A photon rocket is a vehicle propelled by the reaction of light or other electromagnetic radiation. Photons have the highest specific impulse of any rocket propellant and will allow acceleration to 80% of the speed of light with a mass ratio of 3. For higher velocities then a 99.9% of light velocity requires a mass ratio of over 100.
The beam is a radio beam but frequencies above 900 MHz are best and less than 2 GHz. The word "microwave" refers to these frequencies. The dish shape underneath forms the beam.
Only engines that can convert matter into energy can be used.
Referring to my thermonuclear engine I suggest that a similar configuration would be able to contain a plasma at such a high pressure and temperature that 100% conversion of matter into energy would occur.
Using the electrical power generated directly then microwave energy could be directly generated and fed to a parabolic antenna to give a beam. This would result in thrust.
Calculations indicate that a one tonne vehicle would require 10E12 watt to give an acceleration of 1 g. However much lower power levels may be used to accelerate an orbiting spacecraft toward the stars.
Journeys to stars 100 light years away would only take a few weeks. See Relativity.