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

To: <towertalk@contesting.com>
Subject: [Towertalk] Ground the coax braid on LPDA or not?
From: stevek@jmr.com (Steve Katz)
Date: Mon, 9 Dec 2002 13:42:35 -0800
I'd call Chuck at Tennadyne and ask him.

The T8 and other Tennadyne LPDAs have square aluminum booms that are
insulated from their support mast because the booms themselves are the lines
which connect all the elements, and thus conduct all the RF current.  I
would imagine the booms are hot with RF to the tune of the square root of
P*R, all the way along their length.

WB2WIK/6

"If everything seems under control, you're just not going fast enough." -
Mario Andretti

> -----Original Message-----
> From: Chuck Counselman [SMTP:ccc@space.mit.edu]
> Sent: Monday, December 09, 2002 1:09 PM
> To:   towertalk@contesting.com
> Subject:      Re: [Towertalk] Ground the coax braid on LPDA or not?
> 
> 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
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