>> Coax shields won't take very much current before they melt. A large
>> ground
>> wire will lower the impedance between the tower and the common point
>> ground, as well as reduce the voltage drop for these low frequency
>> components.
>
> RG-8 style coax shields won't take much current before they melt, but as I
> mentioned, I have two runs of 1-5/8" heliax running between the tower and
> shack, connected to the tower ground system at one end and the SPG at the
> other end. For those not familiar with this type of cable, the outer
> conductor of each run is nearly 2" in diameter -- basically a big,
> flexible
> copper pipe with a lot of surface area. I don't have a spec on the
> thickness
> of the copper, but it's probably on the order of 1/64". The resistance per
> foot is .14 ohm/1000 feet. I don't know the current handling capacity, but
> the power rating is 310 KW. Although the heliax is jacketed, and therefore
> won't conduct a surge to ground like a bare wire in the trench, it seems
> to
> me that the two large copper tubes present a much more inviting path to
> the
> SPG than the bare wire. True or false?
Wellll...Kinda, sorta...almost...maybe.
There are a lot of variables, but IF that copper tube represents a low
resistance, low impedance, high current capable, insulated from ground, path
into the station it would seem to me it would also be inviting that
lightning stroke in for dinner rather than having it bleed off into the
ground. That means the SPG is going to have its work cut out for it as will
those Polyphasers. The shield is grounded at that entrance point, but the
Polyphaser has to take the brunt of of what comes in on the inner conductor
and that, in this case could be many thousands of volts and a LOT of
current.
Even with the house tied into a single point ground when currents get this
large only tiny differences in resistance between devices and ground can
mean very large voltage differences. I'd prefer to minimize the current and
voltage before it hits the bulkhead ground coming into the station so what
ever protection is used would have it's work minimized and effectiveness
maximized as well.
As has been said several times, IF every thing rises to the same potential
there should be little problem. However with a major lightning strike even a
few inches difference can make a substantial difference in voltage.
Lightning strikes are unpredictable individuals with widely varying rise
times, fall times, peak duration, repetition, and frequency distribution.
Energy normally falls off rapidly with frequency, but the large diameter
conductors may provide a low enough impedance to bring in substantial energy
through a much higher and wider frequency range.
Regardless of the approach taken there are always trade offs.
73
roger (K8RI)
>
> 73, Dick WC1M
>
>
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