A new simple microstrip open-loop resonators filter for high selectivity applications

In this paper the design of a novel planar microwave filter is presented. The structure is designed in microstrip technology, and it is formed by two loop resonators of different lengths. The length of the first resonator is adjusted so that a suitable odd resonance is tuned to the center frequency of the filter. The length of the second resonator is adjusted so that the next even resonance is tuned again to the center frequency of the filter. The change in phase introduced by the resonances of the loops produces a cancellation of energy at a given frequency, therefore implementing a transmission zero for high selectivity. In this paper a design example is provided, and results of a manufactured prototype are discussed. Measured results confirm the theoretical predictions, and validate the new structure for high selectivity applications.