Richard Fry wrote:
"If the Model 43 is unable to make an accurate masuremnt of this, is
that not due to reasons other than not having the right V/I ratio in its
line section?"

Many details of desisn, construction, and application must be chosen and
executed right to get accuracy, but the line impedance is essential.

Bird can adjust the current sample to exactly equal the voltage sample,
both taken from the transmission line at any point. But it must work
with a fixed voltage to current ratio. Bird chose 50 onms.

For a directional meter, it`s necessary to respond to one direction
while rejecting the other. When power is applied to a line, the
resulting current is is in phase with the volts. On reflection, the
volts and amps in the reflected wave are 180 degrees out of phase. The
phase difference of the reflected wave is used by Bird to distinguish it
from the incident wave.

By selecting and adjusting for equal samples of volts and amps in the
forward wave, their total is 2X that of either sample. But, the samples
from the reflected wave, being equal but out of phase, cancel.

To get the value of the reflected power samples, it is only necessary to
reverse the polarity of one of the samples. They are now in phase and
the forward power samples are now out of phase and cancel.

If some other voltage to current ratio is used for the power samples
than that of the design, the samples won`t be exactly equal and
cancellation of the undesired direction does not work.

Best regards, Richard Harrison, KB5WZI

Reg Edwards wrote:
All other methods which purport to measure swr require injection of
additional information. And assumptions form an essential part of the
process. They can hardly be called swr measurements.

They can be called indirect (calculated) SWR measurements
and assumptions indeed do form an essential part of the
process. That's not at all unusual for indirect measurements.

Particularly when they can indicate it on non-existent lines.

One of the assumptions is that a transmission line exists. If
a transmission line doesn't exist, the measurement conditional
assumptions are violated, and the actual values may not be
the desired or expected results. Happens all the time with
various measuring instruments.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
For those who have forgotten how or have never measured SWR.


Hang on, Reg - didn't you spend your career working on VLF cables that
went under the ocean?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Richard Fry wrote:
"Ian White, G3SEK"wrote:
Richard Fry wrote:
"Ian White, G3SEK wrote
The meter measures nothing that involves the source, except
the level of RF that it supplies. It does not respond in any way
whatever to the source impedance.

Not that I said it did in my part of the thread, but nevertheless the
above
statement is not strictly true. In the case where the source Z of the tx
PA
does not match its load Z (which is typical), power reflected from the
load
mismatch will at least partly be re-reflected from the PA -- which then
contributes to the power sensed by a "wattmeter" in the output path.

Sorry, that statement cannot be correct. It would mean that the
impedance you measure at the near end of a transmission line (terminated
by some arbitrary load at the far end) would depend on the internal
impedance of the device that's doing the measuring - and that is not
true, either in transmission-line theory or in the real world. It is a
function only of the line and the load. etc
____________

How, then, do you explain the "ghost image" that can occur* in analog(ue) TV
transmission systems arising from reflections at/near the antenna end of the
station's transmission line?

*with sufficient round-trip propagation time in the transmission line

Yes, that is a true observation, just as true as the one I made... so
now you have *two* different things to explain!

The so-called SWR meter is a steady-state instrument, so it always makes
sense to use that quicker, easier way of thinking. Since you're the one
who chooses to think of this particular situation in terms of multiple
reflections, any difficulties you encounter are entirely yours.

If you ever see a conflict between two different theories that explain
the same observed facts, then there's an error somewhere. If the
multiple-reflection theory is extrapolated to infinite time, so that it
calculates results for the steady state, it *must* give identical
results to the steady-state theory. But whenever the steady-state theory
can be used, it will always get you there much more quickly.

However, when you have finally done it your way, and accounted correctly
for all the reflections and re-reflections, we can predict the outcome
with complete confidence:

1. If you sum the successive reflections correctly to infinity, and
calculate the V/I ratio and phase at the station end of the line, then
the final result will be identical to the impedance given by the
steady-state transmission-line theory. It has to be, because that single
value is the reality.

