Today I completed Experiments #3 and #4.  For details, visit the Antenna Design slack channel.

Experiment 3 was used to determine if enough power could be stored in a capacitor to operate the J-Box RF relay. For this experiment, a variac was used as a variable AC power source to charge the harvesting circuit. The 470 uF capacitor holds enough power to operate the chosen Panasonic 1-coil latching relay at least 4 times – more than enough to establish feasibility of the capacitor to act instead of a traditional battery to power the relay.

Experiment 4 involved charging the exact same circuit, then operating the relay with power harvested from the antenna.  This test also passed.  At just 7 volts, the relay was successfully operated. Note that in a fully-designed harvesting circuit, a voltage doubler circuit will boost the harvested RF voltage up to the full 12 volts.  Here’s a picture of the setup for Experiment 4 below.

In the foreground, we see the end of one leg of the dipole antenna, along with the RF harvest coil and lead wires, which are connected to the white breadboard.  On the left side, there’s a 9V battery, which is used to power a green LED that indicates when the relay has closed.  To the right, is the multimeter.

Below is a close-up of Experiment 4:

Using the energy harvested from the antenna at 10 watts, at just 7 volts (no voltage doubler), the relay operated perfectly and reliably.

You can’t operate the relay while the circuit is charging… the capacitor must charge up to full voltage for a couple of seconds, then there’s enough stored energy to operate the relay.

This concludes Phase 1 validation of the feasibility to harvest enough energy from the ham radio antenna element toward the endpoint where J-Box will be located.
Now it’s a matter of designing proper circuits to create two supplies: 3V to operate the TI MCU and WiFi radio and 12V to operate the relay. Two separate capacitors and a simple zener regulator circuit is all that should be required.

This will enable a completely wireless J-Box with no batteries, powered by the antenna itself.