"Roy Lewallen" wrote
Let me suggest an additional exercise for Richard and anyone else that
believes that source impedance affects the SWR. (etc)
____________________

Just one sec, please. I didn't say that the true SWR connected to the tx
output connector was affected. I said that the RF power measured at the
sample point(s) in the transmitter can be affected by the source and load
impedances of the tx, for the reasons stated.

The true load SWR does not change under these conditions, but it cannot then
be determined by such a meter. Attempting to do so will yield some value,
but it will be wrong.

RF

Sorry, I must have misinterpreted your earlier posting.

But we seem to now have a "true SWR" as opposed to some other kind of
SWR. And "true SWR connected to the tx output" doesn't have any meaning
at all to me. I also have no idea of what "sample points within the
transmitter" might be. So let me explain what I (and virtually all
published literature) mean by SWR.

If we connect a transmitter to an SWR meter, and then to a long piece of
lossless cable with the same Z0 as the SWR meter, and finally to a load,
the SWR meter reading will be the same as the VSWR on the cable, i.e.,
the ratio of maximum to minimum voltages on the line. This ratio of
voltages is, by definition, the VSWR -- which equals the ISWR, and is
often referred to simply as SWR.

If we measure or calculate the impedance seen looking into the line,
then disconnect the line from the SWR meter and replace it and the load
with lumped elements of the same impedance, the SWR meter reading won't
change(*).

Now, I can calculate the what the SWR meter reading will be under this
condition also. In both cases, the source impedance won't affect the SWR
meter reading, the positions or relative magnitudes of the maximum and
minimum voltages on the line, or the voltage or current within the SWR
meter line section. (This last condition assumes that the net power
delivered by the source stays the same; otherwise, the ratio of voltage
to current, and their phase angles, stay constant, regardless of the
power delivered.)

I have no idea how all this relates to your "true SWR". But do you agree
with what I've said above? If not, I'll describe a couple of simple
experiments which will test it against any alternative view you might
propose.

(*) We can also replace them with a load at the end of a line of
different Z0. As long as we choose the load Z and the line length to
make the impedance seen at the line input the same as before, the SWR
meter will read the same as before -- even though it no longer equals
the actual VSWR on the transmission line. The SWR meter is really
indicating the impedance seen looking into the line, not in this case
the actual line VSWR. (That's the essence of Reg's objection to the SWR
meter designation. Of course, if I connect my ammeter across a resistor,
it's not measuring the current through the resistor, either.)

Roy Lewallen, W7EL


Richard Fry wrote:
"Roy Lewallen" wrote

Let me suggest an additional exercise for Richard and anyone else that
believes that source impedance affects the SWR. (etc)

____________________

Just one sec, please. I didn't say that the true SWR connected to the tx
output connector was affected. I said that the RF power measured at the
sample point(s) in the transmitter can be affected by the source and load
impedances of the tx, for the reasons stated.

The true load SWR does not change under these conditions, but it cannot then
be determined by such a meter. Attempting to do so will yield some value,
but it will be wrong.

RF

"Roy Lewallen" wrote

But we seem to now have a "true SWR" as opposed to
some other kind of SWR. And "true SWR connected to
the tx output" doesn't have any meaning at all to me.

My "true SWR" term is used is an attempt to differentiate between the SWR of
the antenna system, and the inaccuracies associated with trying to measure
it with devices that cannot isolate the incident power in the system from
internal reflections of that power. For the conditions and reasoning
outlined in my earlier posts in this thread, and even though the system SWR
is a constant -- the normal SWR meter used in/with an operating transmitter
working into a mismatched load won't have the ability to give strictly
accurate measurement of that SWR. That is all I'm saying.

I also have no idea of what "sample points within the
transmitter" might be.

