enough to avoid problems with the transmitter. Multiple series gaps
only
raises the firing voltage, but the breakdown voltage will be set by the
largest gap in the series. A gap at .035 will fire at about 2-10kv
depending on humidity. You only want it wide enough so that it won't
arc

Humidity doesn't change the breakdown voltage very much. In fact,
increasing humidity increases the breakdown voltage. One correction
table for a "rod gap" with 1/2" square electrodes has a 10% correction
going from 15 torr to 30 torr water vapor pressure and a -16%
correction going from 15 to 2.5 torr, which is nowhere near the 5:1
variation cited above. Interestingly, on this basis water vapor is a
better insulator than nitrogen, since density of the humid air is
actually less. Typical breakdown voltage tolerance on a rod gap is +/-
8%.

That was off the top of my head so I stand corrected but there ARE
variations based on other real world stuff like air pressure and dirt on the
insulators. 5-20kv for inside an engine is real world but that depends on
other stuff too, like what kind of spark-plug, compression, resistors,
mixture, timing, but I digress. The point is the gap may have to be
adjusted for best results and might be too wide for protection. I invited
him to read up on the subject because I have other projects I have my head
into

So what are your experiences with spark gap transmission line arresters?
Perhaps you have the precise gap he should be using? My dim recollection is
of a chart for the transmitter power in use.