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Re: [TowerTalk] Grounding Coax at Tower

To: "'K4SAV'" <>, <>
Subject: Re: [TowerTalk] Grounding Coax at Tower
From: "J. Gordon Beattie, Jr., W2TTT" <>
Date: Wed, 27 Feb 2008 19:34:04 -0000
List-post: <">>
I was reading this thread with some interest.
My tower is next to the house and third floor shack.
My cables come off the tower, to a ground plate that has the lightning
arrestors bolted to them inside the shack wall.  The tower and the plate are
bonded to each other all the way down the ground where there are five ground
rods also bonded together along the side of the house.  These include
grounds for the tower, utilities and an extra one for good measure well
spaced out.
I generally drop my coaxial feeds at the wall in stormy times, so I was
wondering what you guys think of this configuration?
I like the short coaxial feeds myself!
Gordon Beattie, W2TTT

-----Original Message-----
[] On Behalf Of K4SAV
Sent: Wednesday, February 27, 2008 7:00 PM
Subject: Re: [TowerTalk] Grounding Coax at Tower

Having the cables exit the tower above ground is a really bad idea.  
Voltage drop down the tower is very high during a strike.  Anything 
exiting the tower above ground will see a very large voltage impressed 
on the cable at the point it exits the tower.  That translates into very 
high currents on the cable.  Those currents go toward the shack instead 
of into the ground at the tower base.

Other effects:
1. The cable at the point of exit, looses its mutual inductance with the 
tower which increases the currents on the cable.

3.  The cable is now more suceptible to direct strikes.

4.  The cable is also more susceptible to induced currents due to the 
large loop area.

5.  There are no good points.

Jerry, K4SAV wrote:

>TT:The concept of having the coax and other lines leave the tower ABOVE
earth level, is alien to me.   I've always felt that the capacitance of the
coax bundle to earth would tend to limit a strike's current in the direction
of the shack, in favor of the lower impedance tower connection.HOWEVER,  you
COULD have the coax leave the tower at 10' or so,  making a solid connection
to the tower at that level, and letting the tower be the conductor to the
earth ground system.   The problem with flying the cable bundle, however,
would seem to be twofold.   1)  reduced capacitance to earth, due to
increased distance, and therefore reduced strike current along the bundle;
and 2) having the flown cables act as a larger loop, in the event of a
nearby strike.   In the latter case, there would be greater current induced
in the secondary turn of the 1 turn transformer represented by the
tower-cable system.  Does anyone see a flaw in that thinking?N2EA
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