Wideband balun bandpass filter with broadside-coupled microstrip/slotline resonator structure

In this Letter, a new design scheme for wideband balun bandpass filter (BPF) is presented originating from the conventional Marchand-type balun prototype. The designed balun BPF is basically composed of three microstrip lines on the top side of the substrate and two halfwavelength slotline resonators etched in the ground plane. A distinctive wideband performance is attained by utilising the broadside coupling between these microstrip lines and slotline resonators. For demonstration, one fabricated prototype balun BPF operating at 3.75 GHz shows a broad bandwidth of 93% with nice in-band balanced performance. Final results from simulation and experiment are both recorded in good agreement.