Serious question from a beginner in electronics. For reasons I do not fully understand, I have become fixated on the idea of collecting small amounts of electricity from “interesting” sources. I don’t mean “free energy”, instead, I mean things like extracting a few mV from being so close to a AM radio tower using two tuned loop antennas in phase with each other, or getting a few mV from the rain’s kinetic energy with PTFE and using two electrodes which are shorted when a drop of rain hits it. In short, I’ve done small experiments to confirm that I can get a few mV and enough to get me excited but not much more. I know I’m not going to get much power out of this, but I’ve been able to charge a NiMH battery a few mV by being a quarter mile from an AM radio station with my antenna setup. It would be fascinating to me if I could store these small charges in something like a 5V USB power brick eventually.
The smarter idea would be for me to harvest energy with the sun or from the wind or a stream. I’m tinkering with this as well, but larger amounts of electricity scare me for right now. I guess I’ve seen enough experimental sources of harvesting electricity and I’ve gotten the itch to invent, which is a dangerous itch for a newbie like me to have.
The best advice I’ve seen online (ok, it was ChatGPT) is that it’s just not worth it to work with such small amounts of electricity, because the equipment required is too expensive and sophisticated (e.g, devices to read the charge of a capacitor without discharging it) to make anything that’s efficient enough to be worthwhile. Would you agree? Do you know of some other fascinating source of gathering electricity that I should also waste lots of time on?
I just have all these electronic components and magnets and when I move them together the numbers on multimeter get bigger. it’s neat.
I’m skeptical of the “not-spot” claims here. This would suggest that radios also create “not-spots” when being tuned to as well, or that somehow the florescent light tubes were able to “pull” more electrons from the air that were destined to other radios.
Yeah, exactly. This isn't how radio signals work at all. This is a tall tale and never happened. When I searched for such a story, none came up. When I removed the location and searched for the same type of story, thousands of them are floating around the internet. Powering military tents from radar emissions, powering homes from radio tower emissions, all of them are re-tellings of stories someone once heard from the 1930s, 1940s, and so on. They're effectively chain letters for the confused.
I've heard teachers telling the same story, but so it happened locally in a different country entirely. Sounds like a scare legend.
@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...
That is the most plausible explanation I have heard but I still have questions. Say there's an MW tower down the road and I have a 160m tower in my backyard. If I understand correctly, my tower may cause the signal coming from the AM tower to be re-resonated back to the AM tower so the AM tower needs to be detuned. But say I want to harvest the signal and I have tuned my tower to be resonant with the AM tower. Maybe in this case the SWR reading at the AM station is different because it is getting some of that re-radiated power back, and maybe the radiation pattern of the am station has changed slightly, but wouldn't the main AM tower cover any gaps just like how waves spread out in the double slit experiment once they hit my resonant tower?
I get that a tower excites another tower, and I can understand that the AM engineers will likely hate me, but I don't understand how radio reception could be affected. If anything, I might have made the station more directional (like a reflector in a yagi) but probably not.
@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.
@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
It may surprise you to know that in the US as a ham, I have the legal right to hoist an antenna or build a tower so long as it doesn't fall on a power line.
But even then, I don't think this setup will create nulls. Say the antenna is 400 meters away which I think is still in the far field but I could be wrong. Even if I erected an almost resonant tower (160m) and assuming the regulatory bodies gave me the permit to do so, assuming it's not powered and simply is resonant, maybe the radiation pattern changes but not so dramatically that my neighbors on the opposite side of my antenna (from the tower) will get poor reception.
Is the direction of the radiation pattern changing what was meant by the "not-spots"?
@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.
@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.
@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)
Hold the phone! You're telling me now that the government lies to people?!
I have also heard of this tale of "not-spots" but have found no evidence myself. The SWR and near field antennas stuff you mentioned makes a lot of sense, I just didn't understand how I could be stealing electrons meant for others. I mean, if it worked that way wouldn't trees also creat not spots, especially if they get to a certain height?! Anyway, thanks for the info, and.. 73?
@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