Compact Highly Selective Microstrip Filters with Folded Hairpin Resonators

In this paper, we study the possibilities to significantly reduce the dimensions of the microstrip filters structures on co-directional hairpin resonators and at the same time to increase their frequency selectivity. It is shown that in these structures it is possible to form a large number (P = 2N) of attenuation poles at finite frequencies, which makes it possible to discharge the spurious passband spectrum up to the third harmonic. Moreover, the mechanisms of the formation, splitting, joining, and control of the attenuation poles position on the frequency axis are studied in order to ensure the required attenuation level in specified stopbands.