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Does a J-Pole Need a Balun?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N5XI, Jul 7, 2008.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The choke balun may help decouple the feedline from the radiator, but if it's 3 turns of coax 8" in diameter, that shouldn't help at all on the 2m band -- the diameter's waaaaay to big to be effective at such a high frequency. An effective coaxial RF choke balun for 2m is about 1-1/2" in diameter, 3-4 turns.

    If the diameter's larger than that at this frequency, it's just "adding coax" and it's no sort of balun.

    WB2WIK/6
     
  2. K7FE

    K7FE QRZ Lifetime Member #1 Platinum Subscriber Life Member QRZ Page

    The J-pole is one antenna that probably should not have a balun for the following reasons.

    A J-pole needs radials, a counterpoise or ground plane to work properly. Most J-poles use the outside of the coax shield for that purpose. If you deprive it of the coax shield to obtain balance then it will use the mast or tower. A balun or ferrite will block or limit the current that is allowed to flow on the outside of the coax shield.

    One previous poster suggested that the choke balun (ferrite) be placed about 19 inches below the J-pole. That would allow a 1/4 wave counterpoise for the antenna and would be acceptable.

    I cover this and other antenna material here:
    http://www.cvarc.org/tech/antenna_myths/antenna_myths.pdf

    73,
    Terry, K7FE
     
  3. K8JD

    K8JD Ham Member QRZ Page

    I would like to make a J-pole for 80 or 40 M someday !
     
  4. WA7KKP

    WA7KKP Ham Member QRZ Page

    feeding a j-pole

    What you're describing is NOT a balun, but a feedline RF choke, which simply decouples the feedline from radiating RF.

    Read the antenna books, and keep in mind the J pole is nothing more than an en0-fedd Zepp antenna. If you tap up on the J part, you'll need a balun, as that is a balanced feed. I've seen some fed from the bottom of the J (no short) with coax, but I'm not sure if the impedance is anywhere close to 50 ohms.

    Gary WA7KKP
     
  5. K8JD

    K8JD Ham Member QRZ Page

    the bottom of the J acts like an impedance transforming line of sorts.
     
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