Compact quasi-elliptic bandpass filter

This paper presents a novel planar microstrip structure which is analogous to semi-circular open-loop resonators but whose ends form closed loop quadrants folded within the resonator´s inner space, and includes stepped impedance sections. The structure provides phase velocity equalization between the even and odd modes generated within the structure to suppress excited higher order harmonics. Hence the resulting response created by the structure resembles a quasi-elliptical bandpass filter having a relatively low passband insertion-loss and return-loss, and a wide out-of-band rejection characteristic covering a frequency range that spans four times the centre frequency of the filter.