A microwave resonant cavity is a box fabricated from high-conductivity metal (usually from oxygen-free copper; literature value is 5.8 * 105 Ohm-1 cm-1 for the room temperature conductivity) with dimensions comparable to the wavelength. Often it has rectangular or cylindrical form, because in a resonator of complicated shape, it may be nearly impossible to calculate the distribution of electric and magnetic fields. While in the case of right or simple geometrical form these distributions are well-known and calculated for a long time.