Dual-Band Balanced Bandpass Filter Using Slotlines Loaded Patch Resonators With Independently Controllable Bandwidths

A novel dual-band balanced bandpass filter structure is constructed by utilizing patch resonators and meandered slotlines (MSLs). The center frequencies and passband bandwidths of two differential-mode (DM) passbands can be controlled independently. Specifically, the fractional bandwidth (FBW) in a lower band with the level of ${S}_{11}$ better than 15 dB ranges from 2.9% to 19.1%, and the FBW in an upper band with the level of ${S}_{11}$ better than 15 dB is from 2.0% to 13.9%. Furthermore, isolation resistors are added in the middle of the patch resonators to improve the common-mode (CM) suppression across the given frequency range. To verify the proposed filter concept, two dual-band balanced filters with different bandwidths are designed, fabricated, and measured.