Non-linear, or time variant. I'd expect some variation going between
low and high power with typical dummy loads, because the resistance
will change with temperature. In fact, the loss in the transmission
line will change with temperature, too, and it's significant enough to
be able to measure with ham-type equipment if you are careful. But
I'd FIRST suspect other things going on: different calibrations
between different instruments, and the fact that SWR meters that use
uncompensated diode detectors will not read the same SWR at low power
as at high: they will fail to read high enough at low powers.

The resistance of the copper in the transmission line changes with
temperature. If ambient is 20C and you put in enough power to heat up
the line (center conductor) to 70C, that's a 50C change, and will
result in about an 18% increase in resistance. So if you had a line
which had 3dB loss at 20C, it would increase to about 3.5dB at 70C.
If the load end has a 2:1 SWR, then the sending end will have about
1.40:1 SWR at 20C and about 1.35:1 at 70C. It's not a _big_ change,
but it should be observable on an SWR meter that is accurate over a
wide range of powers.

I want to make it clear that this is in support of what Roy and Ian
are saying, as an added detail, and not contrary to the notion that
SWR on a line in a linear, time-invariant system with steady-state
excitation does not depend on the source impedance.

Cheers,
Tom


Roy Lewallen wrote in message ...
Dr. Slick wrote:

As you might know, the input S11 or SWR will change when you go
from an antenna analyzer or network analyzer to measuring with the
actual full power PA and meter hooked up. This may be partly due to
the fact that the meter is usually not a perfect 50 ohm thru, and
partly due to the fact that the analyzers outputs are closer to 50
ohms than the PA.

If S11 or the SWR actually does change, you've either got a nonlinear
transmission line or a nonlinear load. That is, the impedance changes as
the signal level changes. If you *measure* a different SWR or S11, it
means that either the SWR or S11 is actually changing for the reasons I
just stated, or the meter is nonlinear in the sense that its reading
changes with power level (possibly due to RF ingress, but it could be a
host of other things), or you're measuring with two different meters
that don't agree.

It's not because of the different source impedances.

Sure, you can normalize a Smith chart to anything you'd like. That
doesn't make the SWR change with source impedance.

. . .

Roy Lewallen, W7EL

(Dr. Slick) wrote in message . com...

On second thought, i believe we are all wrong to equate S11 with
SWR!

(Of course they are not equal, just related by a formula [which is
usually stated slightly incorrectly])

If your reference impedance for measuring S11 is not the line
characteristic impedance, it's true that SWR = (1+|S11|)/(1-|S11)
won't give you the right answer. But then you haven't really measured
the reflection coefficient on the line, if you've used the wrong
reference impedance, since reflection coefficient is _defined_ as
reverse voltage divided by forward voltage. All this is right at the
heart of why I've been telling you that you should make sure your SWR
meter is calibrated to the impedance you think it is, and in
particular, to the line impedance (or close to that) if you really
want to know the SWR on the line. On the other hand, knowing measured
S11 and the reference impedance for it, and the line characteristic
impedance, you can determine the SWR on that line. But your "SWR"
meter isn't really an S11 meter; at best it's a |S11| meter.

This also brings up another point: do YOU define S11 to be the same
as reflection coefficient?

Cheers,
Tom

(Dr. Slick) wrote in message om...

The problem is characterizing insertion loss using higher power
transmitters, when we know that the 1000 watt cantenna swings from 40
to 70 Ohms (with reactance too) as you get above 80 megs or so. It
become difficult to know if you are moving in the right direction or
not.

So if you have 200 feet of RG-213 and 200 feet of RG-58, put those in
series to the cantenna. Coil them loosely and cool them with a fan if
needed, if you are running high power (or start the chain with larger
coax). That should get you close to 15dB attenuation in the coax at
100MHz and a 1.02:1 or better SWR at the input end for a 2:1 SWR at
the cantenna. At lower frequencies where the line loss is lower, the
cantenna is good enough that you don't need the line loss as
compensation. Do I have you worried about what the actual impedance
of the coax is from earlier postings? If so, good! Now you're in a
position to think about what's good enough, and measure what you have
to see if it meets the desired goals.

