Improved Selectivity in Cylindrical TE/sub 011/ Filters by TE/sub 211/TE/sub 311/ Mode Control

A new method is presented for the design of low loss cylindrical TE ₀₁₁-mode resonators whereby transmission nulls can be placed near the TE ₀₁₁, resonance by controlling the TE ₂₁₁, and TE ₃₁₁, modes that are naturally excited in the same resonator. The frequencies at which the nulls occur are controlled by the angular offset of the sidewall coupling apertures and the relative amplitude of the TE ₀₁₁ mode compared to the TE ₂₁₁ and TE ₃₁₁ modes. It is also shown that a lumped constant circuit model can be used to accurately represent the multimode response of the resonator.