In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote:

If one knows what they are doing, SWR can always be measured.

NOT on a line which isn't there. QED.

In the strong sense of the definition ("A VSWR meter is a meter which
measures, literally, the ratio between the voltage maxima and minima
present on a transmission line"), Reg is correct. This can be done
with a section of slotted line and a probe, of course, tapped into a
section of a transmission line having the same characteristic
impedance.

That's not how a typical amateur "SWR" meter works, though - it's not
either locating or measuring either the maxima or the minima of either
voltage or current on the line. So, the strong, literal, pedantic
sense of the term, I agree with Reg that a standard "SWR" meter is not
truly measuring SWR, and that he's correct in his objection.

However, I also think he's overstating the case.

An "SWR" meter circuit, in the usual sense (e.g. a Monimatch or
similar) can provide an accurate *indirect* measurement of SWR, *if*
the conditions under which it is used are appropriate. It measures
the currents and voltages flowing through it, and derives
(electrically and mathematically) a number which correlates extremely
well to what a true SWR measurement on a T-line of specified impedance
would say. If you build and calibrate this sort of circuit accurately
enough, and then put it in the middle of a section of 50-ohm line (or
whatever it's calibrated for), it'll give the same numbers as a true,
direct measurement of VSWR on a length of T-line at that point.

Now, it's true (as Reg says) that a Monimatch or similar
indirect-measurement meter can give you inaccurate or misleading
numbers, if used in an environment other than what it's designed for.
If you stick a Monimatch at the output of a transmitter or transmatch,
with its output looking into a high-impedance balanced line, then the
numbers it displays won't equal the actual VSWR on the line. You
might be able to come up with a correction factor / curve for it,
though. If you stick it right at the transmitter output, and it reads
1.0:1, then you can be confident that your transmitter is seeing the
load that it wants to see... hence Reg's desire to have it renamed as
a "TLI".

Seems to me, though, that the same is true of a real "VSWR"
measurement system (e.g. a slotted line) if you use it under
inappropriate test conditions. If you take a slotted-line-and-
probe measurement device whose internal line is 50 ohms, and stick it
in the middle of a 75-ohm line, and measure the VSWR on your slotted
line, you'll come up with a number which *does* equal the VSWR in
the slotted line but does *not* equal the VSWR on the actual
transmission line. Same problem, really.

In that sense, even a "real" VSWR meter isn't a "useful" VSWR meter,
if you use it inappropriately.

All that being said: I conclude that there's nothing wrong with
calling a Monimatch (or similar) current/voltage measurement circuit a
"SWR" meter, as it *will* display correct and accurate numbers for the
SWR on the line when used appropriately (within the limits of its
calibration, of course). It's up to the user to understand the
conditions under which this sort of measurement can be made accurately
and usefully... just as it is with every other sort of test instrument
I know of.

Reg, I think you're tilting at windmills.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
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Jim Kelley wrote:



Reg Edwards wrote:

Jim wrote,


If one knows what they are doing, SWR can always be measured.


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

NOT on a line which isn't there. QED.
---
Reg.


Perhaps this is a dumb question Reg, but if the transmission line isn't
there, how does RF get from the transmitter to the antenna?

Thanks, ac6xg



Well there's wave guide, there's printed circuit traces, there's wire.
OPPS.... those are transmission lines.

Dave WD9BDZ

Perhaps this is a dumb question Reg, but if the transmission line
isn't
there, how does RF get from the transmitter to the antenna?

Thanks, ac6xg

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

The 50-ohm line on which the SWR is supposed to be measured is between
the transmitter and the so-called SWR meter.

If the transmitter is connected directly to the meter, (as it usually
is, very often it is inside the transmitter on the front panel) there
is no line and no SWR on it which can be measured.

Neverthess, the meter still provides a reading of SWR. Obviously it
is telling lies and causes confusion and misunderstanding to novices
about what is really happening within their equipment. In fact, just
as it has been doing since Joker, Richard Clark, entered the thread.

Some people force themselves to imagine a 1/4-wavelength transmission
inside the little box in a vain endeavor to explain how an SWR meter
works.

