Earl, K6SE:
> 3) Because the tower is grounded, a lightning strike is conducted
> directly to ground, thus eliminating damage to anything (my tower in
> Houston was hit many times with absolutely no ill effects. One of my
> towers here in the desert was hit about 20 years ago with no damage
> whatsoever).
Per NASA commissioned study back in the 80's, most lightning strikes are
from the ground up. Two metal poles were erected a short distance apart,
one grounded, the other base-insulated. The grounded one always received
the strikes, indifferent to which pole was grounded..
Apparently, when a positive cloud comes over, a grounded structure draws
from the infinite free electron bank, the ground, and becomes highly
negatively charged. Ionized "chains" form in the air between cloud and
tower top. (sharp points encourage this) When the ends of the ionized
chains are within 35-50 feet of each other, there is enough EMF to jump the
gap, and lightning "strikes" the tower, up to 150 separate discharges in
less than a second.
While visiting the late NS5M, Rod, I asked the function of a big knife
switch at the left end of his desk. The switch isolated his tower from
ground and he claimed NO strikes after installation. Of course he also
disconnected his coax from rig. (I disconnect it at tower base.)
Two years ago I supported my Rohn 25G tower base with a 2 X 8, hacksawed 8
inches out of the legs, inserted three solid fiberglass rod insulators, and
bolted them in. There have been NO strikes to the 80 ft tower since. Last
week a bolt struck an 80 ft tall oak tree 150 ft from the tower. If you
have a beam and rotor on your tower, you do NOT want a lightning strike!
The series-fed, top loaded tower has large 2.0:1bandwidth on 160m, works on
80m (3.2:1) with antenna tuner, for 40 zones, and seems immune to lightning
strikes. No capacitors, no coils, no relay switches, no hassle, just coax
feeder and a toroid balun........what more could a low-bander want?
73, Doug / NX4D
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