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Why is the optiamal element length 91% if half the wavelength for a dipole?

Discussion in 'Amateur Radio Antennas' started by RCA Dimensia, Sep 30, 2010.

  1. RCA Dimensia

    RCA Dimensia Member

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    "Actual best length of a dipole antenna is about 91% of that of 1/2 wave in free space, so to be precise about this, the lengths below are already reduced by approx. 9%."

    TV Rabbit Ear



    I meant OF the wavelength.
    #1

  2. KJ6HYC

    KJ6HYC Member

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    Assuming your statement is true. Perhaps the velosity of the wire?
    #2
  3. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    1005/freq gives you wl in free space

    468/freq gives half wl with velocity factor figured in sort of.

    234/freq gives 1/4wl with velocity factor figured in sort of.

    Will this formula hold up everywhere in every antenna build or installation?

    The answer is a big NO, but it will get you in the ball park.
    #3
  4. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    That site in the original post is the only one I have seen that states 9% shortening. Everything else I have ever seen or heard states about 5% which is what I believe to be true unless you are using a really fat conductor on VHF/UHF frequencies. The reason is partly due to the velocity factor of the element material and mostley due to something called "antenna end effect" which is a small capacitance that is formed by the antenna elements in air and has the effect of "detuning" the antenna. In order to compensate the antenna length must be changed slightly.
    #4
  5. pizzaman

    pizzaman Member

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    You answered your own question, "in free space". As you bring your dipole into relationship with the ground the charistics of that antenna change.

    Bernie
    W8NBC
    #5
  6. Kamikaze

    Kamikaze Active Member

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    Optimal for what?
    #6
  7. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    Proper tuning and operation.
    #7


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