On Fri, 6 Mar 2015 13:31:22 -0500
"Jim Wilkins" wrote:

snip
I bought a single-pole relay rated for "only" 75KV once. It was the
size of a desk lamp and opened the contacts several inches.

Ouch!

The way I remember it the product used standard relays. The key was
that in the off/relaxed position everything was shunted together to
common ground point. Lightning could still jump the relay but it would
be greatly reduced with everything shunted/grounded together. It was
a pretty ambitious scheme if I remember correctly. The antenna lines
were all disconnected, shunted too. Depending on the frequency, antenna
switches can be pretty expensive too...

One of the radios I used to work on was the Motorola Micor series. They
had an interesting antenna relay in them. They used two magnetic
reed-switches encased in an aluminum (I think it was aluminum) housing.
One switch had a magnet shrink wrapped to it keeping it in the closed
position (receive side). A small coil (12vdc) went around the metal
case. To transmit they applied 12vdc to the coil which in turn closed
the open reed and opened the one with the magnet affixed... For an
overall image:

http://i.ebayimg.com/00/s/MTA0N1gxMzg5/z/li4AAOSwEeFU4uN5/$_1.JPG

They made a great lightning arrester. Many, many times that is the only
part I would have to replace after a tower lightning strike. If you
took one apart, quite often the two reed-switches would be completely
obliterated, just blown to bits... Motorola had a lifetime guarantee on
those relays. Sent many of them back in for warranty replacement. You
couldn't tell what happened to them without taking them apart

--
Leon Fisk
Grand Rapids MI/Zone 5b
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