Performance enhancement of microstrip open loop resonator band pass filter by defected ground structures

A selective Bandpass filter is presented by using a pairs of open loop resonators, each has a perimeter about half-wavelength. For the improvement of bandwidth, we have used a DGS under coupled area. A prototype filter circuit of center frequency at 1.8 GHz has been fabricated with PTFE substrate. Measured result shows bandwidth of 200 MHz with insertion loss of 0.2 dB at center frequency of 1.95 GHz. We have used defected ground structures under feed lines for extra stopband rejection mode to the 2(nd) and 3(rd) harmonics of the filter with attenuation losses over than -25 dB, without affecting the center frequency and insertion loss of the originally designed filter.