Bandgap characteristics of coupled-line section with periodic corrugation and its application to parallel-coupled microstrip filter design with an extended upper stopband

Coupled sections with periodic corrugation are designed to realize a parallel-coupled line filter a wide upper stopband. The stopband characteristics of the corrugated section are investigated by changing its geometric parameters. Then, corrugated sections with various rejection bands are cascaded to achieve the wide upper stopband. Measured responses of a tested third-order circuit show that the upper rejection band can be extended up to 11.8 times the passband frequency.