[AMPS] Spark-plug gaps

[Tom Rauch]

> I have a nice lightning-arrestor for parallel open-wire transmission-line
> that I bought from (I think) Press Jones, "The Wireman."  This device uses
> a pair of Champion spark plugs, one for each wire, for spark gaps.
> Assuming that I know the RMS RF voltage at this point on on my
> transmission line (between each wire and ground), how should I set the
> spark-plug gaps so that they won't arc over during normal operation with a
> high-power amp? (See, this question does belong on AMPS.)

My other hobby is cars and racing. So I got to see a lot of spark 
plugs over the years. Virtually all spark plugs have internal gaps 
called "booster gaps" or "anti-fouling gaps". These gaps set an 
internal voltage breakdown at many thousands of volts, so if the 
plug gets "wet" the internal gap allows the ignition system to build 
up voltage and energy before it fires across the gap.

After the plug fires across the internal gap, enough current flows to 
develop enough voltage across the tip gap to fire through the fouling 
material.

Most plugs also (except special purpose plugs) have an internal 
series resistor for EMI suppression. 

You have to sort through quite long list of plugs to find a plug that 
does not have a booster gap and/or an internal resistor.     

What plugs does it use? Are you sure they are correct for your 
application, and don't have booster gaps or internal resistors?

As for setting the gap, there is no formula. Spark plugs are 
designed to place sharp edges below the adjustable electrode. The 
sharp corner increases the voltage gradient (just as anything larger  
than a microscopic burr does in a capacitor, relay, or tube) and can 
greatly decrease the voltage breakdown of the gap. Since the 
surface area and shape varies greatly from plug to plug (ignoring 
the internal gaps and resistances), the voltage breakdown will vary 
greatly with the same spacing. You'll have to set the plugs with a 
HV test, or with trial and error.

Compounding that problem most ladder lines are operated with a 
high SWR...the voltage across the line can vary from a few volts to 
many thousands of volts. Many plugs can't be set high enough to 
have the required margin of safety in normal operation if they are at 
a voltage maxima in the line. For example, I have a problem here 
with corona melting the sharp points at half-wave intervals on my 
open wire line feeding a 40 meter collinear antenna. Even modest 
power levels can produce high voltages if line SWR is extremely 
high.

         
73, Tom W8JI
w8ji@contesting.com

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