[AMRadio] Antenna Idea and lightning precautions


Gary Schafer garyschafer at comcast.net
Mon Aug 7 12:21:57 EDT 2006


A square plate buried in the ground makes a poor ground for lightning.
Lightning is dissipated in a sphere around a ground rod. The longer the rod
the larger the sphere of dissipation. Length is what is important, not
surface area.

On the other hand too long a rod does little good either. As length
increases so does inductance. Going down 20 feet with a ground rod will do
nothing more than an 8 foot rod will do unless the soil is very dry and non
conductive.
It is much better to use several 8 foot or so rods spaced by the sum of
their lengths. (16 foot spacing for two 8 foot rods). This is because the
sphere of dissipation around the rods will overlap and not be as effective
if placed closer.


73
Gary  K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Jim candela
> Sent: Saturday, August 05, 2006 5:15 PM
> To: Mike Dorworth, K4XM; Discussion of AM Radio
> Subject: RE: [AMRadio] Antenna Idea and lightning precautions
> 
> 
> 
> 
> Mike,
> 
>    Thanks for the comments on this topic. At this point I am trying to get
> an idea on what to do, and how to calm the XYL about lightning.  I still
> think erring on safety is the way to go, and mount my antenna elsewhere
> away
> from the house. That way I can disconnect the coax, and move the ends
> apart
> 20' or so.
> Still, the tower on the roof idea has technical merit. Heck millions of
> folks had TV antenna's on their roofs before cable or satellite TV was
> popular. It was a rare installation that was properly grounded, and yet
> even
> rarer when there was a lightning strike to the antenna.
> 
>    I was 'googling' around on the topic, and I ran into this neat sight:
> 
> http://www.lightningrod.com/
> 
> This is for DIY lightning protection systems for the home, and they
> provide
> the parts, and guidelines on what you need. I see some Ham radio uses for
> these parts, like the 2 square foot copper ground plate instead of a
> ground
> rod. I notice that their 15/32" diameter copper wire is un-insulated.
> 
> 
> Now If I cut down that tree in the back that provides late afternoon shade
> for my neighbor (not for me), and also rains dead leaves into my pool, and
> replace it with a tower and an antenna....my neighbor's wife will lynch
> me!!! She had a fit last year when I trimmed that same tree (on my
> property), and trimmed a few branches of her tree that hanged over my
> property.  Both trees ar 40' plus Live Oak's. That was after I told her
> husband the day before what I was going to do. Apparently he isn't the one
> wearing pants in that household! :-)
> 
> Jim
> JKO
> 
> 
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net]On Behalf Of Mike Dorworth, K4XM
> Sent: Saturday, August 05, 2006 2:59 PM
> To: Discussion of AM Radio
> Subject: Re: [AMRadio] Antenna Idea and lightning precautions
> 
> 
> 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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