Compact third‐order microstrip bandpass filter designed by the distributed‐ to lumped‐element equivalence

In reality, it is not easy to design microstrip filters for the sake of distributed‐element effect. It is an effective approach to approximate distributed‐element structures through appropriate lumped‐element circuits. In this paper, some basic microstrip structures are discussed, whose equivalent lumped‐element circuits are derived. Then, a novel microstrip filter is obtained by connecting these microstrip structures, according to the similar topology of the third‐order lumped‐element bandpass filter. The equivalent lumped‐element counterpart of the proposed microstrip filter clearly reveals its physical mechanism, ie, how the resonances are created and coupled and how the transmission zeros are created. Furthermore, a set of the design formulas are presented, which can be used to calculate initial structure parameters and then facilitate design process. The proposed microstrip bandpass filter can realize a third‐order elliptic bandpass response with one transmission zero at each side of the passband. In addition, two short‐circuited stubs are added to input/output ports to create the third transmission zero. The filter is featured by good frequency selectivity and out‐of‐band suppression. For demonstration, an actual example was designed, fabricated, and measured.