I think we're confused here about the meaning the words "feedpoint
impedance." It is the impedance of THE ANTENNA at the point where the
feedline is attached, and it is determined entirely by the antenna,
INCLUDING the common mode circuit of the feedline. That common mode
circuit consists of the feedline, any chokes and matching elements, and
whatever it is connected to in the shack. The ONLY contribution of the
feedline to this is as a common mode element, and in the common mode
circuit it looks like a wire connected between the feedpoint and the shack.
We add one or more common mode chokes to minimize the effect if the
common mode circuit, which is radiation and reception, and to do that, a
very good choke must be AT THE FEEDPOINT. I don't know of a way to
EFFECTIVELY choke a feedline that is not matched to the antenna. Yes,
you can put something there, but if it did anything useful it would fry
(unless yo only ran QRP to the antena), and a choke that wouldn't fry
with TX power won't do anything useful!
A few other points: 1) the SWR on a feedline is established BY THE
ANTENNA, not the transmitter or antenna tuner; 2) feeding an antenna
with two-wire line does NOT make it balanced -- most practical antennas
that we think of as balanced are often un-balanced by their surroundings
-- ground slope, variations in the soil under them, trees, wiring in
buildings, unequal height ; 3) off-center-fed antennas are inherently
unbalanced.
73, Jim K9YC
On 1/6/2019 3:51 PM, David Woolley wrote:
he feedpoint impedance DOES depend on the characteristic impedance!
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