In broadcast gear, these are the directional couplers whose pickup probes
are inserted transversely into the coaxial line between the harmonic filter
output and the tx output connector. I haven't been a licensed ham for over
40 years (when I went into the broadcast field), but I expect some ham txs
might have the same setup. Otherwise it could be a Model 43 or the like
inserted between the output connector of the ham tx and the transmission
line to the antenna.

I hope this is understandable now.

RF

Sorry, it still isn't clear.

Richard Fry wrote:
"Roy Lewallen" wrote

But we seem to now have a "true SWR" as opposed to
some other kind of SWR. And "true SWR connected to
the tx output" doesn't have any meaning at all to me.


My "true SWR" term is used is an attempt to differentiate between the SWR of
the antenna system, and the inaccuracies associated with trying to measure
it with devices that cannot isolate the incident power in the system from
internal reflections of that power. For the conditions and reasoning
outlined in my earlier posts in this thread, and even though the system SWR
is a constant -- the normal SWR meter used in/with an operating transmitter
working into a mismatched load won't have the ability to give strictly
accurate measurement of that SWR. That is all I'm saying.

What, then, is "system SWR"? How do you define it?


I also have no idea of what "sample points within the
transmitter" might be.


In broadcast gear, these are the directional couplers whose pickup probes
are inserted transversely into the coaxial line between the harmonic filter
output and the tx output connector. I haven't been a licensed ham for over
40 years (when I went into the broadcast field), but I expect some ham txs
might have the same setup. Otherwise it could be a Model 43 or the like
inserted between the output connector of the ham tx and the transmission
line to the antenna.

In your last posting, you said,

Just one sec, please. I didn't say that the true SWR connected to the
tx output connector was affected. I said that the RF power measured
at the sample point(s) in the transmitter can be affected by the
source and load impedances of the tx, for the reasons stated.

So replacing "sample point(s) in the transmitter" with "Model 43 or the
like inserted between the output connector of the ham tx and the
transmission line to the antenna", you've said that the RF power
measured by the (model 43) SWR meter can be affected by the source
impedance of the transmitter.

Obviously, if we have a voltage or current source of fixed value and
change the source impedance, the power delivered by the source changes,
and any means of measuring the power at the source, load, or in between
should show that change. That follows from elementary circuit theory,
and doesn't require any consideration or knowledge of transmission
lines, waves, or SWR. On the model 43, both the "forward" and "reverse"
powers will change, but by the same fraction. Perhaps that's what you
mean. But if you mean that the SWR reading or the ratio of "forward" to
"reverse" power changes as a result of changing the source impedance,
that's easily shown to be false by the simple experiment I proposed.

I hope this is understandable now.

Almost, but not quite.

"Roy Lewallen" wrote:
Sorry, it still isn't clear.

What, then, is "system SWR"? How do you define it?

System SWR is the net SWR of a component assembly present at its input
terminals. "Antenna system SWR" then is comprised of the net SWR of
everything in the RF path from the output of the SWR meter to and including
the antenna. In a transmitter, the antenna system begins electrically at
the output of the SWR meter -- physically close to the output connector of
the tx.

Obviously, if we have a voltage or current source of fixed value
and change the source impedance, the power delivered by the
source changes,

But the mechanism I've described considers the re-reflection by a mismatched
source of power not initially absorbed by a mismatched load -- not that a
change of source impedance changed the total power flowing out of the
source.

...both the "forward" and "reverse" powers
will change, but by the same fraction...

Agree. I'm not so sure that the Model 43 or equivalent methods used in/with
transmitters accurately preserves the power ratios under these conditions,
though.

RF

The meaning of this paragraph in my last post in this thread is more clearly
understood when two commas are added...

But the mechanism I've described considers the re-reflection,
by a mismatched source, of power not initially absorbed by a
mismatched load -- not that a change of source impedance
changed the total power flowing out of the source.

Richard Fry wrote:
"Roy Lewallen" wrote:

Sorry, it still isn't clear.

What, then, is "system SWR"? How do you define it?


