Dr. Slick wrote:
"Ian White, G3SEK" wrote in message
...

Sorry, that is exactly wrong. S11, SWR and the impedance itself, do
*not* change when you connect a different instrument to the same load.
All the changes you have described are due entirely to instrument
errors.

That's how the instrument errors are determined... by knowing for a fact
that, whatever all the different instruments may say, the impedance
they're trying to measure is the one thing that has *not* changed.


You are right, but i never stated that the impedance we are
feeding ever changes, only the measured SWR.

Oh dear... just when I thought it was safe to go back into the waves...

[On the other points, I'll reply to your second, corrected, posting.]


How would you explain what Cecil wrote?

Who else but Cecil would dare attempt that? :-)

How are some people
improving SWR by changing coax length, when in theory they shouldn't
be able to do this?

There are two possible reasons. One is instrument error - SWR meters are
not perfect.

The other possible reason is that the *outer* surface of the coax has
currents on it, so it has become part of the antenna. In that case,
changing the length of coax is not only changing the length of feedline
(the inner surfaces of the coax), but also is changing the antenna
itself. The voltage and current distributions on all the wires will
shift around, resulting in a different V, I and relative phase at the
top of the coax - in other words, a different feedpoint impedance. Then
the SWR (as measured on the *inside* of the feedline) genuinely will
change.

This SWR change is usually quite difficult to predict, because you
didn't mean there to be any current on the outer surface of the coax in
the first place. The only practical way to see if there could be a
problem is to use a clamp-on RF current meter to see how much surface
current is present.

If the SWR changes with feedline length *and* you have significant
surface current, then you know one probable reason... but in all cases,
these can also be instrument error in the SWR meter.

Do you think the series reactance a system offers a PA may
actually improve it's incident power?

To answer your exact question: I don't think there is a valid general
answer. It depends on the specific PA design, and also on what you mean
by "improve".

What I do know is that changes in the load impedance presented to a PA
will change several of its operating conditions, all at the same time.
Some of those changes will be "improvements" - but others definitely
won't be.

For example, reducing the load impedance will usually make the output
device operate in a more linear way... but the efficiency drops and the
greater heat dissipation and current are likely to shorten the lifetime
of the device. Is that an improvement?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek