This paper presents a design methodology for realizing broadside-coupled microstrip bandpass filters on multilayer substrates to reduce the size of the filter. The new filter configuration consists of broadside coupled split-ring resonators on two layers backed by a ground plane. With the proposed new method, miniaturization to a greater extent can be achieved compared to the conventional method of realizing microstrip multilayer filters. In addition, coupling apertures in the ground plane used to achieve coupling among the resonators in conventional multilayer structures are eliminated. The proposed design is more flexible compared to traditional multilayer filters. Layers can be easily added to increase the filter order. To demonstrate the method, a miniaturized two-layered bandpass filter centered at 728 MHz with low insertion loss is implemented and investigated. Miniaturization of more than 25% is achieved compared to the conventional broadside coupled structure and more than 40% miniaturization compared to the edge coupled structure. The new microstrip filter discussed in this paper can be realized using simple fabrication techniques.
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