On Mon, 6 Oct 2003 20:52:24 -0400, "Tarmo Tammaru" wrote:


"Walter Maxwell" wrote in message
.. .
Tam,

I totally disagree with those who say SWR appearing on a mismatched
transmission
line is dependent on the source impedance.

Walter, I agree with you. But I am making a distinction between SWR and
*measured* SWR. People who think an SWR meter possesses magic properties
should look inside one. It takes one instantaneous sample of the NET voltage
V, and one instantaneous sample of the NET current I. It displays the vector
sum of
K(V + k2I)
and
K(V- k2I)

where K is the sensitivity, and k2 is chosen to make the second equation
equal to 0 for a 50 Ohm load. The approximations made in coming up with what
is printed on the meter scale assumes that for the forward wave the voltage
and current are in phase. I have seen descriptions of how these things work,
but no equations to back these up. I suspect one would start with the two
equations that I listed, but express I in terms of V and deltaZ, where
deltaZ is the deviation from 50 Ohms. K can arbitrarily be 1. I also suspect
that Bird, etc don't really want us to know that.

Tam/WB2TT

Well, Tam, I agree with you also, but your comments only relate to accuracy, not
whether the internal resistance of the source has any influence on the SWR. I'm
well acquainted with the various types of swr meters, the Bruene lumped-constant
directional coupler, for instance, or for a more professional example, the
HP-778D dual directional coupler that I use with an HP-8405A Vector Voltmeter in
my own lab.

The value of the source resistance can be any value, and its reflection
coefficient rho seen looking into the output can be any value from zero to one.
If the value is zero it simply means any reflected power reaching the output is
absorbed and if rho = 1all reflected power is re-reflected. With any combination
of the above the SWR on a mismatched line is the same. The only effect these
parameters have on the line is the magnitude of the signal being propagated. I
know this from years of experience, beginning with slotted lines, and from the
engineering literature. For example, Walter C. Johnson on Page 100 spells it out
specifically.

What I'd like to see is for those who say SWR is dependent on the source
impedance to show how and why this what I call 'misconception' can occur.

Walt, W2DU