Reg Edwards wrote:

Reg, what about Rho = SQRT(P-/P+) and SWR=(1+Rho)/(1-Rho)?

It applies only to a long line lossless line which does not exist but
Zo must be 50 ohms.

Let's say we have the following system configuration with
a 1:1 choke at '+':

100W XMTR---50 ohm coax---+---300 ohm twinlead---...
Pfor1=100w-- Pfor2--
--Pref1=0w --Pref2

What's the forward power on the 300 ohm twinlead?
What's the reflected power on the 300 ohm twinlead?
What's the SWR on the 300 ohm twinlead?
--
73, Cecil http://www.qsl.net/w5dxp

"Reg Edwards" wrote in message
...

"Jerry Martes" wrote
I'm writing this to represent the "other side" of an arguement
that states
that VSWR *cant* be measured. I claim that VSWR *can* be measured
and that
VSWR can be used to identify the impedance terminating the
transmission
line.
========================================

The only way to measure SWR on a transmission line is to run a
voltmeter along it. At least TWO measurements are needed. Not ONE.
And line length is involved.

The voltmeter readings will indeed tell you what the SWR is. But
nothing else. It will be possible to calculate from the readings and
the distance between max and min what the velocity of propagation is.

But it is essential to add extra critical information before anything
else can be deduced. Without this EXTRA information knowledge of the
SWR (if it can be obtained) is useless. The so-called SWR meter does
not and cannot provide this information.

To calculate the terminating impedances from the SWR it is neccsary
also to know the line impedance, its velocity and the exact locations
of the max-volts and min-volts relative to the ends. The meter will
not tell you.

And the foregoing is on a line which exists only in one's imagination.

I am sorry to repeat, the indications of the SWR meter apply only to
the input impedance of the line from the transmitter to the antenna.
The meter, in itself, tells you nothing about what is happening to
conditions along the line. It certainly tells you nothing about the
antenna's input impedance which is of primary interest.

IF, BY SOME MEANS, YOU CAN MEASURE SWR, then there is much more
information needed before the performance of the system can be
predicted.

The funny thing is - the performance of the system can be deduced
from the extra information without reference to the SWR. The whole
business is laughable.

Just change the name of the meter and all will become clear.
----
Reg, G4FGQ.

Hi Reg

I have no understanding of why you find it important to state things that
are not true about VSWR. VSWR *can* be measured. It is clear to me that
you know that the Complex Impedance terminating a transmission line can
determined by measuring the VSWR. Sure, it requires the position of the
voltage mins (or maxs) be identified, with the load and then with a short
ckt., and a Smith chart to be used for quick/easy identification of the load
impedance. But, is that so much calculation that you find it necessary to
state " IF, BY SOME MEANS, YOU CAN MEASURE SWR, then there is much more
information needed before the performance of the system can be
predicted".

The load impedance isnt "predicted". It is actually *determined* with an
accuracy associated with the precision of the test equipment.

I consider measuring VSWR on a transmission line to be an excellent method
of determining load impedance.

Jerry

Jerry

On Mon, 28 Nov 2005 23:52:40 GMT, Cecil Moore wrote:

Owen Duffy wrote:

that resistance. That measurement does not tell you the "impedance
terminating the transmission line" unless the line is of zero length.

A transmission line transforms the impedance in a predictable
manner given the transmission line specifications. One can
backtrack the SWR spiral on a Smith Chart to get a reasonable
estimate for the antenna impedance. The impedances for my dipole

To do that, you need to determine the position of the standing wave
pattern with respect to the load, and a typical reflectometer style
SWR meter does not do that. You could put a ruler to the line, but you
are using another instrument to make a another measurement that the
reflectometer could not make.

It is misleading to suggest that a reflectometer style SWR meter alone
is useful for determining the impede dance of a load connected to the
meter by a length of transmission line, save possibly the case when
VSWR=1 and the line is low loss and Zo is the same as the calibration
Z of the SWR meter.

Owen
--

Owen Duffy wrote:
To do that, you need to determine the position of the standing wave
pattern with respect to the load, and a typical reflectometer style
SWR meter does not do that.

Yes it does, if one has the ability to vary the length of the feedline
until a current maximum point (minimum SWR) is known to be located at
the balun/choke. I do it everytime I get on the air. That's how I tune
my antenna system and I don't use any conventional tuner at all.

