> The addition of lossless transmission line changes the complex impedance,
> but not the SWR.
True as long as the characteristic Z of the additional line remains
constant. In the literature, this footnote needs to be added to the general
statement above. It's been repeated over-and-over again with the
presumption that SWR *must* be the same at any point along the entire
distance of a transmission line, notwithstanding loss.
For example, consider a load (e.g., amp input) that is connected to a
coaxial cable of Zo = 50, and at some random distance down the line, it is
jointed by a line to the source with Zo=75 cable. The SWR on each section
of the overall line will not be the same, even if both sections are
loss-less; SWR will be one value on the 50-ohm section, and a different
value on the 75 ohm section. But the SWR is not the same along the entire
distance of a loss-less, or nearly loss-less line. Because of this,
reference should be made that SWR does not change along a line, provided the
characteristic Z of the *entire line* is constant.
> 2. The meters are sensitive to the load impedance, and are not 'perfect'
> or 'ideal' SWR meters. The internal SWR detection in HF rigs are usually
> pretty bad in this respect. Perhaps try a Bird meter to see what is really
> going on.
My money is on this and also pointed out by Gary, K4FMX. It's been a while
since I've experimented with various directional couplers as the standing
wave builds on the line from the load, but a monimatch, for example, may
behave differently and report a different value than a Thruline section.
Paul, W9AC
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