Measuring common mode current on coax

In Common mode current and coaxial feed lines, I mentioned that common mode current is easily measured,

Clip on RF ammeter.


Above is a clamp-on RF current meter (Measuring common mode current on coax) measuring the common mode current on a coaxial feed line. This coax is between the ATR-30 ATU/balun and transceiver on a G5RV antenna system on 80m using tuned feeders and fed with 100W. The maximum current on the intentional radiator is 3.2A, the common mode current near the transceiver is 20mA, just 0.6% of the maximum radiator current, a good result for this antenna system type.

Note that common mode current is usually a standing wave, and needs assessment at various locations on the feed line, and of course the standing wave pattern will vary from band to band. In the example above, it has been established that measurement at the point used is approximately the maximum on 80m.

The instrument used is a calibrated RF clamp-on ammeter of my design, it was described at but is no longer online. Beware of commercial instruments, a lot of instruments that purport to measure common mode current or indicate line balance are ill-conceived and will not give a correct measurement, likewise for designs in many journal articles. Though I have not tested it, the MFJ-853 appears to use a valid measurement technique, but is not very sensitive in its calibrated ranges.

The bottom line is that common mode current is usually easy to measure, and if you haven’t measured it, then any assertions about the level of common mode current are guesswork, …your knowledge is of a meagre and unsatisfactory kind. It may be the beginning of knowledge but you have scarcely in your thoughts advanced to the state of science. (Lord Kelvin).