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[TowerTalk] Log Periodic Feed

To: <towertalk@contesting.com>
Subject: [TowerTalk] Log Periodic Feed
From: "Jim Thomson" <jim.thom@telus.net>
Date: Thu, 7 Jul 2016 17:40:28 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Date: Wed, 6 Jul 2016 18:25:05 -0400
From: "James Wolf" <jbwolf@comcast.net>
To: "'Kim Elmore'" <cw_de_n5op@sbcglobal.net>,
<towertalk@contesting.com>
Subject: [TowerTalk] Log Periodic Feed
 

Continuing the discussion on how to attach the coax to the ?hot? boom of a 
Tennadyne T8 antenna.

As I assembled this antenna, I became increasingly confused and concerned about 
the feed system.   With this log antenna, instead of running open feedline 
wires crossing over to the elements, it uses two boom pieces and each is 
insulated from the mast and each boom is ?hot?.  The instructions indicate and 
show how to attach the feedline to the bottom boom piece using a Collins type 
balun at the feed point which is at the forward end of the antenna.  

My concern is that of the coax shield separated only by the coax insulation 
alongside one of the ?hot? boom pieces would couple and cause radiation (phase 
shift) problems with the antenna or at the very least radiate as it goes 
vertical down the mast. 

Due to the element mounting method it is not possible to run the feedline 
inside the boom to the mast.    

Does anyone have any thoughts on why running the coax along the ?hot? boom 
would be an OK idea?

And if not, how else would one feed the antenna?

 

Jim ? KR9U

##  http://w8ji.com/baluns_on_log_perodic_antennas.htm    I forget how a 
collins balun is constructed,  but if its just a 1:1  balun, you may be better 
off using a quality CMC device at the feedpoint.    Then another  CMC on the 
coax..where it leaves the boom.... and  then forms the rotor loop.    If coax 
taped to hot boom,  it will induce RF  into the braid....hence the CMC .. right 
where the coax leaves the boom..and forms the rotor loop /splice  ..and heads 
down the tower. 

Jim   VE7RF


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