David Robbins wrote:
SNIP

the most basic one is a spark gap. these have been used since the first
days of long telegraph lines. just position two sharp pointed metal pieces
a small distance apart, connect one side to the cable and the other to a
good ground. you want the distance as small as possible, but wide enough so
your normal transmission power doesn't create an arc.
SNIP

Remember, there is still a significant voltage across the spark gap
before it ignites and during conduction. A 1/16 inch, or a 1.5 mm, gap
will sustain ~1000 volts before igniting and support several hundreds of
volts during conduction.

The best solution, disconnect all antennas when not in use.

DD, W1MCE