>On our large medium wave sites, we have a thing called a prematch coil
>between the mast (which is the antenna and so is live) and ground. These
>things are also called static leaks (direct short to DC and open circuit to
>RF) You would think that a coil made of 30 or so turns of 1/2 inch copper
>pipe would be quite a mechanically strong thing. We recently had one
damaged
>by a direct strike, where some of the turns ended up shorted together,
>pesumably by the huge magnetic fields set up by the currents involved. We
>had to do an emergency repair, and had to use a crow bar to get enough
force
>to put it right!!
>
>Static leaks are definitely the way to go, because they'll always provide
>protection even when the antenna is in use.

late in my apprenticeship I had occasion to visit the radio New Zealand MW
transmitter site at Titahi Bay (pronounced as Tee tah Hee), Wellington. This
has a high power transmitter (some hundreds of kilowatts) at around 550kHz,
and a lower power transmitter (about 100kW IIRC) at around 617kHz fed into a
single mast which is a quarter wave at the mid point between the two
frequencies, and a matching/combiner unit made out of big copper tube coils
etc. The mast suffered a lightning strike some months before our visit, and
the techs there regaled us with stories of molten copper balls rolling
around the floor which had resulted from the coils in these matching units
absolutely melting.

Mind you this mast is real ripe for any sort of lightning strike. It is
built on a peninsula with the sea around about 7/8 of the periphery, and the
highest point on it is about 100 - 200 feet. Then the mast is some 600-700
ft high if memory serves me correctly, and sweet all other high stuff for
miles.

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu