John Ferrell wrote:
. . .
The transmitter final was a military surplus cavity with a 4CX250 that
had been "stuffed" to get it up to 440. As a side effect the output Z
was determined to be pretty low by trying several home brew quarter
wave coaxial matching sections. When a good match was made, a lot of
problems went away. Not only was I managing a better signal, but the
polyethylene cooling ducting was taking a longer time to melt down.

Hence, my position that SWR IS important.
. . .

All this demonstrates is that impedance match is important to the
transmitter final. The quality of impedance match is often indicated as
SWR on an SWR meter when in fact the meter reading often has little or
nothing to do with the SWR on any transmission line. Even when it does,
the problems with the transmitter are due solely to the poor impedance
match and not at all due to the SWR on connected transmission lines.

Let me give an example. Connect your transmitter through a half
wavelength of 300 ohm transmission line to a 50 ohm (resistive) load.
The transmitter sees 50 ohms, so an SWR meter at the transmitter will
read 1:1, even though the SWR on the line is in fact 6:1. The
transmitter can't tell the difference between this setup, a direct
connection to the 50 ohm load, or connection to it through a half
wavelength of cable with any impedance and therefore having any SWR. In
all cases, the transmitter sees 50 ohms, which is all that matters. The
line's SWR makes no difference at all.

If for some reason you were really interested in finding the SWR on the
300 ohm line, you'd have to insert a 300 ohm SWR meter at the
transmitter-line junction. It would correctly read 6:1.

Roy Lewallen, W7EL

Roy, you are, at least, on the right track.

To measure SWR on the feedline, it is necessary to climb up the mast
or a ladder and insert an SWR meter, of the correcct impedance,
between the antenna and the feedline?

Then you have to come down safely to ground level, switch on the
transmitter, and view the meter reading through an astronomical
telescope, bearing in mind that the field of view with an astronomical
telescope is inverted with respect to normal.

In its usual position the SWR meter does not measure SWR on any line.
It merely indicates whether or not the transmitter is correctly loaded
with a resistive 50 ohms. Which is all anyone may wish to know.

After 50 years or more of ignorance, it is about time this hoax was
exposed to the world.

Then, all that is necessary to prevent the instrument from telling
lies, is to leave it where it is and change its name to TLI
(Transmitter Loading Indicator).
----
Reg, G4FGQ.

Reg Edwards wrote:
Roy, you are, at least, on the right track.

To measure SWR on the feedline, it is necessary to climb up the mast
or a ladder and insert an SWR meter, of the correcct impedance,
between the antenna and the feedline?

No. You can insert the SWR meter of the correct impedance at the input
end of the feedline. Stay inside, nice and warm. Of course, if your line
has a significant amount of loss, the SWR will vary along the line, so
you'll have to put the meter at the point where you want to know the SWR.

Then you have to come down safely to ground level, switch on the
transmitter, and view the meter reading through an astronomical
telescope, bearing in mind that the field of view with an astronomical
telescope is inverted with respect to normal.

That's surely a novel way of doing it, although unnecessary. On the one
hand, that method might seem more plausible after finishing off a bottle
of wine. On the other, that would be a bad time to be climbing the mast.

In its usual position the SWR meter does not measure SWR on any line.
It merely indicates whether or not the transmitter is correctly loaded
with a resistive 50 ohms. Which is all anyone may wish to know.

After 50 years or more of ignorance, it is about time this hoax was
exposed to the world.

Then, all that is necessary to prevent the instrument from telling
lies, is to leave it where it is and change its name to TLI
(Transmitter Loading Indicator).

Have you had any luck in selling Agilent (HP), Narda, Anritsu, and those
other ignorant companies into not specifying the input impedances of
their precision RF measurement equipment, terminations, and other
components in terms of SWR? Once you get them to see the light, hams
will surely enlist in your jihad. Otherwise, we'll have postings from
hams that go something like this:

"My TLI says my precision termination resistor has an impedance of 1.02
Reggies. But the manufacturer specifies a maximum SWR of 1.05:1. Is it
ok? Reg says there are 6 dB in an S-Unit, so are there 6 SWRs to a Reggie?"

Roy Lewallen, W7EL

Your reply was very fast. You didn't have time to think about it.

The only way to "measure" SWR is to place the meter at the antenna end
of the line. You know that as well as I do.

The SWR does not apply to any particular point on the line. It applies
to the WHOLE line.
----
Reg, G4FGQ.

Reg Edwards wrote:

. . .

The only way to "measure" SWR is to place the meter at the antenna end
of the line. You know that as well as I do.

The SWR does not apply to any particular point on the line. It applies
to the WHOLE line.

I disagree with both of those statements, and both can be shown to be
incorrect.

If a line is lossless, the SWR is the same all along the line. An SWR
meter of the line's impedance will measure the SWR correctly when placed
anywhere along the line, including at either end.

If a line has loss, the SWR varies along the line, being the greatest at
the load and decreasing toward the source. (The concept of SWR at a
single point is well understood and widely used and accepted, even
though it deviates from the original literal definition.) In that case,
the meter will correctly read the SWR at the position where it's placed.
That position can be anywhere along the line including either end.

