Power Question

I was just wondering, for a transmitter is the power the voltage * current in the antenna (when the radio is in use)?
If so and we say the transmitter has a 5w power, does it actually matter whether:
V=10V, I=500mA or
V=20v, I=250mA
since the power is the same?

The impedance of the antenna is given. Usually is 50 ohm (ideally purely resistive) or in some cases (f.i. TV) is 75 ohms, Then voltage and current are known from the wattage.

you are not considering that no transmitter is 100% efficient, usually much less
therefore as an example you might be supplying the transmitter with 12V @ 10 amps = 120W total power used
IF the TX is 50% efficient then there will be a maximum possibility of 60W of output power
IF the transmitter is say 80% efficient ( not too many are that good) then the best possible output power would be 96 Watts

ALL the rest of the power in both cases is wasted as heat in the circuitry. Hence why any reasonable power ( say 5W or more) needs a significant heatsink

This is all BEFORE losses due to antenna matching/transmission lines are taken into account

cheers
Dave