Microwave Filter Design by Geometrical Tuning of Periodic Microstrip Line Structures

Microstrip line periodically loaded with slits is designed to behave as a filter (either low-pass or band-stop) in the frequency range of 1 - 8 GHz. The filtering characteristic is found to be determined by the geometric dimensions of the slits. Equivalent circuit model describing such periodic microstrip filter as Bridged T-network comprising of series inductance and shunt capacitance is proposed in this paper and the filtering characteristics are computed analytically. The results are compared with those obtained experimentally and also from a commercially available Electromagnetic simulator. The justification of our model with respect to simple series inductance equivalent circuit of slits proposed earlier by other researchers is evident from the comparative study especially in high frequency regime. An empirical formula representing the relationship between circuit elements and slit geometrical parameter is developed. The proposed formula indicates geometrical tuning of the slits can utilize the same compact periodic microstrip line structure either as low-pass or as band-stop filters.