Dick Green WC1M wrote:
>>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?
>
>73, Dick WC1M
>
>
>
True, and you are not going to melt this.
And your numbers look close. If it is 1/64 thick, then each of these
lines would have just slightly less copper than a #00 wire, and lower
skin resistance. DC resistance of a #00 wire is 0.078 ohms per 1000
ft. If it's soft copper, to get a resistance of 0.14 ohms/1000 ft you
would need a thickness of about 0.01 inches. That's close to a #1 wire.
At 10 KHz, the impedance will be dominated by inductance. The impedance
of a solid wire with the same amount of copper as your heliax should be
about 43 ohms/ 1000 ft, and the impedance of your heliax should be about
36 ohms / 1000 ft.
If you have two of these routed next to each other the DC resistance
will drop by a factor of 2 but the impedance at 10 KHz won't drop by
half. It will drop only a small amount, maybe 5 ohms or less. The
wider you separate the cables, the more the impedance will drop (up to a
maximum of 1/2). (The old mutual inductance issue.)
You do loose the ability to dump some of the charge into the surrounding
ground on the way back to the common point ground by having an insulated
wire.
Jerry, K4SAV
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