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impedance matching required at 20MHz?

Discussion in 'General Electronics Discussion' started by rickford66, Dec 26, 2017.

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  1. rickford66


    Dec 26, 2017
    Since 20MHz is such a long wave compared to a tiny PCB, is impedance matching between the transmitter and antenna required if the antenna is a short loop (2"-4" in diameter)? It's really intended as more of a transformer for a very short range radio link. About 6" is the expected range. I'm not terribly worried about transmitting at 100% efficiency either.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Arouse1973 and Cannonball like this.
  3. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Arouse1973 and Cannonball like this.
  4. dorke


    Jun 20, 2015
    Generally yes,
    make provision on your PCB for a T or Pi matching network.
    What is the power you are intended to transmit?
    Arouse1973 and Cannonball like this.
  5. Cannonball


    May 6, 2017
  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Once RF (radio frequencies) are created and launched into free space by any means, deliberately with an "antenna" or accidentally through inadequate shielding of the generating source, they fall under the jurisdiction of the Federal Communications Commission (FCC). Except for certain "special cases" outlined in Code of Federal Regulations, Title 47, Part 15 (47 CFR 15) a license is required. If unlicensed operation is allowed, then it must not interfere with licensed operations on the same frequency. The use of 20 MHz is not advised because this is reserved for broadcasting time information by National Institute of Standards and Technology (NIST) station WWV.

    There is no such thing as "a very short range radio link." Once an EM (electromagnetic) wave is created it continues to propagate with diminishing intensity as a function of distance, but unless completely absorbed by some object it never just "stops" at any particular range.

    There is always a "noise floor" present when trying to detect deliberate radio frequency emissions, but this is just background noise that can be filtered out, discarded, and otherwise "worked around" to recover deliberate emissions buried many decibels below the noise floor. So, you may think your little loop antenna has only a 6" expected range, but it does not. That may be the range your application needs, and you may adjust your power output to reliably receive a signal at that range, but the RF emissions travel far beyond six inches at any power level.

    If your RF emissions cause interference, and someone complains about it to the FCC, they can come to your facility, shut you down, seize your equipment, levy heavy fines, and even (through a court of law) impose prison sentences for flagrant disregard of their rules and regulations. Does this sound Draconian? You bet it is! But it does protect our valuable electromagnetic spectrum from anarchy while providing appropriate, regulated, access to everyone. Suggest you become very familiar with 47 CFR 15 before proceeding.
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