A Novel Dual-Mode Square Loop Passband Filter with Second Spurious Passband Suppression

A novel microstrip dual-mode square loop bandpass filter with suppressed spurious passband is proposed. The degenerate modes are splitted by the embedded spur-lines easily. The spur-lines embedded in the output line suppress the second spurious passband effectively. The two-pole dual-mode filter operating at 2.59 GHz with fractional bandwidth of 4.63% is designed, fabricated, and measured. The measured harmonic suppression ratio >20 dB is obtained