Well, I think this thread went a bit beyond where I had intended
(expected?), but it looks like it's provided some useful discussions, as it
did me. Here's kind of a twist on all this.

I went back to my very old physics book to see what they had to say about
standing waves. Actually, I had more fun reading about waves created by a
string attached to various objects at the remote end. The authors considered
a case with an infinite mass at the end, and another with the end of the
string fastened to a ring, which was place on a pole. Finally, they
discussed what would happen if another string followed the first and the
second the string was denser or less dense than the string on which the wave
begin. Another time. :-)


Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

"He who laughs, lasts." -- Mary Pettibone Poole
--

Web Page: home.earthlink.net/~mtnviews

I forgot to mention the physics book even got into something called virtual
waves, which I believe are the mirror image of an incident wave. The mirror
is perpendicular to the travel, and one can look down it from the side.

Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

"He who laughs, lasts." -- Mary Pettibone Poole
--

Web Page: home.earthlink.net/~mtnviews

..
"Cecil Moore" wrote in message
om...
W. Watson wrote:
A standing wave is the sum of an incident added to the reflective wave.
Isn't it possible to send two incident waves down an xline with different
frequences, and produce two different standing waves by having some
multiplicative relationship between the two incident waves and the xline
length?

Sure, it's possible but one wonders about the application.

Cecil:

Think about a 6 MHz wide analog TV channel. Those antennas aren't flat, and
there are 2 transmitters, visual and aural. Putting an analog TV station on
the air the first time, particularly low VHF, is a real interesting
exercise. Or at least it was back in the stone (vacuum tube) age, the last
time I did one.

73,
George
W5VPQ
My real address is my ham call atARRL.NET The ATTGlobal is a SPAM trap


snip

"Cecil Moore" wrote in message
om...
W. Watson wrote:
A standing wave is the sum of an incident added to the reflective wave.
Isn't it possible to send two incident waves down an xline with
different frequences, and produce two different standing waves by having
some multiplicative relationship between the two incident waves and the
xline length?

Sure, it's possible but one wonders about the application.


Some repeaters use one antenna for two repeaters on 144 and 440 mhz.
Comercial transmitters do this all the time. Usually the transmitters are
in the same band.

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.
==========================================

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

To all and sundry,

The length and directions and off-shoots of this and other threads,
the resulting arguments, confusion and misunderstandings prove my
basic point -

The SWR meter is grossely mis-named.

It leads old-timers, novices, CB-ers and professional engineers
severely astray. It overstretches imaginations.

Dis-educational!
----
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.

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

This is pretty strange.

Suppose Reg has a 50 ohm line of some length connected to an antenna
whose impedance is 100 + j0 ohms. After putting away his evening's
bottle of wine, he climbs the tower and inserts a 50 ohm SWR meter at
the antenna. He climbs back down, gets out his vintage brass telescope
and keys the transmitter. Then, steadying himself, he peers through the
telescope and sees that the SWR meter reads 2:1. (Being a clever person,
he mounted the meter upside down so it would be right side up in the
telescope, obviating the need for the added challenge of mental inversion.)

I have an identical antenna, feedline, and SWR meter. I sit in my warm
shack sipping my moonshine, connect the SWR meter to the input end of
the line, hit the key, and note that the meter reads 2:1. Or perhaps
slightly less if the line is noticeably lossy.

Reg says:

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.

I guess the 2:1 reading from the meter at the input end of the line is
telling Reg nothing, while the 2:1 reading at the antenna is. Strange.
The fact is that it's the SWR on the line, and it can be measured at any
point along the line. I like my method better, but each to his own.

Roy Lewallen, W7EL

Reg Edwards wrote:
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.
==========================================