low power transmitters

Discussion in 'Wireless & RF Design' started by tibbles, Jul 30, 2008.

  1. tibbles

    Thread Starter Active Member

    Jun 27, 2008
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    3
    some years ago i spent an obscene amount of time trying to build some
    xtal 27 mhz radio control transmitters which i think are still legal in the uk http://www.ukrcc.org/27mhz.html i say some, because although pretty basic i could never get the output stage to tune, this was usually a pi type arrangement with one or two variable caps,which i think was high pass filter,ariel matching. i would have thought a band pass filter would have been better.
    found a brilliant site for designing filters http://www.aade.com/
    i had no equipment for checking frequency etc
    now ive a bit more time and a bit more cash im having another go,
    so any suggestions regarding equipment and getting these things to work, particularly anyone whose been down this road would be most welcome.

    the statement on your forum that commercialy available stuff is a far better bet than building your own is perhaps never more true than with this... but its not the same,also if you buy something and it goes wrong its usually scrap but if you build it yourself you can usually fix it.
    regards
    dougal
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    I used to build CB transmitters quite a bit, no freq check needed. Most are single or 2 transistor units. Is this what you are looking for? I doubt I still have the schematics, but it doesn't hurt to look.
     
  3. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    thanks bill
    any info welcome, cb and radio control frequencies and circuits are almost identical, typical layout would be-- xtal oscillator into modulation/driver transistor into power transister and output filter.
    all parts inc toko coils, xtals are still readily available, the only thing unobtainable is xtal holders .
    (i did build a walkie talky using the same circuit)
    main probs were instability and not being able to tune the coils to a peak
    in fact one worked better when i took the wire of the designed coil former and screwed it up into a ball.
    i am treating my self to an ebay frequency meter i live pretty close to an airport flightpath!!!!


    regards
    dougal
     
    Last edited by a moderator: Jul 30, 2008
  4. SgtWookie

    Expert

    Jul 17, 2007
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    As far as building transmitters on the 27MHz band, you can still get in hot water with the authorities for building transmitters. Even back in the 70's, CB radio transmitters were using crystal oscillators with PLL's (phase locked loops) to keep within the bandwidth limits. Production CB units had circuitry to ensure that transmit power was limited to no more than 4W on AM and 12W on SSB.

    In a homebuilt transmitter, you'll have a dickens of a time getting it tuned, and ensuring that sideband emissions are within limits unless you have some pretty sophisticated test equipment. And if you violate your region's laws regarding RF emissions, the authorities will have absolutely NO sense of humor about it.

    You've already stated you live very close to the traffic pattern of an airport. If your sideband or harmonic emissions interfere with aviation communications in any way, you'll shortly find a gaggle of extremely annoyed government employees breaking down your door to confiscate your equipment and throw you in the hoosegow.
     
    Last edited by a moderator: Jul 30, 2008
  5. Wendy

    Moderator

    Mar 24, 2008
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    As long as the transmitter is under 10mw it should be legal, but this is US law. I have no clue what the UK laws are on the matter, you need to check. I liked the fact I could use ready made recievers way back when in the 70's, and one of my uses was trackers for my model rockets.

    You need to check the laws of your country, but a simple crystal transmitter is an easy project. I'll go through my books after I hit post on this reply.

    ****************

    OK, here it is, you have to realize this was over 30 years ago, I drew these notes when I was a teen.
     
    Last edited: Jul 30, 2008
  6. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    thanks sjt and bill
    some pretty sobering thoughts there,i can understand strict limitations on 4 watts,
    but the project i had in mind is under 200mw and would probably just get to the bottom of the garden on a good day, will get back later,
    thanks again
    dougal
     
  7. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    ps

    i would think that the critical factor would be that the transmission was at the correct frequency and any harmonics were blocked hence my emphasis on a good output filter.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    You'll note the design I saved has a tank for the filter on the collector. As long as you're talking simple AM it doesn't need to be too complex. SSB is another story however.
     
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