A dual-mode dual-band bandpass filter using a single ring resonator

A dual-mode dual-band bandpass filter with two transmission poles in both of dual-passbands is presented with the proper stub-loading and external-excitation of a single microstrip ring resonator. By placing two excited ports at the 135°-separated positions along a uniform ring resonator, two 1^st-order degenerate modes, TM₁₀ and TM₀₁, can be simultaneously excited and they are split to produce two transmission poles in the 1^st passband. Meanwhile, two 2^nd-order degenerate modes, TM₂₀ and TM₀₂, are split by attaching four identical open-circuited stubs to the ring in a symmetrical manner with respect to two external ports. Thus, two transmission poles can be generated in both of dual passbands as demonstrated in theory. Finally, a compact dual-band ring resonator filter is designed and fabricated to operate at 2.3 and 4.8 GHz and its predicted results are verified in experiment.