Reg, that can't possibly be you. Someone has hijacked your e-mail.

===========================

Ian, it IS me! Please calm yourself.

Let me put what I said into somewhat different words.

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. It makes the same INCORRECT assumption as a lot of
people do. This should not be surprising because it was people who designed
it.

So SWR meters nearly always give FALSE indications about what actually
exists.

Perfectionists may be upset at the repercussions of this alarming statement.

PS: In the whole of his excellent 236-page exceedingly comprehensive
volume, Chipman, in 1969, makes not the slightest mention of SWR meters.
----
Reg, G4FGQ

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.

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.
--
73, Cecil http://www.qsl.net/w5dxp



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--

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.

=================================

But where have you hidden this remarkable transmission line which is long
enough to mug and hoodwink so-called SWR meters?

It does not exist!

Your argument falls flat at the start.

Reg, G4FGQ

Reg Edwards wrote:

But where have you hidden this remarkable transmission line which is long
enough to mug and hoodwink so-called SWR meters?

It does not exist!

Most resonant 50-ohm-coax-fed dipoles would meet those requirements.
All that is needed is for it to be long enough to force the forward
V/I ratio to Z0. Given the small diameter of a piece of coax compared
to a wavelength at HF, it doesn't seem that it take a very long length.
Waveguides may be a different story.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
But where have you hidden this remarkable transmission line which is long
enough to mug and hoodwink so-called SWR meters?

It does not exist! Your argument falls flat at the start.

I don't know the answer to this question but perhaps some lurker does.

On each side of my SWR meter, I have three feet of RG-400 coax. The
spacing between conductors must be about 0.1 inches. With a spacing
of 0.1 inch between conductors in the coax, what length of coax is
required to force a V/I ratio of 50 ohms? The ratio of 3 feet to 0.1
inches is 360. I suspect that three feet is plenty long enough.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
But where have you hidden this remarkable transmission line which is long
enough to mug and hoodwink so-called SWR meters?

It does not exist!

Your argument falls flat at the start.

Reg, I asked the question over on sci.phisics.electromag and got
the following answer:

So unless almost all the power diverts into an undesireable
mode (by a factor of more than a million to one), one foot
of (RG-213) cable should see pure TEM at the end.

So according to a pretty smart guy, one foot of RG-213 on each side
of a 50 ohm SWR meter will ensure that the SWR meter is in the 50 ohm
environment for which it was designed. I have three feet of RG-400
on each side of mine.
--
73, Cecil http://www.qsl.net/w5dxp



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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.

Walter Maxwell wrote:
Sorry, Cecil, the phase between reflected voltage and current is always 180
degrees, not zero.

Yep, I know better, I just mis-spoke. Did you know that there is no
such convention for light? It's Kirchhoff's current convention that
dictates a 180 degree phase between reflected voltage and reflected
current. EM light doesn't follow Kirchhoff's convention.

For EM light, there is no phase shift in the reflection if the index
of refraction is higher. If the index of refraction is lower, there
is a 180 degree phase shift in both E and H fields.
--
73, Cecil http://www.qsl.net/w5dxp



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Whenever you're dealing with current, you have to pay attention to the
definition of positive direction. If you define the positive direction
of forward current as being toward the load and of reflected current
toward the source, then Vf is in phase with If and Vr is in phase with
Ir. I suspect that a similar caution needs to be heeded when dealing
with optics.

Roy Lewallen, W7EL

Cecil Moore wrote:
Walter Maxwell wrote:

Sorry, Cecil, the phase between reflected voltage and current is
always 180
degrees, not zero.


Yep, I know better, I just mis-spoke. Did you know that there is no
such convention for light? It's Kirchhoff's current convention that
dictates a 180 degree phase between reflected voltage and reflected
current. EM light doesn't follow Kirchhoff's convention.

For EM light, there is no phase shift in the reflection if the index
of refraction is higher. If the index of refraction is lower, there
is a 180 degree phase shift in both E and H fields.

On Sat, 04 Oct 2003 18:00:39 -0700, Roy Lewallen wrote:

Whenever you're dealing with current, you have to pay attention to the
definition of positive direction. If you define the positive direction
of forward current as being toward the load and of reflected current
toward the source, then Vf is in phase with If and Vr is in phase with
Ir. I suspect that a similar caution needs to be heeded when dealing
with optics.

Roy Lewallen, W7EL

Well, Roy, if what you say above is true then why does the phase of reflected
voltage change 180 degrees and reflected current does not change when the
forward waves encounter a perfect short-circuit termination?

And on the other hand, why does the phase of reflected current change 180
degrees and reflected voltage does not change when the forward waves encounter a
perfect open-circuit termination?

How then can the reflected voltage and current be other than 180 degrees
regardless of the load?

If what you say is true then my explanation in Reflections concerning the
establishment of the standing wave must be all wrong. Is this what you're
saying?

Walt, W2DU

Cecil Moore wrote:
Walter Maxwell wrote:

Sorry, Cecil, the phase between reflected voltage and current is
always 180
degrees, not zero.


Yep, I know better, I just mis-spoke. Did you know that there is no
such convention for light? It's Kirchhoff's current convention that
dictates a 180 degree phase between reflected voltage and reflected
current. EM light doesn't follow Kirchhoff's convention.

For EM light, there is no phase shift in the reflection if the index
of refraction is higher. If the index of refraction is lower, there
is a 180 degree phase shift in both E and H fields.