Do dipoles require baluns?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K9PSM, Jan 25, 2010.

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  1. K9PSM

    K9PSM Ham Member QRZ Page

    Hello,

    I am Paul, KC9QYB and my QTH is Peoria Heights, Illinois. I was curious about single band dipole antennas needing baluns. My j-pole antenna that I built at one of our local club meetings has a balun, and the 10 meter dipole here at my house that was put up by some club members has a balun.

    Both of these antennas are single band. When I first discovered the term balun, I was told that they keep RF from getting back into the shack and I knew that this was a very important thing.

    Now I am being told by another amateur that antennas do not need baluns as long as they are single band? What is your opinion on this. Please reply.

    73s de KC9QYB
     
  2. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    Paul, a dipole does not *have* to have a balun. Having one may help prevent RF from coming down the line into the shack and may improve the radiation characteristics of the dipole, but I've run dipoles connected straight to coax without a problem.

    Put one up without a balun and see how it works for you. Part of the fun with antennas is experimenting and finding out what works and what doesn't.

    ..........Bob
     
    W5BUY likes this.
  3. K9PSM

    K9PSM Ham Member QRZ Page

    W4PG:

    Okay, the only other worry is the RF getting to me, but if I put up a dipole without a balun how would I go about connecting coax?
     
  4. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    Connect the center of the coax to one lead of the dipole and the shield to the other. One way to do this is to make a center connection with a piece of plexiglass, mounting a SO-239 connector in the middle of the small piece of plexiglass. Also put a small hole on each end to run the dipole wire through and wrap it around for a firm connection then solder one end of the dipole to the center conductor of the SO-239 and the other to the shield connection.

    An SO-239 is the female equivalent of the PL-259, the standard coax connector that goes on the end of the coax.

    The plexiglass will provide a center connector that will be firm enough to support the feedline and wires without a problem. It needs to be about 1/4 inch thick however, somewhere around that size.

    I've got one I made around here somewhere . . . . :D
     
  5. G3TXQ

    G3TXQ Ham Member QRZ Page

    Paul,

    A dipole without a balun at the feedpoint is actually a Tripole! The antenna forms two of the legs and the outer surface of the coax forms the third antenna wire.

    Of course the length of that third antenna wire, and how it is grounded, varies from installation to installation, which is why some folk who don't use a balun say they've never had a problem, whereas others have serious problems.

    That third antenna wire is probably coming into your shack/home close to TVs, radios, 'phones, computers etc as well as all your radio gear. On Tx it has the potential to cause interference with all those items; on Rx it can pick up noise from them and inject that noise right back at the feedpoint along with your wanted signals.

    I doubt anyone would deliberately connect a piece of wire to one side of their dipole and route it back into their shack; but if you don't use a 1:1 current balun at the feedpoint to "disconnect" the "third wire", that is exactly what you are doing.

    Here's a model of a 20m dipole with a severe example of this problem. Wires 1 & 2 are the dipole and wire 3 is the coax:

    [​IMG]

    See how most of the current which should have flowed into the right hand dipole leg has flowed down the coax. See how much current is flowing (and radiating) at the bottom of the coax where the shack would be. Now insert a mediocre balun at the feedpoint - 100 ohms choking impedance:

    [​IMG]

    The coax current is much reduced. Now insert a good balun at the feedpoint - 1000 ohms choking impedance:

    [​IMG]

    The current is now flowing where it should - into the right hand dipole leg - and the coax current has been reduced to a negligible amount

    73,
    Steve G3TXQ
     
    Last edited: Jan 25, 2010
    M7AVX and AG5DB like this.
  6. K9PSM

    K9PSM Ham Member QRZ Page

    Thanks for clearing that up for me. Now I can't wait to put up my 40 meter dipole I'm going to be building. Also, where is the cheapest place to find an antenna switch?
     
  7. K9PSM

    K9PSM Ham Member QRZ Page

    Hey thanks, Now I have two different ways I could make a dipole :)
     
  8. KD0CAC

    KD0CAC Ham Member QRZ Page

    I posted the other day , on another balun thread .
    My 1st question / point , is , from what I remember / understand from the books , is a balun is serves the 1st purpose , of match feed line to the ohms / impedance to match the radio .
    If using coax , then that matches to radio , but may not match the antenna , so this may be the 1st spot to put a balun , between coax to antenna , or open wire .
    Which brings another idea up , balun , balanced [ ladder line ] or unbalanced [ coax ] .
    After that , I can across [ while looking to buy ] voltage balum & current balun , where this thread is at .
    I should have put question marks at the end of all those statements , because I am still learning and just want to confirm or learn ?
    Thanks John
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    John,

    The primary purpose of a Balun is to interface a balanced part of a system to an unbalanced part. Typical examples are the interface between a dipole antenna (balanced) and coax feedline (unbalanced); or between ladderline (balanced) and an unbalanced tuner.

    In addition, the Balun may perform an impedance transformation; the ratios you are most likely to encounter are 1:1, 4:1 and 9:1 (because of the way they are constructed they tend to be "square numbers")

    Finally, the Balun may be a Current Balun or a Voltage Balun; current baluns tend to force equal currents at their balanced output, whereas voltage baluns tend to force equal voltages. In most ham applications it's equal currents that we want.

    Hope that helps some.

    Steve G3TXQ
     
  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Technically, a dipole antenna requires a balun. However, no one ever told the antenna that it has to have a balun so they just work very well without a balun. Amateur radio operators have been building dipole antennas, fed with coaxial cable, since World War II (when coax first became widely available) and the vast majority of those antennas did not use a balun.

    There have been cases where adding a balun at the antenna actually causes problems. Generally, this is due to having a balun that is either faulty or is of dubious design. if you already have a balun then you can try it. But, I wouldn't go out and purchase a balun if you don't already have one.

    Yes, the "pattern" of the antenna should technically be better using a balun. However, in the real world, you will not be able to tell the difference. There are just too many "other" factors (i.e. nearby objects) that have a considerably more effect than having, or not having, a balun.

    Glen, K9STH
     
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