Its not a simple matter of matching the transmitter to the radiation
resistance. First a short antenna is going to have a capacitive
reactance. You must add an equal amount of inductive reactance (a
loading coil) in order to cancel the capacitive reactance and make the
antenna resonant at the operating frequency. Then what the transmitter
needs to match is the total load impedance of the antenna "system".
The load impedance includes the radiation resistance plus the
resistance of the loading inductance plus the ground losses. The
hardest thing to get a handle on will be the ground losses. The
physical size of the transmitter housing is a small portion of a
wavelength and losses will change as you handle the unit.