The Loop Antenna (A3015) is a circuit board with a BNC socket, support brackets, an antenna, and an attenuator. In the A3015A and A3015B, we make the loop antenna out of stiff, insulated wire that we solder into the holes on the circuit board. In the A3015C, the loop is integrated into the printed circuit board. We designed the A3015 for use with our Subcutaneous Transmitters. Most antennas are designed to have the greatest sensitivity in their most sensitive direction. But the A3015 is designed to have the greatest sensitivity in its least sensitive direction.
The A3015 provides mounting brackets that serve to hold it vertical when resting on a horizontal surface, or allow it to be screwed to any other surface. The photograph below shows the mounting brackets of the A3015C as seen from above.
The A3015 provides an attenuator between the antenna and the coaxial cable. By default, this is a 3-dB attenuator. It absorbs half the power picked up by the antenna and allows the other half to proceed to the coaxial cable. A 3-dB attenuator ensures that the antenna presents a well-behaved load to the antenna cable. The attenuator greatly improves the performance of the antenna in highly reflective environments such faraday enclosures. See the Impedance Matching section below for measurements.
We have the following versions of the A3015. The A3015A is identical to the A3015B, but with a 0-dB attenuator instead of a 3-dB attenuator. The A3015A has problems operating within faraday enclosures, as we describe below.
|A3015A||915-MHz Loop Antenna||Circumference 320 mm, No Attenuator||102 Ω at 915 MHz, reactive or inductive otherwise|
|A3015B||915-MHz Damped Loop Antenna||Wire Loop 320 mm, 3-dB Attenuator||70 Ω at 915 MHz, close to 150 Ω otherwise|
|A3015C||915-MHz Damped Loop Antenna||PCB Track 320 mm, 3-dB Attenuator||70 Ω at 915 MHz, close to 150 Ω otherwise|
The different antenna shapes are sensitive in different directions and to different polarizations. Their behavior is dictated not only by their shape, but also by their interaction with the cable and the circuit at the other end of the cable. Long cables attenuate the signal. Short cables carry reflections back from the other end. A perfect match between the cable and antenna impedances makes the antenna more sensitive in some directions, but much less sensitive in others.
The A3015A uses two L-shaped brackets with holes to provide support on a level surface, and to allow bolting to any flat surface. We use part numbers 15275A51 and 15275A53 from McMaster-Carr.
In the following discussions, we concentrate upon the use of the A3015 as a receiving antenna for our 915-MHz Subcutaneous Transmitters. The transmitters reside in the bodies of animals, and transmit out of the animal and the animal cage to an antenna connected to a receiver such as our Demodulating Receiver A3005C.
S3015_1.gif: Antenna circuit diagram showing connector, attenuator, and layout.
A301501C.zip: PCB Gerber Files for the A3015C Loop Antenna.
A301501B.zip: PCB Gerber Files. Six A301501B PCBs plus one A3021A combiner. A combination circuit board to be cut up with shears.
A302101B.zip: PCB Gerber Files. Six A301501C PCBs plus one A3021B combiner. This circuit board moves the attenuator closer to the connector center.
[20-FEB-16] An antenna combiner takes the signals from several antennas and adds them together to produce one combined antenna signal. The simplest combiner we can use is a BNC T-junction, like this, to join two antenna cables together. The T-junction is inexpensive and readily available. But the T-junction antenna combiner has two disadvantages. When we connect two cables together directly, we lose at least half of our signal power. And we can destabilize the antenna amplifier as well, because the signals propagating along the cables reflect from the T-junction. Our Data Receiver (A3018) has one antenna input, a deficiency of gain, and a tendancy to oscillate. The T-junction combiner does not work well with its antenna amplifier.
Better than a T-junction is a passive combiner that adds the two antenna signals together withoug loss and without reflections. The Antenna Combiner (AC4A) is a four-way combiner made our of a ZB4PD1-2000, which is a four-to-one passive power combiner, two BTRM-50, which are 50-Ω terminators, and a short coaxial cable.
