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.
If you're setting the gap for around 1kV, whether it fires at 800V or
1200V probably doesn't make much difference. The relay survives either
way (since typical hipot for coil to contacts is 2kv or 5kv)
>
> With a spark gap you have the problem of discontinuities in the line (swr)
> where the bulky gap is installed. Gas tubes can also be a problem in getting
> the proper enclosure to maintain a constant impedance to the line.
You can make a pretty compact, low C gap with a couple pieces of wire.
Lower parasitic C than a gas tube, I'll bet. But both are small
discontinuities.
If you were building a PC board with relays, you could probably put
solder in type tubes.. Littlefuse SL1003A, for instance, are 1.5pF
(which is probably lower than the parasitic C of the relay) and handle
5kA. They're available in voltages from 90 to 500V (which, by the way,
correspond to impulse voltages of about twice that, depending on the
rise time). They're about a dollar or two each.
Of course you DO have to replace them eventually (300 shots at 200A, 10
shots at 10kA)
Maybe the SL1003 isn't what you want; like other gas tubes they DO have
a glow discharge characteristic, and for this one it starts at a couple
hundred volts or so. On a matched 50 ohm system, at 1.5kW, you're at
270V rms, so you might be creating IMD that you didn't expect, and then,
there's always the possibility that you have a mismatch (say on an
antenna that's out of band)
>
> It is much simpler to buy a ready made device such as Polyphasers where all
> that work is done for you. But you can do it yourself.
>
> 73
> Gary K4FMX
>
>> -----Original Message-----
>> From: towertalk-bounces@contesting.com [mailto:towertalk-
>> bounces@contesting.com] On Behalf Of Roger (K8RI)
>> Sent: Sunday, September 12, 2010 11:11 PM
>> To: towertalk@contesting.com
>> Subject: Re: [TowerTalk] Lightning Protection Question
>>
>>
>>
>> On 9/12/2010 8:54 PM, Jim Miller wrote:
>>> Jim
>>>
>>> Any construction examples anywhere on the net of the non-sparkplug
>> approach?
>>> Sounds interesting.
>>>
>> All you need are two pointed conductors. One for the ground side and one
>> for the center of the coax. They can be just that simple, or you can
>> machine a fancy coax connector with adjustable points. The conductors
>> should be heavy enough to stand a pretty good jolt, but they can be
>> sacrificial. Just use your imagination. Two points and two conductors.
>>
>> 73
>>
>> Roger (K8RI)
>>> 73
>>>
>>> jim ab3cv
>>>
>>> On Sun, Sep 12, 2010 at 1:21 PM, jimlux<jimlux@earthlink.net> wrote:
>>>
>>>> You can get non-resistor plugs. I've used them as vacuum HV
>> feedthroughs.
>>>> As a spark gap, they'd work ok, but a couple pieces of sheet metal
>> might
>>>> be easier and more compact.
>>> _______________________________________________
>>>
>>>
>>>
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