Actually the meter indicates whether or not the transmitter is loaded
with 50-ohms - and nothing else. It is an HF resistance bridge to
which its circuit reduces.

But, as I say, it is a very useful, almost indispensible instrument.
It is a TLI.
----
Reg.

Reg Edwards wrote:

Perhaps this is a dumb question Reg, but if the transmission line

isn't

there, how does RF get from the transmitter to the antenna?

Thanks, ac6xg


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

The 50-ohm line on which the SWR is supposed to be measured is between
the transmitter and the so-called SWR meter.

If the transmitter is connected directly to the meter, (as it usually
is, very often it is inside the transmitter on the front panel) there
is no line and no SWR on it which can be measured.

What you're saying is that there's a line connecting the transmitter to
the meter, but there isn't a line connecting the transmitter to the
meter. I'd have difficulty defending such a position.

Neverthess, the meter still provides a reading of SWR. Obviously it
is telling lies and causes confusion and misunderstanding to novices
about what is really happening within their equipment. In fact, just
as it has been doing since Joker, Richard Clark, entered the thread.

Some people force themselves to imagine a 1/4-wavelength transmission
inside the little box in a vain endeavor to explain how an SWR meter
works.

Actually the meter indicates whether or not the transmitter is loaded
with 50-ohms - and nothing else.

I don't think the meter really knows what impedance the transmitter is
loaded with. As far as I can tell, it only knows how many volts and
amps gozinta one port and/or compared with how many volts and amps
gozinta the other port.

But I agree that some people read an awful lot into the measurements
they make with one of these devices.

It is an HF resistance bridge to
which its circuit reduces.

True enough.

But, as I say, it is a very useful, almost indispensible instrument.
It is a TLI.

I like to think of it more as a reflectometer.

73 de ac6xg

On Mon, 26 Sep 2005 19:49:00 +0000 (UTC), "Reg Edwards"
wrote:

In fact, just
as it has been doing since Joker, Richard Clark, entered the thread.


Aw Reggie,

You are getting emotional because you were caught spitting on
Kelvinator and telling us it was the London fog.

73's
Richard Clark, KB7QHC

I have not insisted, not even mentioned, that SWR can exist only on
50-ohm lines.

All I have said, somewhere, is that the usual SWR meter gives the
correct answers only on 50-ohm lines.
----
Reg.

Reg Edwards wrote:


Perhaps this is a dumb question Reg, but if the transmission line
isn't
there, how does RF get from the transmitter to the antenna?

Thanks, ac6xg

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

The 50-ohm line on which the SWR is supposed to be measured is between
the transmitter and the so-called SWR meter.

No, the SWR being measured is on the load side of the meter.

If the transmitter is connected directly to the meter, (as it usually
is, very often it is inside the transmitter on the front panel) there
is no line and no SWR on it which can be measured.

Bzzzt, wrong answer. SWR is measured on the output side of the meter,
not the input.

Neverthess, the meter still provides a reading of SWR. Obviously it
is telling lies and causes confusion and misunderstanding to novices
about what is really happening within their equipment. In fact, just
as it has been doing since Joker, Richard Clark, entered the thread.

Some people force themselves to imagine a 1/4-wavelength transmission
inside the little box in a vain endeavor to explain how an SWR meter
works.

That would be inane.

All one need do is realize the meter measures toward the load.

Actually the meter indicates whether or not the transmitter is loaded
with 50-ohms - and nothing else. It is an HF resistance bridge to
which its circuit reduces.

But, as I say, it is a very useful, almost indispensible instrument.
It is a TLI.
----
Reg.



--
Jim Pennino

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Reg Edwards wrote:
I have not insisted, not even mentioned, that SWR can exist only on
50-ohm lines.

All I have said, somewhere, is that the usual SWR meter gives the
correct answers only on 50-ohm lines.
----
Reg.

If I connect a 100 Ohm antenna through a 100 Ohm transmission line to
a SWR meter designed for 50 Ohms and then to a transmitter which expects
a 50 Ohm load, does the meter read correctly with respect to the desired
transmitter loading?

