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Here are my notes on lightning protection for open-wire
feed-lines. This is the original article. It was picked up by ARRL (QST magazine) and
published under HINTS & KINKS -- Notes on Lightning Protection for Open-wire Feed Lines --
WJ5MH.
Reading about lightning, and its disastrous results,
caused me to look into ways of improving my ground system around the house. Because various
publications already adequately explain the grounding of towers, coax fed antenna systems and radio
room equipment, I won't cover it here, but I will write about protection for open-wire or twin-lead
feeders.
 The ARRL Antenna Book has a couple ideas for feeder protection, but
I had one of my own I wanted to experiment with. Since the automobile spark plug is nothing
more than a spark gap, why not use them for feeder protection? They're cheap, and the spark gap
is adjustable.
Out of all the spark plugs available, how does one make the correct choice? Since this was my
experiment, I set the selection criteria based entirely on intuition. (Little technical theory
was actually used to implement this idea.)
First, the spark plug had to be the non-resister type. (Yes, most plugs have a 5 K ohm internal
resistance in series with the main conductor.) Finding a non-resister plug wasn't easy.
Some clearly indicate in the part number (with an R) that it's a resistive plug. Others do
not. Just remember that non-resister types are for older model vehicles.
Second, the spark plug needed to have a threaded tip, so the feeder could be secured with a nut and
lock washer. Without a threaded tip, some other way of connecting the feeder would have to be
devised, and it probably would not be a solid connection.
I actually found the plugs I needed by accident. (Salesmen tried to help, but didn't seem to be
very knowledgeable about their stock. Their first question was, "What kind of car is it for?"
Once I told them it was for an antenna system, the conversation quickly went down hill.) I
happened to look up over the counter and saw a clearly labeled box. Turns out it contained
exactly what I needed.
 The plugs are Japanese AC-Cel high performance, non-resistor
274s. The box indicated they're a replacement for the following:
AC Delco
43TS or 42TS
Old Autolite
AF32B or AF22B
New Autolite
14 or 13
Champion
BL9Y or BL11
(Note: I didn't verify that these plugs are exactly the same as the AC-Cel 274s, so if you
substitute, bring a multi-meter and check for a resistor. Also, the top brass looking connector
should unscrew to reveal the small 4mm threads. The spark plug base should be 14mm.)
My next step was to find a nut to fit the 14mm base of the spark plug. Not an easy task.
The local nut and bolt company came close, but couldn't match the fine threads. I even went
back to the auto parts store, but no nuts were found. However, I stumbled across something
called a "spark plug non-fouler." Not being an auto mechanic, I didn't have a clue as to its
use, but the device fit perfectly on the base of the plug. They come in different depths, so
make sure you get the correct one. These little devices not only secure the spark plug to the
ground buss, they also cover the exposed spark gap, and protect it.
Next, I took a spark plug to the local hardware store and matched a metric nut for securing the
feeder to the top of the plug. A 4mm .7 nut was a perfect fit. (Note: July 17, 2009 - I
found a wing nut at Lowes that fits the spark plug tip. It's manufactured by Hillman and the
part number is M4-.70.)
Before mounting the spark plug to the ground buss, I needed to adjust the gap for best
performance. (I didn't want RF jumping the gap when the transmitter was keyed.) I came
across an article on the WEB written by a broadcast engineer. His research on spark gaps for AM
broadcast towers indicated a need for .029" spacing for a KW station, and .045" spacing for 2.5
KWs. Since I normally run less than 100 watts output, I opted to set the spark plug gap to
.025" (.635mm).
To complete the project, I drilled 9/16" holes in the 1/4" ground buss, and mounted the spark plugs
using a liberal amount of OX-Gard, an anti-oxidant compound for wire connections. I then
grounded the buss to the system ground.
Running 100 watts output has not resulted in any arcing or intermittent changes in reflected
power. No attempt was made to check impedance changes in the line resulting from the addition
of the spark gaps. As expected, very, very minor adjustments had to be made with the antenna
tuner to re-match the antenna system, and this may have been cause by reducing the overall length of
the feeder by 3 feet.
73,
Joe ( AJ8MH )
aj8mh@arrl.net
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