New type of magnetically tunable multisection bandpass filter in ferrite-loaded evanescent waveguide

A description is given of a novel multisection bandpass filter. It is constructed in ferrite loaded evanescent waveguide and can be tuned over a moderate bandwidth by a single knob control of the dc magnetic field applied to the ferrite. The basis of this type of filter is a new principle which states that full transmission of the incident energy through a finite length of waveguide below its cut-off frequency can be effected by using a terminating impedance that is the conjugate of the characteristic impedance of the evanescent waveguide. Briefly, the filter consists of a length of rectangular waveguide operated in the evanescent TE
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mode. A thin slab of ferrite lies along both side walls and in each filter section a capacitive screw is placed for tuning the filter to resonance. The magnetic field on the ferrite controls the cut-off frequency and thereby the resonant frequency of the filter. Design equations are derived and results on a 3-section filter are included.