I don't have access to the manuals, but it is entirely possible that the same VHF transceiver unit can be fitted in either VHF 1, 2, or 3 locations of an aircraft avionics bay, and a 'jumper wire' connection within the control plug for the VHF 3 position switches the transceiver to use lower transmit power in that role, (and therefore draw less current when transmitting).
25-40W is plenty of power for an airborne Comms transceiver, (clear transmission path). In my previous life; a 1W 'walkie-talkie' on 78Mhz worked on the clear path from a hill near Newbury to Crystal Palace TV transmitter station; about 60 statute miles.
The commonly used aerial configuration for fitment to a vehicle or aircraft is a 1/4 wave or 5/8 wave resonant single element 'ground-plane', (the metal body of the vehicle forms the other half of the dipole), and I imagine that is what is inside the shark's fin antennas fitted to aircraft.