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John S · October 8th 15, 10:04 AM posted to rec.radio.amateur.antenna

On 10/7/2015 12:12 PM, wrote:
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, it is implying that his results for a
very short line can be generalized and apply to any length of line. 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.

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.

Jeff

The rule of thumb is that a line less than a tenth of wavelength long
shows negligible transmission line effects and can be viewed simply
as a shielded wire.

He is using a pair of wires, not shielded. However, I'm sure that it
doesn't matter because the line is so short.

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