A compact bandpass filter using dielectric resonators for cellular portable telephones

The microwave dielectric properties of a Ba_{(2 - x)}Sm_{(4 + 2x/3)}Ti₉O₂₆ system (BST) (0.0 ≤ x ≤ 0.3) and the design procedures for a microwave coaxial dielectric bandpass filter are investigated in this work. Dielectric coaxial resonators with excellent dielectric properties of ε_r = 75-80, a Q × f value = 11,000, and great temperature stability at the resonant frequency (τ_f = ∼ 0 ppm/°C) were used. Microwave bandpass filters for mobile communication systems were constructed by using such λ/4 coaxial resonators. Both air-gap coupling and direct coupling methods were applied as the coupling instruments in the transformation of electromagnetic energy. The combline theory was used to adjust the response of filters. The resonant frequency of each resonator, the coupling coefficients between adjacent resonators, and the external quality factor were adjusted to achieve the target specifications. The measured responses were in excellent agreement with the expected results.