On Sun, 2008-04-13 at 22:17 -0500, Bob McGraw - K4TAX wrote:
> The advantage to the spark plug arrestor over the high value resistor is
> that the spark plug offers very high resistance until the arc is ignited
> then the effective value of R goes very low. Almost like putting a dead
> short on the line. Only for an instant or until the arc is extinguished.
> The gap method offers much better protection. High R values resistors stay
> the same value until they explode then they go higher.
>
> 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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