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Re: antenna rig and Lightning



> Date:          Thu, 27 May 1999 23:47:34 +0100
> To:            davemetz@muscanet.com
> Cc:            amsat-bb@AMSAT.Org
> From:          John Charles <m1btr@midmarsh.demon.co.uk>
> Subject:       Re: [amsat-bb] antenna rig and Lightning


> 
> Hope you both stay lucky David.

So do I!

> A year back our local 70cm repeater site took a strike. We never did
> find one aerial, just the metal bottom of the fibreglass colinear which
> was peeled back like a banana skin. Didn't do the rig a lot of good
> either ;)

I had the same thing happed to my 70cm repeater.  Its the UHF
"hub" for a 2M linked repeater system.  As an experiment the club
that supports the repeater purchased a Comet dual band colinear.
The idea was to run both the 70cm repeater and the area 2M repeater
off the same coax and antenna using a diplexer.

Much to my surprise, it worked!  The problem is the construction of the 
fiber glass antenna.  It is not a grounded design and lasted exactly 12 hours 
on the air.  That night it got hit by lighting and turned into what we all called
the "electric nose picker :-)"  We found pieces of it 40' away.  I've heard
several other reports of this type of fiber glass colinear taking hits and
serious damage being the result.

I decided to stop messing around and replaced it the next weekend with
a Antenna Spec. commercial UHF antenna.  It cost me $900!  Note that
this design has a large grounded copper ball on the top.  Since then
(8 years ago) we have not touched the antenna system at the hub.  I'm
sure that it has taken several strikes since then.

The bottom line is that for many of us disconnecting coax constantly is
not an option.  You are either going to forget to do it, or not be around
when the storm comes.  People make mistakes....  Its better to engineer
in a good ground system and use grounded antennas.

You could also take a page from the broadcast engineers handbook
and install a lighting suppression array on your tower.  I idea is to provide
a large number of sharp metal points on the top of the tower.  These act
as static discharge points to bleed off the local static charge.  Thus the
earth to sky potential never reaches the level where lighting can occur.
Some of the discharge arrays look like nothing more then strings of 
barbed wire!  

In my old AM/FM broadcast engineer days I use to sit under the eves
of the radio station and watch my tower get hit after hit by lighting.  We just
kept on transmitting and rarely suffered any damage.  When we did
have damage, it was usually something like the copying machine or
phones, never the broadcast equipment.

--73-- David WA0AUQ

> 
> All my cables have a join (N or F type depending on use, even 3 pin
> types for rotator cables) where they enter the property and I have just
> undone them all with electrical storms forecast for tonight. The plugs
> on the end of each incoming lead then plug into a metal box (full of
> skts with all pins grounded) which has a hefty cable to my earth spikes.
> May not help a lot with a direct hit but should hopefully help a lot to
> dissipate static build up, especially on my beams which have isolated
> driven elements.

The isolated driven elements of the beams can be DC grounded through
RF chokes.  The idea is to keep potentials from building.  In the "good old
days" most HF transmitters had a large RF choke from the antenna connector
to ground just for this reason.
> 
> I've been cautious ever since a friend of mine had a rig 'fried' by
> induced voltages from a near miss.
> 
> 73  John  M1BTR  (Lincolnshire, England)
> -- 
>   m1btr@midmarsh.demon.co.uk         M1BTR@GB7SKG
>   QRV 6m 2m 70cm 3cm   Member: BATC RIG RSGB UKRS


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