A novel compact quadruple-mode microstrip bandpass filter with low harmonic response

A novel compact quadruple-mode resonator is presented in this paper. The quadruple-mode resonator consists of a pair of quarter-wavelength resonators and a pair of L-C type resonators connected to each other through a grounded via. The new resonator has the same size as a dual mode square open-loop resonator published earlier, but has four modes and better harmonic response. The resonator filter has a centre frequency of 0.99 GHz and passband insertion loss of 1.2 dB. The selectivity of the filter and stopband response can be improved by implementing different feed structures. Simulated and measured results are in good agreement.