Roy,

Plot this on a smith chart program. You are correct, your meter reads close to
2:1, however you know nothing about the phase or resonance of the antenna. It
does not tell you if you have a tuned antenna and a poor R match or if your
antenna is way out of tune. (Of course neither does the telescope)

Dan

Roy Lewallen wrote:
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.
==========================================

In this day and age, there is only one meter on or associated with a
transmitter.

It is the misnamed SWR meter.

Consequently and unavoidably, with nothing else left to talk about,
the importance attached to SWR becomes exaggerated.

It is perfectly natural, for CB-ers and professional engineers alike,
to imagine the indicated SWR applies to the one and only transmission
line in the system. That is along the line from the transmitter/tuner
to the antenna.

But the meter does not indicate SWR on any line. It merely indicates
whether or not the load on the transmitter is 50 ohms. Which is nice
to know. But, nevertheless, you have been fooled!

After half a century of being unwittingly misled, it is admittedly
difficult to have to suddenly switch one's ideas about what is thought
to be an important subject.

Carry on arguing!
----
Reg.

dansawyeror wrote:
Roy,

Plot this on a smith chart program. You are correct, your meter reads
close to 2:1, however you know nothing about the phase or resonance of
the antenna. It does not tell you if you have a tuned antenna and a poor
R match or if your antenna is way out of tune. (Of course neither does
the telescope)

What you say is true, but I don't understand what it has to do with the
discussion at hand. No one has mentioned phase, resonance, tuning, or R
match. An SWR meter isn't a suitable tool for measuring any of these,
except that it'll usually indicate the resonant frequency fairly closely
for most typical antennas.

Roy Lewallen, W7EL

Reg Edwards wrote:
. . .
But the meter does not indicate SWR on any line. It merely indicates
whether or not the load on the transmitter is 50 ohms. Which is nice
to know. But, nevertheless, you have been fooled!
. . .
Let's not be fooled by these contrived misstatements.

An SWR meter tells us the SWR on a transmission line to which it's
connected, providing that the line and meter impedances are the same.
This can easily be verified with a couple of simple experiments. So it
does indeed indicate the SWR on a line. It will, of course, still give a
reading under other conditions, such as when the line and meter Z0 are
different or when there's no line at all, in which cases it means only
what Reg says(*). But I'm afraid that the effort to leave a legacy of a
new TLA (three letter acronym) for SWR meters is causing Reg to adopt an
increasingly distorted view of what SWR meters can and can't indicate.

(*) Any kind of test equipment can be misused or the results
misinterpreted. For example, anyone using a 1000 ohm/volt voltmeter to
read voltage in a high-impedance circuit will not see the voltage which
is there when the meter is disconnected. Likewise, measuring high
frequency waveforms with a 10 pF scope probe, even at moderate
impedances. The list is endless. But this doesn't justify renaming each
of those pieces of test equipment to accommodate the most naive user.

Roy Lewallen, W7EL

The so-called SWR meter is just a resistance (not impedance) bridge.

The bridge is at balance and indicates SWR = 1:1 when a resistance of
precisely 50 ohms is connected to its output terminals. It is
arranged within the meter that this 50-ohm resistance, or whatever is
connected to the output terminals, is the transmitter load.

With the meter in its normal location, the load is the input impedance
of the transmission line to the antenna. So when the input impedance
of the line, as determined by Zo of the line and the antenna input
impedance, is 50 ohms then the meter indicates SWR = 1:1 regardless of
Zo, line length and antenna impedance.

As Roy says, in the special case of line Zo being precisely 50 ohms it
so happens that the meter will correctly indicate SWR along the line.
For any other value of line Zo the meter will indicate varying degrees
of nonsense.

At HF, line Zo is frequently anywhere between 50 and 600 ohms and a
tuner is used to transform line input impedance, either up or down, to
the 50 ohms required by the transmitter. But Zo is not affected and
the SWR meter indications remain in error.

Whatever Zo and antenna impedance may be, the meter always indicates
whether or not the transmitter is correctly loaded with a resistive 50
ohms.

Note that the circuit operates independently of transmitter internal
impedance whatever that may be.
----
Reg.

Owen Duffy wrote:
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).

Please reference "Baluns: What They Do and How They Do It" by W7EL.
VSWR causes impedance transformation. Impedance transformation
varies the impedance. Baluns work better with some impedances than
they do with others. Therefore, VSWR can cause balun malfunction
accompanied by feedline radiation. All it takes is one time. And a
possible cause is not necessarily a probable cause.

There is nothing to suggest that you will be injured every time you
ride your motorcycle at 120 mph. All it takes is one time.
--
73, Cecil http://www.qsl.net/w5dxp

Crazy George wrote:
Those antennas aren't flat, and
there are 2 transmitters, visual and aural.

The audio is not mixed with the main carrier?
--
73, Cecil http://www.qsl.net/w5dxp

On Sun, 25 Dec 2005 22:24:20 GMT, Cecil Moore wrote:

Owen Duffy wrote:
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).

Please reference "Baluns: What They Do and How They Do It" by W7EL.
VSWR causes impedance transformation. Impedance transformation
varies the impedance. Baluns work better with some impedances than
they do with others. Therefore, VSWR can cause balun malfunction
accompanied by feedline radiation.

Your example depends on the (mis)behaviour of a component (the balun)
external to the feedline as a vital link in the asserted relationship
between high VSWR and feedline radiation (both properties of the
feedline itself).

As you describe it the balun was not suited to the application, and it
is the interaction of the unsuited balun in the whole topology that
gives rise to feedline radiation.

In addressing the suitability issue, you could:
- change the environment external to the balun until the balun was
suitable; or
- replace the balun with one that suits the external environment.

If the balun were replaced with a balun that was effective, then
feedline radiation would be reduced sufficiently, without needing to
reduce the high VSWR on the feedline.

Excessive feedline radiation is not a necessary outcome of high VSWR,
high VSWR does not, of itself, cause feedline radiation.

If high VSWR does not, of itself, cause excessive feedline radiation,
then finding the root cause of feedline radiation means looking beyond
the myth that high VSWR feedlines radiate.

Owen
--

Owen Duffy wrote:
In addressing the suitability issue, you could:
- change the environment external to the balun until the balun was
suitable; or
- replace the balun with one that suits the external environment.

So if TVI can be fixed, it never existed in the first place?
--
73, Cecil http://www.qsl.net/w5dxp

On Sun, 25 Dec 2005 22:31:09 GMT, Cecil Moore wrote:

Crazy George wrote:
Those antennas aren't flat, and
there are 2 transmitters, visual and aural.

The audio is not mixed with the main carrier?
It can be either way.

If you choose to use separate transmitters the demands on antenna
bandwidth are greatly reduced.
John Ferrell W8CCW