[time-nuts] Lightning arrestors for GPSDO antenna

[ed breya]

Of all device types, I think gas tubes are the best for this sort of 
application - very low C, and high surge current rating. I'm 
picturing the kind that are used in power supplies and such for 
limiting line transients - about 1 cm dia and length with axial 
leads. I don't know what kind are used in "lightning arrestors," if 
they are the same or scaled up in size.

Whether you make it able to take a direct hit depends on how big of a 
hit, your budget, and the environment of the antenna and lines. If 
it's the tallest thing in a huge field in a lightning-prone area, 
then it could be a big issue, but I don't think most people have that 
situation.

You may want to look at the US National Electrical Code (NEC) for 
ideas - I believe that subject is covered there. The main thing there 
would be safety against injuries and fire, even if the equipment is destroyed.

I think what you would want is kind of a pi network - the lowest 
impedance path to ground at the antenna zone that can be practically 
realized, then a high common-mode impedance (or even fusible) line to 
carry the signal to the building, then another low impedance path to 
ground at the building. This means that in my opinion, you should not 
put the feedline in metal conduit unless it's essential for 
protection - or underground, which should improve the grounding. You 
want the antenna zone to absorb the brunt of any discharge, then use 
the higher line Zcm to hopefully give some degree of isolation from 
there to the building.

Ed