In this paper, a novel design method for bandpass filter with attenuation poles (BAP) is presented. The changed inverter element values due to inserting either capacitors or inductors can be optimized using the linear relationship between inverter element values of a conventional bandpass filter (BPF) and those of the BAP using the Touchstone program. A 1800-1825 MHz bandpass filter with attenuation poles for duplexers is designed and fabricated using coaxial dielectric resonators. The validity of this design approach is demonstrated by a computer simulation. The resonators are simulated equivalently as shorted lossy transmission lines. The measured results of center frequency, bandwidth, and attenuation pole frequencies closely agree with the design values.

A three-dimensional analysis of Whispering-Gallery modes (W. G. modes) in a coaxial dielectric resonator is proposed and presented. The coaxial dielectric resonator is constructed from a lossy dielectric disk and ring which have diameters of several tens times as large as wavelength. Eigenvalue equations of the W. G. modes are derived rigorously from field expressions and boundary conditions. The resonant frequencies, unloaded Q values and field distributions are calculated numerically from the eigenvalue equations. These calculated results are in good agreement with experimental ones for an X band model. As a result, it is shown that a considerable quantity of modal energy can be confined in a loss-less gap between the disk and ring, and then the unloaded Q value is higher than that of a conventional dielectric disk and ring resonator.