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It seems there are almost as many "balun" designs as there are web pages about "baluns". There's lots of accusations pointed in all directions ("design Z isn't a balun OR a choke", "design Y is overkill", "design X could lead to spurious emissions/overheating/loss-of-limbs…") and given how hard such an ostensibly simple thing seems to be to understand, I'd like to be able to test any balun I might make or purchase.

How might I go about measuring a balun or a line choke to see if it will work as intended?

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  • $\begingroup$ The ARRL handbook has good reference in many topics. This book has been publishing, with yearly revision, since, at least, year 1947 (or even earlier??) $\endgroup$ – EEd Jun 17 '16 at 9:47
  • $\begingroup$ @JWilliams I suppose the chapter numbers vary between editions, but could you cite a particular section that covers the testing of baluns? (Personally I have the 1988 edition, plus I picked up a much newer one recently, but forget the year…200X something maybe?) $\endgroup$ – natevw - AF7TB Jun 17 '16 at 18:37
  • $\begingroup$ ARRL Antenna Compendium Volume 1, 1985, page 157 and Volume 2, 1989, page 172 also have articles on Balun. For information, ready-made, with advantages of being mechanically strong and waterproof, Ham HF antenna balun/kit is also available. $\endgroup$ – EEd Jun 18 '16 at 10:14
  • $\begingroup$ Reference information on Chapter 20 Balun, Ch 21 on Ant, ARRL Handbook 2014 $\endgroup$ – EEd Jun 18 '16 at 17:39
  • $\begingroup$ ham.stackexchange.com/questions/1271/… $\endgroup$ – Phil Frost - W8II Jul 24 at 21:09
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In the article Measuring HF Balun Performance the author (Ron Skelton, W6WO) states:

The Common Mode Rejection Ratio (CMRR) of a balun is defined in professional literature as the ratio of wanted to un-wanted transmitted power

This sounds like a reasonable start: the claim is that the CMRR is a good measurement of balun performance.

He continues:

Measurements and calculations of CMRR [Common Mode Rejection Ratio] can be made by directly measuring impedance in the common-mode path, or by observing power flows using a power meter or oscilloscope

…but then goes on to say that this is subject to errors due to phase imbalances that would not be detected with simpler methods.

Any balun or line choke can be viewed as a 3-port device.

The unbalanced input would count as the one port, and each of the balanced outputs (referenced to the same ground as the input, afaict) is another port.

He then gives the equation:

CMRR dB =20*log (S21+S31)/(S21-S31)

This basically compares the difference in power transfer from the input to each of the outputs (relative to each other).


So to use this technique, one would need at least a 2-port VNA to measure first the signal transfer to the one output, and then the other. Then a calculated graph based on those two plots could be derived using the above formula.

I don't think this method covers all the things that can go wrong with baluns, however. For example, this measurement doesn't seem to tell you anything about the power handling capability of a balun, nor what happens if you exceed it.

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"How might I go about measuring a balun or a line choke to see if it will work as intended?" (emphasis added)

Steve Hunt (G3RXQ) has measured many chokes, coaxial torroid wound, coaxial air wound, as well as some bifilar wound.

The results of his measurements can be found here: http://karinya.net/g3txq/chokes/

At the bottom of the page he explains how he measured the chokes, using a two port VNA.

VNA setup

Also a good explanation on why resistive chokes (R > X) are better then reactive chokes.

I have used his charts with great success winding my own chokes, using the "black" area's (this is where R > X) of the charts I managed to wind chokes specifically for the bands intended. While in the beginning I did use air-wound-coaxial chokes, the chart is clearly demonstrating that this is not ideal, as they are mostly reactive not resistive.

This explanation may not be as scientific as it seems, it would give a good starting point for your own measurements of chokes.

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