Wideband bandpass filter with wide stopband for microwave imaging system designed for stroke detection

A broadband bandpass filter with wide stopband for microwave imaging system aimed at stroke detection is proposed. The filter is designed using three different types of quarter wavelength resonators. The three resonators are low impedance feeding network, a coupled line with moderate even and odd-mode impedances and a rectangular middle resonator. The low impedance feeding network enhances the bandwidth and improves the performance at the resonant condition of the moderate mode impedance coupled structure. To suppress the harmonics and obtain wide stopband, a two-stage bandstop resonator and open-ended stubs are embedded within the design. Finally, a prototype bandpass filter of dimensions 28 mm× 28 mm is designed with fractional bandwidth 55% (850 MHz) at 1.5 GHz with more than 20 dB of attenuation across a wide stopband extending up to 4.5 of the frequency at the center of the passband.