On Sat, 04 Oct 2003 10:46:41 -0500, Cecil Moore
wrote:

Reg Edwards wrote:

SWR meters are designed to operate and provide indications of SWR, Rho, Fwd
Power, Refl.Power, on the ASSUMPTION that the internal impedance of the
transmitter is 50 ohms.

I believe that to be an incorrect statement, Reg. The assumption is that
a Z0 of 50 ohms exists and the transmission line is long enough to force
the ratio of V/I to be 50 ohms for the forward wave and the reflected wave.
The phase between the forward voltage and current is assumed to be zero.
The phase between the reflected voltage and current is assumed to be zero.
Given all those assumptions, the internal impedance of the transmitter is
irrelevant. I'm not saying all those assumptions are always met.

Sorry, Cecil, the phase between reflected voltage and current is always 180
degrees, not zero. If it were not for this phenomenon the standing wave would
not be established as forward and reflected waves of both voltage and current
pass through each other otherwise undisturbed.

Walt, w2du

Put a 50 ohm dummy load on an SWR meter and feed it with a transmitter
of unknown source impedance. The SWR meter will always read 1:1 because
the dummy load forces the V/I ratio to be 50 no matter what the source
impedance. That's what the 50 ohm characteristic impedance of the
transmission is supposed to do to make the source impedance irrelevant.

PS: In the whole of his excellent 236-page exceedingly comprehensive
volume, Chipman, in 1969, makes not the slightest mention of SWR meters.

In 1969, virtually all ham transmitters had an adjustable pi-net output
so an SWR meter was not needed. When I started out as a ham in the 1950's,
just as many hams used 75 ohm coax as used 50 ohm coax, maybe more. The
pi-net output of a typical ham transmitter back then didn't care what the
Z0 was. I didn't own an SWR meter until the 1980's when I bought an IC-745.

In 1969, the "antenna tuner" was built into the transmitter. If wide-
range antenna tuners were built into transmitters today, there would be
little need for the SWR meter. I don't know of anyone who puts an SWR
meter between an SGC-230 and the antenna.