2. Somewhere in your calculations, any value that you assume for the RF
source impedance is going to cancel right out of your calculations. The
correct mathematical result *must* be independent of that value -
because, again, that's the reality.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Sat, 4 Sep 2004 06:23:28 -0500, "Richard Fry"
wrote:

|"Ian White, G3SEK wrote
|The meter measures nothing that involves the source, except
|the level of RF that it supplies. It does not respond in any way
|whatever to the source impedance.
|_____________
|
|Not that I said it did in my part of the thread, but nevertheless the above
|statement is not strictly true. In the case where the source Z of the tx PA
|does not match its load Z (which is typical), power reflected from the load
|mismatch will at least partly be re-reflected from the PA -- which then
|contributes to the power sensed by a "wattmeter" in the output path.

If you have a directional wattmeter that *perfectly* senses and
displays the ratio of forward to reverse power, what difference does
the absolute power make? Hint: It doesn't.

But let's avoid going any further here. This has been argued about in
at least 10,000 other threads and has absolutely no bearing on Ian's
or my statements regarding the unimportance of source Z in the
discussion of measuring reflection coefficient.

In a laboratory environment, the source is usually well matched at the
system impedance. This is for convenience and improves accuracy only
because is takes less mathematical horsepower to remove the effects of
the "re-reflection" you speak of. This is particularly true when
highly mismatched loads are measured; and two of the most highly
mismatched loads are an open and a short, both of which are often used
as calibration standards.

Re-reflection exists and is actually an undesired thing in laboratory
measurements, primarily because most measurements are swept frequency
and the changing phase of the reflections is a pain in the ass.
Reflections are a source of *measurement error*. Removing the effects
is called *error correction* in the laboratory setting.

But I can tell you that if I calibrate a high-quality network analyzer
using a 50 ohm generator and then add a tee and second 50 ohm load to
the generator output, making it a 2:1 mismatched source, there will be
*no* significant change in the answer presented when typical
mismatches are measured.

Any differences represent measurement *error*, not measurement
*reality*. Reality is the interaction of the line and load and the
source plays *no* role.

Ian White, G3SEK wrote:



Yes, that is a true observation, just as true as the one I made... so
now you have *two* different things to explain!

The so-called SWR meter is a steady-state instrument, so it always makes
sense to use that quicker, easier way of thinking. Since you're the one
who chooses to think of this particular situation in terms of multiple
reflections, any difficulties you encounter are entirely yours.

If you ever see a conflict between two different theories that explain
the same observed facts, then there's an error somewhere. If the
multiple-reflection theory is extrapolated to infinite time, so that it
calculates results for the steady state, it *must* give identical
results to the steady-state theory. But whenever the steady-state theory
can be used, it will always get you there much more quickly.

However, when you have finally done it your way, and accounted correctly
for all the reflections and re-reflections, we can predict the outcome
with complete confidence:

1. If you sum the successive reflections correctly to infinity, and
calculate the V/I ratio and phase at the station end of the line, then
the final result will be identical to the impedance given by the
steady-state transmission-line theory. It has to be, because that single
value is the reality.

2. Somewhere in your calculations, any value that you assume for the RF
source impedance is going to cancel right out of your calculations. The
correct mathematical result *must* be independent of that value -
because, again, that's the reality.



This is correct. If you divide the formula for voltage, at any point on
a transmission line, by the formula for current, the generator impedance
cancels.
73,
Tom Donaly, KA6RUH

Pardon the extensive pasting, but it will allow a clearer post, and save a
lot of click time. / RF

"Ian White, G3SEK" wrote :
The meter measures nothing that involves the source, except
the level of RF that it supplies. It does not respond in any way
whatever to the source impedance.

Richard Fry wrote:
Not that I said it did in my part of the thread, but nevertheless the
above statement is not strictly true. In the case where the source Z
of the tx PA does not match its load Z (which is typical), power
reflected from the load mismatch will at least partly be re-reflected
from the PA -- which then contributes to the power sensed by a
"wattmeter" in the output path.

