Building a UHF FM audio transmitter

Question

I'd like to build a UHF (~800 MHz) FM audio transmitter. I have no experience in electrical engineering but I study physics and I'd like to acquire practical experience in electronics. In fact my goal is (almost) entirely didactical, with the purpose of self-learning.

I suppose I will need an oscillator, an FM modulator (let's say I pick the audio signal from a computer through an audio cable, and not from a microphone) and some sort of antenna. I'm asking you how to build all these elements, possibly from scratch.

I guess the biggest problem would be the oscillator, because 800 MHz is really a high frequency and even the function generator we have in the laboratory isn't capable to produce such a signal. On the other hand computers and microprocessors have oscillators in the range of gigahertzes so it should not be an impossible task, either.

If the problem of the oscillator is settled I will also need to interface it somehow with a FM modulator and get a frequency-modulated signal. I don't know how to do this but I am willing to learn

Then comes the antenna. In class we have learned about the dipole antenna, but I guess for my purposes I need another type of antennas. I'm saying this because the length of a dipole antenna should be the wavelength of the generated frequency, at least according to my textbook, and I see that the a UHF transmitter has an antenna shorter than 30 cm.

The reason I want to build a UHF transmitter is that I'd like to transmit a signal to a receiver that is already tuned to that frequency. If this reveals to be too difficult I will do something other instead.

Answer 1

This is a very broad question and will probably be closed, so I'll only answer briefly.

Find something called a voltage controlled oscillator (VCO). There are probably some available in your frequency range.

As for the antenna, a dipole should be 1/2 wavelength, which is about 176 mm or 7 inches in your case.

Answer 2

You are better off buying a mini circuit VCO, a PLL chip, a ripple value crystal (10.24, 2.56,5.12MHz) as reference and a prescaler. You need op amps as summation amplifier network to combine audio and error voltage to the VCO, split the VCO output into two one goes to the prescaler and the other half to an amplifier (there are several in the market).

Personally I prefer MC145152 chips as they are parallel loaded(you can use switches) takes almost all dual modulus prescalers from any manufacturer and almost any ripple value crystal. it can be built in one hour.

Here are the links

PLL and prescalers, still available in ebay:

http://f5ad.free.fr/Docs_ATV/MC145151.pdf

MCL VCOs you can use:

http://www.minicircuits.com/products/vco_coax_5v.shtml

Similar implementation:

http://www.qsl.net/kb9mwr/projects/uwave/850-870%20MHz%20PLL%20RF%20Oscillator.pdf