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Thread: A balun for a coax fed dipole.

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  1. #1

    Default A balun for a coax fed dipole.

    So.... in my continuing education into antennas I see that I may have made a mistake in constructing my dipoles. I originally connected each half of a 1/2w dipole to the two conductors on the coax. the shield to one leg, and the center to the other. Now correct me if I am wrong, this setup allows RF to follow the coax shield back to the radio, instead of staying in the dipole. If I read correctly I would need a choke or bauln to keep the RF in the antenna, and not come back down the coax shield, so..... How do I calculate what type of torrid I should use, what size wire, # of turns etc... It's for 20 meters and 100W +/-
    Thanks
    KC2ZPK
    John
    [URL="http://www.kc2zpk.com"]KC2ZPK.com[/URL] a blog in progress, new, still needs work, but up and running.

  2. #2

    Default

    If you haven't noticed any problems with your dipoles you do not need a balun. Before the balun craze every dipole was fed directly with coax.

    As to the balun, the balun choking impedance should be 1000 ohms or more. A lossy balun composed of ferrite is preferred. The "ugly" baluns made of rolled up coax I don't recommend. This forms a high-Q inductor that can tune the coax outer shield and actually increase common-mode current.

  3. #3
    Join Date
    Dec 2010
    Location
    Wembury UK
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    4,057

    Default

    My first dipole for 20 was just as you describe (connected directly to coax) and it worked fine!

  4. #4
    Join Date
    Apr 2001
    Location
    Shropshire. England.
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    17,140

    Default

    God forbid that anyone can say that a dipole works without a balun.! To prevent any further repercussions of this all sinners will be severly lashed with a cat o'nine RG58's

    G0GQK

  5. #5

    Default

    Quote Originally Posted by WX7G View Post
    If you haven't noticed any problems with your dipoles you do not need a balun. Before the balun craze every dipole was fed directly with coax.

    As to the balun, the balun choking impedance should be 1000 ohms or more. A lossy balun composed of ferrite is preferred. The "ugly" baluns made of rolled up coax I don't recommend. This forms a high-Q inductor that can tune the coax outer shield and actually increase common-mode current.

    What problems would I e looking for? I do have some RF buzz in my computer speakers. I thought if I could reduce the RF on the shield coming back to the radio I might clear that up. My other dipoles are balun-less. I suppose there are no performance gains by using a balun?
    KC2ZPK
    John
    [URL="http://www.kc2zpk.com"]KC2ZPK.com[/URL] a blog in progress, new, still needs work, but up and running.

  6. #6

    Default

    Quote Originally Posted by KC2ZPK View Post
    What problems would I e looking for? I do have some RF buzz in my computer speakers. I thought if I could reduce the RF on the shield coming back to the radio I might clear that up. My other dipoles are balun-less. I suppose there are no performance gains by using a balun?
    John,

    The most obvious symptoms would be RF interference caused by common-mode current flowing on the coax, and increased noise pick up on receive. Other effects such as pattern modification might well go unnoticed.

    One thing is indisputable, if you feed a dipole with coax and no balun you have created a "tripole": the outside surface of the coax braid acts as an additional dipole "leg". That may or may not be a problem, depending on many factors unique to your installation.

    There are some choke/balun designs here:
    http://www.karinya.net/g3txq/chokes/
    Choose a design which has high choking impedance and is resistive across the bands of interest.

    73,
    Steve G3TXQ

  7. #7
    Join Date
    Aug 2001
    Location
    College Park, MD
    Posts
    429

    Default

    Quote Originally Posted by G0GQK
    God forbid that anyone can say that a dipole works without a balun.! To prevent any further repercussions of this all sinners will be severly lashed with a cat o'nine RG58's
    Like many of the rest of us I've routinely used dipoles without baluns and I've yet to burst into flames... but this experience, however common, is really beside the point.