It is misleading to suggest that a reflectometer style SWR meter alone
is useful for determining the impedance of a load connected to the
meter by a length of transmission line, save possibly the case when
VSWR=1 and the line is low loss and Zo is the same as the calibration
Z of the SWR meter.

Not misleading at all. I do it all the time. I know the exact length,
velocity factor, and Z0 of my feedline. I know an SWR current maximum
point is located at my choke. I know if it is greater than, less than,
or equal to 50 ohms. It is a rather simple-minded process to accurately
estimate the antenna feedpoint impedance given everything I know. You
should try it sometime. Even if I didn't know if the current maximum
impedance was lower than or higher than 50 ohms, there would only be
two possible antenna impedances. EZNEC has a perfect track record in
predicting which of those two antenna impedances actually exists.
--
73, Cecil http://www.qsl.net/w5dxp

Have it your way Cecil...

Owen
--

Cecil, there's no need to rack your brains for hours trying to deduce
the antenna input impedance from the input impedance of the
transmission line. There's a computer program which will tell you the
exact answer in milliseconds. Download program ZL_Zin from website
below.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Cec,

The meter indicates SWR on the 50-ohm coax between meter and
transmitter. It HAS to be 50-ohm coax. Any other impedance and you
get the wrong answer. Without measurement of Zo = 50 ohms it can only
be assumed.

You then include in the calculation the measurement or assumption of
the Zo of the 50-ohm coax, and the measurement or assumption of Zo of
the twin-line, and the forward and reverse powers, and the SWR on the
twin line can be deduced or assumed.

But if you think you are measuring SWR on anything you are cheating
and fooling yourself.

In your particular case an assumptiom of Zo = 450 ohms for the
twinline would be very much in error because both you and I know you
have measured Zo to be 380 ohms. I can tell you what the SWR is on
YOUR feedline without getting out of this armchair. I don't need to
know your meter readings. ;o)
----
Reg.

"Jerry Martes" wrote
I have no understanding of why you find it important to state things
that
are not true about VSWR.

===================================
Jerry,

It is important because the SWR meter is EDUCATIONAL. It is more than
a pair of red and green LED's on our automatic tuners.

All along I have stated that the name of the so-called SWR meter
should be changed. Other more technical statements have been made to
convince they whose state of mind prevents agreement.

Remarkably few people disagree with my technical statements but offer
no reasons for disagreement or prove me to be incorrect.

SWR meters are by far the most prevelent topic on amateur radio
newsgroups. It appears time and time again in contexts which
demonstrate it to be a source of misunderstandings, arguments and
general confusion.

I maintain that the instrument's name is the root cause of the
problems. It does not do what its name says it does. This inevitably
leads people, not just novices and CB-ers, into incorrect channels of
thought which become deeply ingrained. It unnecessarily introduces
SWR into discussions which actually have nothing to do with SWR. And
worst of all, when operating equipment, it causes people to have
problems which either don't exist or are different to what people
imagine they are. Mis-education is the keyword.

Re-naming should begin in amateur radio handbooks and similar
publications. Editors should be the first to be educated.

SWR meters are seldom mentioned as such in professional text books.
They are given other more correct names. Terman manages very well
wthout them. But there's nothing wrong with his bibles. (Yes, I know
they probably hadn't been invented in his day.)

Perhaps when our Chinese friends enter the amateur radio market,
manufacturers' wisdom will allow the light of reason to shine through.
But they will have to get a move on. I can foresee the time when
automatic tuners are universal and the only meter on black boxes will
be the S-meter.

I don't doubt that you thoroughly understand how the so-called SWR
meter works. But even the present discussion is enough to demonstrate
that a simple change is needed. In the end it all reduces to
economics and survival of the fittest argument.
----
Reg, G4FGQ

Gerry,

The load is the antenna - about which the SWR meter knows absolutely
nothing. All the the meter has to work with is the input impedance of
the tuner or the transmission line.

Line input Z = R+jX and to aggravate matters the meter discards all
information about X.
----
Reg.

Cecil, I note you have changed the name from "SWR Meter" to "Forward &
Reverse Power Meter", a procedure I have been recommending for years.
Congratulations!

Although I am not altogether happy with your choice of new name.
----
Reg.