Reg Edwards wrote:
Roy, to cut things short, why don't you just say SWR meters don't
measure SWR on anything. All they do is indicate whether or not the
transmitter is terminated with its correct load resistance. So they
are quite useful.

They won't even tell you what the load resistance actually is unless
the load is exactly correct.

Stop fooling and confusing yourselves. The solution to everybody's
problems is simple - just change the name of the thing to TLI.
(Transmitter Loading Indicator).

Or - recalling that what the meter actually measures is the reflection
coefficient - why not go back to the old name of "Reflectometer"?


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Thanks very much to Owen for pointing out the following errors in my
recent posting:

Roy Lewallen wrote:
. . .
Next experiment: Connect the SWR meter through a *quarter* wavelength of
50 ohm line to a 100 ohm load. Now the impedance looking into the line
is 25 ohms instead of 100. But the SWR meter reads 2:1 when it sees 25
ohms as well as 100, so it still reads 2:1, which is also still the SWR
on the 50 ohm line. You can change the length of the 50 ohm line all you
want and, if it's lossless, the line's actual SWR stays the same -- but
the impedance at the input end of the line changes. For a 100 ohm load,
when the line is any even number of half wavelengths long, the input Z
is 100 ohms. . .

That last sentence should be "For a 100 ohm load, when the line is *any
whole number* of half wavelengths long, the input Z is 100 ohms."

Likewise,

Now instead of a 50 ohm line, let's connect a half wavelength 100 ohm
line to the output of the same 50 ohm SWR meter and hook that to a 50
ohm resistive load. The line's actual SWR is 2:1 and, just like any
lossless line, the SWR stays the same regardless of its length. If the
transmission line is an even number of half wavelengths long we'll have
50 ohms at the input and the SWR meter will read 1:1, since it's a 50
ohm meter and interprets 50 ohms as 1:1.

"an even number of half wavelengths" should be "any whole number of half
wavelengths".

I appreciate the corrections, and encourage anyone who spots errors to
bring them to my attention, or the newsgroup's.

Roy Lewallen, W7EL

Steveo wrote:
if you have over a 2:1 standing wave you can do damage to your finals
or linear

Depends on what one is running. My IC-706 folds back
automatically and protects itself. My SGC-500 linear
is advertised to tolerate an SWR of up to 6:1.
--
73, Cecil http://www.qsl.net/w5dxp


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Roy Lewallen wrote:
Some people can put up a huge
smokescreen and waving of hands about reflected waves of one kind or
another, but at the end of the day the SWR meter can't tell the
difference between a resistor and a transmission line terminated with a
load, if the impedances the meter sees are the same. It's sensitive only
to impedance;

A 20K ohms/volt Simpson may yield an irrelevant screen voltage
reading for a pentode because it loads the circuit down. Hand
waving aside, any instrument can be misused.

An SWR meter designed and calibrated for a Z0=50 standing-wave
environment may yield an irrelevant reading when operated outside
of a Z0=50 ohm standing-wave environment.
--
73, Cecil http://www.qsl.net/w5dxp


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Reg Edwards wrote:
Roy, to cut things short, why don't you just say SWR meters don't
measure SWR on anything. All they do is indicate whether or not the
transmitter is terminated with its correct load resistance. So they
are quite useful.

Reg, how about my 450 ohm SWR meter? It just sits there
reading somewhere between 6:1 and 12:1.
--
73, Cecil http://www.qsl.net/w5dxp


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Ian White GM3SEK wrote:

Reg Edwards wrote:
Stop fooling and confusing yourselves. The solution to everybody's
problems is simple - just change the name of the thing to TLI.
(Transmitter Loading Indicator).

Or - recalling that what the meter actually measures is the reflection
coefficient - why not go back to the old name of "Reflectometer"?

Trouble is, during steady-state, they only measure the virtual
reflection coefficient which is itself confusing since it is not
the same as the physical reflection coefficient measured by a TDR.
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" wrote:
An SWR meter designed and calibrated for a Z0=50 standing-wave
environment may yield an irrelevant reading when operated outside
of a Z0=50 ohm standing-wave environment.
___________________

Elaborating, an SWR meter will produce ~ accurate readings for an unknown
termination connected to it via a lossless transmission line of any length,
as long as that line has the same Zo as the sample section in the SWR meter.

It is only when the transmission line Zo varies from the Zo of the SWR meter
line section that accurate measurement of load SWR is problematic.

Selecting line lengths and line impedances to make an SWR meter and/or tx
"happy" when connected to an antenna doesn't necessarily mean that the all
components in the r-f output system have low SWR. The tx may be able to
deliver more power to the net load under those conditions, but SWR may still
exist on the transmission line capable of causing its failure.

RF

"Ian White wrote -
Stop fooling and confusing yourselves. The solution to everybody's
problems is simple - just change the name of the thing to TLI.
(Transmitter Loading Indicator).

Or - recalling that what the meter actually measures is the
reflection
coefficient - why not go back to the old name of "Reflectometer"?

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

It does NOT read the reflection coefficient. It reads only half of
it. At least half of the information lies in the angle of the RC -
which is disregarded, ignored, by the so-called meter.

The magnitude of the RC without its angle is just another worthless
number. It can't be used for anything except to calculate a fictional
SWR.
----
Reg, G4FGQ

Reg Edwards wrote:
The magnitude of the RC without its angle is just another worthless
number. It can't be used for anything except to calculate a fictional
SWR.

Actually, it is pretty useful for a Z0-matched system
since there is one and only one unique solution at the
Z0-match point.

In a Z0-matched system, all forward and reflected voltages
and currents are at the reference zero degrees or at 180
degrees so all the phase angles are known without measuring
them. The physical reflection coefficient is a function of
Z01 and Z02 at a Z0-match point. The sign of the reflection
coefficient corresponds to either zero degrees or 180 degrees
and depends on whether (Z01 Z02) or (Z01 Z02).

Since the great majority of amateur radio systems are close
to a Z0-match, this becomes a useful analysis tool for the
most common cases.
--
73, Cecil http://www.qsl.net/w5dxp


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What is the reason a 2:1 SWR can cause such havoc?

How can I avoid this catastrophic condition?

I feed my dipoles with 450 Ohm ladder line, but the last 20 feet or so is 50
Ohm coax, I guess that makes it work ok. I haven't blown up my finals yet.

Lions and tigers and bears Oh my...

"Steveo" wrote in message
news:nceoaqqpc0a3yzz.280620052102@kirk...
if you have over a 2:1 standing wave you can do damage to your finals
or linear