Each of our faraday enclosures contains at least one antenna. We can connect four such antennas to a single receiver input with the AC4A antenna combiner. Or we could use a ZAPD-1 to connect two antennas to one receiver input. These passive combiners attenuate our antenna signal less than 1 dB. But if we are going to have a circuit combining antenna signals, we can add amplification to the combiner as well, which iw shat we have in the Active Antenna Combiner (A3021B), which amplifies each of four inputs before adding them with a 4-way passive combiner.
The Octal Data Receiver (A3027) has eight antenna inputs, each of which provides ample gain and is unconditionally stable. We can combine antenna signals with T-junctions and see no significant degradation of reception. We present a comparison of passive, active, and T-junction combiners in the antenna chapter of the A3027 manual. The implication of these tests is that a T-junction joining two antenna cables is just as good as any other combiner, but three T-junctions joining four antenna cables is significantly less effective than an AC4A or A3021B combiner. Thus we should feel free to connect up to 16 antennas to an Octal Data Receiver (A3027) using eight BNC T-junctions.
The following table summarizes the properties of common coaxial cables.
|A 100 MHz
|A 900 MHz
|A 2400 MHz
|RG-58A/U||50||5.0||0.18||0.70||1.3||Stranded Sn-Coated Conductor|
|RG-58C/U||50||5.0||0.16||0.67||1.2||Stranded Sn-Coated Conductor|
|RG-59B/U||75||5.9||0.11||0.37||0.65||Cu-Coated Steel Conductor|
Stranded Ag-Coated Conductor
Solid Ag-Coated Conductor
Solid Ag-Coated Conductor
We obtained our data from diverse sources, but in each case we give a link to one of the sources. Sometimes we were forced to interpolate between existing data points to obtain our attenuation values, or to use an on-line calculator. By default, all conductor materials in the table are copper.
Our older laboratory BNC cables are RG-58A/U. As shown in the table above, these cables attenuate by 0.7 dB/m. A 1-m antenna cable gives us plenty of opportunity to place the antenna between two animal cages. Nevertheless, we ordered some longer RG-213/U cables, and compared their performance to those of RG-58A/U cables.
We used the same power-measurement apparatus we describe in our RF Combo Manual to measure the attenuation of cables. Our source of RF was −4 dBm at 910 MHz, provided by an A3016MT. We connected this source of power to the RF input of a ZAD-11 mixer with our test cable. For our LO we used a +10 dBm 864 MHz A3016SO. The IF amplitude and power are given below. For the raw data, download this.
There must be more than just attenuation taking place in the cables. Our IF power is lower for our 12-inch RG58C/U cable than it is for our 36-inch RG58C/U cable, and almost equal at lengths 12 inches and 132 inches.
Nevertheless, we conclude that we can expect a power loss of no more than 3 dB as a result of inserting a 96-inch RG58C/U cable between the mixer and the antenna. Given the flexibility and range of the 96-inch cable, and the small power loss, we see no reason to use a shorter cable with the A3015.
[14-DEC-12] We purchased ten 1.8-m RG-58/U BNC cables from Jameco, part number 205-527BK. We connected five of these together to make one 9-m cable. We applied +22.0 dBm (8 V p-p) of 146 MHz from our Command Transmitter (A3023) to one end, as measured by our 300-MHz oscilloscope. The signal at the other end was 18.5 dBm (5.3 V p-p). We lose 0.39 dB/m, which is three times the 0.11 dB/m we expect for RG-58/U from our table of cable properties. We replace the 205-527BK with 1.8 m of our Amphenol RG-58C/U and get 20.9 dBm out the other end, for 0.12 dB/m, which is slightly less than the 0.16 dB/m we expect for this type of cable.