System SWR is the net SWR of a component assembly present at its input
terminals. "Antenna system SWR" then is comprised of the net SWR of
everything in the RF path from the output of the SWR meter to and including
the antenna. In a transmitter, the antenna system begins electrically at
the output of the SWR meter -- physically close to the output connector of
the tx.

You've still lost me. Let's say the "component assembly" is a half
wavelength of 75 ohm transmission line terminated with a 75 ohm
resistor. What is its "net SWR"? How about a half wavelength of 75 ohm
line terminated with 50 ohms? Or a plain 75 ohm resistor? You surely
have an equation you use to calculate "system SWR" or "net SWR" -- can
you share it with us?

This is getting more complicated rather than simpler. We now have "true
SWR", "antenna system SWR", and "net SWR". Quite a step from the ratio
of maximum to minimum voltages on a transmission line.

. . .

Roy Lewallen, W7EL

"Roy Lewallen" wrote

If we connect a transmitter to an SWR meter, and then to a long piece of
lossless cable with the same Z0 as the SWR meter, and finally to a load,
the SWR meter reading will be the same as the VSWR on the cable, i.e.,
the ratio of maximum to minimum voltages on the line.

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

It is at this point where impressionable novices are led astray by old
wives, never again to return to logical thought on the subject.

They imagine that because the meter happens to indicate the swr on the line,
the meter is actually responding to the swr on it. Whereas the meter is
actually responding to the modulus of the reflection coefficient caused by
the line's input impedance regardless of what its Zo may be. The act of
making the line's Zo and the meter's resistance both equal to the
transmitter's designed-for load resistance, has put additional infomation
into the system. Cooking the books!

If there's an SWR to be indicated it is on a long line between meter and the
transmitter. In the absence of such a line the meter wastefully discards
half of the information it is presented with and indicates the modulus of
the reflection cofficient. A more appropriate name is TLI.
----
Reg, G4FGQ

In message , Reg Edwards
writes

"Roy Lewallen" wrote

If we connect a transmitter to an SWR meter, and then to a long piece of
lossless cable with the same Z0 as the SWR meter, and finally to a load,
the SWR meter reading will be the same as the VSWR on the cable, i.e.,
the ratio of maximum to minimum voltages on the line.

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

It is at this point where impressionable novices are led astray by old
wives, never again to return to logical thought on the subject.

They imagine that because the meter happens to indicate the swr on the line,
the meter is actually responding to the swr on it. Whereas the meter is
actually responding to the modulus of the reflection coefficient caused by
the line's input impedance regardless of what its Zo may be. The act of
making the line's Zo and the meter's resistance both equal to the
transmitter's designed-for load resistance, has put additional infomation
into the system. Cooking the books!

If there's an SWR to be indicated it is on a long line between meter and the
transmitter. In the absence of such a line the meter wastefully discards
half of the information it is presented with and indicates the modulus of
the reflection cofficient. A more appropriate name is TLI.
----
Reg, G4FGQ



Would this help?

On the subject of whether the TX impedance affected the SWR reading, I
propose the following practical test:
Using standard CATV bits and pieces, connect up the following-
Signal source directional coupler #1 directional coupler #2 load.
DC#1 picks off the forward signal, DC#2 picks off the reverse.
Use a spectrum analyser to measure signal levels.
Beforehand check the DCs for go directivity, and chose a frequency where
it is best (at least 25dB). This will probably be around 20MHz.
With good load, measure forward and reverse signals.
Repeat with known load mismatch.
Screw up source impedance (eg add T-piece at source o/p, and
double-terminate).
Repeat the above.
Think about what the results mean.

Ian.



--

What's a directional coupler?
What do they look like?
Don't bother answering those questions.

Why do the arguers, when caught in a tight corner, always escape to UHF for
help from directional couplers?

There are NO directional couplers at HF. They are as scarce as real swr
meters. So they cannot be used in futile attempts to explain what really
happens at HF.

You're next move will be to drag in scattering-matrices.
---
Reg ;o)