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