Head-Mounting Transmitter (A3040)

© 2021-2022, Kevan Hashemi, Open Source Instruments Inc.

Contents

Description
Warranty
Versions
Connector
Antenna
Analog Inputs
Synchronization
Input Noise
Power Measurement
Battery Life
Wrapping
Design
Modifications
Development

Description

[08-APR-22] The Head-Mounting Transmitter (HMT, A3040) is a head-mounting telemetry sensor for rats and mice. It provides high-fidelity amplification and recording of four biopotentials. The experimenter cements a connector to the animal's head, and wires biopotential and reference signals to the connector. The HMT mates with this connector, which serves both to hold the HMT to the head and to deliver the signals to its amplifiers. The connector carries up to six voltages: four signals and two reference potentials. The HMT transmits its wireless signals using its integrated flex-circuit antenna, which will hover over the back of the animal. The HMT uses the same telemetry system as our Subcutaneous Transmitters (SCT). Its design is almost identical to that of the A3028 subcutaneous transmitter. We intend for the HMT to replace the crude head-mounting versions of the A3028.


Figure: Bottom View of A3040D3 Wrapped in Teflon Tape. A flexible antenna that hovers over the mouse's back transmits the EEG signals. A connector mates with another on the mouse's head. Battery is CR1225, 2000 μA-dy, total mass 2.2 g.

The HMT needs to be splash-proof and dirt-resistant, but need not be water-proof or corrosion-resistant. We wrap the circuit in some kind of tape during use, so as to cover the electronics, but allow the connector and antenna to protrude. When the HMT's battery runs down, we disconnect it from the animal's head, remove the silicone tape, remove the battery from its retainer, load a new battery, and wrap in fresh tape before re-mounting on the animal. By this means, the HMT can be used for continuous recordings of indefinite length, so long as we can tolerate five-minute interruptions for battery replacement every two or three weeks.


Figure: Head-Mounting Transmitter Sketch. The transmit antenna protrudes behind the body of the device. The mouse's nose will be to the left in the drawing, and its tail to the right. The connector axis is parallel to the mouse body axis.

Unlike the SCT, the HMT has no magnetic switch. When we slide the battery into the retainer, it starts running and it keeps going until either the battery runs down or we remove the battery. We can deduce the state of the battery from the telemetry signals, so we will always have a day or two warning before we need to replace the battery if we want to maintain a continuous recording.

PropertyValue
Volume of Transmitter Body1.6 ml
Mass of Transmitter Body with Teflon Wrap2.2 g
Maximum Dimensions14 mm × 14 mm × 8 mm
Maximum Operating Current235 μA
Battery TypeCR1225 Coin Cell
Battery Capacity2000 μA-days
Minimum Operating Life8.5 days
On-Off Switchload or remove battery
Number of Inputs4
Input Impedance10 MΩ || 2 pF
Sample Rate (Each Input)512 SPS
Sample Resolution16-bit
Input Dynamic Range27 mV
Input Bandwidth0.3-160 Hz
Input Noise≤8 μV rms
Total Harmonic Distortion<0.1%
Absolute Maximum Input Voltage±4.3 V
Table: Specifications of the A3040D3 Four-Channel 0.3-160 Hz Head Mounting Transmitter.

The HMT provides four amplifiers, with either one or two reference potentials. Current consumption is linear with total sample rate, with intecept ≤25 μA and slope ≤0.12 μA/SPS.


Figure: Top View of A3040P Wrapped in Parafilm Tape.

Any water-proof tape that adheres well to itself is a likely candidate for wrapping the HMT during use. Self-fusing silicone tape is rugged and water-proof, but adds 0.5 g to the transmitter mass, see here. Plumber's teflon tape is thinner, and adds only 0.2 g to the mass. The British variety of plastic wrap, which they call clingfilm adds only 0.2 g, see here. Parafilm adds 0.2 g see here. Gardener's tape adds only 0.2 g as well, see here.

Warranty

[02-MAY-22] The HMT comes with a one-year warranty against unfortunate events. And unfortunate event is any event during the course of an experiment that damages the HMT but which appeared at the start of the experiment to be unlikely. In the case of an unfortunate event, return the HMT to us. We will either repair your HMT and return it to you promptly, or replace your HMT and send the replacement promptly. The replacement will be covered by the warranty of the original HMT.


