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FM Transmitter antenna

Discussion in 'Electronic Design' started by mook johnson, Jun 20, 2004.

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  1. mook johnson

    mook johnson Guest

    I'm looking at some of the 2 transistor FM transmitter circuits on the
    internet and trying to determine the optimum antenna length (single wire
    only) to get a range of about 500 - 1000ft. with a 100MHz carrier.

    It will be mounted to a small wood or fiberglass object floating on a lake.
     
  2. Ken Smith

    Ken Smith Guest

    Assuming the small object floats right at the surface and doesn't stand
    much above it:

    Put a whip up to about 29 inches (1/4th of a wavelength of the carrier)
    above the water. ie: if the object stands 2 inches above the water use 27
    inches. "ground" the transmitter by AC coupling the other side to the
    water.

    If the object is larger make the top of it conductive and measure the 29
    inches from there.
     
  3. mook johnson

    mook johnson Guest

    OK, what would be the next shortest length that would be acceptable?

    1/8 wavelength? 1/16th?
     
  4. Tim Wescott

    Tim Wescott Guest

     
  5. Ken Smith

    Ken Smith Guest

    Put back in proper order

    Thats easy, the next shorter is:

    (N-1)/(4xN)

    Where N is a very large number.

    If you go under a 1/4 wave whip, you need to put a coil in series with it.
    The shorter the whip the larger the inductance and the worse the
    performance.

    If you increase the diameter, you can get away with shorter lengths.
     
  6. K Williams

    K Williams Guest

    Of course the longet the better, but the "rule of thumb" for EMI is
    that anything less than 1/20th of a wavelength isn't an efficient
    radiator may be of use here. In reality there is no answer.
    You're going to have to "load" the antenna for any mismatches.
     
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