On 10/7/2015 12:12 PM, Jeff wrote:
On 07/10/2015 14:33, John S wrote:
On 10/7/2015 1:45 AM, Jeff wrote:
On 07/10/2015 04:50, David Ryeburn wrote:
I recommend http://owenduffy.net/blog/?p=5442#more-5442, a new
posting
by Owen Duffy (who in days of yore when rec.radio.amateur.antenna had a
number of people like him posting regularly here, was one of the
best of
them).

Often the correctness of ideas can be tested by pushing their
applicability to extremes. He considers a two-wire air-insulated line
operating at 10 MHz, wonderfully short, just 30 mm long. Using it, he
shows that predictions made by some popular models for transmission
line
loss cannot possibly be correct. To understand his argument you do not
need partial differential equations or Smith Charts or anything much
more complicated than Ohm's Law. This is what is nice about really
short
lines, where for all intents and purposes the current and voltage do
not
change along the line.

Caution: His argument clearly shows that loss is less in a mismatched
line with high load impedance than in a matched line, and more in a
mismatched line with low load impedance than in a matched line, for
very
short lines. Do not apply his reasoning to longer lines. But his
argument does demolish the theory that additional loss depends only on
SWR.

He used to have a very nice line loss program on his old website, but
it's gone now. For a good one, one that does not just enter in the SWR
but instead uses the actual complex load impedance directly, see a Java
based program at http://fermi.la.asu.edu/w9cf/tran/ from Kevin
Schmidt, W9CF. Java can be dangerous to your computer's health, but his
program is OK. However opening it up on-line using Java will expose
your
computer to evil things from other people while Java is running. You
can
instead download his program, and then remove your computer from the
Internet while you run it.

David, VE7EZM and AF7BZ


I post is pretty much hogwash,


This conclusion seems a bit strong for you, Jeff.


it is implying that his results for a
very short line can be generalized and apply to any length of line.


I cannot find where he implies that. Can you point to the particular
section where he alludes to this?


I
have not had time to go through his workings to see what other flaws
there are in them, BUT trying to draw conclusions from the analysis of a
short line is flawed anyway. A spice analysis may show the correct
results.


Why is a short line analysis flawed? I kinda thought physics was the
same everywhere. Yes, please check it out and let us know your findings.


Consider a very short discontinuity in the middle of a long 50 ohm line,
say a PL259/239 connector pair. The impedance of a PL259 is not 50 ohms
and usually somewhere nearer 100ohms. Do you see a 2:1 vswr when you use
it at 7MHz? No of course you don't, but you may well at 23cms. The
reason is the length of the discontinuity compared to the wavelength.


But, that is not part of the analysis. Can you provide a disagreement
with his analysis under the same conditions?



The analysis relies on taking a line that is 1/1000th of a wavelength
long which has pretty much a constant voltage on the line.

In the case of a line that short the analysis is correct because the
load impedance dominates over the characteristic impedance of the line.
However, when a source is connected to a load via a “long” transmission
line, the line’s own characteristic impedance dominates over load
impedance in determining circuit behaviour. In other words, an
electrically “long” line acts as the principal component in the circuit,
its own characteristics overshadowing the load’s.

Because of this the claim in the Conclusions that "A very simple
transmission line scenario that could be solved accurately using basic
linear circuit analysis was designed as a basis for evaluation of some
published techniques for predicting TL loss under mismatch" is untrue as
the analysis is ONLY valid for a very short line.

Okay, I accept that. However, since you agree that his analysis is
correct for his scenario, then you must admit that the equations and the
statements in quotes are not correct as they do not give the same answer
in his scenario.

And of course the conclusion that "Reflections II did not reconcile with
the linear circuit analysis solution, and showed gross error" is correct
because of the flawed analysis.

See above.

Jeff