, ceiling mat, and floor mat respectively. The vets have trouble undoing all the clips and fastening them up again. We assemble the canopy shown below, which will replace the ceiling mat and curtains.

Figure: Canopy with Door, Designed for IVC Rack. The sides of the door frame will clip to the absorber walls on the side of the IVC Rack. The ceiling will clip to the absorber walls on the sides and back of the IVC rack. The bottom of the door frame will be overlapped by the floor mat, which will extend farther out from the IVC rack than the door.

The zippers are overlapped by a 50-mm flap of mesh fabric, to reflect interference away from the narrow strip of non-conducting fabric on either side of the zipper. The zippers unzip from bottom to top. Once one zipper is open, we can step inside the door and remove cages from the rack. If one zipper jams, we have a spare. If we want to open the rack up completely, we undo both zippers and throw the door over the top of the rack so it is out of the way.

[03-APR-18] We have our first FE3A enclosure. We place two antennas inside and obtain 98-100% reception from an A3028E taped to a toy rolling around on the metal floor. With the door open we see −41 dBm of interference at 930 MHz. With door closed, this peak drops to −63 dBm, giving us 22 dB of isolation.

[04-SEP-18] Our 915-MHz radio signals are attenuated by roughly 1 dB (10%) for each centimeter they travel through water or body tissue (see here). The dielectric constant of water at 915 MHz is roughly 90, which means the wave velocity is around ten times slower than in air. When 915 MHz tries to move from air to water, this mis-match in propagation velocity results in 60% of the incident power being reflected. After that, the incoming power spreads out by diffraction and attenuates at 1 dB/cm. If we want to record from aquatic animals, we can use a sufficiently large body of water as a means of isolating our recording system from ambient interference.

Figure: Water Tank Chamber. Here we have a 33-nH inductor used as a receiving antenna, suspended in a 30-cm deep reservoir of water. Two transmitters, one with a 13-mm helical antenna and another with a 30-mm loop antenna are 20 cm away. All antennas are at depth 15 cm.

We fill our sink with water and set up the reception test shown above. A 33-nH pick-up coil receives −68 dBm of ambient interference when pressed against the wall of the reservoir, or when in the center of the reservoir. An 80-mm loop antenna picks up −61 dBm in the same places. We move the 30-mm transmit antenna at random within 30 cm of the pick-up coil and get roughly 80% reception. The 13-mm helical antenna gives us roughly 70%. When we add a coaxial 3-dB attenuator to the base of the receiving antenna cable, we get roughly the same performance. A 20-mm quarter wave wire in place of the 33-nH coil gives approximately the same performance. We present further work on immersed reception in Aquatic Antennas.

[05-JAN-19] We have twenty-five days of recordings from seven A3028P2 transmitters with 13-mm helical spring antennas implanted in mice, operating within an IVC rack with isolation chamber. We obtain hourly average reception for all seven devices and plot.

Figure: Reception from Seven A3028P2 Implanted in Mice in IVC Rack versus Time. For detail of eight days see here.

Average reception is 97%. Short drops to 80% or below are shared by all seven devices, suggesting that the curtains of the chamber were open for cage cleaning and animal feeding. The final drops at day 19, 22, 23, and 24 are exhaustion of the transmitter batteries. The final three devices are still running. These devices are equipped with a 13-mm long antenna made out of a 500-μm helical steel wire, which emits less power than our usual loop antennas.

[25-JUN-19] The hearing range of mice is 1-100 kHz, and that of rats is 0.2-80 kHz. We place a microphone on the shelf of an FE3AS enclosure, just inside the steel mesh of the front door. Just outside the mesh, we place a speaker. By moving the speaker out of the way, and replacing it in the same location, we measure the attenuation of sound by the steel mesh. We use this procedure for 200 Hz to 20 kHz. At 40 kHz we use a 40-kHz ultrasonic transmitter and a 40-kHz ultrasonic receiver, with the transmitter 90 cm from the receiver so that we can open the door without moving either device.

Figure: Steel Mesh Sound Attenuation (dB) versus Frequency (Hz). Attenuation of +10 dB is a factor of ten reduction in power, and 0 dB is no reduction in power.

The attenuation of sound by steel mesh is negligible at all frequencies 200 Hz to 40 kHz. We leave our 40-kHz transmitter and receiver in place and measure attenuation by various barriers placed between them.

Figure: Barrier Sound Attenuation (dB) at 40 kHz. Attenuation of +20 dB is a factor of one hundred reduction in power, 30 dB is a factor of one thousand.

Clear polycarbonate plastic, such as the shelf we use in the FE3AS enclosure, provides 32 dB attenuation. A single clear vinyl sheet 120 μm thick provides 26 dB attenuation at 40 kHz, which is a factor of 400 reduction in the power. We cover the three steel mesh walls of an FE3AS with 120-μm vinyl sheet. We place our 40-kHz receiver inside, and move our 40-kHz transmitter around outside. Compared to direct transmission through 90 cm of air, the volume inside the enclosure is 50 dB lower with transmitter just outside any aluminum wall, 30 dB lower just outside the two side walls made of steel mesh and vinyl sheet, and 12 dB lower when the transmitter is pressed up against the front door mesh and vinyl sheet with the receiver 5 cm away on the inside. In this latter arrangement, the received signal is 35 dB less powerful than the same geometry without the vinyl sheet. We cut the vinyl sheet on the front door into 10-cm wide vertical strips, hanging from the top edge of the door. This curtain provides attenuation of 26 dB at 40 kHz.

[28-MAY-21] We are shipping our FE3AS enclosures as flat-pack kits for our customer to assemble.

Figure: Three FE3AS Enclosure Kits Delivered to United Kingdom by Air Freight.

