One idea is to simply arrange the coax feeders so that at a Y junction you
can feed a 20m beam say, thru half wave of coax, and feed a dipole say for
40m thru another half wave of coax. At 20m, the 40m antenna should look
like something other than a low impedance, while the 20m antenna looks like
the low impedance path and most of the power will go there. (At 20m, 40m
half wave of coax looks like a full wave, and its end impedance is high for
20m.)
If you have more than 2 antennas, a home made switch is not hard to make,
using a stepping solenoid to power the switch shaft of something like one of
the MFJ 6 way antenna switches. The whole must be placed in a better water
tight box, however, but you could place it under a cover of plastic box open
at bottom to receive the cables and allow for air circulation and exit of
condensation.
-Stuart
K5KVH
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