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This is a question about power dissipated due to radiation and ohmic heating caused by common mode current in the coaxial shielding, specifically at VHF frequencies.

When a balanced antenna (such as a yagi) is fed with unbalanced feed line (such as coax), common mode current is induced in the shield of the coax. Even with a perfect SWR (no power reflected to transmitter) common mode current results in some of the power delivered to the antenna being radiated by or dissipated in the coax shield, rather than the antenna, reducing the power radiated by the antenna itself.

My question is: in practice, for VHF frequencies, are these losses due to common mode current negligible or can using a balun noticeably decrease losses due to common mode current? For the sake of this discussion, assume a near-perfect SWR (e.g., 1:1.01).

If there is an antenna efficiency reason to use a balun, what are the pros and cons of sleeve baluns vs gamma matches vs other types?

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  • $\begingroup$ It comes down to what you mean by "efficiency". What is the measurable average SWR now, and what are your aims for improving that? $\endgroup$
    – user21417
    Commented Jan 21, 2022 at 22:59
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    $\begingroup$ By efficiency here, I mean the fraction of the transmitter power that is radiated by the antenna. Specifically I'm asking about minimizing losses due to radiation and heating by common mode current in the coax shielding. SWR is a different question. $\endgroup$
    – elplatt
    Commented Jan 22, 2022 at 23:33
  • $\begingroup$ It's not really a different question, as it is certainly related. But my point is how are you measuring any of this? You can't improve an efficiency of any kind unless you know where you are starting. $\endgroup$
    – user21417
    Commented Jan 23, 2022 at 1:12
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    $\begingroup$ You're asking a lot of questions here. :-) Please edit your question to include what type of feedline you use, and a description of your "stub match". Did you mean a gamma match, or something else? A drawing or photo would be helpful. TIA. $\endgroup$
    – Mike Waters
    Commented Jan 24, 2022 at 4:16
  • $\begingroup$ Small changes in antenna system efficiency are very hard to notice, so in practice almost nobody would notice a difference of a decibel or two caused by using a properly-designed balun. Other consequences of not using a balun, such as mismatched SWR, changes in the radiated pattern, and RF in the shack, are easier to notice. $\endgroup$
    – rclocher3
    Commented Jan 27, 2022 at 15:46

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Most antennas don't actually need a balun, but the results of not having one might be undesirable.

Without a balun, the feed coax can become part of the antenna, effectively moving your feed point. If the length of the coax is not resonant, the added length of wire will give wrong impedance and thus higher SWR.

Even if it is resonant (and some antennas do this on purpose), it will distort your antenna's radiation pattern. Depending on the layout of the coax, this might manifest as adding a bit of an omni-directional pattern on top of your existing pattern -- which could be good or bad. It may also distort the power going into the antenna; with something like a yagi, this might shift the highest gain lobe off to one side somewhat.

A gamma or delta match can help balance the antenna when fed with coax, but I don't know if it is a complete solution.

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You ask a few too many questions in one question post, so rather than running down the many answers, I will make a general observation: small changes in antenna system efficiency are very hard to notice, so in practice almost nobody would notice a difference of a few decibels caused by using, or not using, a properly-designed balun. Other consequences of not using a balun, such as changes in the radiated pattern and RF in the shack, are easier to notice.

When choosing a balun, I concern myself with two things. First, does it work? Some badly-designed baluns don't do much. Here's one way to measure common-mode current, which a balun seeks to eliminate. Such a ferrite detector could be used to measure whether a balun is working properly or not. Second, can the balun handle the power without overheating?

Common types of baluns include common-mode chokes, which can be as simple as ten turns of coaxial cable on an air core, and baluns that use ferrites, such as sleeve baluns. Gamma matches are specialized devices that are more intended to transform impedances than act as baluns.

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  • $\begingroup$ I forgot about baluns preventing RF in the shack --- that's pretty important. No fun getting RF burns touching your radio because you have too much common mode. $\endgroup$
    – user10489
    Commented Jan 27, 2022 at 23:30

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