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)
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