Notebook Series - Surge Protection for Open-wire Feed-lines
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. Remember, not much will protect you from a direct hit, so disconnect!
[ Austin, TX ] 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 Accel high performance, non-resistor 274s. (They were still available on the Internet in December of 2011.) The box indicated they're a replacement for the following:
43TS or 42TS
AF32B or AF22B
14 or 13
BL9Y or BL11
(Note: I didn't verify that these plugs are exactly the same as the Accel 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. There are other replacements listed on
the Internet, but buyer beware.)
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.
ex: WPE8EUM, WN8AQL, WB5FCO and WJ5MH