Design of bandpass filter using star loop dual mode resonator

This paper presents bandpass filter designs using a star type loop resonator. Both single mode and dual mode microstrip filters are designed. A small triangular patch element is used to perturb the fields and couple the dual degenerate modes for constructing dual mode bandpass filter. Two single mode filters are designed and results show chebyshev like characteristics. The dual mode filter exhibits elliptic response having transmission zeros on either side of the center frequency which improves the selectivity of the filter. The respective electric field patterns generated by the full wave simulator are given. The first single mode filter designed shows an insertion loss of 1 dB and return loss of 18.5 dB at 4.1 GHz, with fractional bandwidth (FBW) of 4.87%. The second single mode filter gives an insertion loss of 1 dB and return loss of 22.5 dB at 4.15 GHz with FBW of 6.097%. The dual mode filter achieves an insertion loss of 2 dB and return loss of 34 dB at 4.1 GHz with FBW of 4.93%.