Hi Don,
> Dropping lines down a pull or push-up is not feasible (I understand the
> reason for putting them inside). Suggestions for we who must use pull or
> push up towers?
At broadcast stations, properly designed installations have lightning
retarding loops in cables or feed wires whenever possible. The idea is to
add a high common-mode series impedance, since that makes any shunt path
through a MOV or gap look much more "attractive". Thousands of AM BC towers
sitting on insulators stay on the air despite direct hits, without anything
more than strategically placed air gaps and lightning retarding coils in
feedlines.
If I had a tower where control cables had to run outside the tower,
especially away from the face, I'd use some sort of cable with a shield
that could be grounded at the top and bottom. Lightning contains very
little steady state (dc) energy, most of the energy is in the rapidly
changing potential. That makes current inside a closed conductor almost
zero, as long as the shield is thicker than a few skin depths.
A nice solenoid coil of several turns a few inches in diameter at the
shield exit points would confine most of the energy to the shield.
MOV's certainly help, but we must consider how well they will *really*
handle hundreds or thousands of amperes for the duration of a lightning
stroke. IMO, they need all the help they can get. Damage is more affected
by grounding and lead dress than any tiny little components. With all this
worry about welding heavy ground connections so they don't come "unglued"
and insistence on large diameter grounding conductors, I wonder how many
MOV's are welded to the super large number 16 wires they "guard"?
By the way, I don't have a single MOV anywhere in my station except across
the power line and telco cables where they enter the room (at a common
point). I don't have a single gas tube across the feedlines. Despite
several hits (I have an insulated base 260 foot tower and an insulated base
200 foot tower on the highest point within several miles, you can see my
260 ft tower for miles) I haven't lost a single component that wasn't on an
actual antenna.
My cables (including control) are all shielded, and the shields are all
properly grounded. I also use lightning retarding loops in the cables, and
common sense. I unplug and disconnect power and antenna cables during
storms, despite false rumors they are best left connected to equipment.
73 Tom
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