Yeah, a 2 square foot x 7/16" thick fiberglass plate would
be about 600pF of shunt capacitance to the concrete base.
That amounts to about ~150 ohms shunt Xc at 1.8 MHz.
If you cut out the center of the insulator (the part that isn't
under the legs) and put a metal plate under the bottom of
the insulator so you are shunting RF current back to the
shield of the feedline instead of the lossy concrete, then it
might work okay, but you would still have the lightning issue
to contend with. You could put spark gaps on each tower
leg that are setup to break down before the insulating plate
to take care of that.
73 de Mike, W4EF...................................
.
----- Original Message -----
From: "Donald Chester" <k4kyv@hotmail.com>
To: <jz73@verizon.net>; <topband@contesting.com>
Sent: Saturday, October 02, 2004 6:17 PM
Subject: RE: Topband: Tower insulator thickness?
>
>
>
> >I'm planning on using some insulating material under BPC25 and BPC55 Rohn
> >base plates (and around the pier pin in the center) so the towers may be
> >series-fed on 160.
> >
> >Is 7/16-inch thick enough for this purpose?
>
> I think you would end up with a pretty good capacitor from tower base to
> ground. Plus, what if a lightning surge breaks down the thin sheet of
> insulating material?
>
> I would opt for a real base insulator. If you can't find a used one
removed
> from a broadcast tower, you could fabricate one from a large electric
power
> insulator. I have seen ones about 6" in diameter and anywhere from 6" to
> over 12" tall, with metal castings with tapped screw holes at each end.
>
> -k4kyv
>
>
> _______________________________________________
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> Topband@contesting.com
> http://lists.contesting.com/mailman/listinfo/topband
>
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