Analysis of dual-band bandpass wireless filters using hairpin resonator by transmission line model

This paper presents a microstrip dual band band pass filter using two hairpin resonators with symmetric and asymmetric feed lines. A simple transmission line model is used to determine the two transmission zeros for the two hairpin resonator structure. The locations of transmission zeros are at the fundamental and higher mode resonances. The analysis is done through MATLAB and the equivalent circuits have also been verified. The dual-band bandpass filters with the bandwidths of 50MHz and 100 MHz have been designed with centre frequencies of 5.7 GHz and 6.9 GHz respectively. The filter shows different bandwidth for different lengths with the same coupling gap. More bandwidth is available for high insertion loss and vice-versa. Also, this filter with compact size shows a sharp pass band and lower insertion loss due to less number of coupling gaps. To obtain high selectivity of the filter, cascading structure of the filter can also be further developed.