The coaxial cable we use to connect the antenna to our receiver or transmitter circuit presents its own impedance to the antenna. The most common values for coaxial cable impedance are 50 Ω and 75 Ω. If we know the polarization of our incoming signal, and the signal is weak, we benefit from matching our antenna and cable impedance closely. The close match allows us to extract more power from the antenna, or deliver more power to it. But if we do not know the polarization of the incoming signal, a poor imedance match makes the antenna less discriminating. It's maximum sensitivity drops, but its minimum sensitivity rises. In the case of loop antennas, such as the A3015A or A3015B, the mis-match between the cable and antenna dampens the asymmetric resonance of the loop in the vertical direction, which would otherwise render the loop insensitive to horizontally-polarized waves. We present measurements of the maximum and minimum sensitivity of various antennas here.
[17-NOV-10] Our original A3015A loop antenna had no attenuator between the antenna and the coaxial cable. This antenna perfomed well out in the open, but reception was intermittent when we used it inside faraday cages. It took us a year to identify the cause of poor reception, but when we did so, our observations were repeatable and compelling.
We had been going into the OSI office every morning to work for an hour on intermittent data corruption and reception problems. One this particular morning, we arranged the cables and observed poor reception from the faraday cage. We exchanged the data receiver, moved the transmitter within the cage, re-seated the cage lid, and removed the antenna combiner, but always we observed poor reception from within the cage. We began a series of experiments with coaxial attenuators, which we can insert into the antenna system at any BNC junction.
|FE2B Lid Off, antenna-coax-wall-coax-receiver||69%|
|FE2B Lid Off, antenna-coax-wall-3dB-coax-receiver||94%|
|FE2B Lid Off, antenna-3dB-coax-wall-coax-receiver||94%|
Empty FE2A, antenna-6dB-coax-wall-coax-combiner-receiver
|Transmitter in Open, antenna-3dB-coax-combiner-receiver||59%|
These results convinced us that we should add an attenuator to the A3015 circuit board. We see that 3 dB is always adequate. Any larger attenuation would weaken our signal without any benefit from better impedance matching.
[18-NOV-10] Today we discovered the importance of shielded cable for the connection between the Data Receiver and the LWDAQ Driver. An unshielded cable picks up interference and greatly aggravates the system. We switched to a shielded LWDAQ cable and performed tests upon this system. We have transmitter No7 in the lower enclosure, close to the antenna, so we are certain of a strong signal. We start with an A3015A with a 3-dB attenuator and record the number of messages in channel No7 each second, as well as the number of bad messages per second. After 200 s we remove the attenuator and record for another 200 s.
In the graph we see interference coming and going as some external source of 915-MHz power turns on and off. Interference generates bad messages. When we remove the attenuator, we see ten times as many bad messages during interference, and ten times as many such messages being accepted into the transmitter data.
The following series of tests explore the effect of attenuators, terminators, and interference. We start with the system shown here, with 3-dB antenna attenuators. In each case, we calculate reception by taking the average of ten one-second intervals. When we show the number of bad messages, we wait until the bad message rate jumps up and record at that time.
|1||No7 in FE2A near antenna||99.3%|
|3||Terminate open socket||99.3%|
|4||Remove 3-dB antenna attenuator||100%|
|5||Restore 3-dB and rotate antenna loop||97.2%|
|9||Remove 2 terminators on unused combiner sockets||94.9%|
|10||Restore 1st terminator||99.3%|
|11||Restore 2nd terminator||99.9%|
|12||Remove 1st terminator||99.5%|
|13||Remove 2nd terminator||94.4%|
|14||Restore 2 terminators||99.9%|
|15||Remove 2 terminators and 3-dB antenna attenuator||99.8%|
|16||Move No7 to FE2B||99.9%|
|17||Restore 2 terminators||99.9%|
|18||Remove 3-dB atenna attenuator||99.9%|
|19||Move transmitter until reception is weak||76.6%|
|20||Put 3-dB attenuator at cage wall||0%|
|21||Remove combiner and connect directly to data receiver||99.7%|
|22||Remove 3-dB attenuator||99.4%|
|23||Restore 3-dB attenuator to cage wall||99.2%|
|24||Remove 3-dB attenuator, restore combiner||67%|
|25||Replace combiner with 6-dB attenuator||68%|
|26||Restore combiner, unshielded root cable||42%|
|27||Add 3-dB attenuator to antenna, unshielded root cable||0%|
|28||Remove combiner, unshielded root cable||15%|
|29||Remove 3-dB attenuator, unshielded root cable||90%|
|30||Move transmitter close to antenna, unshielded root cable||88% + 500 Bad/s|
|31||Add 3-dB attenuator to antenna, unshielded root cable||97% + 40 Bad/s|
|32||Remove 3-dB attenuator, unshielded root cable||90% + 800 Bad/s|
|33||Restore shielded root cable||100% + 100 Bad/s|
|34||Add 3-dB to antenna||99.7% + 5 Bad/s|
Tests 1 to 18 show that when our signal is strong, unused combiner sockets should be terminated. In one case we see reception drop from 99% to 94% because of poor termination. In tests 19 to 25, the transmitter signal is barely strong enough for detection by an antenna without an attenuator. Adding any form of attenuation in the antenna cable causes reception to drop. The combiner affects the signal like a 6-dB attenuator.
