A new design of a miniature filter on microstrip resonators with an interdigital structure of conductors

A microstrip bandpass filter of new design based on original resonators with an interdigital structure of conductors has been studied. The proposed filters of third to sixth order are distinguished for their high frequency-selective properties and much smaller size than analogs. It is established that a broad stop band, extending up to a sixfold central bandpass frequency, is determined by low unloaded Q of higher resonance mode and weak coupling of resonators in the pass band. It is shown for the first time that, as the spacing of interdigital stripe conductors decreases, the Q of higher resonance mode monotonically drops, while the Q value for the first operating mode remains high. A prototype fourth-order filter with a central frequency of 0.9 GHz manufactured on a ceramic substrate with dielectric permittivity $\varepsilon$ = 80 has microstrip topology dimensions of 9.5 $\times$ 4.6 $\times$ 1 mm$^3$. The electrodynamic 3D model simulations of the filter characteristics agree well with the results of measurements.