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Re: [TowerTalk] Ferrite Balun on an LPDA?

Subject: Re: [TowerTalk] Ferrite Balun on an LPDA?
From: Jim Jarvis <>
Date: Fri, 15 Jun 2007 10:39:17 -0400
List-post: <>
Comments, on Tom's observation, Below:

From: "Tom Rauch" <>
Subject: Re: [TowerTalk] Ferrite Balun on a LPDA?
To: <>, <>

The Tennadyne log has two hot booms. Each boom is used as a 
conductor in a balanced transmission line. The elements are 
attached to those transmission line conductors which also 
serve as a boom.

Many people install the antenna by placing a balun out at 
the end of the booms. This divorces the boom to coax shield 

Then they run the same coax they decoupled right along the 
boom, often taping it right to the lower boom.

Now they have the coax shield they painstakingly decoupled 
from the boom being tightly coupled to a hot boom! The 
mutual coupling is so high it undoes what the balun did, and 
they are back to square one with terrible common mode on the 
coax. It's just about as bad as using a balun at the 
feedpoint of a dipole and then taping the coax to one leg of 
the dipole for 15 feet after the balun.

None of this applies to any other system, unless that system 
has a "hot boom".

73 Tom

Tom made this observation several years ago, when I had my
T8 in Vermont.  Tennadyne, at the time, recommended running
the coax along the lower element of the boom, thereby unbalancing
the system, and coupling current onto the feedline.

I ran a pattern test with a neighbor, and found things weren't 
symmetrical, and were far from spec.  Then, I dropped the feedline
down vertically from the boom.  It cleaned up nicely, but it
looked ugly and tended to snag on things.  So, I re-ran the coax
along the lower boom element to the mast, and added a second ferrite balun,
and additional ferrite chokes at that point.  Pattern remained



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