Of course it does.

--
Jim Pennino

Remove .spam.sux to reply.

On Mon, 26 Sep 2005 20:38:01 +0000 (UTC),
wrote:

Reg Edwards wrote:
I have not insisted, not even mentioned, that SWR can exist only on
50-ohm lines.

All I have said, somewhere, is that the usual SWR meter gives the
correct answers only on 50-ohm lines.
----
Reg.

If I connect a 100 Ohm antenna through a 100 Ohm transmission line to
a SWR meter designed for 50 Ohms and then to a transmitter which expects
a 50 Ohm load, does the meter read correctly with respect to the desired
transmitter loading?

Of course it does.

Jim, that seems inconsistent with your earlier statemetn "No, the SWR
being measured is on the load side of the meter."

In the example you quoted with a 100 ohm load on a 100 ohm line, were
the line loss low, and the line long enough to be sure to sample a
fully developed voltage maximum and voltage minimum it would be found
that the VSWR was 1:1.

Yet we would expect the "SWR meter designed for 50 Ohms" to which it
is connected (on the load side) to read VSWR=2:1, so is it measuring
the SWR on the load side of the meter as you earlier stated?

Perhaps typical SWR meters are actually sampling current and voltage
on a very short section (almost a point sample) of transmission line
that is part of the instrument, and they are indicating what the
observed SWR would be on an extended line of that type, provided that
attenuation was insignificant, and that the extended line was
sufficiently long to allow full development of the standing wave
pattern.

The inference of what is happening on adjacent line is ours, not the
instruments, as demonstrated by your example above.

Owen
--

Owen Duffy wrote:
On Mon, 26 Sep 2005 20:38:01 +0000 (UTC),
wrote:

Reg Edwards wrote:
I have not insisted, not even mentioned, that SWR can exist only on
50-ohm lines.

All I have said, somewhere, is that the usual SWR meter gives the
correct answers only on 50-ohm lines.
----
Reg.

If I connect a 100 Ohm antenna through a 100 Ohm transmission line to
a SWR meter designed for 50 Ohms and then to a transmitter which expects
a 50 Ohm load, does the meter read correctly with respect to the desired
transmitter loading?

Of course it does.

Jim, that seems inconsistent with your earlier statemetn "No, the SWR
being measured is on the load side of the meter."

The load side is the side with the load, i.e. the antenna, on it.

In the example you quoted with a 100 ohm load on a 100 ohm line, were
the line loss low, and the line long enough to be sure to sample a
fully developed voltage maximum and voltage minimum it would be found
that the VSWR was 1:1.

Not for a 50 Ohm system, i.e. a transmitter expecting 50 Ohms and a
meter calibrated for a 50 Ohm system.

Yet we would expect the "SWR meter designed for 50 Ohms" to which it
is connected (on the load side) to read VSWR=2:1, so is it measuring
the SWR on the load side of the meter as you earlier stated?

Yep.

Perhaps typical SWR meters are actually sampling current and voltage
on a very short section (almost a point sample) of transmission line
that is part of the instrument, and they are indicating what the
observed SWR would be on an extended line of that type, provided that
attenuation was insignificant, and that the extended line was
sufficiently long to allow full development of the standing wave
pattern.

The SWR that most people care about is that of the antenna.

Usually you have a 50 Ohm transmitter connected to 50 Ohm line, and
then to an antenna you hope is 50 Ohms.

To accurately measure the antenna SWR without any error being introduced
by line losses, you have to put the SWR meter at the end of the line
adjacent to the antenna.

This is usually impractical and we normally put the SWR meter near the
transmitter.

In this case the meter measures the SWR of the entire system, i.e. the
line going from the meter to the antenna and the antenna.

The net practical effect of the line loss for real line and real antennas
is that the observed SWR will indicate a lower value than if the meter
were directly connected to the antenna.

The inference of what is happening on adjacent line is ours, not the
instruments, as demonstrated by your example above.

Owen
--

--
Jim Pennino

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