[TowerTalk] Log Periodic Feed

[Jim Thomson]

Date: Wed, 6 Jul 2016 18:25:05 -0400
From: "James Wolf" <jbwolf at comcast.net>
To: "'Kim Elmore'" <cw_de_n5op at sbcglobal.net>,
<towertalk at 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