Electronics > RF, Microwave, Ham Radio

weird "energy-harvesting" broadband (?) antenna

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I recently tore down the "CleanSpace" tag "powered by Freevolt", a rather large device which contains a CO sensor, a BLE interface and of most interest a broadband antenna which is claimed to allow trickle-charging the battery from RF sources in the environment: https://store.clean.space.

The antenna is exceedingly weird to me. There is a set of two concentric copper foil circles on the inside of the casing. Opposite that are two radial slots in the groundplane of the PCB. On the opposite side of the PCB is a thick trace (antenna?) which intersects the radial slots on the other side. This track is shorted to ground through an inductor and a stub, followed by what looks like some lumped element filters, a sot-23 device with two pins connected (presumably a rectifier diode) and a pi filter network (?).
The PCB material feels unusual, a little Teflon-like, perhaps Rogers?

Does that antenna arrangement make sense to anyone? My naive assumption would have been that the ground plane would absorb most of the RF energy that could be received by this arrangement but I'm far from an RF wizard...

Camouflaged listening device. Works as expected, Beeper run as microphone, second Antenna broadcasts picked-up audio somewhere in the GHz range :-)

The chip near the antenna is a harvester http://www.ti.com/lit/ds/symlink/bq25504.pdf

There is a lab at Georgia Tech that has done a great deal in this area. Those TI chips can run off of almost nothing. They implement a joule thief, and then store the power. So its for real. They use printed broadband antennas.

Just don't try to run anything that takes a lot of power off of it.

Going to guess it relies on the store WiFi system to provide the battery life extension, harvesting the 2.4GHz RF energy to power it. the CO detector and the BTLE  themselves are very likely run at a very low duty cycle, so that in normal non power harvesting use they likely would run for a year or so off the Lipo cell in any case. Add the energy harvester, and provide it is near to a wireless AP or a cellular repeater it likely will last an indefinite time as the cell gets a slow pulsed charge to charge up in the intervals between transmit pulses. It probably only pulses the CO sensor every minute, as it is a slow sensor, and the BTLE is likely being run at lowest energy use.


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