nuckolls.bob(at)aeroelect
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 Posted: Tue Jan 25, 2011 8:11 am Post subject: Comm Antenna & SWR Reading -- Too High? |
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At 06:18 AM 1/25/2011, you wrote:
| Quote: |
<Tom(at)CostanzaAndAssociates.com>
Bob,
In a previous life, I was a seat-of-the-pants, self-taught
technician. I was told that the length of the coax didn't affect
the SWR. But intuition, and more importantly, your test clearly
shows that it does.
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Yeah but your teachers were right and so are you.
The length of the coax between signal source and
the load has losses. From a practical perspective,
those losses are not significant in setting performance.
In other words, we pick a coax that has an acceptably
low loss for the system we're building.
In the wild and wooly world of RF and transmission
lines, most investigations are carried out with
transmission lines of very low loss such that while
the number is not zero, it's so small as to be
ignored.
| Quote: | Extrapolating from this, it seems it would be possible to reduce
the SWR to near zero by varying the length of the coax.
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I think you missed an important point in the data.
Yes, SWR varied with frequency because the load on
the far end of the transmission line waves around
in the frequency spectrum. Adding coax MIGHT make
SWR appear better but only if there are losses in
the coax. Losses reduce the magnitude of energy that
makes it to the far end where it suffers reflection
due to mis-match. The same losses reduce magnitude
of energy reflected back to the observer's instrument
at the input end. Yeah, the SWR 'appears' to improve
while in fact, losses have served only to mask the
truth from an otherwise reliable measurement system.
Check out this tutorial on Smith charts:
http://www.microwaves101.com/encyclopedia/Smithchart.cfm
As a practical matter, while investigating of any generator/
transmission line/load combination the observer can
ignore the effects of losses because the transmission
lines are short (less than 360 electrical degrees
or one wavelength). When you plot the characteristics
for any combination of hardware on the Smith chart, they
form a circle of constant SWR. There's an infinite
number of resistance/reactance values represented
by points on that circle . . . but for any given
frequency, the SWR is constant for any length of
transmission line.
When you investigate a range of frequencies, then
the circle explodes and you can get some really
curious graphic art forms. That's because the
antenna at the far end is optimal at one frequency
only. But an size dummy load at the far end
and SWR is zero only when load matches the line.
When the load does not match the line, you get
SWR circles other than 1:1 but they are still
smooth circles and they do not change magnitude
or shape by adjusting line length.
The data I offered supports this. Yes, SWR did
appear to become lower by adding coax but the
real SWR value for the two conditions were identical.
| Quote: | But would this affect performance? Your response seems to indicate
that SWR is not a defacto predictor of performance. So why bother
measuring it, unless it's to rule out an open or short, for which
there are much easier measuring techniques?
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Yes, it affects performance . . . but probably
not in observable ways. One reader noted that an
SWR value on the order of 6:1 implied loss of
50% of radiated energy. That's a 3db drop of
signal strength at the receiving site. The guy
listening to that signal would be unlikely to
hear any difference. At the same time, we COULD
put 100' of coax on the antenna being investigated.
The SWR would get MUCH BETTER but the power situation
would get worse. Not only is REAL SWR unchanged,
we've added losses to the system that mask real
SWR and further reduce power delivered to the
antenna.
There WAS a time when practitioners of the
arts and sciences of aircraft antennas counseled
cutting coax cables to some specific length to
maximize performance. They cited experiments
with a field strength meter to support their
assertions. You could not argue with their
demonstrated success. You could argue their
lack of understanding of what was going on.
In these cases, SWR values in the system had
to be high. Once a piece of transmission line
becomes mis-matched, it becomes part of the
impedance transformation system that attaches
the power generator (tube or transistor) to
the antenna's feedpoint. By fiddling with the
length of a poorly matched feedline, they were
in effect "tuning" the combination of reactances
that participated in that power transfer.
If they put a dummy load on the antenna end
of the transmission line, length would have
no effect on SWR or the transmitter's ability
to deliver power into that transmission line.
| Quote: | Also, the Comant specs for 2:1/3:1 probably assume an aluminum
airplane. Would the fact that this is installed on a glass plane
not make a difference?
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You bet. Not only do we need to satisfy minimal
RF requirements for size and geometry of ground
plane, achieving a good RF bond to that ground
plane is important. He cited a 2' x 3' ground
plane. That's the size I used in my experiment.
While not "ideal" (1/4-wave radius disk), it
was certainly adequate to the task. I would not
expect there to be observable difference in
performance between infinite ground plane, aluminum
structure, or glass structure backed up by
6 square feet of aluminum sheet.
| Quote: | Last question: since I started studying antennas and transmission
lines as a kid, it has always seemed like they were more art than
science. Can you point me to some study material that someone with
less than a PhD in physics can understand?
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They are VERY much science. The experiments
have been repeated many times with great
consistency. There is also an art of making
sure we understand and minimize the conditions
that affect the science. I'm sure that's
what we're dealing with here.
There ARE good reasons why we both got the
measurements reported. The goal is to figure
out their veracity and significance. That's
the art of sifting simple-ideas for answers
that define the results.
| Quote: | As always, thanks for everything.
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You're most welcome . . . and thanks for asking!
Bob . . .
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