Non-linear, or time variant. I'd expect some variation going between
low and high power with typical dummy loads, because the resistance
will change with temperature. In fact, the loss in the transmission
line will change with temperature, too, and it's significant enough to
be able to measure with ham-type equipment if you are careful. But
I'd FIRST suspect other things going on: different calibrations
between different instruments, and the fact that SWR meters that use
uncompensated diode detectors will not read the same SWR at low power
as at high: they will fail to read high enough at low powers.

The resistance of the copper in the transmission line changes with
temperature. If ambient is 20C and you put in enough power to heat up
the line (center conductor) to 70C, that's a 50C change, and will
result in about an 18% increase in resistance. So if you had a line
which had 3dB loss at 20C, it would increase to about 3.5dB at 70C.
If the load end has a 2:1 SWR, then the sending end will have about
1.40:1 SWR at 20C and about 1.35:1 at 70C. It's not a _big_ change,
but it should be observable on an SWR meter that is accurate over a
wide range of powers.

I want to make it clear that this is in support of what Roy and Ian
are saying, as an added detail, and not contrary to the notion that
SWR on a line in a linear, time-invariant system with steady-state
excitation does not depend on the source impedance.

Cheers,
Tom


Roy Lewallen wrote in message ...
Dr. Slick wrote:

As you might know, the input S11 or SWR will change when you go
from an antenna analyzer or network analyzer to measuring with the
actual full power PA and meter hooked up. This may be partly due to
the fact that the meter is usually not a perfect 50 ohm thru, and
partly due to the fact that the analyzers outputs are closer to 50
ohms than the PA.

If S11 or the SWR actually does change, you've either got a nonlinear
transmission line or a nonlinear load. That is, the impedance changes as
the signal level changes. If you *measure* a different SWR or S11, it
means that either the SWR or S11 is actually changing for the reasons I
just stated, or the meter is nonlinear in the sense that its reading
changes with power level (possibly due to RF ingress, but it could be a
host of other things), or you're measuring with two different meters
that don't agree.

It's not because of the different source impedances.

Sure, you can normalize a Smith chart to anything you'd like. That
doesn't make the SWR change with source impedance.

. . .

Roy Lewallen, W7EL