High-Selectivity Microstrip Filters Based on Structures With a Limited Number of Hairpin Resonators [Application Notes]

Microstrip filters on half-wave hairpin resonators are widely used in different microwave applications because they are very compact structures and do not contain short circuit elements, characteristics that make them relatively easy to manufacture and use over a wide frequency range. In this class of microstrip filters, three basic structures can be defined: one based on hairpin resonators with alternating orientations [Figure 1(a)], another based on codirectional hairpin resonators [Figure 1(b)], and a third based on filters using hairpin resonators with supplementary electromagnetic couplings between nonadjacent resonators [Figure 1(c)]. These structures are fundamentally different both in their operating principles and in their achievable characteristics despite layouts that, for two of them, seem similar.