Could anyone tell me what sort of voltage is induced by radio waves in the coil of a basic tank circuit like you would find in a cheap transistor radio? How about with a small telescopic aerial, and how about with a huge antenna?

I thought it would be easier to find this info on Google, but haven't had much success.

 

Good question. The coil on a small radio (crystal radio, transistor, etc) will have a very small voltage induced, Toby. Millivolts! You can measure this with a home-brew crystal radio set.

I'm not as sure re. a big broadcast or powerful HAM rig during transmission, though. I believe there is not much voltage as we discuss it per se, but RF ENERGY can be/is emitted at pretty powerful levels! That would be measured in Watts, I believe, rather than volts, since when talking about antennas it's usually about the current flow rather than any voltage that's present. Since the intelligence is mixed with a carrier before transmission, the frequencies we're talking about are quite high...and yes, enough energy is there to zap you!

Someone will read this and give you a better answer, but that's the gist of it.

 

If you are talking about receiving antennas, then the voltage typically varies from the low microvolt region up to perhaps a few mV if you are close to the transmitter. Good FM receivers are rated for a minimum sensititivity of a few microvolts at the input, for example. Receiver gain is usually in the neighborhood of 100 to 120dB so that gives you an idea of the minimum signal at the antenna.

 

Thanks for the replies. So we are looking at microvolts or a few millivolts at best.

So does that mean that a working crystal radio needs to pull in at least 2 or 300mV on it's aerial, in order to forward bias the diode?

 

Put simply, Yes. There are "things that go on" in the tank, too, but for all practical purposes, you're on the right track. You could try putting together a very simple crystal receiver, and you can actually measure the voltage at the tank circuit with your DMM, if your is sensitive enough.

Strangely enough, you DO pull in enough voltage to overcome the diode Vf, and can even receive some distant stations

For a LOT of fun, couple the output after the diode to a voltage amplifier > current amplifier (like a 741 to an LM386), and you just made your own 'transistor radio'. Change the standard coil to a 3-30 MHZ coil (50 turns magnet wire on a film canister with a paper clip armature to select ranges), and you're listening to shortwave and DX.