An RF filter includes: an electrical conductor defining an outer sphere; a dielectric material defining an inner sphere disposed within the conductor outer sphere; and at least a first electrical probe and a second electrical probe. Each probe extends through the conductor and is electrically insulated from it. A spherical shape of one or both of the inner and outer spheres is interrupted by: a) a first localized discontinuity in said spherical shape disposed along a first axis passing through a geometric center of the one or both of the inner and outer spheres; and b) a second localized discontinuity in said sphere form disposed along a second axis passing through the geometric center, the second axis perpendicular to the first axis. There can be more than these two discontinuities, implemented as chamfers, tuning screws, and the like. Series and parallel coupling of the spheres is detailed.