[TowerTalk] spark gaps & GDT's for lightning/static
jim.thom at telus.net
Mon Apr 10 11:11:26 EDT 2017
Date: Sun, 9 Apr 2017 19:01:00 -0400
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk at tm.net>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] spark gaps & GDT's for lightning/static
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.
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.
73, Roger (K8RI)
## What if you have a hairpin or beta match on a yagi ? A beta
match is just a hairpin with the mid point bonded to the boom.
The hairpin would bond both sides of the DE together. The beta
match would do that..plus bond the midpoint to the boom..which grounds
both the center conductor + shield..to the boom. Boom is bonded to mast or
tower. I cant see the XL of a tiny hairpin being too high at 1 mhz, where lightning
## Then bond braid of coax to top of tower. Bond braid of coax to base of tower.
On tall towers, they will also bond braid to tower at the half way point as well, so 3
places its bonded to tower.
## Use your favourite gas discharge device at base of tower..along with the usual myriad
of cad welded rods at base of tower. Run a 2 ga bare stranded cu wir, buried, from base of tower
to SPG setup in the basement wall etc...with some more cad welded rods. Then the potential at spg
is the same as the base of the tower..or close to it. Then another gas discharge device at the spg.
## You dont see am, fm broadcast or even cell sites with coax disconnected during lightning storms,
they all stay on the air. The broadcast ants and towers on the rooftops in chicago + NYC get hit all
the time with lightning...so they must have it figured out.
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