Fundamental characteristics of microstrip resonators loaded with dielectric rods for suppression of spurious responses

Abstract

Characteristics of a class of microstrip resonators which are loaded with dielectric rods, are investigated by a finite-difference time-domain (FDTD) method. Dielectric rods to be inserted between a strip conductor and the ground plane have higher relative permittivity than that of the substrate. When a basic half-wavelength (λ/2) microstrip resonator is loaded with two dielectric rods, the electric length of a loaded half-wavelength (λ/2) resonator becomes longer than λ/2, which makes its fundamental resonant frequency (f′r) to be generated on the region lower than that of an unloaded λ/2 resonator (fr) and its first spurious response fsp1 is generated on the region higher than 2f′r. Therefore, to shift f′r back to fr, the resonator's length is to be reduced, and this, in turns, suppresses the spurious responses. A tapped resonator employing the proposed method is capable of realising the improved spurious responses together with an attenuation pole in the stopband, and of reducing the resonator's length as well. A resonator employing the presented method is fabricated using low-temperature cofired ceramic (LTCC) materials, and the analysis is verified by comparing the computed results with the measurement.