Figure: An Unfortunate Event. Connector broke off the HMT while removing from animal.

The HMT is designed to be used repeatedly. We replace the battery and wrap it again. Unlike our implantable transmitters, the HMT is not encapsulated in epoxy, and is not hidden away beneath the skin of an animal. The HMT is vulnerable to scratching, abrasion, and possible accidents involving water and urine. If an HMT becomes detached from an animal during an experiment, it might be chewed and destroyed while it is loose upon the floor of the animal cage. While cutting off our wrapping, we might inadvertently cut the antenna, or we could crack one of the HMT's tiny, ceramic components. The vulnerability of the HMT to physical damage during its multi-use lifespan introduces a financial risk into any experiment we plan with the device. In order to relieve our customers of this financial risk, and instead take this risk upon ourselves, we warranty the HMT for one year after shipping against inadvertent damage as well as manufacturing defect. If, despite taking reasonable precautions, you find that six of your ten HMTs have been dismounted and chewed within a year of your purchasing them, send them back to us. We will confirm the nature of the damage, repair them if possible, and replace them otherwise.

Versions

[30-MAR-22] All versions of the A3040 have the same profile: 14 mm × 14 mm × 8 mm with 14-mm rear flex antenna. But they may be programmed to enable one to four input channels at various sample rates and bandwidths. We can equip them with CR1025, CR1220, or CR1225 batteries, but we doubt we will ever see the device deployed with anything other than the largest battery, the CR1225. The CR1225 provides 1.6× the capacity of the CR1025, at the cost of an increase in mass of only 10%. The table below gives example versions with their signal bandwidths and operating life.

Version W X Y Z Battery
Capacity
(μA-dy)
Mass
(g)
Minumum
Operating
Life (dy)
A3040A1 0.3-40 Hz, 128 SPS, 0.41 μV/cnt Disabled Disabled Disabled 2000 (CR1225) 2.2 50
A3040A2 0.3-80 Hz, 256 SPS, 0.41 μV/cnt Disabled Disabled Disabled 2000 (CR1225) 2.2 36
A3040A3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Disabled Disabled Disabled 2000 (CR1225) 2.2 23
A3040A4 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt Disabled Disabled Disabled 2000 (CR1225) 2.2 14
A3040B3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Disabled Disabled 2000 (CR1225) 2.2 14
A3040C3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Disabled 2000 (CR1225) 2.2 11
A3040D3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 2000 (CR1225) 2.2 7.4
A3040P1 0.3-40 Hz, 128 SPS, 0.41 μV/cnt Disabled Disabled Disabled 1250 (CR1025) 2.0 31
Table: Primary Version Codes of A3040 Head-Mounting Transmitters. Minimum operating life at 25°C in days. Typical operating life is 10% higher.

In the table above, we specify mass with Teflon or Clingfilm wrap. A silicone wrap is more rugged, but adds another 0.3 g to the mass.

Connector

[30-MAR-22] The A3040A provides a nine-way, dual-row, 0.025" pitch, surface-mount socket with a single guide post for connection to the electrode interface. We use custom Omnetics NSD-09-VV-G, which we order with part number A78538-001.


Figure: The NSD-09-VV-G Socket Mounted on A3040AV1. Note guide pin and nine socket tubes. Pin one is front-left.

We use the pin numbering given by the manufacturer. The figure below is an exerpt from the S3040A_1 schematic, showing pin numbering as seen looking down on a soldered socket. We add pin "G" for "guide post" so that we have a label for the tenth pad on the printed circuit board. This pad is not used by the connetor, because there is no connection pin below the guide post.


Figure: Pinout of NSD-09-VV-G Socket Manufacture's Pin Numbering. View as seen when holding the A3040A and looking down on the connector.

The NSD-09-VV-G socket mates with any NSD-09-G plug, provided the plug's guide post hole is in the G position. We designed the A3040A to mate with Neualynx's EIB-8 electrode interface board. In this figure, we see the top of the EIB-8's nine-way plug, with the guide post hole in position G on the top-right. The EIB-8 provides ten connection pads. The two GND pads are connected to P1-1, which the A2040A connects to VC, the common voltage of the amplifiers. The eight electrode connections A1-A8 are connected to P1 like this: A1/P1-2, A2/P1-3, A3/P1-4, A4/P1-5, A5/P1-9, A6/P1-8, A7/P1-7, A8/P1-6.