Roy Lewallen, W7EL

On Thu, 22 Dec 2005 16:07:56 -0800, Roy Lewallen
wrote:


If a line has loss, the SWR varies along the line, being the greatest at
the load and decreasing toward the source. (The concept of SWR at a
single point is well understood and widely used and accepted, even
though it deviates from the original literal definition.) In that case,
the meter will correctly read the SWR at the position where it's placed.
That position can be anywhere along the line including either end.

.... and for most practical purposes, with knowledge of the matched
line loss, the VSWR at any other point on that line can be estimated
with reasonable accuracy from the measurement at a point on the line.

Owen
--

On Thu, 22 Dec 2005 16:07:56 -0800, Roy Lewallen
wrote:

Reg Edwards wrote:

. . .

The only way to "measure" SWR is to place the meter at the antenna end
of the line. You know that as well as I do.

The SWR does not apply to any particular point on the line. It applies
to the WHOLE line.

I disagree with both of those statements, and both can be shown to be
incorrect.

If a line is lossless, the SWR is the same all along the line. An SWR
meter of the line's impedance will measure the SWR correctly when placed
anywhere along the line, including at either end.

If a line has loss, the SWR varies along the line, being the greatest at
the load and decreasing toward the source. (The concept of SWR at a
single point is well understood and widely used and accepted, even
though it deviates from the original literal definition.) In that case,
the meter will correctly read the SWR at the position where it's placed.
That position can be anywhere along the line including either end.

Roy Lewallen, W7EL

I am absorbing this, but slowly.

I have understood that a "matched line" would indicate the same SWR at
every point you might measure it with a directional coupler. The Swr
we are discussing is that which we can measure with a directional
coupler, is it not?

The SWR on a mis-matched line will vary with the position you choose
to measure it. This can be indicated by varying the transmission line
length to get an acceptable match for the system. This will satisfy
the need to match a transmitter for a given frequency. A directional
coupler placed at different places on the line will still indicate a
non uniform SWR. Any feed line losses due to insulation or radiation
are effectively hidden from the transmitter end.

This is why I have gone to the antenna/feed line to measure the power
level and the SWR. If your feed line has become an effective dummy
load or a better radiator than your antenna it would nice to know.

I don't have an answer as to how to measure the instruments insertion
effects.

Please tell me where I am in error!
John Ferrell W8CCW

John Ferrell wrote:

I am absorbing this, but slowly.

I have understood that a "matched line" would indicate the same SWR at
every point you might measure it with a directional coupler.

A matched line is one which is terminated with its characteristic
impedance. The SWR on a matched line is 1:1 at all points along the line.

The Swr
we are discussing is that which we can measure with a directional
coupler, is it not?

Yes and no. To measure the SWR requires an SWR meter or directional
coupler which is designed for the particular characteristic impedance of
the line. If a directional coupler is the proper impedance, it can be
used to calculate the SWR from the forward and reverse powers. If it
isn't, it can't.

The SWR on a mis-matched line will vary with the position you choose
to measure it.

No, it won't, unless it has loss. If it has loss, the SWR will be
greatest at the load and will monotonically decrease toward the source.

This can be indicated by varying the transmission line
length to get an acceptable match for the system. This will satisfy
the need to match a transmitter for a given frequency. A directional
coupler placed at different places on the line will still indicate a
non uniform SWR. Any feed line losses due to insulation or radiation
are effectively hidden from the transmitter end.

It appears that you're assuming that what you measure with an SWR meter
or calculate from directional coupler readings is the SWR. Unless the
coupler or meter is designed for the Z0 of the line, it isn't. If the
coupler or meter isn't of the proper impedance for the line, you'll get
different readings as you move along the line. Those readings aren't,
however, the line's SWR.

. . .

Roy Lewallen, W7EL

Roy, you surprise me. Try a jug of Moonshine.

Placing the SWR meter at the start of the feed-line terminated by the
antenna, will tell you NOTHING about the SWR on that line.

It is the antenna input impedance which determines the SWR on the
line, and the meter doesn't have the foggiest idea what THAT is.

The unknown antenna impedance is at the other end of a line of unknown
length, unknown impedance and unknown loss. Unknown, that is, to the
meter.

YOU might have that knowledge. But then you can CALCULATE what the SWR
is on the line. Meter readings having been discarded as useless.

I repeat - the meter tells you only whether or not the transmitter is
loaded with a resistive 50 ohms. No more and no less. If it is not
50 ohms the ambiguous meter will not even tell you the actual value of
Z.

Intoxicated or not, if you insist on a meter reading, there is no
alternative to climbing the antenna mast.
----
Reg, G4FGQ.

PS. The use of SWR by American plug and socket manufacturers to
describe unrelated characteristics of their products is a small
indication of the abysmal depths to which engineering has descended.
Technical specifications are reduced to Camm's Comics. But they look
good to the uninitiated.
----
Reg.
==========================================

"Reg Edwards" wrote
Technical specifications are reduced to Camm's Comics. But they look
good to the uninitiated.
==========================================

Insert between "they look good" and "to the uninitiated" -

"and sell".

----
Reg.