[Towertalk] Ground the coax braid on LPDA or not?

[Chuck Counselman]

At 2:19 PM -0600 12/9/02, Louis Sica, Jr. wrote:
>I have a T8 8 element LPDA on top of a 50 foot tower.... The problem 
>is that since the LPDA boom is electrically isolated from the tower, 
>I don't get the advantage of the top loading that antenna would 
>bring, and that makes the performance of the DX-A sloper not where 
>it could be.

I'm not familiar with that LPDA.  Do you know _why_ its boom is 
isolated?  I would expect that, to preserve the balance of the 
antenna (I'm thinking primarily of electromagnetic balance, but 
mass/weight balance as well), the boom is located symmetrically 
(centered horizontally) with respect to the radiating elements that 
are perpendicular to the boom, and also with respect to the two 
conductors of the array feedline that runs parallel to the boom. 
Therefore, no RF current should flow longitudinally on the boom, and 
it should not matter at all whether the boom is connected 
electrically to the mast, and the mast to the tower.

I wonder whether the manufacturer isolated the boom to cover the 
possibility of the antenna being mounted offset horizontally from a 
tower.  In this case the isolation would break the path along which 
RF current _would_ otherwise flow between the tower and the boom. 
Since you said that your LPDA is "on top of" the tower, I assume that 
your mast, and tower, are centered below your antenna, in which case 
I don't see why the boom needs to be isolated.


>My question is, can I electrically connect the LPDA to the tower by
>grounding the braid of the coax to the tower?

I assume that your coax, being an unbalanced transmission line, is 
connected through a balun of some kind to the balanced two-terminal 
feedpoint of the LPDA.  The balun may act as a common-mode current 
choke.  If so, it would prevent the coax braid from serving your 
intended purpose, of connecting the tower to the LPDA.

In any case, your mast should be a better RF connector than your coax.

I'd concentrate on finding whether there's a reason not to connect 
the LPDA boom to the mast.


73 de Chuck, W1HIS