Figure: EIB-8 Connections for HMT Inputs (X1-X4) and Reference Potential (GND)./

Within the HMT circuit, we refer to the reference potential as "VC", the "common voltage". Once we connect this potential to an animal body, we say it is "grounded". Our assumption is the the GND pads on the EIB-8 will be used for a low-impedance connection to the animal body.

Antenna

[30-MAR-22] The antenna on the A3040 is a flexible circuit with a rounded end. Counting the neck at its base, the antenna is 14 mm long. It is 4 mm wide. It carries a zig-zag conductor of total length 70 mm, which is slightly less than one quarter of the wavelength of our telemetry transmissions. We transmit in 902-928 MHz, for which the wavelength is around 330 mm.


Figure: Transmit Antenna. Ruler marks are millimeters.

The A3040AV1 provides footprints for a series-parallel matching network between the circuit's radio-frequency (RF) oscillator and the antenna. But the AV1 does not attempt to match the antenna to the oscillator: the two are coupled with a single 1 nF capacitor.

Analog Inputs

[23-MAR-22] The A3040D3 is equipped with four amplifiers with gain ×100 and frequency range 0.3-160 Hz. The A3040D3 samples all four inputs with sample rate 512 SPS. Each sample is a sixteen-bit number 0..65535, where the number zero corresponds to the bottom of the input dynamic range, and 65535 corresponds to the top of the range. The plot below shows their response to a 15-mVpp sinusoidal input with 10-MΩ source resistance as we increase the sinusoidal frequency from 0.25 Hz to 1 kHz. Because the input resistance of the amplifiers is 10 MΩ, we see 7.5 mVpp on each input.


Figure: Frequency Response of Two A3040AV1. The first provides X1-X4 with channel numbers 242-245. The second provides X1-X4 with channel numbers 1-4.

The dynamic range of the inputs is equal to the battery voltage divided by the amplifier gain. For the A3040D3 at 10 Hz, the gain is ×100, so the dynamic range is 27 mV. All inputs are referred to a single reference voltage, VC = 1.8 V. The A3040D3 provides a high-pass filter at its inputs. The average value of any of its input is zero. The amplifiers use VC = 1.8 V to represent an input of zero. With battery voltage 2.7 V, an input of zero will be converted to 1.8 V / 2.7 V × 65535 = 43690 cnt (ADC counts). Count zero corresponds to −18 mV at the input, and count 65535 corresponds to +9 mV.

Synchronization

For advice on how to synchronize video, stimuli, and environmental phenomena to the HMT signals, see the Synchronization section of the A3028 manual.

Input Noise

Input noise from the A3040D3 in a Faraday enclosure with its leads in water is 20 cnt rms. We multiply by 0.41 μV/cnt to get 8 μV rms input noise. The plot below shows the spectrum of this noise.


Figure: Spectrum of A3040D3 Input Noise, All Four Channels. Vertical 0.41 μV/div. Horizontal 10 Hz/div.

Power Measurement

For advice on how to calculate the power of a biometric signal, see the Power Measurement section of the A3028 manual.

Battery Life

The HMT current consumption from a 3-V Lithium Primary cell will be no greater than:

Ia = 25 μA + (R × 0.12 μA/SPS)

We divide the nominal battery capacity by the maximum active current to obtain our minimum operating life. The typical operating life is 10% higher, because the typical active current is 10% lower than that given above.


Figure: Active Current Consumption of A3040AV1 versus Total Sample Rate. Straight line fit gives intercept 17 μA and slope 0.110 μA/SPS.

Wrapping

[30-MAR-22] The A3040A has no permanent epoxy or silicone encapsulation. Before loading the transmitter on the animal, we fold the circuit as shown in our sketch. We take a 40-mm long, 25-mm wide piece of self-fusing silicone tape, such as that made by XFasten. We place the center over the connector and press the tape down on either side of the connector until the tape tears and the connector breaks through. We trim the tape around the connector with iris scissors. We wrap the tape around the circuit and fuse on the opposite side from the connector. The antenna is sticking out of one of the open ends of the wrap, and the battery is visible through the other. We squeeze the edges together to close both ends. When they are well-adhered, trim excess with iris scissors so as to reduce the HMT mass.

