That's interesting, David. If you google the specific term "isolating spark gap", you find descriptions of a modern device that limits the voltage of a lighting strike on pipelines by arcing across the device to ground. These are used on pipelines where each section is electrically isolated from the next, for corrosion resistance I think. A lighting strike would damage the isolating gaskets, bolt sleeves, etc., so the "gap" device limits the voltage to what the isolation can handle.
So in the case of a radio antenna, maybe that glass "fuse" is to limit the voltage fed back down the antenna by a lighting strike to what the circuitry can handle?
Rather reminds me of the "safety gap" built into the booster coil. To prevent HT coil damage during the times the magneto isn't supplying a spark plug, the generated voltage sparks across the safety gap to ground.
Malcolm
|