[TowerTalk] Ferrite Balun on a LPDA?

[Gary Wescom]

> Tom, how would you recommend feeding/decoupling the feedline on a Tennadyne 
> LP?
>
> Mark, N5OT

I'm not sure this question ever actually got answered.  I'll give it a try.

As Tom said, the Tennadyne twin booms act as an open 'wire' transmission line feeding the antenna elements.  They are not at RF ground.  I spotted this when I first bought my T6.  What I did was to directly connect the coax, without a balun to the feedpoint with the shield connected to the lower boom and the center condutor to the upper.  I used black tie-wraps (zip-ties) to strap the coax to the bottom of the lower boom.  At the point where the coax needed to come off the boom and to run down the mast, I mounted a short piece of 4 inch PVC pipe.  I wrapped six turns of the coax around the PVC pipe, hold the turns in place with more tie-wraps as a current choke for the shield.  A ferite line isolator could just as well have been installed at that point.  The continous run of coax without connectors idea seemed to me like it might have some maintenance advantage though.

The point is that to maintain good null depth off the sides of the Tennadyne antennas, a choke 'balun' should be installed where the coax leaves the lower boom.  If you do that near the boom the mast mounting plate, no balun is needed at the feedpoint.

Gary
N0GW