Design of Fourth Order Microstrip Filter Using the Open Loop Resonator with a Novel Interdigital Loading Element

Design of the fourth order microstrip filter with high selectivity is presented in this paper. Proposed microstrip filter circuit is carried out using the microstrip open loop resonators with a novel loading element consisting of interdigital unit cells. Dual mode is obtained by an open loop resonator with this type-loading element. Thus, the number of resonator is reduced and miniature and compact circuit is designed to be used in proposed structure. The interdigital loading element allows changing the frequency of the even mode and transmission zero, sensitively. Also, the transmission zero is arbitrarily placed using this type loading element on both side of the passband without change in the surface area of loading element. Multi-mode filter is achieved by coupling of the open loop resonators with interdigital loading element. The proposed circuit has been fabricated to validate. The simulated and measured results are compared and they have shown good agreement.