The sharpness of the roll-off response of bandpass filters is a major concern for wireless communication systems. Bandpass filters with attenuation poles provide sharp roll-off. This paper investigates the performance of a ceramic comb-line filter with attenuation pole resonators (APRs), and studies the effects of the attenuation pole resonators on the filter response. The presented APRs are improved versions of previous ones and they are modeled here. The obtained results show that the length of APRs can be miniaturized via the loading capacitance. The resultant volume is about 400 mm3, which is very small comparing to coaxial type filters with the same attenuation rate in the stopband. With attenuation pole resonators added, skirt attenuation is greatly improved. Narrow bandwidth bandpass filters with attenuation poles in the stopband are designed and tested. Two designed examples demonstrate the flexibility of the attenuation pole resonator in the filter configuration. Experiments show good agreement with simulation results.

An extended configuration of a stepped impedance comb-line filter is presented. The parallel stripline sections of stepped impedance resonators are coupled electromagnetically and a coupling capacitor is introduced. The creation of an attenuation pole near the passband was detailed. A design procedure for the two-pole extended filter is derived from an analysis using even-and odd-mode impedances. Experimental filters were constructed by ceramic lamination technique. They exhibited excellent performances suitable for portable telephones.