When we replace the battery, we cut open the tape with iris scissors. We want to avoid cracking one of the circuit components with the tip of the scissors, so we make our cut on the top side, around the battery. As soon as we have access to the battery, we remove it, cut the tape some more, and remove the tape as well. We now we have the opportunity to wash the circuit if we like. We rinse in hot water while scrubbing gently with a tooth brush. We blow dry with compressed air. Now we soak the circuit and its next wrapper in ethanol for a few minutes, blow dry, load battery, and wrap with sterile gloves.

Design

[30-MAR-22] The following files define the A3040 design. Note that we distribute all these files under the GNU Public License. Any design that incorporates any part of our work must itself be distributed under the same GNU Public License.

S3040A_1.gif: 4×0.3-160 Hz, BGA-64, flex antenna.
A304001A: Gerber files for A3040A PCB.
A304001A_Top: Top view of A304001A.
A304001A_Bottom: Bottom view of A304001A.
Code: Logic chip firmware library.
CR1225: Diameter 12 mm battery electrical data sheet.
A78538: NSD-09-VV-G, nine-way, dual-row, 0.025" pitch, surface mount connector with guide post.
BC-2009: Battery retainer for CR1025, CR1220, and CR122, showing cuts we make before loading.

The A304001A is a rigid-flex circuit board that provides two rigid areas for transmitter components, one rigid area for programming and calibration, which we cut off once the transmitter is configured correctly, and a flexible antenna.


Figure: Rigid-Flex Printed Circuit Board for Prototype HMT.

The A304001A's battery holder supports the CR1220, CR1225, and CR1025 coin cells.

Modifications

[16-MAR-22] To make the A3040AV1 with the A304001A PCB:

  1. Remove R7, connect U3-7 to D0, a logic output on Bank 0. This output is accessible by wire modification, but provides only 1.8-V logic. The 1.8-V HI level proves sufficient to drive the ADC when powered by a 2.7-V coin cell.
  2. Remove L1 and replace with 1.0 nF.
  3. Remove C26.

[01-MAR-22] When updating A304001A to A304001B we should perform the following modifications.

  1. Add polarity to U5 silk screen and permanent footprint silk screen.
  2. Connect SCK through A4 to B5, an output in Bank 1 that provides 2.7-V logic output.
  3. Need a ground pad on the programming extension.
  4. Need a silk screen white area on programming extension.
  5. Add ground pad to top layer next to analog multiplexer.
  6. Remove J1 from solder paste stencil so it is flat gold.
  7. Add pull-down resistor on TCK on programming extension.

Development

[19-DEC-21] Sketch of prototype here.

[03-JAN-22] Finish first draft of A304001A rigid-flex circuit board, equipped with four amplifiers. We are using the ADG804 four-to-one analog multiplexer to select one of four signals for the ADC. We are using the LTC1865L in SOP-8 package because we want to conserve our stock of MSOP-10 packages for our smaller transmitters. We find that the op-amp and passives for each amplifier weigh 34 mg.

[06-JAN-22] We receive battery retainers and modify them by cutting off end flanges and solder to an existing 12.5-mm square transmitter board. The corners of the retainer protrude by 0.5 mm. We prefer to bend the retaining spring so that it pushes down on the center of the battery. After these modifications, the battery is secure in the retainer, with its negative tab pressed on the center of the circuit board, where we expect to have our gold-plated negative battery pad.


Figure: CR1225 Mounted in Battery Retainer.

[03-FEB-22] Change pinout of our Omnetics socket connector, J1, to match the manufacturer's A78538 drawing, modify all sketches, schematic, and printed circuit board files.

[16-FEB-22] We have A304001A rigid-flex boards in panels of five. Load Omnetics connector we burn off an EIB-8 onto the footprint of J1. Trim battery holder, bend down its spring, insert CR1225. Wrap in blue, self-adhesive silicone. Mass is 2.3 g.

[28-FEB-22] Kit for 15 of A30304A on 10-day turn is ready to ship.

