Alan -
You're forgetting the loading coil has to be switched out when using the
loop at HF, so the relay has to withstand the full RF voltage when using it as
a vertical at LF/MF.
So you'll need a vacuum relay with a contacts rated at something like
10kV. The current will only be an amp or so. I tried this once using FR
/ Crydom reed relays that were certainly rated at that sort of voltage.
Trouble was that rating was DC. At 137kHz it died with a flash from
inside and melted contacts as soon as a decent amount of RF was applied.
A knife switch on a ceramic former would probably work, otherwise find
someone working for
NatGrid and try to get a 33 or 132kV contactor :-)
Andy G4JNT