A balun for a coax fed dipole.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC2ZPK, Feb 10, 2011.

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  1. KC2ZPK

    KC2ZPK Ham Member QRZ Page

    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
     
  2. WX7G

    WX7G Ham Member QRZ Page

    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. 2E0OZI

    2E0OZI Ham Member QRZ Page

    My first dipole for 20 was just as you describe (connected directly to coax) and it worked fine!
     
  4. G0GQK

    G0GQK Ham Member QRZ Page

    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. KC2ZPK

    KC2ZPK Ham Member QRZ Page


    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?
     
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    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. N3OX

    N3OX Ham Member QRZ Page

    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.

    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. KA4DPO

    KA4DPO Ham Member QRZ Page

    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......;););)
     
  9. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    You flat out don't need it for a half wave dipole running a 100 Watts. Waist of time money and effort.
     
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    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.;)
     
  11. WA4OTD

    WA4OTD XML Subscriber QRZ Page

    I have 5 dipoles in the attic just above my head, none with a balun. The problem may just be poor design in your speakers, amp, or power supply. ie a bypass cap or ferrite bead somewhere on your computer speaker may be a better solution. The field strength will be strong near you antenna.
     
  12. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    With most consumer electronics, I'd say that's a given. They're all "unintentional receivers" that per Part 15 should be able to withstand electromagnetic interference, and most cannot do it well.

    I solved the computer speaker problem here by not using any amplified speakers anywhere, and we have 7 computers in the house that are never turned off except accidentally.
     
  13. K1BQT

    K1BQT Ham Member QRZ Page

    And just a few short years before that, we burned witches at the stake. I use 'em and every antenna I design has one. It's cheap insurance that the antenna is going to work the same no matter how long the feed line or what it's routed next to. BTW, the statement that a balun can't change an antenna's SWR is simply wrong.
     
  14. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I agree, and I use them also.

    BTW how's burned witch steak taste? I'd probably make 'em medium rare and use some A-1...
     
  15. N5YPJ

    N5YPJ QRZ Moderator QRZ Page

    No need to seek dispensation, some folks need baluns and some don't. For feeding ladder line I highly recommend a 1:1 balun, and to not use the junk found in tuners. For coax fed antennas it depends whether or not you have an installation where a balun would make things work better.
     
  16. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    So, when he adds the balun to his coaxial fed half wave dipole what improvements will he see? His rfi problem will go away in the speakers? His signal will be improved? His swr will be improved? I think the only difference that will be noticeable in the weight of his wallet. I certainly am not saying the use of a balun is a bad thing but I seriously doubt he will notice any difference unless adding the balun fixes a poor connection. I do use baluns of one form or another on my antennas but for 30 years or so never used one. I have added them to existing antennas and I have had them removed, can't say I ever noticed a difference on a coaxial fed dipole.
     
  17. KA4DPO

    KA4DPO Ham Member QRZ Page

    OK, care to back that up with some facts?

    A Balun will not impact the SWR on an antenna period.

    That is a fact so unless you can prove otherwise.....:confused:
     
  18. VK1OD

    VK1OD Ham Member QRZ Page

    Ways in which a balun may alter line VSWR

    1. If the feedline carries common mode current, any change which results in a change to that common mode current may result in a changed V/I ratio at the load end of the feedline, and that may result in a different VSWR on the line.
    2. Baluns are not perfect devices. A nominal 1:1 balun does not transform impedance exactly 1:1 over a wide impedance and frequency ranges, so that transformation may result in a change to line VSWR when the balun is inserted.
    Owen
     
  19. 4L4AGU

    4L4AGU Ham Member QRZ Page

    I was also getting RF buzz in computer speakers. Installing balun on radio coax haven't helped much. Then I discovered, that RF was coming back to my computer, via the satellite and ethernet cables, which were coming down from the roof. So I had to install choke filters on them also. It haven't killed RF buzz completely, but reduced it to barely noticeable level.
     
  20. W8JI

    W8JI Ham Member QRZ Page

    The notion that a balun is unnecessary and a waste of money is wrong. Notions that any balun works in any system are wrong also.

    There are cases with a dipole (or with any other antenna) where a balun doesn't do anything.

    There are cases where a balun does a great deal.

    There are even baluns sold as baluns that don't act like baluns and actually force unbalance, like single core 4:1 baluns. Any single core 4:1 transmission line balun is a ripoff because it ALWAYS unbalances a balanced system.

    There are cases where adding a certain type of balun will make things worse, yet the same balun in a different system will work fine and help.
     
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