[TowerTalk] Lightning Protection Question

[Roger (K8RI)]

On 9/13/2010 8:58 PM, jimlux wrote:
> Gary Schafer wrote:
>> Simple spark gaps are not so simple. Temperature, humidity, 
>> contamination by
>> dirt etc will all make a difference on what voltage they arc at. Gas 
>> tubes
>> are much simpler and more reliable for repeatability of breakdown 
>> voltage.
>
> Indeed, gas tubes are nice for that reason, BUT.. they do cost more 
> than a couple pieces of wire (not much more), and I'm not sure that in 
> this application you need that kind of control.  After all, the 
> original intent here was for installation at the tower, *in addition 
> to* the high quality protection at the shack entrance.  All you're 
> really doing here is protecting the relatively cheap relay, or, more 
> accurately, saving yourself the hassle of replacing it on a big strike.
>
> And, the breakdown voltage doesn't change all that much.  It goes as 
> density of the air, so from -20C to +50C, you're talking about 20% 
> variation or so. Humidity effect is very small.. maybe a couple percent.
>
The problem from humidity comes from condensation and that condensation 
is far from pure, just check the PH.  It'll conduct like crazy and with 
a .020 to .030" gap it's likely to bridge the gap creating a conductive 
path.  OTOH after a slight burp in the impedance at a KW it'll 
clear...or initiate an arc.

We were able to get get a constant breakdown voltage, but the paperwork 
was pretty heavy.  We did two things.  We sealed them and made them 
radioactive so the ionization potential was a constant. Not exactly 
handy with all the regulations.

73

Roger (K8RI)