A magnetically tuned band reject filter for millimeter wave frequencies crising a barium ferrite sphere embedded in the upper surface of a semi-insulating gallium arsenide waveguide transmission line element located on a conductive ground plane and forming thereby an image guide. A pair of high energy rare earth permanent magnets are located on the upper and lower surfaces of the waveguide element at the location of the ferrite sphere with a single tuning coil surrounding both the magnets and the waveguide and operates to bias the ferrite sphere for selective absorption of signals propagating along the transmission line. With the magnetically tuned ferrite sphere located therebetween, both an input signal and a local oscillator signal, for example, are coupled to one end of the waveguide element while receiving electronics apparatus, such as a Schottky barrier diode mixer, is located at the other end of the waveguide element.