[AMRadio] Antenna Idea and lightning precautions


Gary Schafer garyschafer at comcast.net
Mon Aug 7 12:37:21 EDT 2006


Hi Mike,

While the sharp pointed rods are ok at the top of the towers to try and make
them the place where lightning strikes first, the insulated down lead is not
a good idea.
It is better to connect the lightning rod directly to the tower. The tower
will have much less inductance than the down lead that is supposed to carry
the strike current. The chances of an arc over from the down lead to the
tower are great because of the high inductance of the lead. Voltages can
become very high on it.

Also the down lead will induce current into the tower and coax lines running
down the tower anyway. So there is nothing to be gained by the insulated
down lead except higher cost and the invitation to an arc somewhere you
don't want it. 
This is why it is recommended for all cables on the tower to be bonded at
several intervals along the length of the tower, to prevent voltage
differences between cables and tower.

A ball or rounded end type lightning rod does not bleed off energy but
prevents corona so that early streamers do not form as they do from pointed
rods.

A "real strike" is survivable with no damage if things are properly
protected and grounded. It happens many times on many many installations.
There is always the possibility for damage to occur but it is rare on good
installations.

73
Gary  K4FMX


> -----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
> > --
> > No virus found in this outgoing message.
> > Checked by AVG Free Edition.
> > Version: 7.1.394 / Virus Database: 268.10.7/409 - Release Date: 8/4/2006
> >
> > ______________________________________________________________
> > AMRadio mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/amradio
> > Help: http://mailman.qth.net/mmfaq.html
> > Post: mailto:AMRadio at mailman.qth.net
> > AMfone Website: http://www.amfone.net
> > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb
> >
> 
> ______________________________________________________________
> AMRadio mailing list
> Home: http://mailman.qth.net/mailman/listinfo/amradio
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:AMRadio at mailman.qth.net
> AMfone Website: http://www.amfone.net
> AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb





More information about the AMRadio mailing list

This page last updated 12 Dec 2017.