There are videos on YouTube that show how to make a 1:1 current balun. When the balun is done some people connect a 100W 50 Ohms resistor to the balun and measure SWR say in 7.000-7.200 MHz range. If SWR is low they conclude that the balun works as expected.

Why SWR, i.e. a measure of reflected power, has anything to do with common mode current that the balun suppose to cut off? Or maybe these hams just don't know what they are doing and the test is worthless?

I tried to ask the authors of corresponding videos, but unfortunately they didn't reply.

  • $\begingroup$ I agree with your concerns. I could repeat that forever: a good SWVR just means that power is properly sunk into a device. It doesn't mean that e.g. an antenna radiates that power, a transformer passes that power through, or that any modes are properly cut off. The capacitive coupling between primary and secondary side can be (very much depending on the type of balun) significant. $\endgroup$ Jan 24, 2019 at 9:37
  • $\begingroup$ for other types of baluns, you simply don't need to worry: common mode can't pass. $\endgroup$ Jan 24, 2019 at 9:52
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    $\begingroup$ I don't think they are trying to demo the balun's effectiveness for common mode rejection (its purported purpose). They are just trying to show that it's assembled correctly and if so, assuming the performance as a common mode choke is good. I don't think the average ham any other device than a SWR meter to test anything like this. $\endgroup$
    – mike65535
    Jan 24, 2019 at 13:15
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    $\begingroup$ @MarcusMüller don't forget choke baluns, which is probably what a ham means by "balun" $\endgroup$ Jan 24, 2019 at 14:28
  • $\begingroup$ @PhilFrost-W8II a true, I was stuck in my mind with transformer baluns. $\endgroup$ Jan 24, 2019 at 16:02

2 Answers 2


A balun has two functions:

  1. present a high impedance to common-mode currents
  2. present a low impedance to differential-mode currents

A balun that does just the first but not the second is easy: an open circuit fits the bill.

Testing with an SWR meter doesn't test the first, but it does test the second. That does mean the balun is doing at least half its job. It means the balun has been constructed without any shorts, etc. It may not be a complete test of the balun, but it's better than no testing at all, and it doesn't require any advanced equipment.


A 1:1 choke-balun has a design impedance, usually 50 ohms. It also has an electrical length, e.g. mine is 60 degrees long on 28 MHz (4 feet of RG58 on two FT240-52 cores). If I load it with a 50 ohm dummy load on 28 MHz, it will read ~50 ohms on the input because the SWR is 1:1. However, if I test it with a 600 ohm dummy load, it will read 5.8-j26 ohms on the input. A lot of hams would expect a 1:1 balun to read 600 ohms with a 600 ohm load. Here is a graphic of actual measurements made with an AIM4170D. The violet curve is the measured impedance at the balun output looking into the ladder line going to a ZS6BKW antenna. The green curve is the measured impedance seen by the transmitter through eight feet of RG8x connected to the balun input. I was surprised to learn that my "1:1 balun" is only 1:1 with a 50 ohm load. Input/Output Impedance for ZS6BKW 1:1 Choke-Balun


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