Standing Waves (and Impedance)
 

John Ferrell wrote:
I really don't know why there is more TVI with a high swr. But my
experience has been that there is, especially on 6 meters.

SWR can have an indirect effect on common-mode currents by causing
a malfunction of the balun. For an SWR of 1:1, the balun is probably
functioning in the impedance environment for which it was designed.
That balun may cease to choke properly when exposed to the impedances
present in an SWR 1:1 environment and allow more common-mode signals
to develop and radiate.
--
73, Cecil http://www.qsl.net/w5dxp

Standing Waves (and Impedance)
 

Roy Lewallen wrote:
But the antenna doesn't have an SWR, ...

Actually it does, if it is a standing-wave antenna like
a 1/2WL resonant dipole. The reflections from the ends
of the dipole are what lowers the virtual feedpoint
impedance so that coax is a good match.
--
73, Cecil http://www.qsl.net/w5dxp

Standing Waves (and Impedance)
 

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

Standing Waves (and Impedance)
 

On Thu, 22 Dec 2005 15:07:45 GMT, Cecil Moore wrote:

Owen Duffy wrote:
Lots of people operate feedlines at high VSWR by design, and they do
not necessarily cause TVI.

Here's one example where a high VSWR might cause more TVI.


Your example suggests a simplified method of analysing the
effectiveness of a balun in reducing common mode current under two
different load scenarios (that happen to have a transmission line
operating at different VSWR).

If the line length of your example was an even number of quarter
waves, then by your own analysis method, in the second case, the balun
with 50 ohm load and "3000 ohm choking impedance" would be more
effective at 12:1 VSWR than the flat line?

So that goes to the meaning of "high VSWR *might* cause more TVI".
Perhaps there is some other factor, and perhaps VSWR is not a root
cause at all.

There is nothing in what you have said that suggests to me that VSWR
is the cause of TVI (or feedline radiation in the more general case).

Owen
--

Standing Waves (and Impedance)
 

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.

Standing Waves (and Impedance)
 

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

Standing Waves (and Impedance)
 

"Reg Edwards" wrote
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).
==========================================

This will have the effect of reducing SWR on the line to a more
appropriate and realistic level of importance.

Who cares what is the SWR on the transmission line provided the
transmitter is loaded with its correct load resistance?
----
Reg, G4FGQ.

Standing Waves (and Impedance)
 

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.

Standing Waves (and Impedance)
 

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

Standing Waves (and Impedance)
 

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
--