Should I use a balun for a G5RV?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0CPH, Oct 14, 2018.

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  1. KE0CPH

    KE0CPH Ham Member QRZ Page

    So from some of the diagrams I've seen, it looks like one connection of a ladder line connects to the feed point of the coax while the other connection connects to the shield. Coax is unbalanced while ladder line and the dipole are balanced.

    If I shouldn't using a 1:1 balun, or a any kind of balun, why?

    If I should use a balun, should it be 1:1 or something that matches the resistance of the ladder line?
    KA2RRK likes this.
  2. KK5JY

    KK5JY Ham Member QRZ Page

    You should use a coaxial choke, which some people call a 1:1 balun, at the point where the coax connects to the ladder line.

    The kind of coaxial choke commonly used for other dipole types. such as flat-tops and inverted V configurations, is fine. The only difference with a G5RV (or any similar ladder-line-tuned dipole), is that the choke doesn't belong at the antenna feedpoint itself, but at the junction between the ladder line and the coax. This is the point closest to the antenna where unwanted I3 current can start flowing on the coaxial cable.
    WB5YUZ and KD6RF like this.
  3. KE0CPH

    KE0CPH Ham Member QRZ Page

    Nah, a 1:1 balun is different than a choke I think. I don't even think a choke is technically considered a balun.
  4. KK5JY

    KK5JY Ham Member QRZ Page

    It depends on whose terminology you choose to use. That's why I said "some people call." Nonetheless, what you want is a coaxial choke, as explained above.

    Examples of such devices can be found here:

    The ones built with coax are the ones you want for the installation you described.
  5. KE0CPH

    KE0CPH Ham Member QRZ Page

    Hmm, so just the choke, no balun? Why no balun though?

    I mean I know a choke raises the impedance on the shield to help prevent current traveling back into the shack. But I don't think it converts unbalanced to balanced, does it?
  6. KK5JY

    KK5JY Ham Member QRZ Page

    Balanced vs. unbalanced is widely misunderstood. So is common-mode current. As a result, the terms are somewhat misused.

    What you actually have in this situation is a cable with two conductors connected to one that has three conductors at RF. In order to prevent the third conductor in the coaxial cable (the outside surface of the shield) from conducting current, you choke it. It's that simple.
    NQ1B likes this.
  7. G8ADD

    G8ADD Ham Member QRZ Page

    An alternative approach is to dump the coax and extend the ladder line all the way to the tuner. This removes the coax losses and makes the antenna work on all bands, 160m to 10m, which is a big improvement over the handful of bands that the more normal arrangement will give a workable match on.
  8. KE0CPH

    KE0CPH Ham Member QRZ Page

    How long would the ladder line have to be in total?
  9. KK5JY

    KK5JY Ham Member QRZ Page

    Just note that the choke is still needed unless one uses a tuner with a floating transformer output, since the I3 problem still exists with any tuner or other device designed for coaxial output.
    EA1DDO likes this.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If you have a good, wide-range balanced line tuner, it won't matter what the ladder line length is; no specific length would be good for "all bands," anyway.

    The advantage of the G5RV or ZS6BKW or various strains of this design is that they allow coaxial cable to feet a non-trapped single doublet of fixed length on at least a few bands, and use coaxial cable as the transmission line. This can be a huge advantage when you can't properly route balance line. But if you can suspend balanced line in such a way that it works properly and run it all the way to your tuner, that's generally a lower-loss configuration.
    WB5YUZ, NQ1B and KD6RF like this.

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