On Sun, 23 May 2004 16:39:19 GMT, "Lord Snooty" wrote:

Indeed so, because the whole idea of characterising my SWR meter is towards
the goal of measuring the output impedance of my RF amp!
I agree that it isn't 50 ohms - it's R + jX, and I want a bulletproof
procedure to find R and X. Got one?

Cheers,
Andrew
G3UHD

Hi Andrew,

You have asked an inescapable question that will lead to a deluge of
scribbling commemorating the best attempts of Houdini.

There are several many methods to determine exactly what you want to
know. The simplest and certainly the one that contains as much
information necessary is called "load pulling." To even mention this
time and bench proven method will result in hoots from those who would
be the last to offer you a fixed answer; however, we shall proceed.

This requires that you have access to known, but non-standard value
loads capable of sustaining the power you will perform your
measurement at. This is not a trivial requirement. It also requires
that you can in some way defeat your ALC which will attempt to offset
the pull of the non-standard load.

It is simplicity itself that only demands you consider the elements of
a Thevenin model and how to determine the model's source Z (or
likewise, the Norton model's source Z). You will need a means to
measure the voltage across the load, or the current through it. Even
here, proportionality is all that is required as long as the Load is
characterized and thus the tools can be rather spartan.

In the long run, this will mean you have to construct and verify your
own non-standard loads. Take care that through your verification you
confirm their value across all power applications (resistors are very
susceptible to drift with temperature). You should also take care to
insure that all paths and leads are as short as possible. Loading
directly at the terminals will save grief of complex compensation math
(and reduce introducing other errors). However, you can choose to
employ remote loads if you take care to characterize the lines through
which they are attached (this means you should be adept at the Smith
Chart).

There is more to be said, but this enough to offer you a significant
lead to find that, yes, the source exhibits nearly 50 Ohms (the common
Ham transmitter running at rated power will fall between 30 and 70
Ohms) - as specified and designed.

73's
Richard Clark, KB7QHC