Parallel-coupled microstrip filters with periodic floating-conductors on coupled-edges for spurious suppression

This paper presents a new technique for suppressing the spurious response of parallel-coupled microstrip bandpass filters at twice the design frequency. Floating conductors loaded into periodic slots on the inner edges of each coupled section are employed to compensate the difference between even and odd-mode phase velocities. This technique is marked by maintenance of planar structure and ease of design due to the symmetrical coupled line section. The designed filter has a satisfactory suppression level of about 40 dB and a symmetrical frequency response of the pass band. The validity of the design concept is confirmed both numerically and experimentally.