  A new approach for designing the single-band and dual-band balanced bandpass
filters (BPFs) based on the circular patch resonator is proposed in this work.
By modifying the output balanced ports of the balanced BPFs to inhibit or
transport TM31 mode of the circular patch resonator under the differential-mode
(DM) excitation, a single-band or dual-band balanced BPF can realized. As the
odd modes could be restrained under the common-mode (CM) excitation, the
balanced BPFs could achieve high CM suppression in the DM passbands. Meanwhile,
several perturbing ways are introduced to improve the performance of the
balanced BPFs. Specifically, perturbation vias are introduced to disturb the
electric field distribution to improve the CM suppression in the DM passbands,
and slots are cut on the circular patch resonators to perturb the current
distribution to modify the second DM passband of the dual-band balanced BPF.
Furthermore, the proposed single-band and dual-band balanced BPFs are
fabricated and characterized. Good agreement can be achieved between the
simulated and measured results of the fabricated balanced BPFs, which validates
the feasibility of the proposed design method.