[16-MAR-22] We have fifteen A3040AV1 assembled circuits. We have assigned an input-only pin on the logic chip to the SCK output. We remove R7 and use the D0 tuning bit for SCK, connect the R7 pad to U3-7 with a wire link. The D0 bit is U4-G1, on Bank 0 of the logic chip, which runs on 1.8 V, so the HI level will be only 1.8 V. We program as D3: all channels enabled, 512 SPS. Active current is 231 μA, less than the 270 μA we predict with our current calculation formula.


Figure: Top Side of A3040AV1. Red wire corrects design error.

We load a battery holder and insert CR1225 battery. Obtain robust reception in FE3A enclosure with one pick-up antenna in a few locations, no systematic test of reception. Right now, the antenna matching network is our first guess: 10 nH series inductor with 1 pF parallel capacitor.


Figure: Bottom Side of A3040AV1. Connector J1 not loaded.

Our nine-way Omnetics connectors with guide pin, A78538, are due to ship to us on 21-MAR-22. We touch the pads of the connector and see voltage induced in all four input channels on our Receiver Instrument display. The ADC appears to be working despite the fact that SCK is being driven by only a 1.8 V logic signal when the ADC expects 2.7-V logic.

[20-MAR-22] We clip the programming extension of our working, programmed A3040D3, No224. We wrap in silicone and trim. We are still missing J1. Mass is 2.51 g. Of this 1.10 g is the A3040AV1 assembly, 0.87 g is the CR1225 battery, and 0.54 g is the silicone wrap. When we load a CR1025 battery, the mass of the circuit without wrap is 1.76 g, the mass of the battery is 0.65 g. Apply wrap, total mass 2.30 g. Running off the CR1025, we record from all four channels and see VBAT = 3.0 V (fresh battery). We prepare two more AV1s and program as D1..D4, measuring current consumption, which we find to be linear with sample rate, 17 μA + 0.110 μ/SPS. Our A3040A02 firmware brings the transmit channel numbers and input channel numbers into agreement. If the base transmit channel number is b, the channel numbers for X1, X2, X3, and X4 inputs will be b, b + 1, b + 2, and b + 3.

[22-MAR-23] We solder silicone leads to our No224 AV1 and connect these to a 15-mVpp, 10-MΩ signal source so as to measure gain versus frequency. With 10-MΩ input resistance, we expect to see 7.5 mVpp at X1..X4. Assuming VB = 2.7 V and gain ×100 in the middle of the amplifier pass-band, we expect amplitude roughly 6.5 kcnt rms, with cut-off frequency 160 Hz, and this is indeed what we see.

[23-MAR-22] We experiment with our antenna matching network, L1 and C26 in schematic. We program an A3040AV1 to turn on its transmitter (U6) continuously. We connect an external tuning ramp 0-5 V to the TUNE input of U6. We place the antenna of the A3040AV1 near the coil antenna of an A3038DM-C detetector module in which we have replaced the 900-930 MHz bandpass filter with a wire link. We look at the logarithmic power measurement, P, on the detector module. The peak power received is a strong function of the location of the transmitter. The variations in power received are a weak function of location. We place the transmitter in a petri dish with the antenna sticking up in the air, about 10 cm from the detector antenna.


Figure: Three Tuning Networks. Blue: tuning ramp. Yellow: logarithmic power measurement, 500 mV/div = 16 dB/div. Cursors mark 900-930 MHz band. Left: L1 = 10 nH, C26 = 1.0 pF. Center: L1 = 10 nH, C26 = 2.0 pF. Right: L1 = 0 nH, C26 = 0 pF.

