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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73
Roger (K8RI)
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