nanoVNA - Testing the Common Mode Attenuation of a DG0SA 1:1 Current Balun by VE6WGM

This balun was built for use in an 80 meter dipole antenna. I wanted to ensure that it is actually behaving as a balun transformer.. matching the balanced load of the dipole to the unbalanced coax feedline. A true balanced system naturally has an excellent CMRR. In this case, if the balun is doing it’s job, most of the energy being supplied by the radio and feedline will stay on the antenna even if there is a bit of mismatch on the antenna. This ensures no unwanted RF appears in the shield of the coax and the antenna’s radiation pattern remains that of a dipole.

I make no claims at being an RF engineer, so if you happen to notice any faults or inaccuracies in what I have presented here, please feel free to correct me. There’s one thing with Amateur Radio.. just when one thinks they have something figured out, along comes some new information and former ‘truths’ must then be re-examined and new ‘truths’ formed :-)

March 2021 - I now realize that I did not take into account the loss of signal introduced by the resistor power divider used to apply common mode signal to the input of the choke, and in the combiner used to measure the output of the choke. I likely will be approaching the subject of testing CMC’s again in another video... :-)

November 2024 - At 2:45 I mention that I am trying to block common mode currents from an antenna impedance mis-match. I have since learned that this concept is not quite correct.

Impedance mismatch alone does not cause common mode currents. It causes a portion of the EM wave arriving at the antenna feedpoint to be reflected back down the coax cable in the same manner that it arrived. This causes standing waves on the inside of the coax. If the standing wave pattern happens to be such that there is increased current at the antenna feedpoint, then this may increase common mode currents on the outside of the coax.

Common mode currents are covered in a later video here…    • nanoVNA - What does a 1:1 current balun do...