A resonance isolator in which microwave ferrite bars are positioned in and in contact with a waveguide to achieve unilateral absorption and in which ceramic permanent magnet bars are positioned on the outside of the waveguide adjacent to the ferrite bars and are insulated by a thin sheet of thermally insulating material that also acts as a spacer to adjust the magnetic field. The waveguide may be made with nonmagnetic sheet metal and the ferrite bars may be used as extruded. The isolator is adapted to connect between a microwave generator, such as a magnetron, and a microwave load, such as a heating chamber, and forms an integral feed unit. The isolator effectively reduces the peaks of the standing wave caused by the power reflected by a load and protects the generator against premature degradation and extends its useful life time. Preferably, the isolator structure is mounted on the heating chamber which forms a wall of the waveguide and a set of permanent magnets may be on the inside of the heating chamber and coated with a conductive layer to prevent absorption of energy. Magnetron cooling air may also be drawn through a waveguide for cooling and additionally for driving a mode stirrer in the heating chamber.