On Sun, 17 Aug 2003 15:01:44 -0500, W5DXP
wrote:

Richard Clark wrote:
Feel free to attempt it. You are one of the more qualified to do this
by having a variable transmission line like mine.

Unfortunately, my variable transmission line is Z0 = 388 ohms and I
wouldn't trust my homemade 388 ohm SWR meter for wattmeter accuracy.
The lengths of my T-line sections are also not super accurate.

Let me get straight what you are saying. As I understand it, you are
saying that if you chop off one foot of coax at a time when driving
a fixed amount of forward power into a 3:1 mismatched load, the SWR
will change from, e.g. 3.8:1 to 1.9:1. Is that correct?

Hi Cecil,

Your trepidation about the accuracy is well taken, but perhaps
overstated for a first pass approximation. I will leave you to judge
that, but I would point out, that if you can conspire to load both
ends to mismatch as I have described (or worse), you should see the
same symptoms. That your lines do not fall on cardinal points; I have
faith in that your craftsmanship is of high enough caliber that it
won't make any difference if you can replicate the same wavelength
interval (which means you will probably need to work this out above 20
MHz). Error is not inescapable, but your regression analysis should
reveal the same trend.

If you in fact do not know the characteristic source Z of your
transmitter, then forcing a known value imparts a basic maximum. In
other words, what ever unknown value in parallel with a known value
could never exceed that known value. A simple 50 Ohm load at the
transmitter output guarantees that the source could never exhibit any
higher value. This feeding your 388 Ohm line is by far and away more
severe than any of the reports or tests I have offered. With the far
end terminated in another 50 Ohm load, then you should see extremely
wide variation. Your regression analysis should reveal the same
pattern as you found with my data (what you call nonlinearity). You
may then compare your regressions' chi-squares.

Or simply measure the line loss in your standard configuration and
compare it to the severely mismatched arrangement I describe above
(both ends terminated in 50 Ohm loads).

Accuracy demands a lot of front work, but a first pass test such as I
describe should present a quick resolution to whether or not you want
to commit beyond to engage in that labor. The worst of this is how
much you can (or your transmitter can) tolerate working into these
deliberate mismatches.

73's
Richard Clark, KB7QHC