Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #3

Third part in the series..


Common mode current measurement

Direct measurement of common mode current in an antenna system is the best indicator or whether there is a common mode current problem.

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


Above is measurement of Icm using a clamp on RF ammeter around both wires on the input side of the prototype balun, the wires from the lower side of the balun are attached to the open wire feeder on my G5RV. Measurement is being made on 80m with 100W of tx output, in this installation 80m is the worst case for Icm.

The maximum current on the intentional radiator is 3.2A, the common mode current near the transceiver is 32mA (some parallax error in the pic), just 1.0% 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.

Incidentally, the current in each of the wires stand alone is 3.5A, to close to be able to differentiate with the meter, and since the common mode current is 32mA, the currents in the two wires are almost equal in magnitude but opposite in direction, ie they are well balanced. (Note that measuring the magnitude alone does not prove balance, a common mistake.)

Knowing from measurement that Zcm is 50+j2000Ω, we can calculate the loss in the balun core due to Icm as 0.032^2*50=50mW. When the choking impedance is high enough to reduce common mode current to a small value, the power lost in the balun core can be quite low.

A similar test was conducted on 40m, with measured Icm at 100W of 30mA, quite acceptable.

So, the balun design passes the common mode current test in a real world antenna system.

Next installment: Design / build project: Guanella 1:1 ‘tuner balun for HF' – #4.