On 1/25/2026 7:58 PM, David Gilbert wrote:
Yeah, I assumed one at the feedpoint. I was referring to the situation
where you might want to add another one. I still think that should go
at a current maximum if possible.
Good question. The times I've added a second choke were the examples
I've mentioned several times, breaking up the line to prevent it acting
as parasitic to a nearby antenna.
There's another mechanism that can couple common mode shield current
that is quantified as the "transfer impedance" of the shield. It's
defined as the differential voltage induced inside the coax by that
common mode current. Ott observed that the resistance of the shield at
the frequency of interest is the lower limit, and non-ideal shield
construction increases it. For example, irregularities, a less dense
braid. I would expect typical CATV coax to have fairly high transfer
impedance at low frequencies. Hard line gets close to ideal.
So yes, at a common current maxima forced by the minima at the first
choke, surrounding objects, and termination at the premises would make
sense. But I would expect this to be a pretty low level event except
when the potential interference was quite strong (like a high power
multi-transmitter environment of near a high power broadcast transmitter.
73, Jim K9YC
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