Abstract: In a helical resonator, sometimes called a coaxial resonator with helical inner conductor, new capabilities are achieved by providing a double winding with the turns interspersed and of like pitch. One pair of respective opposite ends of the two portions of the double winding is connected directly to the cavity or ground conductor of the resonator, sometimes called the shield. Coupling to the cavity is achieved by any prior coupling technique. The new resonator can be included in compound helical resonator filters as would other helical resonators. One of the principal advantages of the new helical resonator is its ability to suppress odd harmonics both in its own resonator characteristic and in the characteristics of a composite filter in which it is only one of several helical resonators.