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

Roger (K8RI) on TT K8RI-on-TowerTalk at tm.net
Sun Apr 9 19:01:00 EDT 2017

Agreed, lightning is an RF event and the feedpoint is probably the best 
place to start.

My thoughts on the direct path to the amp, were not toward a DC event, 
it's just a lot easier for the residual voltage to traverse. I say 
residual voltage as reducing the potential to around 300 volts, give or 
take means a lot less for the buried coax to reduce.  I'd also ground 
the coax shield with a network of ground rods (at least three spaced at 
120 degrees and at least 6' to 8' spacing) where the coax enters the 
ground under the antenna.

Perhaps a spark gap (with fine points) at the feedpoint followed by a 
gas discharge device where the coax enters the ground, lowering the 
pulse entering the coax to less than several hundred volts and something 
like a polyphaser where the coax enters the shack at the common point 
ground.  The inductance and capacitance of the 100' of coax should 
greatly reduce that voltage before it gets to the polyphaser/rig.

IOW, tackle it in steps, or stages instead of one brute force method.

Although I have the big antennas down at present, the center fed, half 
wave slopers for 75 and 40 still present a lot of wire for strikes and 
induced voltage (90' at the top).  All the coax shields are grounded 
back at the tower with polyphasers at the building entrances.

With the many direct lightning strikes to the tower and antennas, I've 
only had one event that took out a polyphaser.  OTOH a NEARBY strike 
induced enough voltage into my "buried" CAT5 network cables to create an 
expensive mess that still rears its ugly head now and then, two years 
later. SS devices can be "hurt" without failing immediately.  Computer 
motherboards, network interface cards (NICs), routers, switches, and who 
knows what else.
At least both stations (also hooked to the CAT5) survived.  CAT5 is no 
longer connected to the shop

73, Roger (K8RI)

On 4/9/2017 1:45 PM, Grant Saviers wrote:
> 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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Roger (K8RI)

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