Multiple FM frequency transmitter

OFDM

Try picoChip (Bath, UK) for suitable processors to perform the IFFT

Near-field antenna

Trying to take 3000A from your PC PSU will melt it long before it gets to your target output.

Do you mean any frequency at a time, or ALL the standard frequencies simultaneously. Ramsey Electronics has a couple of kits that do the first. I don't even WANT to help you do the second :-(

Ramsey Electronics has a line of AM and FM transmitters at

Don Lancaster did a bit on this when he had his column in Radio Electronics (or maybe it had morphed to Electronics Now by the time he covered this specific topic).

Apparently there was a need for a transmitter that output on multiple channels. The example I remember was for real estate, where apparently they used low power transmitters to send a recorded message to passing cars, the sign saying something about turning on your radio, so they get the sales spiel without even stopping or dropping in. And they wanted multiple frequencies so nobody had to fuss over tuning to the proper frequency.

My memory says the column mentioned commercial products that did that, but it's been a while. He went into some ideas on how to do it, none of which I can remember now.

Michael

An article I read somewhere (quite possibly the same article you're referring to) described doing this by generating a comb using logic gates to produce a sufficiently harmonic-rich signal, bandpass filtering it to the FM band, and then either mixing it with a baseband signal or varying the oscillator that produced the comb, in order to modulate all the individual carriers with FM audio.

Generate the comb (200 KHz square wave fed into an ECL ff to divide by two to get 100 KHz with really fast edges, which means plenty of harmonics), bandpass filter that, amplify, and feed it into a mixer. You'll probably want a 20 MHz spread above 200 MHz or so, to make the bandpass filtering easier, and also so that the tilt on the comb spectrum isn't that great over the 20 MHx band you want.

Rather than a fixed LO going into the mixer, you want to FM the LO, so that the output of the mixer (bandpass filtered once again) is the FM'd combs 100 KHz apart.

Amplify to taste (but you're going to need a very good amplifier).

The amps have to have a *lot* of headroom -- each comb is fairly small, but you're amplifying all of them at once. That's a different problem...