Dr. Slick wrote:
How would you explain what Cecil wrote? How are some people
improving SWR by changing coax length, when in theory they shouldn't
be able to do this?

If I were using a 450 ohm SWR meter on 450 ohm Z0 feedline, the SWR
meter would indicate no change in SWR as I change the feedline length.
Assuming the 450 ohm SWR meter is accurate, the indicated SWR and the
actual SWR are the same.

When I use a 50 ohm SWR meter on 450 ohm Z0 feedline, the SWR meter
is NOT indicating the true SWR on the 450 ohm feedline. It is indicating
what the SWR would be if the line were Z0=50 ohms. All it is indicating
is the impedance seen by the transmitter which indeed does change with
feedline length in a system with reflections.

Theoretically, the SWR on a feedline changes only slightly with changing
feedline length and that is because of additional losses in the feedline
as the length is increased.
--
73, Cecil http://www.qsl.net/w5dxp



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I wrote a little BASIC program.
================================

The value of a computer program resides not in the source code language but
solely in the Reliability of the programmer.

And Reliability is a measure of Quality versus time.

And Quality is the extent to which a product conforms to pre-specified
requirements.

On Sat, 16 Aug 2003 14:50:29 -0500, W5DXP
wrote:

Richard Clark wrote:
Do you have a question about it following that point in time?

I wrote a little BASIC program to digest your data and it decided
that all your data is perfectly consistent with a constant SWR on
a Z0=~40 ohm coax between the 50 ohm SWR meter and the 16.67 ohm
load. What is the Z0 of your "RG-58 type hardline"?

Hi Cecil,

50 Ohm. The BVT has been calibrated long before this application (8
years ago the first time) without surprises. Methods are described in
Walt's Book, Reflections. I suggest you use his software rather than
roll your own. I would also suggest you consider his methods as well.
Point in fact, I followed his advice to measure at odd 8th wave
intervals if I recall his instructions correctly. If not, it was "by
the book." I also performed the calibrations at the remaining
cardinal points. Tedious perhaps, but having done it through maybe
three or four times without outliers, I don't expect there is any need
to visit that again.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
I suggest you use his software rather than roll your own.

Funny that you request responses, nay, demand responses, and then dismiss
them out of hand. The data indicates an impedance incompatibility between
the transmission line and the measuring instrument. Your results are
perfectly compatible with an SWR spiral on a 40 ohm Smith Chart including
the loss in the feedline.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
I wrote a little BASIC program.
================================

The value of a computer program resides not in the source code language but
solely in the Reliability of the programmer.

And Reliability is a measure of Quality versus time.

And Quality is the extent to which a product conforms to pre-specified
requirements.

As demonstrated by reference to what external standards? I wouldn't walk
too far out along that plank, Reg...


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

An additional source of VSWR error exists. It is of interest to the purist.

If I provide a perfect 50.000 ohm dummy load for typical JAN-C-17 RG
type cable, example RG-9913, I have to design for a maximum VSWR of 1.06
to 1 due to tolerances in the cable!! So, I'll have a minor variation in
observed VSWR as a function of length.

Same applies to the design impedance of your VSWR meter. It has a
tolerance on it's 50 ohms!!!!!

Deacon Dave, W1MCE

W5DXP wrote:

Dr. Slick wrote:

How would you explain what Cecil wrote? How are some people
improving SWR by changing coax length, when in theory they shouldn't
be able to do this?


If I were using a 450 ohm SWR meter on 450 ohm Z0 feedline, the SWR
meter would indicate no change in SWR as I change the feedline length.
Assuming the 450 ohm SWR meter is accurate, the indicated SWR and the
actual SWR are the same.

When I use a 50 ohm SWR meter on 450 ohm Z0 feedline, the SWR meter
is NOT indicating the true SWR on the 450 ohm feedline. It is indicating
what the SWR would be if the line were Z0=50 ohms. All it is indicating
is the impedance seen by the transmitter which indeed does change with
feedline length in a system with reflections.

Theoretically, the SWR on a feedline changes only slightly with changing
feedline length and that is because of additional losses in the feedline
as the length is increased.

Cecil,

Suppose I do the following experiment: a source of unknown impedance,
connected to a shorted 1/4 wave line through a ~50 Ohm series resistor. When
looking at the voltage on the two ends of the resistor with a 'scope, I
expect them to be the same value, with no phase difference. Do you think
that would be enough to satisfy the nay sayers that the source is delivering
no power? I am *not* talking about you.

In retrospect, my posting here is probably a variation of what Roy posted a
few months ago. Didn't mean to rip him off. BTW, I was in error when I said
the line charges up in 1/2 cycle; that would be true only if the source
impedance was 0.

Tam/WB2TT
"W5DXP" wrote in message
...
Tarmo Tammaru wrote:
I think the only alternative in un nice; namely that there is no
reflection
in steady state

There are an infinite number of possibilities between the rails of 100%
re-reflection and zero re-reflection. I suspect a PA obeys the rules of
the wave reflection model set forth in Ramo & Whinnery.
--
73, Cecil http://www.qsl.net/w5dxp



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Richard Clark wrote:
What does your software report for calibrating a software 40 Ohm line
feeding a software 50 Ohm load?

It doesn't report that. It is a data reduction, curve fitting program.

What does your software report for calibrating a software 50 Ohm line
feeding a software 50 Ohm load?

It doesn't report that. It is a data reduction, curve fitting program.
The only input to the program is your data.

Consider that with your 16.67 ohm load, the impedance 1/2WL away from
the load should repeat. But the minimum reflected power point indicates
a load of about 25 ohms. So there's one error already, perhaps caused
by the diode. If you plot your data as SWR points on a 50 ohm Smith
Chart, it is obvious that the SWR circle (spiral) is compressed at the
lowest reflected power readings. It is also obvious that the center
of the SWR circle is not the center of the 50 ohm Smith Chart. That's
a pretty clear indication that everything is not 50 ohms. If full scale
on the Bird is 5 watts, the readings can be off by 0.25 watts. 0.45
watts plus or minus 0.25 watts is not very accurate.
--
73, Cecil http://www.qsl.net/w5dxp



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Richard Clark wrote:
What does your software report for a software 40 Ohm line feeding a
software 50 Ohm load? That is, when all data points (that is each
increment of 1 foot) exhibits 0 Watts returned for 5W applied?

My software takes a list of reflected powers, assumes the forward
power is five watts, calculates the 50 ohm SWR for each point, and
plots them every 0.03WL on a Smith Chart. It was written to accept
your data points. If the reflected power above was always zero watts,
all those points would be the same point, i.e. a circle of zero radius
at the center of the Smith Chart.

Take time to plot those 50 ohm SWR points on a Smith Chart with the
0.45 watts reflected point assumed to be the lowest purely resistive
point. A picture is worth a thousand words. I have seen that picture
before. It was when I designed my SWR meter for a Z0 of 450 ohms and
my window-line turned out to have a Z0 of 388 ohms.
--
73, Cecil http://www.qsl.net/w5dxp



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Richard Clark wrote:
Thank you. I see that for the 50 Ohm load that the line exhibits a
characteristic of 50 Ohms as described by its concentricity about the
reference.

This is no surprise given the cable is premium short run (length, not
reels) material specified out to 20 GHz (from Boeing's precision
electronics facilities). I have several hundred feet brand new.

Please plot your data on a Smith Chart. Since your data is
obviously nonlinear, I would suggest diagnosing the nonlinearity
problem before concluding anything. For instance, what is the
one tenth scale RF RMS voltage before it is rectified by the
diode?
--
73, Cecil http://www.qsl.net/w5dxp



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