Forget about trying to protect from a direct lightning strike. That is not
what I am saying or implying. Simply too much energy to dissipate. Won't
happen. I've seen sections of Rohn 45G from a single tower and strike
burned in to multiple pieces with the leg ends balled as a result of heat
that actually melted the metal. Thus they burned and separated. The
separations did not occur at the joints but in mid section. The tower came
down. Also the original 500 ft self supporting tower at WSM-TV in
Nashville took a hit of such intensity that one leg, being a 10" x 10"
angel steel some 3/4" thick, bowed due to heat. Fortunately the tower didn't
fall and was repaired. So the #10 or #12 or #14 wire that we use for
antennas and feedlines will simply vaporize under these conditions. And
this needs to take place OUTSIDE of your house.
The idea with the spark plug is to use the "small engine" type plug not
automotive plugs. There is a difference. Most small engine plugs do not
have series resistors thus they are suitable for dissipating static
electricity, the result of near-by lightning strikes or static buildup from
wind or rain or snow. This can induce several thousand volts on the antenna
which can be dissipated by the use of a spark gap made from a small engine
spark plug.
Of course commercial systems such as those by Polyphasor work great and are
highly suggested. This assumes your budget will tolerate such and that they
are correctly installed. With hard-line for feed line, add the cost of 2
more connectors in addition to the Polyphasor. This then gets to be about
$150 for lightning protection for each coax.
For the "do it yourselfer" the spark plug approach works for balanced
feedlines and can be done for under $10. If you wish to buy ready to
install, these are available from The Wireman for $24.95.
http://thewireman.com/ground.html#878
73
Bob, K4TAX
An application hazard with spark plugs for the gap is that most plugs
sold today have series resistors, often 5 or 10K to minimize radio
interference from the running engine.
Then the plug is made to handle a few amps. Maybe a 14 gauge but steel
conductor on the ground side of the gap. A direct lightning hit tends to
run a kiloamp to ten kiloamps. The plug won't stand that. For that
matter, wire antennas and feed lines tend to not survive direct hits,
especially feed lines.
A little noticed rating for resistors is voltage. Not voltage based on
power dissipation, but voltage based on internal arc over. 1/2 watt
resistors are often rated at 250 volts. Unfortunately depending on
resistor construction and available energy that internal arc can either
short the resistor with a carbon path or blow it into two pieces.
The benefit of resistive grounding is that static charge is dissipated
before the voltage rises to the arc over point of the spark gap
protection. And so receiving is quieter until there's nearby lightning.
Polyphasors are expensive, but repeaters with Polyphasors on power and
antenna leads have run for decades without lightning damage. Polyphasors
have lower breakdown voltages than practical air gaps, but can handle
quite a bit of current. That lower breakdown voltage protects the
equipment much better than air gaps.
I bring all my antennas to a patch panel with all the coaxes on type C
connectors. (Large bayonet). Its grounded outside. I unhook all
feedlines when there are storms in the neighborhood. The tower takes a
couple direct hits a year. And some of the equipment has been damaged
when I didn't maintain a large enough air gap between the outside
antenna wiring and the inside wiring.
It is possible to develop good grounding and Polyphasor protection for
the radio station, part of which involves tying power, phone, and
antenna grounds together with a few feet of 12 or 18" wide copper strap
and running all coaxes and power wiring through Polyphasors.
73, Jerry, K0CQ
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