"Richard Harrison" wrote in message
...
Jerry wrote:
"I know about some articles on the ARRL site, but was wondering if
anyone else had some ideas or pointers on how to pratically do
this?"

Coax helps protect your radio from lightning. It rejects common-mode
currents inside which might otherwise damage the radio.

Thunderstorms often produce lightning from clouds charged to 100
million
volts with respect to the earth. Current may oscilate up to 200
thousand
amps in a lightning discharge. Temperature inside the stroke may
reach
30 thousand degrees C (5x the temperature of the sun`s surface).

A stroke starts and stops abruptly, so it contains r-f in addition
to
d-c. The discharge may take up to 150 milliseconds and consist of
several flashes in both directions. It may include a path
miles long, so it has a pretty good ionization trail for an
antenna.

If your antenna is struck by lightning, it is best to bypass the
energy
aroundb people and equipment.

Medium wave stations have arc-gaps around the tower base insulators,
Faraday screens between primary and secondary of tower r-f coupling
transformers, and tower lighting chokes which keep both r-f and
lightning out of the power mains.

High frequency stations often use balanced wire lines, and these
have an
arc-gap from each wire to the earth at a point outside the station.

VHF, UHF, and microwave stations use grounded antennas and coax.
Towers
which support the antenna generally have each tower leg separately
grounded by a heavy cable to its own ground rod near the tower base.
R-F
cables and waveguide are grounded at the antenna and at least at the
base of the tower. Coax nay be coiled with several turns between the
tower base and the shack to discourage lightning on the outside of
the
coax from entry to the shack. Waveguide is solidly bonded to the
tower
but not usually coiled to make a lightning choke.

The solid-state VHF, UHF, or microwave station often needs
additional
surge protection because of the difference in potential between
electric
service and antenna system grounds
This takes the form of husky r-f chokes in each power wire to the
r-f
equipment. Each choke is shunted at each end to ground with a
capacitor
and with a voltage limiting device or devices, often MOV`s. There
are
ready-made brute force coil and capacitor low-pass pi-filters which
need
only addition of MOV`s to make them effective lightning suppressors.
I
made mine in an earlier time using Miller Coil Company tower
lighting
chokes and they worked well. You could wind 2 or 3 dozen turns of
#12 or
#14 insulated wire in an 8-in. dia. circle to make your own 0.1
millihenry chokes. The standard choke used to be 2.5 millihenry, but
it
is not critical.

The same wiring techniques required for noise reduction apply to the
biggest noise of all, lightning.

Best regards, Richard Harrison, KB5WZI


An interesting fact that many don't know is that lightning actually
strkes upwards as the clouds are negatively charged.


--
Woody

harrogate2 at ntlworld dot com