"Richard Harrison" wrote
IF you were to insert the Model 43 into most 75-ohm transmission
systems, the precision 50-ohm meter line of 5.125 inches would not
likely enforce the 50-ohm V/I ratio and the meter reading would be in
error.
________________

Yet a 50 ohm RF bridge or network analyzer with a 75 ohm termination applied
directly at its output port has no trouble showing the true SWR. These
measuring devices are looking at the same transition plane from 50 to 75
ohms as the Bird 43 would see with a 75 ohm load at its output port.

If the Model 43 is unable to make an accurate measurement of this, is that
not due to reasons other than not having the right 50-ohm V/I ratio in its
line section?

RF

On Fri, 3 Sep 2004 14:14:57 +0000 (UTC), "Reg Edwards"
wrote:

|Richard Fry wrote -
| The generic function of this meter is to measure the degree of match
|between
| a source and a load.
|
|--------------------------------------------------------
|
|Exactly!

Not!

The source plays no role at all. The degree of match that is
indicated is that between the line (or system Zo) and the load Z.

A 50 ohm instrument with a 50 ohm termination shows a reflection
coefficient (or whatever mathematical equivalent you want to use) of
zero regardless of the source impedance.


|So let's call it a TLI.

Let's don't. That's just more bafflegab. Suppose the source impedance
is 25 ohm and the load is 50 ohm. By this new monstrosity of a
definition, the source should be delighted when the "transmitter
loading indicator" says---well---I'm not sure what it says, but I
think the desired number is either 0 or 1. And another source
(transmitter) with a source Z of 100 ohm should be equally happy.
Right?


|Which is what it actually is. Abolish
|the source of confusion and the arguments on what it does.


I'm all for abolishing the confusion too, so why did you add to it?

Richard Fry wrote:
The coax sampling sections for RF frequencies at least as low as 540
kHz. is around 9" in length.

The guys over on s.p.e said it has something to do with conductor
spacing Vs conductor length. They said a 100/1 ratio is plenty
long enough for Z0 to assert itself.
--
73, Cecil http://www.qsl.net/w5dxp


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"Wes Stewart" wrote (clip):

"Reg Edwards" wrote:
|Richard Fry wrote -
| The generic function of this meter is to measure
| the degree of match between a source and a load.
|--------------------------------------------------------
|Exactly!

Not!
The source plays no role at all. The degree of match that is
indicated is that between the line (or system Zo) and the load Z.
A 50 ohm instrument with a 50 ohm termination shows a reflection
coefficient (or whatever mathematical equivalent you want to use) of
zero regardless of the source impedance.
__________

I wrote "BETWEEN a source and a load," not OF the source
and a load. There is a difference.

RF

Richard Fry wrote:
"Wes Stewart" wrote (clip):

"Reg Edwards" wrote:
|Richard Fry wrote -
| The generic function of this meter is to measure
| the degree of match between a source and a load.
|--------------------------------------------------------
|Exactly!

Not!
The source plays no role at all. The degree of match that is
indicated is that between the line (or system Zo) and the load Z.
A 50 ohm instrument with a 50 ohm termination shows a reflection
coefficient (or whatever mathematical equivalent you want to use) of
zero regardless of the source impedance.
__________

I wrote "BETWEEN a source and a load," not OF the source
and a load. There is a difference.

The only difference between those two terms is that "match between" is
normal and grammatical technical usage; and "match of" ain't neither.

Wes is correct. What the meter measures is the match (expressed as
reflection coefficient, SWR, whatever) between the system Zo for which
that meter was designed and calibrated, and the load Z.

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.


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

"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.

RF

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.

Others can probably explain why the statement is also incorrect
according to the concept of "forward and reflected power waves". Myself,
I prefer avoid that concept completely, because it so easily leads into
this kind of mess.


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

"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

RF

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"Richard Fry"
How....do you explain the "ghost image" .... TV

Sigh - moth + lamp (not you RF specifically, the newsgroup...).

LOOK - Discuss a simple step function (rising edge) - not RF. All of your
disagreements about SWR and reflections will be revealed as silly semantics
and the mixing up of the transient versus the steady state. A step function
makes it so simple that there is no room for arguments.

There is NOTHING in the endless (and now repeating) discussion other than
semantics and the above mention lack of discernment (initial transient
versus steady state).



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For those who have forgotten how or have never measured SWR.

Two separate voltage measurements are needed at two different places along
the line.
Slide the voltmeter along the line until a maximum is found.
Remember the reading, Vmax.
Slide the meter along the line again until a minimum is found.
Remember the reading, Vmin.
Before you forget, divide Vmax by Vmin.
You are left with a single number.
It has no dimensions.
It is the TRUE swr.

NOTE: In the above description and calculation there is no mention of Zo,
terminating impedance, source impedance, reflection coefficient, forward
power, reflected power, reflected volts, reflected current, Smith charts, or
conjugate matches. All these things are superflous to the determination. No
information other than the two voltage measurements is needed.

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