Buck wrote:

Would the changing of the coax lead to moving the SWR meter to a
different voltage point on the coax?

Sort of, but not exactly. Let's take an example. I'll keep the values
purely real to help folks who aren't familiar with complex math, but
keep in mind that these are special cases and a full treatment would be
somewhat more involved. I'll also make all the transmission lines
lossless to simplify things.

An SWR meter really just provides another way of reporting the impedance
it sees. You can verify this by connecting pure resistances of various
values to its output. For example, a (properly calibrated and operating)
50 ohm SWR meter will report 1:1 if you connect its output to a 50 ohm
resistor. If you connect it to either a 25 or 100 ohm resistor, it
reports 2:1. It does this despite the fact that there's no transmission
line at all connected to its output. Some people can put up a huge
smokescreen and waving of hands about reflected waves of one kind or
another, but at the end of the day the SWR meter can't tell the
difference between a resistor and a transmission line terminated with a
load, if the impedances the meter sees are the same. It's sensitive only
to impedance; it has no way of knowing even if a transmission line is
connected to its output, let alone what the transmission line's SWR or
even characteristic impedance is.

Now put a half wavelength piece of 50 ohm coax between the SWR meter and
those resistors. The SWR meter will still see the same impedances as
before, so it'll report the same SWRs. Now, though, there really is a
transmission line connected to its output. And because the meter is a 50
ohm meter and the line has a 50 ohm Z0, the SWR meter reading is the
same as the actual SWR on the line. When the load is 50 ohms, the line's
SWR is 1:1 and the meter sees 50 ohms so it reports 1:1. When the load
is 25 ohms, the line's SWR is 2:1, and the meter sees 25 ohms and
reports 2:1. When the load is 100 ohms, the line's SWR is 2:1, and the
meter sees 100 ohms and reports 2:1.

Next experiment: Connect the SWR meter through a *quarter* wavelength of
50 ohm line to a 100 ohm load. Now the impedance looking into the line
is 25 ohms instead of 100. But the SWR meter reads 2:1 when it sees 25
ohms as well as 100, so it still reads 2:1, which is also still the SWR
on the 50 ohm line. You can change the length of the 50 ohm line all you
want and, if it's lossless, the line's actual SWR stays the same -- but
the impedance at the input end of the line changes. For a 100 ohm load,
when the line is any even number of half wavelengths long, the input Z
is 100 ohms. When the line is any odd number of quarter wavelengths
long, the input Z is 25 ohms. At other lengths, the impedance is both
resistive and reactive, but the line's SWR is always 2:1. And the SWR
meter interprets all these possible impedances as 2:1, and that's what
it reads. The line SWR doesn't change as you change its length, and the
SWR meter reading doesn't change, either.

Now instead of a 50 ohm line, let's connect a half wavelength 100 ohm
line to the output of the same 50 ohm SWR meter and hook that to a 50
ohm resistive load. The line's actual SWR is 2:1 and, just like any
lossless line, the SWR stays the same regardless of its length. If the
transmission line is an even number of half wavelengths long we'll have
50 ohms at the input and the SWR meter will read 1:1, since it's a 50
ohm meter and interprets 50 ohms as 1:1. If we change the line length to
a quarter wavelength, the input impedance will be 200 ohms, which the 50
ohm SWR meter will interpret and report as 4:1. So by changing the line
length from a half to a quarter wavelength we've changed the SWR meter
reading from 1:1 to 4:1, even though the line's actual SWR was 2:1 all
along. That's what I was talking about. The SWR meter makes assumptions
about the SWR on the line from the impedances it sees. The line
transforms the load impedance in a different way than a 50 ohm line
would. The SWR meter then assumes an incorrect SWR value for the line,
and this incorrect value changes as the line length changes.

The same thing happens if the line has a 50 ohm characteristic impedance
and the meter is designed for some other Z0. The lesson is that an SWR
meter shows the actual SWR on a transmission line connected to its
output only if the SWR meter is designed for the same Z0 as the line.

Too often, people say "The SWR is. . .", but really mean "The SWR meter
reading is. . .". As you've seen, the two can often be very different.
When you see the SWR reading changing as you change the line length, it
doesn't necessarily mean that the line's SWR is actually changing.

Remember, in the preceding discussion I've assumed for simplicity that
all lines were lossless. In the real world, no line is, so the actual
line SWR will always be higher at the load than the source (unless of
course it's 1:1 at the load).

Roy Lewallen, W7EL