[AMRadio] Antenna Idea and lightning precautions


Mike Dorworth, K4XM k4xm at arczip.com
Sat Aug 5 15:58:59 EDT 2006


I can see this is the beginning of a long thread since everyone has their
own ideas. In commercial work a sharpened spike above the thing to be
protected is to DRAW the lightning to a well insulated and very well
grounded ground system. This is to protect the equipment below it. To
dissipate,  the ball should be rounded like a car radio antenna to gently
discharge the corona.  We put up a series of 150 foot towers at work with a
21 foot stainless sharpened lightning spike above the tower top to draw the
lightning. All of our ( 92 each) microwave towers had a 3 or four inch
diameter sharpened brass rod 2 feet above the tip top of  the tower. It's
ground cable was insulated from the tower all the way down. Of course the
tower and all the guys were also grounded to the common ground. A dipole can
easily discharge static build up with a 100 k ohm resistor of at least 1
fourth watt. This keeps the system equalized. Lightning usually hit the
HIGHEST ( though noy always) spot, so if there are taller trees they would
get it first. I like insulated wire instead of bare since the damp wind will
not build up thousands of volts when it blows over..just before a storm. For
fun take the antenna connector and put in a mason jar and place near ground
and watch the 4 inch long blue firs just before a storm on a hilltop. A
Johnson Matchbox sounds like a fourth of July celebration if left connected.
I guess, in the end a direct strike is bad news in every case. Most of us
are really talking about big static discharges I think. A real strike will
blow every receptacle in the house out and the wire on on side of every
power cord will vaporize and the fuse box will be blown off the wall. Let
the tall trees take that!.. 73 Mike
----- Original Message ----- 
From: "Jim candela" <jcandela at prodigy.net>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Saturday, August 05, 2006 2:36 PM
Subject: [AMRadio] Antenna Idea and lightning precautions


>
>
> Hi All,
>
>    I am contemplating putting up an inverted Vee antenna where the center
> point is above my house suspended with a 30' Lowes push up mast attached
to
> my roof with a tripod mast base made for roof mounting. This would make
the
> apex at almost 50', and with the trees around my home, the ends at about
> 30'. Other locations that I might have the antenna apex at will be densely
> surrounded by trees, and I am trying to avoid that.
>
>    My question is about lighting concerns with this approach. I would have
> multiple 12 awg ground straps from the mast base to earth ground via
copper
> ground stakes at least 5' long. This would act as a counterpoise for the
> antenna, and provide a DC ground reference for the 30' mast. My fear is
that
> the antenna would attract a lightning hit (direct) and that would cause my
> home to burn up in a flaming fireball.
>
>    Then I was thinking about how lightning rods work, and when done
> properly, don't lightning rods work by having a sharp point at the tip,
> where they bleed the static (a corona discharge) to prevent a lightning
> strike? If so, why can't I take a 1/8" stainless 8' whip with a point on
> top, mounted above the inverted Vee apex, and use that as a lightning rod?
I
> guess I'd need to beef up my ground wiring scheme just in case of a direct
> hit. Any suggestions?
>
>    I am hoping for having more lightning protection with my antenna in
place
> over that of no antenna at all? Is this possible?
>
> Regards,
> Jim Candela
> WD5JKO
> --
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