Novel wideband bandpass filter design based on two transformations of coupled microstrip line

This paper presents a novel wideband bandpass filter with wide stop-band. The filter is realized by using two transformations of coupled microstrip line and four dumbbell-shaped defected ground structures. The stub loaded coupled microstrip line with two dumbbell-shaped defected ground structures is used to generate wide pass-band, and the end-connected coupled microstrip line with dumbbell-shaped defected ground structure is employed to implement low-pass and wide stop-band. The equivalent circuit of end-connected coupled microstrip line with dumbbell-shaped defected ground structure is derived. A wideband bandpass filter with wide stop-band using coupled microstrip line with defected ground structure has been designed, fabricated and tested. The filter has 2.76GHz bandwidth centered at 4.89GHz, and 56.4% relative bandwidth has been achieved. It is also illustrated by measured results that the filter possesses a wide stop-band from 0GHz to 2.3GHz and from 6.73GHz to 20GHz, in which the insert loss is more than 20dB. Moreover, the measured insert loss is more than 28dB from 6.97GHz to 19.03GHz. Finally, it can be found that the measured results are in agreement with the simulated results.