On Sat, 2006-05-06 at 09:37 -0700, Randy Russe3ll wrote:
> Even if the #6 wire only connects other gnd rods to
> the central ground, and is buried bare? Having a hard
> time seeing the inductance problem, Running a two foot
> wide strip of flat copper is a nice idea, but when you
> eventually connect to your equipment, you still have a
> conductor only as wide as the connection bolt. Should
> I build my operating table out of a solid cast copper
> block extending directly ten feet into the ground
> below my shack, with a few hundred strips of flat
> copper running in all directions from that? Bolt the
> entire chassis of each radio to it and call it good?
> Or would a faraday shield be of added benefit? Hi Hi
>
Yes to both the copper table and thee faraday shield, though as a
practical matter covering a wooden table with a copper sheet works as
good as the solid block because the rapid rise of lightning current
tends to keep it very near the surface of any conductor. I've built such
a bench for use making low level measurements in a lab surrounded by
100KW and 250 KW HF transmitters. There I soldered a 2" wide strip of
copper at each equipment location and bolted it to the coax connector
mounting on the test equipment. Then I ran everything from a Sola to
reduce the effects of line voltage variations and was still annoyed by
them.
Inductance is a severe problem in keeping voltages low between pieces of
apparatus hit by lightning and the 18" wide straps DO WORK to keep the
voltage drops down. Wire of any practical size doesn't NOT WORK because
its surface area is so much smaller and its inductance is much higher.
The copper strap needs to connect the grounds and lightning protectors
at the power and telephone service entrances as well as the antenna and
radio equipment grounds. In commercial setups that do survive lightning,
the radio building has that strap on all sides and all wires in and out
of the building go through that strap. Where they shortcut the full
strap, they regularly loose equipment to lightning. With Polyphemus and
the wide strap, they don't loose equipment.
You can't get the whole of the equipment and external power and phone
line feeds to go to ground potential when the antenna is hit by
lighting, but if you surround the equipment with a low enough inductance
ground connection that connects all the elements you can keep the
equipment at close enough to thee same potential to not be damaged. But
shortcuts in the form of wire in that ground cage defeat its purpose.
Maybe a ribbon of #6 a couple feet wide may work but flashing copper
hasn't been all that much more expensive until recently.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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