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Topband: 160 Vertical

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Subject: Topband: 160 Vertical
From: (Tom Rauch)
Date: Sat, 19 Oct 2002 20:05:37 -0400
> I am aware that the 5 element beam on the top will further lengthen
> the effective height from the 200 feet by (just estimating here)
> around 20-25 more feet.  Will the other anten

Maybe I missed it, but I don't think anyone addressed the insulated 
elements. The voltage between insulated element and the boom can be 
very high when you shunt feed the tower. It can also be VERY tough on 
baluns, as a matter of fact I haven't found ANY popular core-type 
balun out of dozens tested that did not heat significantly with even 
low levels of common-mode. That could easily be the situation with an 
insulated driven element group like the KLM uses when the tower is 
shunt fed.

So you have two worries...the balun heating and the parasitic 
elements arcing to the boom.

I'd ground the parasitic elements to the boom through a heavy wire 
air core RF choke that has a few hundred ohm reactance at the lowest 
operating frequency of the yagi.

Your tower is an excellent candidate for elevated shunt feed, and may 
even work quite well with a reverse feed like the N4KG feed. The 
reason your tower is better is most likely the feed point could be 
placed 100-150 feet above ground, and the tower BELOW that point 
would look like a very high impedance at the feed point. That would 
minimize unwanted and undesirable current into the tower below the 
feed point, something that is especially problematic with the N4KG 

The only significant problem I see is the effective height of the 
radiation center would produce a pattern that is not particularly 


       73, Tom W8JI 

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