Varactor-tuned helical resonator filter

Claim 1

A helical resonator filter comprising: a plurality of conductive helical coils, a conductive shell having a plurality of cavities, each cavity housing one of said helical coils and separated by conductive walls, the wall between adjacent cavities having an aperture for providing electromagnetic coupling between adjacent helical coils; a feedthrough capacitor provided for each of said cavities and positioned through said conductive shell in the axis of said helical coil separating the cavity from exterior thereof; a varactor positioned within the helical coil in each cavity along the longitudinal axis thereof so as not to interfere with the non-axial electromagnetic fields in the cavity, said varactor having two conductive leads, one lead connecting said varactor to said feedthrough capacitor and the other lead connecting said feedthrough capacitor to a tapping point on the helical coil within the cavity, and biasing means for applying different biasing voltages to said varactors through to said respective feedthrough capacitors, for electrically tuning said filter to different resonant frequencies.