It's easy to drive one or two relays, to select one of two or three
antennas. Use a capacitor at each end to feed RF through, and a choke
at each end to feed DC through. For the two-relay case, from the
control end of the coax, apply no DC to energize neither relay, or,
say, +12V to energize one relay, or -12V to energize the other. At the
mast end, use a series diode after the choke to each relay, in opposite
directions. Obviously, use 12V relays. Omit the diodes for only one
relay. If the run is very long, it's better to feed the DC as a
current--the rated current of the relay--to account for resistance of
the line. One way to do that is to use, say, 24V and put a series
resistor (appropriate power rating) at the power supply end, equal to
the nominal relay coil resistance. That can also protect somewhat
against the effects of a DC short across the line. Pick capacitors
that work well for the RF frequency you're using, and have low
reactance (say less than 5 ohms) at the lowest frequency of interest.
Pick chokes that have high impedance over the range of operating
frequencies, say 500 ohms minimum. If you don't have much experience
building RF things, check out some references for RF construction
practices. Without that experience, don't expect to get good
performance much above 30MHz.