Re: And is illegal in the UK
"As in the celebrated case of a a farmer using fluorescent tubes with some wire attached to the ends to light his cowshed. He was in the near field of a some large (IIRC BBC) transmitter."
You're right but I was told during a lecture that it was in the U.S. and it was signals from the ultra long wave submarine service (in the 10s of kHz). Perhaps, that's just another case.
Anyway, the net result was that it's considered stealing electricity (by longstanding case law). Seriously, this is a potential problem:
(a) with enough RX antennas absorbing energy, the effective service area will be reduced, and;
(b) increasing the TX power to overcome the increased 'absorption' will lead to excessive and necessary power levels, which, in the extreme, will increase the radio spectrum noise floor. (Increasing noise floor in spectrum management is already a significant issue.)
This getting-power-for-nothing idea has been around for some considerable time. Except for induction charging batteries (a la electric toothbrushes etc.) it's not been very effective. Always guaranteeing sufficient Volts/m to power devices is a problem as signal strength can fluctuate wildly. Whilst signal strength fluctuations are unlikely to cause problems to the the RF link (with AGC, limiting etc.), that cannot be said for devices which have to absorb power from surrounding RF to work.
These devices aren't absorbing low frequency stuff, rather UHF (from the antennas). UHF, of course, is much more prone to nodes and anti-nodes thus more unpredictable/unreliable than the ultra low freq. case to which I referred.