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

[Michael Tope]

I would put a choke balun (coax wound into coil) between the
point where the coax leaves the boom and starts down the mast
and then ground the coax shield to the mast on the transmitter
(shack) side of this choke balun. The lower boom is RF hot
at the point where the coax turns away and starts down the tower.
Since the coax is in intimate contact with the lower boom there
will be some capacitive coupling between lower boom and the
outer shield of the coax which I would expect to excite the
outer shield of the coax. Since the coax is in intimate contact
with the tower along its vertical span, I would expect the hot
coax to capacitively couple to the tower even if there is no
physical connection. In other words, the shield of the coax
is already connected to the mast and tower thru the distributed
capacitance between the tower and the coax. In fact even if
the whole tower is made of an insulating material (like a
telephone pole), you have the conductive coax running its
entire length, which begs the question what difference can it
make if you connect the tower and the coax at the top
(its like the guys who tape coax along the length of a
fiberglass cross boom)?

My bet is that you are already exciting the tower and coax
outer shield in the factory recommended configuration. Adding
a choke balun to the feedline right at the boom to mast clamp
and then grounding the coax shield to the mast on the transmitter
side of the choke shouldn't hurt anything. In fact it might even
improve the pattern of the log periodic by reducing the
aformentioned feedline radiation and you'll get the top loading
you need for the sloper.

73 de Mike, W4EF.................................................

----- Original Message -----
From: <K7LXC@aol.com>
To: <lsica@attbi.com>; <towertalk@contesting.com>
Sent: Monday, December 09, 2002 5:49 PM
Subject: Re: [Towertalk] Ground the coax braid on LPDA or not?


> In a message dated 12/9/02 4:48:25 PM Pacific Standard Time,
lsica@attbi.com
> writes:
>
> > Chuck was the one who recommended to me to ground the coax at the top of
the
> >  tower, but other local hams who've had Tennadyne LPDA's (T10's) have
told
> me
> >  how they've noted performance degredation after grounding the coax. So
I'm
> >  not quite sure who to follow, the manufacturer, or real-world data from
> >  someone whose opinion I respect.
>
>     Huh?!? There's a question? The LXC Prime Directive is to "DO what the
> manufacturer says".
>
> Cheers,
> Steve    K7LXC
> TOWER TECH
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