Usually a double pole knife switch used to switch antennas would have a
double throw. That is in one direction it made the circuit tot he
radio, in the other direction it either connected to the receiver, or to
ground fot dissipate any static build up on the antenna.
To protect parallel line feeder, you can take a pair of used spark plugs
and put the points close enough to not arc over with your transmitter's
maximum power, but to still arc over if static build up happens. Wind
induced static can build quickly to over 600 volts and reach thousands
of volts in the presence of a nearby lightning event.
There are also telephone type line protectors for nominal 600 ohm lines.
The modern versions use a Zener diode on each line leg, that breaks
over to conduct to a ground rod, at a peak voltage higher than the radio
RF wave voltage. The diodes that come for telephone service may not be
as resistant to constant RF, and may need to be changed to a higher
break down diode.
-Stuart
K5KVH
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