nuckolls.bob(at)aeroelect
Guest
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 Posted: Tue Jan 25, 2011 10:50 am Post subject: Comm Antenna & SWR Reading -- Too High? |
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At 10:07 AM 1/25/2011, you wrote:
| Quote: |
<nuckolls.bob(at)aeroelectric.com>
At 06:18 AM 1/25/2011, you wrote:
>
><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.
> 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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Permit me a further expansion on this idea. See:
http://www.antennex.com/preview/New/quarter.htm
Here we are tutored on a a really cool feature
of high SWR in a transmission line. The article
talks about two cases were two low SWR systems,
like a 100 ohm antenna and a 50 ohm coax would
like to be joined. It is the nature of a 1/4 wavelength
of transmission line to do a 'mirror image' of
not-perfect SWR terminations at each end.
In the case cited, putting a piece of 75 ohm
transmission line in series with a 100 ohm
load 'mirrors' that 100 ohms into a 50 ohm
value at the other end. Viola! Hooking your
50 ohm coax to a 100 ohm load would give
you 2:1 SWR. Putting a short section of
hi SWR coax in the gap gives you a nice place
to tie 50 ohm coax and have a good match to
100 ohms at the other end.
This condition optimizes at one and one
frequency only. SWR in the matching stub
is never 1:1 and SWR on the feed-point
end is 50 ohms resistive only at one
frequency.
This is a small insight into what appears
to be experimental success in "lowering the
SWR by fiddling with feed line lengths."
In fact, adjusting the feed line length
only alters the combination of R+X for
the combined feed line and antenna such that
the transmitter works a bit better (stronger
radiated signal). In no way does it go to
"lowering SWR" . . . in fact, just as SWR in
the 1/4-wave matching section is never 1:1,
diddling with feed line length depends
on SWR within the feed line being anything
BUT 1:1.
If it WERE 1:1, length would have no difference.
Something other than 1:1 allows you to slide
out along the line to find to optimize the
combination of R+X for your particular
situation.
Bob . . .
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