Novel microstrip coupled-line bandpass filters with shortened coupled sections for stopband extension

Novel microstrip coupled-line bandpass filters with extended rejection band are proposed using the transmission zeros inherently associated with the shortened coupled sections to suppress the odd spurious harmonics at (2n+1)f_o (n=1,2,...), where f_o is the passband center frequency. The proposed filters feature compact size and low insertion loss, and have two cross- coupled-induced transmission zeros near the passband edges for improving the filter selectivity. To demonstrate this novel technique of shortening coupled sections, the second-order filter with the stopband extended up to 4.6 f_o is implemented and carefully examined. To further improve the filter response, the fourth-order filter with the rejection extended up to 7.4 f_o and better than 30 dB is also implemented.