[TowerTalk] Antenna/Tower Grounding (Lightning Protection)

[Jim Lux]

At 06:42 PM 1/3/2007, Richard W. Solomon wrote:
>Looking for advice on what to use to protect the Tower, Antennas and
>Rigs from lightning strikes. I guess I need Tower Ground(s) and some
>kind of Coax grounding that can survive.
>I looked at the Polyphaser site, but unless you know the Model Number,
>it could be a long night.
>
>What would you suggest ?
>
>Thanks,
>
>73, Dick, W1KSZ


#1.. decide what your risk strategy is, ranging from
Most Conservative: Must operate with 5 9's availability, even after 
receiving multiple direct hits from positive 300kA strokes.
to
Least Conservative: Shrapnel from failed components doesn't fly more 
than 1000 ft from the system.

Realistically, you'll be somewhere in between. It's important to 
figure out, though, what you want to protect against.  Is it a direct 
strike?  Is it induced pulses from nearby strikes, etc.?

And then, you need to figure out how conscientious you are (or want 
to be) about things like disconnecting coax.  If you're like me, 
there will be times you forget or get busy, and wouldn't you know it, 
it's when you ran outside to cover up the furniture so the rain doesn't get it.

part of this is assessing the liklihood of the event.  Here in 
Southern California, you would probably be better off investing in 
seismic strapdowns and such than protecting against a direct hit by 
lightning.  Most overvoltage impulses around here are from things 
like power line crosses onto phone and other wiring or MV 
distribution falling on LV lines, often from the wind blowing the wires down.


#2... decide how much you want to spend on lightning/impulse 
protection vs what the equipment is worth vs getting a decent 
insurance policy (say, via ARRL).  No point in spending $10K on 
lightning protection to protect a $1000 rig. (I've seen it done, 
though, in a remote telemetry system that needed 24/7 kind of 
availability.. the cost of the protected equipment was insignificant 
in comparison to the value of having the data.)  What do you want to 
survive?  Is it ok if the coax is damaged, but the rig 
survives?  What's your "time/money to repair" situation? Can you 
afford to wait til the insurance company cuts you a check, and the 
new stuff arrives?
(The insurance policy might require some protective measures, too.)


Recognize that advice you get from others (including me) is colored 
by previous experience and the advisor's personal risk acceptance and 
required availability/reliability strategy.
What's reasonable and appropriate for an Airport Traffic Control 
Tower might not be so reasonable for casual HF operating.

#3.. look at the regulatory requirements (the "code"), but those are 
actually fairly easy to meet and are more for electrical safety than 
lightning protection.  The coax shield can serve as the required 
grounding bonding conductor for instance.  The Mike Holt "Low Voltage 
Handbook" (free for downloading on the Mike Holt website) covers all 
this stuff quite nicely.


Then, have at it.

Most likely, some sort of transient suppressor at the Coax entry, 
with a good grounding system, and disconnecting the coax when you're 
not on the air, would be all you need.  In this scheme, stuff 
"outside" is essentially sacrificial in the event of a direct hit.

Jim, W6RMK