"Ian White, G3SEK" wrote :
Sorry, that statement cannot be correct. It would mean that the
impedance you measure at the near end of a transmission line
terminated by some arbitrary load at the far end) would depend
on the internal impedance of the device that's doing the
measuring - and that is not true, either in transmission-line
theory or in the real world. It is a function only of the line and
the load. etc

Richard Fry wrote:
How, then, do you explain the "ghost image" that can occur* in
analog(ue) TV transmission systems arising from reflections
at/near the antenna end of the station's transmission line?

*with sufficient round-trip propagation time in the transmission line

Ian White wrote:
Yes, that is a true observation, just as true as the one I made... so
now you have *two* different things to explain!

The so-called SWR meter is a steady-state instrument, so it always makes
sense to use that quicker, easier way of thinking. Since you're the one
who chooses to think of this particular situation in terms of multiple
reflections, any difficulties you encounter are entirely yours.

This reads to me as though you know they are there, but choose
to ignore them...?

If you ever see a conflict between two different theories that explain
the same observed facts, then there's an error somewhere.

We agree on the subject of conflict resolution, but apparently not on
the location of the error.

If the multiple-reflection theory is extrapolated to infinite time, so
that it calculates results for the steady state, it *must* give identical
results to the steady-state theory. But whenever the steady-state
theory can be used, it will always get you there much more quickly.

This is true only to the extent that all the power ever generated by the
transmitter eventually either is radiated by the antenna or is dissipated by
losses somewhere.

For simplicity, let's assume a tx with a source impedance of zero ohms feeds
a lossless transmission line of uniform impedance throughout its length to a
mismatch at the far end. The mismatch reflects a percentage of the incident
power back down the line to the tx, and continues to do so as long as the
transmitter generates power. The tx will re-reflect the reflected power
back to the far end -- in this case all of the reflected power it ever sees,
in fact. To this easily-seen, real-world reality you agreed above ("Yes,
that is a true observation, ...").

The re-reflections combine with the power generated by the tx at that
instant to create a vector sum at the sample point used by the meter. The
typical tx meter is a frequency-domain device, and cannot by itself separate
the RF output of the transmitter from re-/reflections of it. That requires
a time-domain device. So the magnitude of the transmission line samples
driving the tx RF metering circuits during normal operation under these
conditions become a function of both the source impedance and the
load impedance.

The defense rests.

RF

On Sat, 4 Sep 2004 13:59:43 +0100, "Ian White, G3SEK"
wrote:

depend on the internal impedance of the device that's doing the measuring - and that is not
true, either in transmission-line theory

Hi Ian,

I see you have yet to respond to this very matter attended to quite at
length by Chipman.

73's
Richard Clark, KB7QHC

Please see my post in this thread of 4 Sept 2004 at 16:54 UT (shown above).

RF

Richard Fry wrote:
This is true only to the extent that all the power ever generated by the
transmitter eventually either is radiated by the antenna or is dissipated by
losses somewhere.

The fly in the ointment is a definition.

If a signal generator is sourcing 100 watts and 20 watts of reflected
power is being dissipated in a circulator load resistor, we say the
source is sourcing 100 watts and 20 watts of reflected power is being
dissipated in the circulator load resistor.

If the identical thing happens in a ham transmitter, we say that the
source is sourcing 80 watts, BY DEFINITION. What's wrong with this
picture? Ham transmitters NEVER re-reflect anything, by definition.

The reason that the source impedance doesn't enter into the forward/reflected
power values is that it has been defined out of any relationship to them. By
definition, there is zero power re-reflected from a ham transmitter NO MATTER
WHAT THE IMPEDANCE OF THE HAM TRANSMITTER MIGHT BE. Never mind that we can see
those reflections with our own eyes in TV ghosting. We must be crazy because
they have been defined out of existence. How dare we have the gall to observe
them!
--
73, Cecil http://www.qsl.net/w5dxp


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