one would hope different feed lines would have only marginal impact on the
the final
measurement.
Only when the line is matched to the load and line loss is low. The moment
you introduce a mismatch between line and load, the impedance at points
along the line begin to change and the amount of that change is a direct
function of the magnitude of the mismatch.
For example, let's say the SWR at the antenna is 2:1 and the antenna is 100
ohms resistive, no reactance. Let's terminate the antenna with 100 feet of
RG-58 line. At 14 MHz, the SWR at the line input is 1.6:1 and not 2:1 as
it is at the antenna. Also, the impedance seen at the line input is a
complex 43 ohms (39-j18) and not 100 ohms resistive as it is at the antenna.
So, here's a case where SWR and impedance can be quite different between the
line/antenna and the line input.
Some would argue that they would never use RG-58 on 20m. Let's use LMR-400,
all other factors remaining equal. SWR is 1.8:1 and the impedance at the
end of the line is still a complex 36 ohms (31.5-j17). Again, there's a
disparity between the SWR and impedance between measurements taken at the
antenna and the end of the line.
OTOH, assume you have a precise electrical half-wave of nearly lossless
line. Since the impedance at the line-to-antenna connection will repeat at
an electrical half wave of line, the measurement at the end of the line will
be nearly identical to that taken at the antenna.
Perhaps the moral of the story is that one can attain reasonably accurate
results of what is occurring at the antenna by measuring at the end of the
line, but it requires computation of the loss, type of line, and distance.
Some of the better analyzers, like the RigExpert, may allow the user to
change those variables by choosing line type and physical distance in the
menu.
Paul, W9AC
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