Microwaves are generated by circulating or traveling electron currents in a magnetic field that excites modes of cavity which take the form of magnetron, klystron, or gyrotron. The microwave can be channeled in a waveguide and can radiate through a horn antenna. The effective resonant frequency of the cavity, waveguide, and antenna system can be calculated and described here. It is shown how to change this resonance using various impedance matching techniques.