That's what is generally used at Broadcast Radio transmitting sites ...
(Copper strap, like 4-12" wide0 ... Cheers!
-----Original Message-----
From: kingpop <kingpop@sanasys.com>
To: Roderick M. Fitz-Randolph <w5hvv@aeneas.net>
Cc: Towertalk@contesting.com <Towertalk@contesting.com>; antennas@qth.net
<antennas@qth.net>
To: <towertalk@contesting.com>
Date: Thursday, November 06, 1997 3:31 PM
Subject: Re: [Antennas] Braid as an Effective Grounding Material
>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
>
>
>
>
>------
>Submissions: antennas@qth.net
>
>
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