Antenna Matching

Today we try a split capacitor antenna matching circuit to see if we can amplify the RF signal on our antenna before presenting it to our detector diode. Such amplification is, in theory, possible, because the source impedance of the antenna signal is of order 100 Ω, this being the resistance of the antenna wire to 146 MHz, while the load impedance of our detector diode is of order 10 kΩ. We describe our analysis and testing of the split-capacitor network in detail here. The following oscilloscope trace shows the dual resonance of the matching circuit we built.


Here we see the detector diode output as a function of frequency when we have a two-loop stainless steel antenna picking up power from a nearby transmitter. The top trace is a frequency sweep voltage. The bottom trace is the detector diode output. The first peak is at 146 MHz and corresponds to a gain of 20 generated by the split capacitor tuning network. Thus the signal applied to our detector diode is twenty times larger in amplitude than the signal on our antenna. The second peak is at 170 MHz and corresponds to the resonance of the antenna inductance with our split capacitor.

We equip an Implantable Lamp (A3024) with the split capacitor circuit and test its reception when transmitting pulses of 146 MHz power at 1.6 W into a half-wave antenna. We find that reception is 100% up to range 3 m, and remains 50% at 14 m. The matching network has increased the effective range of our command receiver by a factor of six. We can now expect 100% reliable reception within a faraday enclosure at up to 1.5 m, which will be sufficient to communicate with the ISL.

Faraday Canopy

We tried out our new FE3A faraday canopy today. The photograph below shows the canopy hanging from the walls of our office. Inside you can see an implantable lamp flashing in response to power transmitted by a half-wave antenna.



The walls and ceiling are made of a transparent veil fabric woven from thin wire. The base is a layer of absorbent sheet on top of reflecting foil. For a better idea of the size and shape of the canopy, we invite you to watch the following video.



The implantable lamp is an A3024Y, which has no tuning capacitor or inductor, and is therefore sensitive to all RF frequencies. The white lamp flashes when it is within a few centimeters of our 2.4 GHz wireless router. Inside the the canopy, however, it never turns on unless we stimulate it with our own RF power. We transmit 146-MHz with a half-wave, telescoping antenna driven by a Command Transmitter (A3023CT) with booster amplifier. The booster amplifier delivers 1.6 W to the antenna and is only 10% efficient, so its black heat-sink vanes get hot to the touch.

We hold the A3024Y between thumb and forefinger and move it at random 30 cm from the transmitting antenna. We obtain 100% reliable reception. We move to range 50 cm and obtain 95% reliable reception. If we hold the A3024Y in our hand, enclosing its two-loop receiving antenna, reception at 30 cm drops to 95% and at 50 cm drops to around 80%. We expect that reception from an implanted A3034Y would be somewhere in between these two tests.

We measure the isolation provided by the canopy enclosure with respect to external interference in the 900-930 MHz range, this being the range used by our SCT system. We are hoping to find that interference power drops by a factor of one hundred within the enclosure, but instead we observe a factor of ten or twenty. For more details and discussion see here.

We are gratified to find that the ISL command transmission works just as well inside the enclosure as outside. We will work on improving the isolation the enclosure offers at the SCT data frequency. We might, for example, try adding a layer of absorber to the ceiling.

Implantable Lamp Encapsulation

Here is our first encapsulated Implantable Lamp (A3024Y), equipped with a BR1225 battery with capacity 48 mA-hr. This device is equipped with a white LED rather than an implantable head fixture.



The volume of the main body of the device is roughly 1.7 ml. The volume of the Subcutaneous Transmitter (A3019A), meanwhile, is 1.3 ml, and that of the Subcutaneous Transmitter (A3019D) is 2.4 ml. The former is tolerated by mice and the latter by rats. Our hope is that we can implant our 1.7-ml Implantable Lamp along with a 1.3-ml Subcutaneous Transmitter in a rat to monitor EEG and apply optical stimulus.

The A3024Y's 146-MHz command antenna is 30 mm in diameter compared to only 20 mm in diameter for the A3019A and A3019D 915-MHz data antenna. We do not know how well any animal will tolerate the larger antenna.

We move the A3024Y around at random near our 1.6-W source of 146-MHz radio-frequency power. At range 50 cm, the lamp flashes in roughly 95% of orientations. We are hoping for over 98% reliability at range 50 cm. We should be able to increase the reliability of reception by improving the tuning circuit and by increasing the power of our radio-frequency transmission.