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OCF Dipole Dimensions

Discussion in 'Amateur Radio Antennas' started by tahrens, Oct 19, 2011.

  1. tahrens

    tahrens Member

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    Hi Folks,

    I want to put up an OCF Dipole (off center fed). I want to put up the 'long' one that will also do 160.

    What are the true dimensions of each leg? I've read 180' on one, and 90' on the other, but was wondering if there is a 'true' dimension.

    I'm going to use a 4:1 balun at the feed point.

    Thanks,

    Tim W5FN


    #1
  2. RatsoW8

    RatsoW8 Supporting Member

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    You can build anything but I don't think the OCF will work very well for 160 though.

    I used this to build mine but you can plug in ant freq. you need and it works the same.

    The formula:

    For a dipole antenna cut to 3.6 MHz, take 468 / 3.5MHz and you get 130'. That is the overall length of a standard dipole. Normally you divide that in half to find the center feed point; however, we are feeding this 14% off-center to add 14% to 50% and you get 64% for the longest leg and 36% for the shortest leg.

    So, 130' multiplied by .64 (64%) gives the longest leg length of 83.2 feet. the shortest leg length is the remainder, or 46.8 feet. You can double check this by reversing the math, 130' multiplied by .36 (36%) for the shortest leg is still 46.8'.
    #2
  3. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Ratso is Dead on the dimensions, Same way I built one for a blind ham friend of mine, needed a no tune antenna that required no switches to operate.

    I did find I got a better match hanging it from the center of the dipole and left the balun hanging at the feed point, looks like an inverted V feed OC which it is.

    Then I built him a fan dipole for 20/40/80 meters works 15 off the 3rd harmonic of the 40 meter leg.

    Fan beats the OCF on TX and RX.
    #3
  4. W5LZ

    W5LZ Crotchety Old Bastard

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    There really isn't a 'definite' length, too many variable like height, what's around it, etc. Start with a 1/2 wave length at the desired frequency, feed it at about 33% - 38% from one end, and adjust for best match as you would any antenna. There's no good reason to use that balun, an OCF antenna isn't 'balanced' to start with. The point in feeding the think off center is to get the input impedance closer to what you want, so that '4:1' ratio isn't needed either.
    Have fun.
    - 'Doc
    #4


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