If that's all too kludgey for you, go buy a good load. Or just tune
the cantenna for bands of interest. You could make a set of small
boxes with L networks, each of which compensates the cantenna on a
particular band. Or look up one of the articles about how to make a
better "cantenna", probably starting with the parts you have. There
are lots of options. Why settle for a setup which will only lead you
deeper into confusion? Short of really precision measurements,
there's a lot you can do trading off time and careful thought for the
high cost of commercial equipment.

Cheers,
Tom

Roy Lewallen wrote in message ...

I stand by my statement.

I, and I'm sure many others, likely including the authors of certain
cited papers, stand beside you.

Cheers,
Tom

Tom Bruhns wrote:

So if you have 200 feet of RG-213 and 200 feet of RG-58, put those in
series to the cantenna. Coil them loosely and cool them with a fan if
needed, if you are running high power (or start the chain with larger
coax). . .

One thing to keep in mind when you use coax as an attenuator or dummy
load is that the portion of the cable nearest the transmitter dissipates
most of the power. If you had, say, 6 dB per 100 ft attenuation and a
200 ft cable, the first 50 ft dissipate 1/2 the power (and that's
concentrated toward the transmitter end), the next 50 ft dissipate 1/4
the power, the next 50 ft 1/8, and the final 50 ft 1/16. So do as Tom
says and put the heavier coax up front, and allow for more air
circulation for the coax nearest the transmitter if dissipation becomes
a problem.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
Dr. Slick wrote:
How would you explain what Cecil wrote?

Who else but Cecil would dare attempt that? :-)

In my example, the SWR on the ladder-line is not changing (except
for losses in the ladder-line). Changing the length of the
ladder-line changes the 50 ohm SWR meter *reading*, i.e. it
changes the impedance seen by the transmitter.
--
73, Cecil http://www.qsl.net/w5dxp



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Tom Bruhns wrote:
On the other hand, knowing measured
S11 and the reference impedance for it, and the line characteristic
impedance, you can determine the SWR on that line.

That's true for a one-port load but not usually true for a two-port
impedance discontinuity in the transmission line.

This also brings up another point: do YOU define S11 to be the same
as reflection coefficient?

S11 is the (physical) reflection coefficient when a2 equals zero. When
a2 is not zero, the physical reflection coefficient, S11, will not usually
equal the measured reflection coefficient, the square root of Pref/Pfwd.
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen, W7EL sed...

"The transmitter output impedance has no effect whatsoever on the line's SWR."
---------------------------------------------

Of course.

Good grief, is there still someone out there who does not know this???

Jack K9CUN

--
=======================
Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================
"JDer8745" wrote in message
...
Roy Lewallen, W7EL sed...

"The transmitter output impedance has no effect whatsoever on the line's
SWR."
---------------------------------------------

Good grief, is there still someone out there who does not know this???
-----------------------------------------------

Here comes the $64,000 question . . . . Of what possible use is the SWR
when they think they've got it ???

Dr. Slick wrote:

I disagree on this point. You are caught up in the 50 Ohm world,
which i admit is easy to do. The SWR is based on the ratio of the
forward to the reflected power.

That's not correct. The SWR (more correctly VSWR) is, by definition, the
ratio of the highest to lowest voltages which appear on a line long
enough to have both a maximum and minimum. It can be calculated from the
forward and reverse voltage waves. ISWR, the current standing wave
ratio, is numerically equal to the VSWR.

If you had an analyzer that was
calibrated to 20 Ohms (the same as normalizing the Smith for 20 Ohms
in the center) you would certainly have reflected power and high SWR
going into a 50 Ohm load.

And a 20 Ohm load would have a 1:1 SWR.

Loads do not have an SWR, only transmission lines do. The fact that you
get a reading on an SWR meter when it's connected to a resistor doesn't
alter that.

You have to realize that an SWR meter isn't really measuring SWR, as Reg
has repeatedly pointed out. It's actually measuring an impedance, and
reporting that on a scale marked SWR. So you have to be careful to avoid
making the mistake of confusing an SWR meter reading with the SWR on a
cable it's connected to. The two correspond only if the cable's Z0
equals the SWR meter's.

Likewise, you have to realize that you don't change the SWR or
"reflected power" when you change the normalization of your network
analyzer or Smith chart. Those things are a function only of the load
and transmission line impedance, not on your measurements or calculations.

Roy Lewallen, W7EL