I recommend that anyone interested in this subject get a copy of
Polyphaser's "The Grounds for Lightning and EMP Protection". Of course, the
case presented is self-serving, but there's a lot of good information in the
book.
There seem to be three schools of thought -- 1) disconnect everything, 2)
try to prevent a strike by mucking with the corona, 3) try to dissipate a
strike. Some advocate combinations of these approaches (i.e., one solution
would be to disconnect all cables, use porcupines at the top of masts, and
use lightning arrestors connected to a superior ground system.) But due to
the random nature of lighting strikes (i.e., the impossibility of conducting
reproducible tests), I doubt that this debate will ever be resolved.
In the ham community, all we seem to be able to do is report anecdotal
evidence. You will find many hams who flat out refuse to consider any
solution other than disconnecting the cables. You will find others who say
that they experienced lots of damage until they installed their porcupines
and/or lightning arrestors. Seems as though few, if any, of those in the
dissipation camp have installed one of Polyphaser's strike counters to
verify that they've been struck and that the charge has been dissipated (and
then one couldn't measure the power of the strike anyway.)
I, for one, haven't heard many stories about commercial B/C stations with
proper lightning protection being knocked off the air, especially in my
local area. The ones I have heard concern towers at the tops of mountains
where it is difficult to install a proper ground system.
I opted for the full Polyphaser solution -- surge protectors at both ends of
every condutor coming into the shack, including coax, rotor, control,
telephone, 110VAC, 220VAC, etc., with a single point ground connected by a
heavy buried conductor to an extensive ground system at the base of the
tower (12 rods bonded with heavy Cadwelded radial wires, plus the 240-radial
field for the 4-square.)
Why? Well, I took a look at what it would take to fully disconnect all those
wires every time a thunder storm came into the area. There are literally
dozens of signal wires, low power conductors, AC plugs, and ground wires in
my shack. Even with quick-disconnect connectors, it would take a
considerable amount of time to do it -- and I don't want to have my hands on
a metal chassis or partially disconnected wire when lightning is striking in
the area! Also, it makes no sense to disconnect those wires in the shack --
you don't want lightning coming in the house and jumping from those
disconnected cables to anything that's grounded. A freind of mine with a
100' tower uses the disconnect method and last year he heard arcing in a
closet where a bundle of cables comes into the house. Evidently, the surge
was jumping from a cable shield to some grounded piece of metal (maybe a
water pipe.) I think you have to disconnect the wires outside and pull them
well away from the house. And then there are the Telco, cable TV, and AC
power conductors that enter your house... do you disconnect them too?
Outside the house?
The disconnect method only works if 1) you do it religiously, 2) you can do
it well in advance of lightning getting close enough to induce surges (have
you ever been surprised by an unforseen lightning bolt?), 3) you do it
outside the house, and 4) you can be absolutely sure that you got all the
cables disconnected (there have been many stories about hams forgeting that
one cable -- like the phone line to the modem -- with disasterous results.)
There were a few other reasons I went with the Polyphase solution. First,
even if you disconnect everything, there is still a possibility that
"floating" coax can be damaged by a surge. I think the danger is that the
charge can jump from the coax to a nearby grounded structure, such as the
tower. Apparently, this results in small, invisible pinholes burned in the
jacket. Replacing a coax run going up the tower isn't my idea of fun.
Second, I figured that doing what commerical B/C stations do (or as close as
I can come to it), made the most sense. Third, I wanted a system that might
be effective at draining off unpredictible static charges (so far, haven't
had any problem with that.)
Most hams say that nothing will protect you from a direct hit. Ployphaser
claims that a proper installation can be protected. I really don't know. I
guess I'm relying on the relatively low probability of a catastophic direct
hit (especially where I live in inland New England.) However, I do believe
that a properly installed lightning protection system can be effective
against the far higher probability of strong surges induced by nearby
lightning strikes. Those strikes may not burn your house down, but they are
quite capable of damaging delicate electronic equipment.
By the way, all sorts of electronic gear in my house, especially phones,
used to beep from the surges everytime we had a storm. Since installing my
improved ground and protection system, they don't do that anymore. This
weekend, we had electrical storms in the area during CQ WPX CW. I could see
the lightning storms coming over the western hills in the distance as I
operated the contest. I heard no clicks or pops in the radio when I saw very
big strikes. Of course, once the clouds got within a few miles of my house,
I shut down and moved away from the radio room. I did pull the main coax to
the amp and the rotor controller (because it was easy), but these were just
short jumpers from the surge protectors at the single-point ground. I doubt
that this would have been enough to avert damage (because of all the other
conductors), but it gave me peace of mind to disconnect the two cables that
come directly from the tower. In retrospect, I probably should have left
them connected to avoid danger to myself while in the process of
disconnecting them.
I'll let you all know if I ever have any equipment damage at my system.
73, Dick, WC1M
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|