Ian White GM3SEK wrote:

Another option would be to make two identical transformers, one for each
wire, and connect both secondaries to the same terminating resistor.
Reversing one of the secondaries gives true vector addition or
subtraction of the two currents, and then you can detect and measure the
resultant in the normal way.

Out-of-phase connection of the two secondaries indicates the
differential line current, or the in-phase connection gives the
common-mode current (hopefully much smaller).

I think that should work...


Excellent idea. But be sure to use a fairly low value of terminating
resistor for this and any time you run just one of the conductors
through the core. Otherwise you'll be putting a significant impedance in
series with your feedline conductor, causing mismatch and/or loss. The
insertion impedance, that is, the effective series impedance of the
core, will be Rt / N^2, where Rt is the terminating resistor connected
across the secondary winding and N is the number of secondary turns.
Lower values of terminating resistor will reduce the detector's
sensitivity, but this won't be a problem at moderate or even quite low
power levels.

Roy Lewallen, W7EL