If you are trying to construct a transmitter or amplifier you will probably
need a mains power supply. Most RF amplifiers need power supply rated at a voltage of 12-15V, 48V or lately also 62-65V. Check your transmitter, pallet or amplifier specifications and make sure you are using mains power supply of correct voltage rating.
Stronger amplifiers need more power and current. You can calculate power rating by multiplying current and voltage: P = U * I. For example a power supply rated at 48V that can supply 10A of current has a power rating of: 48V * 10A* 480W. However be carefull, a 300W FM band amplifier will typically need mains power supply rated at 450-600W. Why? Amplifiers are not 100% efficient, a part of power is lost as heat.