> If you want to do it right, go to this link. I make my living as a
> two-way land mobile dealer and tower operator. We use polyphaser
> products exclusively, and have had virtually no lightning damage,
> other than to antennas, in 20 years.
Hi Ron and all,
I have a slightly different perspective on this. While I agree the best
practice is to disconnect antennas, the real secret is in cable
routing and grounding, not in lighting suppressors across shielded
cables.
While light protectors certainly do no harm with low power, I often
see equipment damage from use of lightning suppression devices
with kilowatt HF amplifiers. All it takes is a momentary transient to
fire a gas tube, and the coax is effectively shorted. I've also seen
"pills" break down and fail at lower than designed voltages. Any
momentary flash-over can damage bandswitches and other
expensive components if you happen to be transmitting when the
fault occurs.
For example, a high dipole I have will occasionally (even in clear
weather) arc a gap that is 1/10th of an inch from charge buildup.
Same with my insulated towers.
If I happen to be transmitting when the gap fires, the RF sustains
the arc and the feedline is effectively shorted. A gas tube would do
the same. The cure of course is RF chokes across the cables, but
I wonder how many people just stick a gas discharge tube device
across a feedline on an antenna without a dc ground path for static
drain??
I don't see many web sites warning about that!
To safely handle a typical 1500 watt amplifier into a 2:1 SWR and
allowing for tolerances and aging, gas tubes must generally be
rated to hold-off over 800 volts. Voltages well below 800 volts will
zap almost any front end, so these little gas discharge tubes result
in a false sense of security....which is often worse than being
paranoid enough to remove cables and go off the air!
Remember, if the arrestor lets 1500 watts out it will certainly allow
more than 1500 watts back into the front end!
My antenna layout has miles of feedlines and control cables, and
the tallest tower is 318 feet high. With so many control lines and
cables, it is impossible to disconnect antennas and control cables.
I don't use a single lightning suppression device in my transmitting
or receiving antenna systems, other than occasionally using a
good old fashioned spark gap (like at the feedpoint and resistor end
of Beverages). Same with repeaters here. The sum total of damage
to my system has been a few small transistors that directly tie to
the control cables (which are up to a mile long and are *never"
disconnected), and of course I've had some cables shields melted
from direct hits.
I bury all feedlines and control cables (which are shielded cables),
have very good radial systems on all my tall towers, and use RCS-
8V antenna switches to isolate antennas when the power is off. I
use single-point grounding.
The vast majority of lightning damage comes from common-mode
currents that almost exclusively flow on the shield of cables. If a
person grounds shields properly, and follows advice on polyphaser
and other web sites (or better yet in the National Association of
Broadcasters handbook) regarding grounding and cable routing,
99% of the battle is won.
Of course the very best thing is to disconnect outside the building
if you can, but the real key to protection is in grounding and cable
routing.
73, Tom W8JI
W8JI@contesting.com
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