The best thing I know and have never heard disputed is wide stripes of copper
flashing available from roof supply stores De N4YFX (Tim)
Roderick M. Fitz-Randolph wrote:
> >HI Rod...I've never thought braid was good for a ground, and now that
> >I understand the mechanics (OK< the electromechanics) of the situation
> >my gut feel was right. You need the ground wire to conduct gobs of current,
> >and it needs to be THICK because of skin effect. Braid, even tinned, still
> >doesn't have near the effective thickness of good old solid copper wire.
> >The energy in lightning (last measurements I saw from a research team in
> >the late 70's) goes way up into the VHF region, but there's plenty of
> >energy below that to sizzle your braid.
> >
> >Just to give you an idea...in nuclear stations they use copper BAR (abt
> >1 in by 3 in as I remember) for grounding.
> >
> >No, no, no...do not use braid for a ground. You'll have plenty of people
> >say "I use it, and nothing happens." I say: "You haven't seen 'real'
> >lightning yet."
> ___________________________________________________________________________
> I considered the braid because it has less inductance than copper. My
> particular situation here is not quite the same as for most situations in
> that I have a 130 foot tower with a 20 foot mast (7 down and 13 up) and a
> lightning rod that extends to 150 feet above the ground. Each leg of the
> Rohn 25G is connnected to two 8 foot lightning rods, 16 feet apart... for a
> total of 6 lightning rods. The coax cable that connects the antennas to the
> ham shack is buried for 250 feet from the base of the tower to the hamshack.
> Some of the coax is 3/4" CATV aluminum shield cable that is buried direct (we
> have acidic soil here so direct burial of CATV aluminum jacket coax is
> acceptable).
>
> Only a small amount of voltage has reached the ham shack during a lightning
> strike. Two of the three strikes did not hurt the Alinco VHF transceivers
> that remain connected to my PacketCluster node 24 hours a day. I have
> installed PolyPhaser IS-50US-C1 devices in both of those coax lines now and
> intended to run the 3/4" braid 6 feet (from the 1/8" thick aluminum plate upon
> which the PolyPhaser devices are mounted) to ground rod system at the ouside
> wall of the hamshack (which is a different ground system than that of the
> tower because of the distance involved and PolyPhaser's statement that if
> the tower is more than 100-150 feet away from the hamshack, a common point
> ground is not necessary; rather two separate, but adequate grounding systems
> should be used).
>
> There are two prime concerns: (1) The braid must have low enough DC
> resistance to effectively short the current to ground and (2) the braid
> (or whatever is used) must be of small enough inductance to assure that
> L di/dt does not cause the sort of build up over its length that allows
> a 100 foot well grounded tower (that has only 0.1 ohm DC resistance from
> top to bottom) to achieve > 200,000 volts from top to bottom, instantaneous
> at the moment of the typical 18,000 Ampere strike. The only two viable
> choices are a copper strip 6" to 9" wide running from the common tie point
> at the station console to the grounding system 8 feet away (and through the
> wall via a 3" PVC) on the outside of the house.
>
> Copper strip is out of the question (even though it exhibits much less
> inductance than either a wire or braid) because of the small aperture of the
> PVC. This leaves only large copper braid or tinned copper braid as a viable
> choice. Based on the excellent protection the 7 feet of 3/4" copper braid
> afforded at the digipeater site at which every other piece of radio/electric/
> communications equipment was destroyed, the electrical distrubution box
> was blown to pieces, the repeater telephone line input box was nearly
> vaporized, the ground lug on my power supply was almost arced in half, etc.,
> yet my equipment protected by the 3/4" braid to two ground rods and the
> PolyPhaser device inline with the coax, leaves me to the inescapable
> conclusion that the small charge remaining at 250 feet from the well-grounded
> tower will be successfully "bled" or grounded by braid that is even more
> massive than that which successfully protected the equipment from the effects
> of a direct strike at the digipeater site.
>
> My question is: Is the tinned copper braid to be avoided because of any
> oxidation problems with the tin?.... i.e., should I only use large copper
> braid as opposed to large tinned copper braid? Does the tin on the outside
> of the copper pose a problem in light of the well connected and Penetrox A
> protected joints at either end?
>
> Comments gratefully accepted.
>
> Rod, N5HV
>
> ------
> Submissions: antennas@qth.net
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