5 watts should transmit 5 km with a 1/4 wave antenna but a lot less power would be needed using a parabolic antenna. You could also use a Yagi array antenna.
The power supply provides DC (direct current) for the transmitter and amplifier.
An RF amplifier could boost the power of a low power transmitter. The amplifier would be class C with a tuned output circuit and harmonic filters before the antenna. You could modulate the B+ of the amplifier for amplitude modulation.