In tests 26 to 29, we have a weak signal and an unshielded root cable. Between 24 and 26 the only change is removing the root cable shield. Reception drops from 67% to 42%, which may or may not be significant. But we see no dramatic increase in the bad message rate when our transmitter signal is weak and we remove the root cable shield. With no attenuator and no combiner, we get 90% reception.
In tests 30 to 34 we move the transmitter close to the antenna so that we have a strong signal. We start recording the bad message frequency. The source of these bad messages is suppressed both by the 3-dB attenuator and the shielded cable, and most of all by both acting together.
We conclude that a 3-dB attenuator in the antenna will improve reception when the transmitter signal is strong. When the transmitter signal is almost too weak to receive, the additional attenuator will degrade reception. We plan to overcome the degradation caused by the 3-dB attenuator in the future with the help of an active antenna combiner: the A3021.
The above experiments are closely related to our experiment with shielded and unshielded LWDAQ cables in the presence of a 3-dB antenna attenuator. A shielded cable decreases the bad message by a factor of ten. From the above experiments we see that a 3-dB attenuator on the antenna also decreases the bad message rate by factor of ten. When we remove the attenuator and the shield, the bad message rate increases by a factor of a hundred.
[19-NOV-10] Today we measured reception with random movement on the end of a stick within our FE2A enclosure. Our objective was to determine how the 3-dB attenuator affects reception and to better understand the source of bad messages in the OSI office. We have two transmitters: an A3013A without encapsulation, No7, and an A3019A with encapsulation, No1. We move and rotate the transmitter inside the cage for a minute and calculate the average reception, the robustness, and bad message rate.
|35||A3013A, 0-dB antenna, no combiner||93%||90%||<1/s|
|36||A3013A, 3-dB antenna, no combiner||93%||89%||<1/s|
|37||A3013A, 3-dB attenna, combiner||84%||74%||<1/s|
|38||A3013A, 3-dB attenna, combiner, FE2B added||87%||70%||<1/s|
|39||A3019A, 3-dB attenna, combiner, FE2B added||78%||53%||<1/s|
|40||A3019A, 0-dB antenna, no combiner||92%||82%||<1/s|
|41||A3019A, 3-dB antenna, no combiner||94%||92%||<1/s|
|42||A3019A, Turn absorber grey side up||88%||92%||<1/s|
|43||A3019A, Absorber black side up,|
put absorber fragments in receiver box
|46||Remove absorber from receiver. Turn off transmitter.||0%||0%||500/s|
|48||Restore FE2A, 6-dB attenuator at receiver box,|
3-dB at antenna
|49||Remove 6-dB attenuator||0%||0%||400/s|
|50||Remove antenna leaving 3-dB attenuator||0%||0%||0/s|
|51||Remove 3-dB attenuator||0%||0%||0/s|
|53||3-dB at antenna enclosure||0%||0%||0/s|
|54||Remove 3-dB from antenna enclosure||0%||0%||10/s|
We see the bad message rate going up and down as we watch without altering the system. We try to wait until we see the rate go up before we make our measurement. Today we see high bad message rates even when the transmitter is off. The bad messages stop when we disconnect the antenna. They stop if we put 9 dB of attenuation between the antenna and the receiver, but not when we insert only 3 dB. Our observations are consistent with powerful interference entering the faraday enclosure and resonating between its walls. The poor performance of the FE2A is evident in our earlier experiments. The FE2A absorber is half as thick as the ones we settled upon for the FE2B, and is on the floor instead of glued under the lid.
