In a message dated 3/13/2006 4:02:02 PM Eastern Standard Time,
From: "firstname.lastname@example.org" <email@example.com>
Subject: Re: [TowerTalk] remote antenna switch
What are the cases for a remote coax switch with unused antennas being
Grounded, or UNgrounded? For example, I have a 5-band quad with an RCS-8V
coax switch. So there are four unused bands on the quad with one active, since
have five separate feedlines.
In general it is best to have the unused quad loops appear "OPEN" to minimize
some of the potential interaction. Regards "static" or EMP dissipation as
long as you are feeding direct with coaxial cable the shield will always
a DC path.
Now the problem is achieving the "OPEN" condition because it is a complex
relationship between the length of the feed lines, VF and the frequency of the
band in use. Each unused feed line will have a different "electrical" length
depending on those three variables, Cable Length, VF and TX frequency.
As you know a "quarterwave" cable terminated in a SHORT reflects an OPEN at
the other end. On the other hand a HALF WAVE cable length would reflect a
SHORT at the other end. Given the harmonic relationship of some bands the feed
line could look like a Half wave, or some fraction which will transform some
different complex Z to the antenna termination.
The answer is not easy to find - first try it (shorting jumper), observe the
results, compared to being open. Antenna patterns (forward lobe shape and
F/B) can be effected by the different cases). If there is an obvious "bad"
effect, then change the length of the feed line to that loop.
There is some analytical work being done on just what are the optimum
individual feed line lengths would be to minimize interaction. First drafts
been completed, but it still needs some refinement before it can be published.
"The Power of a Cubex Quad"
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