2
\$\begingroup\$

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.

\$\endgroup\$
  • 3
    \$\begingroup\$ This will be illegal in most countries. \$\endgroup\$ – Leon Heller Apr 24 '12 at 21:46
  • \$\begingroup\$ This question is too broad. Ask more specific targeted questions. \$\endgroup\$ – Olin Lathrop Apr 24 '12 at 21:50
  • \$\begingroup\$ @LeonHeller Why should it be illegal? If the producer of the transmitter acts legally why shouldn't I? \$\endgroup\$ – Fiat Lux Apr 24 '12 at 22:42
  • 2
    \$\begingroup\$ Check the regulations for your country. It would be illegal here in the UK. \$\endgroup\$ – Leon Heller Apr 24 '12 at 22:47
  • \$\begingroup\$ @LeonHeller OK, thanks for pointing it out. I live in Italy and the regulations are not so clear, but it seems it should not be a problem if I operated with a low power, because it wouldn't interfere with radio broadcasting \$\endgroup\$ – Fiat Lux Apr 24 '12 at 23:02
3
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ If you feel my question is too broad act accordingly. Anyway, thank you for your tip. \$\endgroup\$ – Fiat Lux Apr 24 '12 at 22:49
  • \$\begingroup\$ @Fiat: Huh? I did act accordingly. \$\endgroup\$ – Olin Lathrop Apr 24 '12 at 23:08
  • \$\begingroup\$ maybe you want to buy a couple Qualcomm QSC6010 phones @ 800Mhz and use these. But if you want to experiment. Use an FM generator and Spectrum analyzer with any wire, patch or yagi antenna then you can listen to the output of the spectrum analyzer when tuned to source on slope of filter. This is a good way to learn. \$\endgroup\$ – Sunnyskyguy EE75 Apr 25 '12 at 1:49
0
\$\begingroup\$

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

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.