[Towertalk] Air Terminal Placement

[Tom Rauch]

Hi Jeff,

I'm mostly echoing what others have said.

> I intend to put an aluminum air terminal at the top of my 100 foot
> tower to help reduce localized charge accumulation and hopefully lower
> the possibility of a direct lightning strike. I have an omni at the

Be sure to include a rabbit's foot, it will probably reduce the 
chances of a lightning hit as much as the "air terminal"...and costs 
less (except to the rabbit).

> top with a fiberglass radome that extends 11 feet above the tower.  Do
> I need to get the tip of the air terminal some distance above tip of
> the omni before it will do any good?  I guess my concern is that
> charge accumulation on the fiberglass omni might provide a conduction
> path, effectively making my air terminal useless if the tip of the
> terminal is below the tip of the omni.

Like other person said, the only way to protect a fiberglass antenna 
is to mount it below the tower top or below a taller metal mast.

Of course if you mount a metal rod along side the fiberglass antenna 
or if you mount the wire brush up there and run a ground lead down, 
the antenna pattern and performance will be affected.

The problem is not "charge accumulation", it is voltage gradient 
between the cloud and earth. There really is not a sure way to reduce 
chances of a hit other than lowering the antenna.

I have a metal mast 4-bay dipole sticking 20 feet above the top of my 
300 foot tower, and it takes lightning hits just fine. I'd suggest 
you use your fiberglass antenna as is, and if it ever gets hit 
replace it with a metal antenna like a multi-bay dipole.73, Tom W8JI
W8JI@contesting.com