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

[Grant Saviers]

No, the protection is for a 160m 90' high "T" with 8x 120' elevated 
radials in a mostly treed area.  That's a lot of exposure to induced or 
trees to wires strikes.  Since the elevated radials can't be grounded at 
160m the protection problem is a bit different than buried radial 
verticals.  The trees average 110' and the vertical is about 100' from 
the shack entrance panel, with buried RG8 and a remote tuner control line.

The T is fed with a 50:25 ohm TLT which does DC connect the vertical to 
the radials but given lightning is an RF event that leaves some missing 
protection, maybe blowing up the TLT.

The plan is to shunt any current to ground at the feedpoint from either 
radials or vertical to a ground rod field and some TBD current path.  A 
high Z RF choke can provide a DC connection from the radials to ground, 
but again there is inductance to consider.

The amp is solid state.

re GDT's, the spec on the axial leaded devices is 1.5pf.

Grant KZ1W

On 4/9/2017 0:07 AM, Roger (K8RI) on TT wrote:
> I'm assuming this is for open wire feed line?  That brings up the 
> question of the rig. Is it SS, or tube.
> Is the tuner link coupled, or with a transformer?  Is there a direct 
> path from the antenna "through" the tuner to the rig?
>
> 73, Roger (K8RI)
>
>
> On 4/6/2017 5:35 PM, jimlux wrote:
>> 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 spark.
>>
>> 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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