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Re: Lightening and mast mounted preamps



The thread of lightning protection comes up quite a bit on the Towertalk
reflector (towertalk@contesting.com).  There's a LOT that many hams do not
do.

First off, the idea of disconnecting coax from rigs is subject to debate.
Lightning will find the shortest path to earth (note, I did not say
"ground") and sometimes that could literally be an arc through the air to
some other piece of equipment.  That can happen when you disconnect.

The safest and best thing to do is have a surge arrestor prior to the
preamps.  Additionally, since most VHF antennas are DC grounded (this
helps), ground the shield of the coax to the tower at the top of the tower
and at the bottom of the tower.  This way, the tower helps to shunt some of
the strike current to earth.

The tower itself needs to be grounded on each leg with a network of 8 foot
ground rods.  The rods extend radially from each leg of the tower every 16
feet.  Connect the rods in each leg with #4 copper wire or better yet,
copper strap.  The tower and the surge arrestors all need (ideally) to come
to a single point prior to entering the house.  This is ideally next to the
electrical service ground.

ALL GROUNDS MUST BE CONNECTED!!!  Including electrical ground to the
tower!!!

For the surge supressors themselves, Polyphaser and ICE both make very good
units.  The ICE units are a little cheaper and are of an excellent design,
but not waterproof.  Polyphaser makes some waterproof units you would not
need to put in a box.

A good web site for lightning info and examples is Bill Hider's (N3RR) site:

http://users.erols.com/n3rr

I learned a tremendous amount from talking to Bill.

Pholyphaser also has a set of must read notes on their website:
www.polyphaser.com.

Grounding for lightning protection is not trivial.  Roof mounted tripods are
an additional challenge.  I'll admit, while I am fully protected on my 55
foot tower, my satellite tripod on my roof has more work left.  Hopefully
this spring.

Good protection will cost a few hundred dollars for all the antennas.
Considering the cost of your home and equipment, having the right
suppression techniques will save you a heck of a lot of money and grief!
Don't be cheap when it comes to proper protection.  It's HUGE area where
hams like to scrimp, but it's just not worth it.

73,

Jon
NA9D


on 3/8/03 3:55 PM, John Geiger at johngeig@yahoo.com wrote:

> I have a question that I have not seen addressed
> before.  Asked my friend Don, NL7CO, who is a big VHF
> operator, and he didn't know but said put it on the
> reflector, so here it is.
> 
> We generally unhook our antennas during thunderstorms
> because a nearby lightening strike can cause enough
> energy to burn out the receiver's front end-or at
> least important components in the front end, like the
> RF amp.  What happens with mast mounted preamps?  You
> usually cannot unhook the coax from them during a
> thunderstorm.  Do they have some sort of protection
> device built in to handle surges down the coax?
> 
> WOuld be interested to know, as we get pretty good
> thunderstorms around here.

-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Citizen of the People's Democratic Republik of Illinois

Life Member: ARRL, NRA
Member:  AMSAT, DXCC

http://www.qsl.net/na9d   <- Updated on 1/22/03!!!

"A life lived in fear is a life half lived."


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