vfrmedia

vfrmedia, 10 months ago

@drdabbles @rarely

A few years back some farmer living in Droitwich, England (where the Radio 4 longwave transmitter is situated) lit his barn by connecting an antenna to fluorescent light tubes.

It worked, but also created a "not-spot" in the radio reception which the BBC really didn't like (its part of critical national infrastructure!) - officers from Ofcom turned up at his door, made him take the lot down and ordered him to use more "normal" power sources..

vfrmedia, 10 months ago

@rarely a radio receiver uses much less of the power than lighting up the fluorescent tubes would (it wasn't just one lamp) and this incident happened close enough to the TX that it could upset the SWR of the transmitter output stages - if it /was/ possible to do this without creating problems elsewhere then every tall transmission tower would use the RF to power their aircraft warning lamps rather than a separate power supply...

vfrmedia, 10 months ago

@rarely if the link below federates correctly, here is a receiver that uses just the power of the signal, but from a 4kW transmitter 32km away there are only a few tens of milivolts, enough to be amplified by the line in of a desktop PC to listen to the audio but certainly not enough to light any lamps (even an LED). Maybe I could light an LED from our wifi signal close to the access point, but I don't have any RF detector diodes to hand that work at 2,4 GHz

https://social.tchncs.de/@vfrmedia/110873682071137732

vfrmedia, 10 months ago

@rarely if you are /that/ close to the antenna an extra tower, or any large amount of metal making the station more directional will definitely be unwanted, both by tradio station engineers and the Communications Ministry (licenses often require a particular directional pattern). But this is more an issue with LF and MF where waves are larger. At UHF/SHF frequencies for wifi harvesting could work but at present the component count required makes it less viable than other power sources.

vfrmedia, 10 months ago

@rarely this is why things behave differently when very close to the TX (how close will of course depend on the TX power and frequency/wavelength)

https://en.wikipedia.org/wiki/Near_and_far_field

vfrmedia, 10 months ago

@rarely the historical reports of issues I've read about are from mid-late 20th century in areas near high power LF/MF stations that would be in the nearfield - from the Wiki article

> absorption of radiation in the near field by adjacent conducting objects detectably affects the loading on the signal generator (the transmitter).

so it would be noticeable, and viewed as an undesirable thing. Harvesting (small) amounts of power in the far field would not cause issues.

vfrmedia, 10 months ago

@rarely Temporarily lighting small lamps from nearfield RF with a TX power of some kW is definitely possible, a family friend who was the engineer at Radio Caroline in the 1960s did it on board the ship as a demonstration to visitors; but didn't use any antenna nor leave the lamps around to light up the deck (it would have created hassle with unwanted stray RF, and there was plenty about already!). Its not common these days as TX sites are designed to keep people out of the nearfield for safety.

vfrmedia, 10 months ago

@rarely the claims of "poor reception" caused by "large scale" nearfield power harvesting are from Communications Ministry officers from some decades ago (I mistakenly referred to modern Ofcom rather than the British Post Office which investigated these things until the 1980s), it is possible they just wanted to discourage this practice for the safety of those involved whilst not also opening a can of worms about human exposure to RF (it was Cold War era and much info was classified)