woolly thinking on the nature of feed line common mode current

An online expert somewhat exasperated that the audience hasn’t absorbed his wisdom elaborated in apparently many previous posts said:

We’ve been round and round on this discussion but in a current mode balun aka a common mode choke the losses due to the windings and core are common mode not differential mode losses. You DO NOT dissipate your transmitted and received signals, which are carried as differential mode signals, as choke losses.

I know you’ve been reminded of this many times and don’t expect you to accept it now but that’s how common mode chokes work.

Now there is a sense in ham radio forums that repetition transforms assertions to fact, but setting that aside, let’s look at the assertion from an energy conservation point of view.

There is a claim that:

  1. heating of a common mode choke is mostly due to common mode current; and that since
  2. transmitted and received signals (currents) are (purely) differential signals (currents), ie with zero common mode current component; therefore
  3. no transmit or receive signal power can be converted to heat in the common mode choke.

Those of us who live in the real world and have experience with common mode chokes know that they can get quite hot (scenario dependent) when the transmitter operates so the simple explanation is that the heating is due to conversion of transmitter RF output to heat. If 1. is true, then the heating evidences significant common mode current due to the transmitter operation (ie 3. is clearly wrong). There is no doubt that 1. is true, and that the power that heats the core is due to and from the transmitter, ie some transmitter output power is converted to heat in the balun (and not radiated as RF energy).

So assertion 1. is sound, 3. is plainly wrong and that implies that 2. is wrong.

2. is wrong, there are several ways in which common mode current may result from ‘antenna current’, be it receive or transmit, and the online expert’s thinking is woolly, his claims 2 and as a result 3 are demonstrably wrong, no matter how many times he states them.

There is a failure on the online expert’s part to reconcile the reality that common mode chokes can get hot with his assertions, had he had the experience to know that heating can occur, he might have realised his assertions were not soundly based.