[TowerTalk] spark gaps & GDT's for lightning/static protection

jimlux jimlux at earthlink.net
Thu Apr 6 17:35:37 EDT 2017

On 4/6/17 12:29 PM, Grant Saviers wrote:
> I've seen pictures of all sorts of gaps,  two wires almost touching,
> interlocking rings, adjacent spheres, Jacobs ladder like rods with
> tapered spacing , etc.  One amateur radio product uses automotive spark
> plugs (non resistor) and is sold as a pair on a copper plate for open
> wire feeders.  However, a copper plumbing sweat fitting is soldered over
> the gap so it is impossible to measure the gap.  My question is, if I
> use a non resistor spark plug as a arc gap, is there any experience that
> can be shared about what the gap distance should be set to?  I plan to
> have a removable cover over the gap end for WX protection .
> Are there better designs for a gap that are easy to fabricate and
> weather/bug resistant?

breakdown of air is 70 kv/inch. That's in a uniform field at sea level, 
and a sparkplug or wires or whatever will breakdown at a lower level.

A typical auto gap of, say, 0.030" will break down at about 1.5-2kV.

No matter how small the gap, or how low the pressure, an air gap will 
not breakdown at less than 327 volts (the minimum sparking voltage) - 
you might get significant field emission or corona, but you won't get a 

Argon has a much lower minimum sparking voltage - 137V

vacuum gaps have entirely different behavior, and it's a bit of an 
arcane art to design a consistent breakdown voltage.

The commercial transient suppression gaps might have fairly high 
parasitic C  - they're basically a couple electrodes, a spacer, and a 
fill gas like neon or argon- I'd spend the $3 and measure one at your 
frequency of interest to see what the RF properties are.

But maybe it's a few pF, and you don't care?

> And a second question:  Gas discharge capsules are used in coax
> lightning protection devices.  A large variety of gas discharge
> components are stocked at Mouse, Digikey, etc in various voltages and
> Kamps.  Is there any difference in the RF properties of the tube in the
> in-line coax devices vs what I can buy a lot cheaper as a component?  eg
> Littlefuse and TDK 800v @ 10Ka around $3ea.
> http://www.littelfuse.com/~/media/electronics/datasheets/gas_discharge_tubes/littelfuse_gdt_cg_cg2_datasheet.pdf.pdf
> Grant KZ1W
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