[31-DEC-10] We left two transmitters running in two faraday enclosures for three hundred hours. Antennas are A3015Bs with 3-dB attenuators installed. Instead of an antenna combiner, we use a BNC T-junction at the Data Receiver. We obtain the following reception, averaged over each hour with the help Neuroarchiver Analysis.
We also use analysis to search for periods when reception drops below 90%, and find that this occurs during only one twenty-second period during the three hundred hours. During this twenty-second period, reception drops to zero for both transmitters.
[10-JAN-11] We incorporate an Antenna Combiner (A3021B) into our system. This combiner has amplification before combination. We plug the antennas into the combiner. We do not terminate the unused combiner inputs. We obtain the following reception. We note that No9 has not moved since the T-Junction experiment.
We see that reception is substantially improved with the introduction of the active antenna combiners. We searched for periods of poor reception and found two. We examined the raw data in the Neuroarchiver and found that both eight-second periods of poor reception contain thousands of bad messages.
We see that the antenna attenuator, the active antenna combiner, and the shielded root cable together reduce the frequency of these bad-message failures from once every few minutes to once every few days.
Using the power-measurement apparatus described above, we mixed 864-MHz LO with 910-MHz RF to obtain a −14 dBm IF signal. We passed the RF signal through a 240-cm cable on the way to the mixer. We unplugged the cable from the RF source and inserted an 80-mm whip antenna into its BNC socket output. We plugged an A3015A Loop Antenna into the free end of the cable that had previously been connected to the RF source. At range 1 m, we observed transmission from the whip antenna to the loop antenna. We rotated the A3015A loop antenna in all directions, and our IF signal varied from −46 dBm to −56 dBm. Transmission and reception across 1 m presents a minimum loss of 32 dB with respect to direct connection. In other words: we receive 0.1% of the power available at the RF source.
We discuss transmission efficiency here. To the first approximation, our loop antenna gathers all RF power that enters its effective aperture. At range 100 cm, the power radiated from our quarter-wave antenna is distributed over a ±45° solid angle perpendicular to its length. This solid angle has surface area roughly ½4π1002cm2 ≈ 60,000 cm2. The effective aperture of a well-terminated loop antenna is roughly equal to its diameter squared, which in the case of our 900-MHz A3015A loop, is 100 cm2. We expect the power loss going from the RF source to the antenna to be roughly −28 dB.
But our loop antenna is not well-terminated. It is terminated with 50Ω when its own impedance is 100Ω. We discuss poorly-terminated omni-directional antennas here. Our loop antenna is 3 dB less sensitive in one direction than a well-terminated loop, but 6 dB more sensitive in the perpendicular direction. With our poorly-terminated antenna, we expect the one-meter transmission loss to be −31 dB. We observe −32 dB.
[19-FEB-16] An antenna that detects transmitters only up to ranges of one or two centimeters would be useful as part of an animal tracking system. A video camera above the animal cage can monitor movements of blobs, each blob being one or more animals moving or resting together. When a blob is near the short-range antenna, we receive data from the transmitters implanted in the animals, and so we can identify which animals are in the blob. We took an A3015B and cut the wire back until it was a loop of diameter 10 mm. We measure reception versus range with three different attenuators placed in series with the antenna cable. All measurements are within an FE2F faraday enclosure. In our first test, we place the transmitter in 50 ml of water to simulate imlantation in a 50-g rodent. The antenna is vertical.
Reception does not always increase as range decreases: reception dead spots cause drops in reception even when the transmitter is near. We pour the water out of our 50-ml beaker and repeat, making sure the transmitter is still standing up and oriented identically.