Balanced Band-Notched UWB Filtering Circular Patch Antenna with Common-Mode Suppression

This letter presents a balanced band-notched ultra-wideband (UWB) filtering circular patch antenna fed by a microstrip line. The proposed circuit is composed of a λ/4 slotline-resonator-based balanced band-notched UWB bandpass filter (BPF) and a stub-loaded circular radiating patch. The compact balanced BPF is new in that the slotline resonator is fed judiciously at its open end so that differential-mode fields can be effectively excited in the slotline resonator. Moreover, it can achieve an all-stop-like common-mode rejection because the slotline resonator is incomplete in structure when in common-mode operation. The way the BPF is coupled to the circular radiating patch is novel in that a good impedance match can be easily obtained with the aid of two tuning open stubs. A pair of spur-lines was implemented in the microstrip feedline near the plane of symmetry to produce a notch band around 5.5 GHz. Measurement shows that the differential-mode operating band with return loss greater than 10 dB covers 2.95-10.75 GHz except for the notch band of 5.01-6.19 GHz. Very good common-mode suppression is observed with the measured common-mode reflection coefficient greater than -1.78 dB in 1-12 GHz.