Bill Tippett wrote:
> First make sure you're using an isolated transformer with
> separate grounds for input (Beverage) and output (coax). Ground
> the Beverage but leave the coax floating and ground it at your
> shack. If you're paranoid, you can also make a feedline choke
> at the Beverage (14 turns of mini-coax like RG-176 around a mix
> 31 toroid should work).
>
> 73, Bill W4ZV
>
Good advice signal-wise, however, if the feedline is long and you happen
to have a lot of lightning, you will probably find that close strikes
cause the transformer to arc-over, destroying the transformer and other
things, because the far end of the coax line is floating (even though
the coax line is buried).
Notice there are two possible sources of the problem, induced currents
on the cable, and ground surge where lightning may hit a tower elevating
the ground around the tower while the ground at the receiving antenna is
still low. Because the shack ground is also elevated in potential, and
all these cables get tied together there, currents flow down the coax to
the receiving antenna. If it is floating there can be a huge voltage
impressed between the coax and the ground at the far end.
There are a few different ways to fix that. Some people just ground the
coax line at the far end and use a VERY good choke. I don't like that
one but it will work if the choke is very good. Another way is to put a
very low voltage (85V) gas tube between the coax shield and the ground
rod. Another way is to add a ground rod just for the coax line. That
is my preference because it helps dump common mode currents, as well as
helps with lightning protection.
Jerry, K4SAV
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