MIC Bandfilters using Open-Ring Resonators

Well-known microstrip filters, using ¿/2 straightline resonators, offer problems with integration, viz. 1) their frequency characteristic depends on the packaging; 2) parasitic coupling occurs towards the remainder of the circuit. A usual solution of this problem is either to place the filter in a separate box or to use absorbing material. Our approach has been to chose the configuration of the resonator and remainder of the filter such, that minimum radiation and surfacewave excitation occurred. The ¿/2 open-ring resonator satisfied this condition. Narrow (¿ 2%) as well as moderate bandwidth (¿ 10%) bandpass - and bandstop filters have been developed in this manner (at X-band on Alumina) with better performance than the well--known straightline ones.