    A good choke balun at a balanced-to-unbalanced junction is an important system component in many situations. It can be omitted in others and we shouldn't be convincing people they've made some unforgivable sin by going without. But the fact of the matter is that it's a simple and useful antenna system component and it can actually be really rather cheap to try it.

    People do need to keep plugging away at the fact that the right balun for the job, especially for simple coax-fed dipoles at low power, is a couple dollar ferrite core (or maybe a few of them) with the feedline wound on it. Pick something from G3TXQ's chart and wind it and go. No need in most cases for hundred-dollar monster 20kW job from DX Engineering or whatever.

    Quote Originally Posted by KC2ZPK
    What problems would I e looking for? I do have some RF buzz in my computer speakers. I thought if I could reduce the RF on the shield coming back to the radio I might clear that up. My other dipoles are balun-less. I suppose there are no performance gains by using a balun?
    If your dipole is pretty far away from your computer speakers, the balun will probably cure the buzz. As far as performance goes, you just won't know until you try it. For coax-fed single-band half-wavelength dipoles, it's unlikely that you'll have basic efficiency or pattern problems to the point where it "doesn't work" on the air. For other antennas where you really care about the pattern, like beams, baluns can be really critical. And reception is a different matter.

    When I started out in the hobby, I had lots of space at my parents' place that was surrounded by woods and had a bare minimum of electronic devices inside. I never bothered with baluns on my fixed dipoles (though I did use them on beams and rotatable loops, etc). That worked fine. No received noise problems, partially because it was a very RF-quiet house. No RF in the shack problems: antennas were thirty to fifty feet from the house and coax was buried (this is a type of lossy choke and helps with conducted noise and RF ingress to the shack). These were fixed antennas and I wanted to work in all directions anyway, so the pattern didn't matter much. Feedline radiation would be fine.

    In my current house, I strung up a 10/15m trap dipole squeezed in between the house and a big vertical (http://n3ox.net/pictures/backyard_lg.jpg) and a balun on that antenna's feedpoint dropped the noise floor by a huge amount, probably around 8-10dB. That's a BIG performance increase on reception. Some of the problem was stringing a vertical feedline next to a big vertical antenna.

    These days, I'll never put up a fixed-direction home station dipole, or loop without at least trying a balun. Rotatable antennas here have a good one installed and keep them, period. It's just some coax and a couple dollar ferrite. A balun is only useless addition if you're lucky, especially with all the noise-generators we have plugged in to our power outlets these days.

    There are lots of situations where a balun will make minimal improvement. But there are others where it will be very helpful. I don't really agree with the idea that you should only try a balun if you notice problems. Sometimes the problems aren't going to pop out at you. Steve's chart is good.

  8. #8
    Join Date
    Feb 2002
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    Right here
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    12,919

    Default

    It is always good practice to use a one to one current Balun on a single band dipole connected to coaxial cable. The dipole is a balanced antenna and the coaxial cable is an unbalanced feed line connected to an unbalanced transimitter output (at least in modern rigs). In the majority of installations you can get away without the Balun and never know the difference. A Balun will not reduce loss or change the SWR of an antenna so if everything is working then leave it alone.

    If however, you experience noise or feedback into stuff in the shack then a Balun is probably the first thing you should do. Just be aware that many hundreds of thousands DX contacts have been made using dipoles that had coax connected directly to the center without a Balun.

    If you can afford one use it. If you can't afford one then don't and see what happens. If you get lucky life is good......
    I'm sorry you don't have the experience or understanding to realize that others possess a skill set that you seem to dismiss as fantastical.

  9. #9

    Default

    You flat out don't need it for a half wave dipole running a 100 Watts. Waist of time money and effort.

  10. #10

    Default

    Quote Originally Posted by WX7G View Post
    If you haven't noticed any problems with your dipoles you do not need a balun. Before the balun craze every dipole was fed directly with coax.
    Heck, before the coax craze, every dipole was fed with twin lead. And before the twin lead craze, antennas were fed with a single wire.

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