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

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 (f_r) and its first spurious response f_sp1 is generated on the region higher than 2f′_r. Therefore, to shift f′_r back to f_r, 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.