Thompson’s coax common mode explanation

A recent online discussion on common mode feed line current was directed to Thompson’s article with the recommendation that is ALL basically needed to discuss the common mode current.

Above is Thompson’s diagram of currents in a feed coax, and it contains two significant errors that could / would lead to formation of the wrong concepts in a learner’s mind.

The red is my deletion of an obvious error in designation of currents. on his diagram.

The second flaw is the implication that currents I1, I2, and I3 are uniform along the coax. They are not necessarily uniform, I1 and I2 may be the result of a differential mode standing wave (ie on the inside of the coax), and I3 a result of a common mode standing wave (ie on the outside of the coax), each of these modes having their own velocity factor and so the phase and location of the standing wave patterns are independent.

The likelihood that I3 is a standing wave has important practical implications, eg measuring common mode current at a single point does not imply common mode current elsewhere on the coax.

Above is a diagram from Thompson’s article, a distribution of magnitude of current on a dipole and common mode conductor from an NEC model. It can be seen that in this scenario, the common mode current I3 is a standing wave of quite high standing wave ratio, and it should be obvious that the current contributes to total current moment and to radiated fields.

Readers might note the similarity of Thompson’s diagram to that discussed at DK7ZB’s balun. These are so similar it seems likely that one ‘inspired’ the other, Thompsons image is dated earlier and may be the ‘original’. DK7ZB’s graphic is dated later, prettier, but equally misleading.

There is a fundamental problem in applying classic lumped circuit linear  circuit theory and analysis to this problem because:

  • the assumption that there is negligible phase shift in current through a single element is not valid;
  • simple component models to not capture the field coupling between system components (eg coax shield and dipole elements in this case); and
  • radiation due to element currents is not captured.

So, it turns out that despite the poster’s recommendation, Thompson’s article is NOT that is ALL basically needed to discuss the common mode current.