Compact balanced bandpass filter with the fractal defected structures

In this letter, the fractal defected structure (FDS) is firstly used to design miniaturized balanced bandpass filter (BPF). A novel quarter-mode substrate integrated waveguide resonator (QMSIWR) with etched symmetrical FDSs is presented, which has a lower resonate frequency (f0) and acceptable unloaded quality (Qu) at TE201 mode. Based on this novel resonator and eighth-mode substrate integrated waveguide resonator (EMSIWR) with etched the FDS, a compact balanced BPF with a reduced size over 72.5% is designed, which can implement one transmission zero (TZ) on each side of the differential-mode (DM) passband by using cross-coupling technology. Meanwhile, the common-mode (CM) signal is suppressed below −23.6 dB in the DM passband. The measured results agree well with the simulated ones.