Dual-Mode Dual-Band Bandpass Filter Using Balun Structure for Single Substrate Configuration

A novel dual-mode dual-band bandpass filter (BPF) for a single substrate configuration is proposed that uses a balun structure. The outer and inner loop resonators are simultaneously excited by the microstrip line and slotline balun structure. In spite of the microstrip feed network, the proposed dual-mode BPF can achieve dual-band operation. Two proper degenerate modes in the lower and upper passbands can be generated and controlled by installing an inductive cut and a capacitive patch in the corner of two loops, respectively. This letter deals with the analysis and design of the proposed dual-band BPF as well as the experimental validation of the predicted dual-band performance.