[Corvettaholic]

There's plenty of homemade radio kits that will receive AM radio and output it with a simple speaker. There has to be some quantity of energy behind a radio wave, and I'm wondering if I can use it. I think I touched on this a long time ago, but this is now fresh on my mind after thinking about the nuclear battery thread. So if I had an antenna, and dumped the output into an AC capacitor, think it'd work? Dump the capacitor into an LED with a flip of a switch of something.

[kwhubby]

Hi, well yes you can use it. Just like in a crystal radio set you can listen to the radio station without any batteries - the radio energy is enough to power the circuit and sound through one of those super sensitive ear piece speaker things. The problem is, unless your right next to a big broadcasting station, your not going to get much energy at all. The energy from the station is indirectly proportional to the distance from it and therefore tenths of microvolts are measurments for detecting radio waves. Lets say you were 1 inch away from a large 50kw FM broadcaster antenna and you could somehow capture 5kw of the radio energy. Five miles away from this station with the same capturing device (1/(5*5280*12)^2 = 9.96e-12 * 5000= 4.98e-8 ) you would get about 4.98x10^-8 watts of energy or 4.98 hundredmillionths of a watt. This little amount of energy would really be completly usless to try and power things with. If you could build a very large resonant antenna near a very powerful broadcaster you can capture significant amounts of energy(take a flourecent light bulb near a very large television broadcast tower and watch it glow) that with rectifiers and a capasitor you could capture. But this in most cases is not practical.

[Corvettaholic]

Practical isn't the issue here, just proof of concept. Proof to myself. If I can charge up a 1nF capacitor in a week, i'll be happy. Think I'll tackle this after the nuclear battery project.

[kwhubby]

You may succeed if your close to a high power broadcaster, but most capasitors usually loose there charge over time without drain and so you would need to find a suitable capasitor that won't drain faster then its charged.