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Antena question

Discussion in 'Electronic Basics' started by Claude, Apr 14, 2008.

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  1. Claude

    Claude Guest

    Greetings dudes.

    I am building a 3 volt FM transmitter, just strong enough to scare the
    bejesus out of the wife when she is listening to her crappy music :eek:)

    The project suggests using a half wave antenna instead of a single wire. I
    understand the use of a 50 Ohm coax , it is the wire they hook up in a loop
    between the signal and the ground ( respectively the centre of the coax and
    the shield) every site that I have checked states that this loop should be
    150/Mhz. Here comes the stupid question, what are the units? Do they mean
    150 mm per Mhz? I will be attempting transmissions between 99.0 and 100 Mhz
    providing I can properly tune the Cap. does this mean my antenna would have
    a 15 meter antenna in a loop shape? This sounds wickedly big?

    Mathematically challenged old geezer.
    Montreal Canada
  2. Bob Eld

    Bob Eld Guest

    The units are Meters. The wavelength of 100 MHz is 3 Meters. Therefore a
    half wave length is 1.5 meters or 59 inches.

    W.L. = c/f where WL = wavelength in meters. C = speed of light, 3 X 10^8
    meters/sec. and, f = frequency in Hertz.

    The width of the loop should be 1.5 meters, not the length of the wire. The
    loop is usually squashed flat and is called a folded dipole. BTW, the
    impedance of such a loop is 300 ohms not 50 ohms so you'll need a balun or
    other impedance matching device.
  3. Claude

    Claude Guest

    Thanks a lot. Wave lenght, this is all starting to make sense ( I stress the
    word starting :eek:)
    RF seems to be akin to black magic.
    So far every little wire in my projects involving oscillators have caused
    havoc by causing spurious transmissions ( feed backs that shouldn't be there
    etc. Seems like the atmosphere in my shop is laughing at my capacitors :eek:)
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