New coupling topologies of microstrip bandpass filters with strongly asymmetric characteristics

Simple, compact low-cost microwave bandpass filters with strongly asymmetric characteristics, based on new topologies, are investigated, designed, fabricated and measured. A classical fourth-order filter or quadruplet, for example, has two transmission zeros located symmetrically, one on each side of the passband. A filter of the same order and, consequently, with a similar insertion loss, can offer a higher attenuation of one of the two adjacent channels when the filter has both transmission zeros in the same (upper or lower) stopband. Such strongly asymmetric characteristics require however an extra coupling between resonators, consequently require a different topology of the microwave filter. In this paper novel filter structures are proposed for quadruplets with strongly asymmetric characteristics with two transmission zeros located both on the same side of the passband, in prescribed positions. These filters can provide a very high attenuation not only at a single frequency, but in a large adjacent frequency band.