Jim, NN7K wrote:
"When lightning strikes, it is going where ever it wants to, in spite of
the best grounding intentions."

Yes, the trick is to send it to where you would rather it goes. You do
this with your grounding techniques.

The coax itself is important protection against lightning. Opposition
within the coax to lightning is severe. Lightning is a common-mode
phenomenon. Lightning takes the easy path on the outside of the coax.
All coax has a flashover potential which depends on its spacing and the
dielectric of the coax. Once it flashes (not far within the coax) the
voltage is clamped to the sustaining potential of the arc. This is a
steady low potential with a very high conductance in the arc.

Lightning on the outside of the coax must be properly diverted to ground
to avoid damage.
A few twists and turns in the coax increase its opposition to the
lightning (a transient) on the outside of the coax. Such a coil is
advisable between the antenna support and your operating position. Let
the antenna tower carry the lightning to the earth.

Best regards, Richard Harrison, KB5WZI

Richard- the point I make-and the company I worked for, until I retired
on disability- has Thousands of base stations- and yet still sustains
major damage every year-- they have a manual on grounding, and a 2 week
program on grounding/insulateing and , in fact have so many grounds that
the wonder is they dont have major problems with ground loops! That
company is Union Pacific Railroad. If you can find a employee of the
communications dept, you might ask to borrow a copy of it. It is
interesting to read, just for information's sake. One thing that I
DIDN'T point out (as it has no bearing to grounding antennas an a ham
(or normal business) environment, is that because of Federal Railroad
Administration rules, that in many applications (primararily involveing
switching equipment), that any GROUNDING is strictly prohibited! As
most signals are now either radio controlled, or soon will be (the old
open wire "Code Line" is being rapidly retired), this brings new meaning
to ISOLATION for the radios/antenna , via the RS-232 bus to the Coder
logic-- a forest, if you will, of Opto-Couplers at each site. This also
applies to "Talking hot box,dragging equiptment, weight in motion, and
Automatic Car Identification eqpt! Even where phone cables cross
under/over the tracks brings new meaning to arcing in a lightning storm.
Had one here in the yard, that was a dead cable (cut off at both ends)
when lightning struck (not even anywhere near the cable/tracks) you
could see arcing between the pairs (talk about induction!) The length of
that cable was about 500 feet! The protectors that U.P. recommends (and
buys by the hundreds is poly-phasor , tho for my own use like the Alpha
Deltas. The Alpha-Deltas use standard gas protectors as are in
telephone circuits, and break down at 200, and 300 volt levels- while the
polyphasor measure a wide open at the center conductors of your 2 pieces
of coax The difference is the Alpha Delta is a relatively broad band
device, while there are several different models (depending on freq. for
the Poly-phasors. The point that I was trying to make (perhaps poorly),
is :1) WHEN you sustain a DIRECT HIT, something is going to happen - and
that wont be good news, except to the parts jobbers/manufacturers and 2)
Even a nearby strike can leave great damage if no preventive measurs
have been taking to isolate the antenna from the radio. If you open the
preamp of a 150 MHz motrac, you see a neon lamp-- that was for
protection of a nearby strike, but for a direct hit-- doesnt do a lot of
good! as info, Jim NN7K




Richard Harrison wrote:

Yes, the trick is to send it to where you would rather it goes. You do
this with your grounding techniques.

The coax itself is important protection against lightning. Opposition
within the coax to lightning is severe. Lightning is a common-mode
phenomenon. Lightning takes the easy path on the outside of the coax.
All coax has a flashover potential which depends on its spacing and the
dielectric of the coax. Once it flashes (not far within the coax) the
voltage is clamped to the sustaining potential of the arc. This is a
steady low potential with a very high conductance in the arc.

Lightning on the outside of the coax must be properly diverted to ground
to avoid damage.
A few twists and turns in the coax increase its opposition to the
lightning (a transient) on the outside of the coax. Such a coil is
advisable between the antenna support and your operating position. Let
the antenna tower carry the lightning to the earth.

Best regards, Richard Harrison, KB5WZI