Re[2]: [TowerTalk] Lightning GROUND switch wanted RE:DE K0FF

[Tom Rauch]

> the storm.  Any ideas from the group to minimize problems??  One thing
> Polyphaser suggested was to side-mount a 2M antenna on a tower, and put a
> sort of lightning rod above and below the antenna that sticks out father
> than the antenna itself.  Doing this would not affect the performance of
> the antenna, and would allow a "hit" to go to ground via the tower legs. 

The biggest single problem is always common mode current.

Common mode current is not stopped at all by any of that advice. 
The only that advice will do is greatly reduce chances the antenna 
is damaged by a direct hit to the antenna.

Side mounting the antenna below the tower top a distance equal to 
three or more "spacing to tower" distances will do all you need. If 
you really want damage immunity, use a dB products dipole array 
on an aluminum mast or something similar, bracket it to the tower 
at the top and bottom, and run the feedline down the inside of the 
tower.  

> Polyphaser explains that a lighting strike hits in an arc radius of about
> 150', and if the lightning rods above and below the side mounted antenna
> protrude at least 6" beyond the antenna, the antenna has a 96% chance it
> will not be struck by a direct hit.

Something is missing from that data. What if I have a 100' long 
antenna spaced 20 feet from the tower? Would that 6" protrusion 
beyond the antenna make that antenna "96% less likely to get 
hit"??  What about a two inch long antenna spaced 1/2"  from the 
tower? Now the extra six inches completely hides the antenna!

See my point? Something is missing in that "equation".
It would be interesting to see the formula or statistical data that 
predicts 96% reliability.

The key to staying on the air, once you reduce the odds of the 
antenna being vaporized, is grounding and layout of the system. I 
had dozens and dozens of repeaters on towers, and never had a 
single failure from lightning hits without a single lightning arrestor in 
line anywhere because cables and equipment were grounded 
properly and routed properly.

Think about an AM radio station. They almost always survive direct 
hits on towers, which actually ARE the antennas, and in the 80's 
an earlier no one had any idea what an "in-line" RF  gas-tube 
lightning arrestor was! Their idea of an arrestor was a ball gap at 
the tower base, a lightning impeding single turn loop of copper 
tubing, a gap at the feedline or tuning unit, and another gap at the 
transmitter.

The key to success is layout and grounding, far more than 
anything else you can do. I'm not saying other measures don't 
help, just that it's a good idea to take care of major issues that are 
most likely to be problems first.

The most likely problem is a ground loop through your equipment 
from the power lines and telco lines to your tower and antenna and 
ground system. Make sure you correct that path first.


73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com