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[TowerTalk] Grounding unused feed lines -

Subject: [TowerTalk] Grounding unused feed lines -
Date: Mon, 13 Mar 2006 17:49:16 EST
List-post: <>
In a message dated 3/13/2006 4:02:02 PM Eastern Standard Time, writes:
From: "" <>
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.

Norman W4QN

"The Power of a Cubex Quad"

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