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QUOTE (Jeff, G8HUL): Also with a 50ohm coax it will not be possible to
alter the length to achieve a 1:1 match.
Changing the length will just cause the observed impedance to travel around
a constant
VSWR circle on the Smith Chart, (plus or minus a very small deviation due
to the changed loss
from the different length of coax) ie the same VSWR but with a different
phase. It is only
if you used a length of coax of a different impedance that you might see a
move towards
the centre of the chart.

All true. What I neglected to mention follows. The typical amateur SWR
measuring device only taps into either voltage or current at a given
(fixed) position along the feedline. If there are standing waves /
reflections within the coax, Ohm's Law tells us that when the voltage
decreases, the current increases, and vise-versa, such that the power is
constant. Therefore, the average amateur SWR measuring device will show
different reading when placed at varying positions along the XMSN line. If
power were measured instead of individual voltage or current, this would
not show.

Dave - W?LEV

On Tue, Jan 21, 2025 at 9:47?AM G8HUL via groups.io <g8hul=
[email protected]> wrote:

If one measures SWR in the shack considering the antenna plus feedline
(assumed coax) as a system, there is a good way to verify there is no
standing waves or reflections on the line - A totally matched system.

This

can occur if the feedline is just the right length to transform the
impedance at the antenna feedpoint to 1:1 over its length.

If that is the situation then there WILL be standing waves on the line!

The fact that the impedance is transformed at the end of the line to
50ohms does
not alter what happens on the line at the mismatch to the load. There will
still be a
reflected signal due to that mismatch. The only way that there can be no
reflected signal,
and no standing waves on the line, is if the load matches the impedance of
the coax.

Also with a 50ohm coax it will not be possible to alter the length to
achieve a 1:1 match.
Changing the length will just cause the observed impedance to travel
around a constant
VSWR circle on the Smith Chart, (plus or minus a very small deviation due
to the changed loss
from the different length of coax) ie the same VSWR but with a different
phase. It is only
if you used a length of coax of a different impedance that you might see a
move towards
the centre of the chart.

73
Jeff G8HUL

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*Dave - W?LEV*


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Dave - W?LEV