Each enclosure uses over a hundred screws to secure the walls to the frame, so as to create a tight seal against radio frequency interference. We recommend a torque-controlled battery-powered screwdriver to drive the screws to within one turn of tight, then tightening by hand the last turn.

Figure: A Team of Scientists Assembling an FE3AS Faraday Enclosure at SUNY Albany. The enclosure is upside down right now, having its floor screwed in place. The black foam on the ceiling is microwave absorber.

Assembly takes roughly two hours.

[14-DEC-21] Our canopy enclosures enclose an entire shelf or rack of cages, providing isolation within the entire area. The enclosure floor is made of aluminum sheet metal. We tape the sheet metal to our laboratory floor with aluminum tape. The enclosure comes with its own support frame, which is visible in this photograph of the FE5A enclosure. The stainless steel mesh canopy fits over the frame, and provides two magnetically sealed seams. We have four absorbers hanging from the frame on either side. These absorbers could equally well hang against the back wall. The enclosure works well provided we bring all electrical signals through the enclosure wall with shielded feedthroughs. We make coaxial and Ethernet Enclosure Feedthroughs (A3039) for use with our canopy enclosures. We fasten these feedthroughs to the floor using aluminum tape. The steel mesh wall rests between the input and output rows of connectors. We connect cables on either side of the feedthrough. The 8-to-4 Coaxial Feedthrough (A3039C) provides antenna combiners so that we can connect eight antennas within an enclosure to four coaxial cables outside, and run these to four antenna inputs on a data receiver such as the Octal Data Receiver (A3027).

of Antennas
1 (unpaired)75%
2 (unpaired)85%
3 (unpaired)97%
4 (unpaired)99%
5 (paired with 1)100%
6 (paired with 2)100%
7 (paired with 3)100%
8 (paired with 4)100%
Table: Average Reception Throughout a Faraday Canopy (FE5A) versus Number of Antennas. We use an 8-to-4 Coaxial Feedthrough (A3039C) to bring antenna signals out of the enclosure.

With four unpaired antennas in an FE5A, we move an A3028E transmitter all around the volume of the enclosure on a glass rod and obtain 99% reception. With four more antennas, With five or more antennas, we see 100% reception. In our experiment, we hung the antennas from the frame, placing them in front of the absorbers. They could be placed on a shelving unit, or taped within the structure of a cage rack. For measurements of the isolation of the FE5A with the A3039 feedthrough, see the A3039 development reports.

Early versions of the canopy enclosure provided adequate reception when equipped with nine absorbers and thirty-two interior antennas. These canopies were not equipped with feedthroughs. Some laboratories placed their data receivers, Ethernet switches, and power supplies inside the enclosure. It turns out that these cables were radiating interference within the enclosure. When we introduced the feedthroughs, and insisted on shielded Ethernet cables within the enclosure, the performance of the enclosures improved dramatically. We now need only four absorbers and eight antennas to provide robust reception from dozens of animals.

[14-MAR-22] We place an A3015C loop antenna on center of the floor of an FE2F enclosure. We support another A3015C antenna outside the enclosure, in front of the door, at a range of 100 cm from the first antenna. We connect +10 dBm to the external antenna through 2 m of coaxial cable. The two antenna are facing one another. The internal antenna is upright, as defined by its mounting brackets. At first, the external antenna is at 90° to the internal. The internal antenna we connect to the back wall of the enclosure, and then to our hand-held spectrometer. We apply 400-3000 MHz and for each frequency measure power received by the internal antenna with the FE2A door open and closed.

Figure: FE2A Isolation versus Frequency. For two A3015C aligned (0°) and mis-aligned (90°).

When the antennas are at 90°, their coupling efficiency is 12 dB lower when the door is open, but not much different when the door is closed. Thus the isolation of the FE2A appears to be lower for the 90° experiment. The 0° experiment provides stable isolation in the 900-930 MHz band of around 25-30 dBm. Outside the 900-930 MHz band, isolation varies greatly with frequency, and can be as low as 5 dB.

[02-DEC-22] We install an FE5A at the Royal Free hospital in London. Reception in this bricked-in basement room is near-perfect with only two antennas. We arrange absorbers and antennas on the back wall and slide the IVC rack in easily. We make one cross cut near the ceiling for a first ventilation tube, leave the second cross cut for others. See IVC rack inside, one tube connected here.

[13-APR-23] We have a new version of the FE3A, which we call the FE3B. It is easier to manufacture, has easy-to-replace side mesh, no hinges that rust, and is easier to assemble because it uses ordinary Phillips head screws instead of self-tapping screws. We compare the FE3A and FE3B isolation for an interference source 3.6 m (12') from an antenna inside the enclosure. We measure isolation by closing the door and measuring the drop in power.

Figure: FE3A versus FE3B Isolation at Various Frequencies.

[11-SEP-23] We attempt to measure the isolation provided by a new FE5AS Faraday Canopy. Nathan writes, "We placed the transmitting antenna outside of the canopy, about 40-80cm away from the front door of the canopy. Inside the canopy was an antenna connected to a BNC feedthrough going to a spectrum analyzer outside the canopy. The transmitting antenna output +10dBm of 0=915MHz. We then measured the power on the antenna inside the canopy with the spectrum analyzer when the door was closed and when it was open. We found that averaging the isolation given from 5 different transmitting positions gives us about 25dBm of isolation. That is to say the signal inside the canopy is about 25dB weaker than the signal outside."

[18-SEP-23] We are assembling another FE5A frame for shipping. The frame consists of 8 struts of 40", 2 struts of 28", and two struts of 62", as well as four corner joints, four straight joints, and four feet. The outer dimensions of the frame are 82" high, 64" wide, and 30" deep.