Abstract: A variable bandwidth tunable directional filter to directionally route a broadband signal between an input and an output microwave transmission line for controlling and processing signals in data handling systems. The transmission lines are waveguides directionally coupled to a ring resonator having frequency response characteristics at resonance of a band-pass filter. The ring resonator directionally coupled to transmission lines is capable of resonating with waves progressing unidirectionally. Tuning to resonance is achieved by adjusting the guided wave in the ring by the insertion into the ring of flat dielectric material which also provides for a broadband impedance match of the coupled circuits. Broadening of the filter bandwidth is achieved by additional attenuation in the ring resonator which also decreases the insertion loss due to the coupling of the resonator to the waveguides.