Our original tuning newtork was 10 nH with 1.0 pF. This network produces a 20 dB reduction in transmit power in the 902-930 MHz band when compared to no network at all (0 nH and 0 pF). When we increase the tuning capacitor to 2.0 pF, we see a slight increase in power emitted in our 900-930 MHz band compared to no network. We equip one A3040D3 with 10nH/2pF and another with 0nH/0pF. Both have had their programming extensions removed. We fold up and wrap in silicone. We place on a petri dish in FE2F enclosure and move around, watching reception from a single A3015C damped loop antenna connected to an Octal Data Receiver. We try different arrangements of the antenna and transmitters. In each arrangement, we have both transmitters oriented the same way and close to one another. With transmit antennas vertical, we obtain robust reception from both transmitters in ten arrangements. With antennas horizontal, we obtain robust reception from the 10nH/2pF network in 18 of 20 arrangements, and from the 0nH/0pF network in 10 of 20 arrangements. We damage No5. We load 10nH/2pF onto No224 and No1. We clip No1, fold both, and compare. We obtain robust reception in only half the arrangements, and equally for both. We try 100pF/10nH (100 pF in place of L1, 10 nH in place of C26) on No1. Reception is far inferior to No224 with 10nH/2pF. We try 100pF/0pF on No1. Reception from No1 100pF/0pF and No224 10nH/2pF robust with antennas vertical. With antennas horizontal, we see robust reception from 100pF/0pF in two thirds of arrangements, and robust reception from 10nH/2pF in only one third of locations. Load No224 with 1nF/0pF. Reception from No1 100pF/0pF and No224 1nF/0pF is equally robust with antenna horizontal.

[24-MAR-22] We receive 15 Omnetics connectors. Solder one to No1, takes about five minutes. Wrap with 40-mm tape and CR1225 battery, mass 2.6 g. The connector weighs 57 mg. We solder leads to an EIB-8 and plug our A3040D3 into the EIB-8. We connect the EIB-8 leads to a four-way SCT-BNC interface, and by this means we apply a frequency sweep to all four inputs and measure frequency response. At first, amplitude is low and varies from channel to channel. We heat up and blow dry, now obtain uniform response.

[28-MAR-22] We wrap No1 in Saran Wrap. The wrap does not stick to itself well enough to provide a lasting seal, but mass with three layers of Saran Wrap is 2.1 g. Some residual wrap material adheres to the tubes of socket J1.

[30-MAR-22] We have five A3040D3 ready to ship to UCL. Frequency response D1_1.gif. We ship with ten CR1225, ten CR1025 batteries, and one meter of silicone wrapping tape. Calibration constants below.


Figure: Calibration Constants and Active Current for Batch D1_1. Six A3040D3 HMTs.

During calibration, we measure transmit spectrum of several devices before and after we replace the A3040AV1's 10nH/1pF antenna matching network with 1nF/0pF. In all cases, we first perform the SCK/R7 modification. The transmit center frequency is stable to ±1 MHz, and power stable to ±3 dB as we change the matching network and re-measure the spectrum.

[08-APR-22] We wrap an HMT in plumber's teflon tape, and are well-satisfied with the result. Mass with CR1225 battery is only 2.2 g.

[11-APR-22] We receive this picture from ION of an A3040D3 wrapped in British clingfilm.

[05-MAY-22] We have four hours of recording from ION of No17 and No37 mounted on mice. In the first hour we nave No37 running. In the second hour we have No17 and No37 running.


Figure: Reception from Mounted HMTs in IVC Rack in London. Average is 95% from No37 and 89% from No17.

[06-MAY-22] We receive another six hours of recording from the IVC rack at ION. There are half a dozen A3028C transmitters running along with two A3040D3 HMTs. We measure reception from HMT channels No17 and No37, and compare to SCT channels 201, 203, 205, and 210.


Figure: Reception from Transmitters in IVC Rack in London. A3040D3 HMTs are 17 and 37. The others are A3028C SCTs.

Reception from all transmitters drops dramatically from 2.2-2.5 hr. We assume the enclosure door was opened for access. From time 2.5-6.0 hr, reception from each channel is as follows.

Channel   17   37   201  203  205  210
Average 95.2 97.6  99.4 99.1 99.2 97.0

In the period 0.0-2.2 hr, reception from the two HMTs is inferior to reception from the SCTs. After 2.5 hr, reception from HMTs is slightly inferior on average (97% compared to 99%). When we implant an SCT, the presence of water around the device slows down the electric waves propagating back and forth along the antenna. We find that antennas of thirty to forty millimeters are ideal for subcutaneous operation. The HMT antenna is operating in air, where the resonant length of a straight quarter-wave antenna is 80 mm. The HMT antenna is straight, but it is only 14 mm long. The antenna conductor is 80 mm of zig-zag track, but this zig-zag does not behave the same as a straight wire. It was our hope that the 14-mm zig-zag antenna would work fine without any matching network at its base. But it now appears that we are going to have to spend a few days figuring out what inductor and capacitor value we should load for L1 and C26 in order to get the antenna to resonate.