[TowerTalk] Feedline (choke) question

[Howard Hoyt]

>>Date: Tue, 30 Sep 2025 21:22:23 -0700
>>From: Jim Brown via TowerTalk wrote:

>>The shells and center conductors of connectors splicing two sections of line
>>exhibit skin effect just like any other conductor. The only difference are 
>>the dimension of the shield as it goes through the connector(s).
>>73, Jim K9YC

Exactly, following EM theory the energy is contained in the interstitial space between the inner and outer conductor.  The CM currents are due to potential difference between the outer surface of the shield and the capacitance with the environment.  This is why EM fields in the environment cause CM currents to flow.  CM Chokes therefore reduce external pickup from the coax in receive.
The reciprocal is true in transmit.

If you look at the magnitude of CM currents using an inductive pickup on feedlines as I have you will see both CM chokes and grounding can reduce CM currents at the shack, but the grounding is perhaps more problematic given it's variable impedance as part of the changing environment.  A well designed CM choke as Jim well describes in his documentation can provide reliable CM attenuation regardless of ground impedance  I have such chokes on my single feedline at the feedpoint, where it hits ground level and in the shack.

Cheers & 73,
Howie / WA4PSC