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Ladder-line dipole feedline length calculations

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by ND5ND, Jul 17, 2011.

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  1. ND5ND

    ND5ND Ham Member

    How long should I make a 20 meter 1/2 wave dipole feedline, if I am using 450 ohm ladder line terminated to a short length of coax to connect to a tuner?
     
  2. W5DXP

    W5DXP Ham Member

    Assuming the 1/2WL dipole is resonant and feedline VF=0.9, make it an integer multiple of 1/2WL, i.e. N*443/f. So for 14.2 MHz, the feedline could be 31.2', 62.4', or 93,6' to reproduce the resonant dipole's feedpoint impedance. Be sure to put a 1:1 choke at the ladder-line to coax junction.
     
  3. N5YPJ

    N5YPJ QRZ Moderator

    W5DXP has a nice little program on his web site that will let you plug in your values and gives you the approximate best / worst case length. Google W5DXP.

    To answer your question, the feed line should be a half wave length long approximately 30.5 ft long.
     
  4. K9ZMD

    K9ZMD Subscriber

    The Slow Typer's Reply (hello echo echo echo)

    Although this calculator was designed for coaxial feedlines, it also works for balanced line length from antenna to transmatch, or from antenna to short coax jumper connnected to a transmatch. You simply process the calculator results as follows:

    (Resulting coax length in feet x 450 ohms)/450 ohms = ladder line length in feet​

    The final figure will give you a balanced line length that should easily be handled by a good external transmatch.

    Be sure that you scroll to the bottom of the page after using the calculator; there you will find an easy-to-understand explanation of the concept. :)

    Built-in antenna tuners are generally less tolerant of a mismatch, so you'd want a feedline length in multiples of a half-wave (current maximum points). That would be approximately 32', 64', 96', etc, although some slight adjustment might be required to compensate for variations in velocity factor.

    And, quite seriously with full intent to be very helpful, you really can find a wealth of information at Cecil's web site: (W5DXP)
     
    Last edited: Jul 17, 2011
  5. W5DXP

    W5DXP Ham Member

    Thanks Gary, I also have a QuickBasic program that runs in a DOS window that shows the optimum lengths of feedline for any length dipole for all of the HF bands. It runs under XP and earlier Windows versions but does not run directly under Windows 7. For Win7, I believe it will run under DOSBox, available free on the web. Here's what the display looks like for a 90' ZS6BKW dipole. Note how five HF bands line up at ~41 feet of ladder-line. (The red dot on 80m means that a 90' dipole is too short for 80m.)

    http://www.w5dxp.com/IMAXGRAF.EXE
     

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