A microstrip bandpass filter using coupled lines loaded by open stubs

A compact simple structure bandpass filter based on terminated three coupled line structures with high selectivity is introduced through odd and even-mode analysis. The proposed filter consists of three coupled lines loaded by open and short circuit stubs. The parallel-coupled lines structure is used to obtain high selectivity with transmission zeros near the passband. Meanwhile, simultaneously tuning on bandwidth and center frequency are introduced by coupled line. One type of bandpass filter based on terminated coupled line structure is theoretically analyzed, simulated, and measured. The stepped impedance structures at the input and output ports have been used to improve the attenuation level at the stopband region and create the multiple attenuation poles. The center frequency of 1.3 GHz, the fractional bandwidth (FBW) of about 65%, the low insertion loss of 0.25 dB, and the high return loss of 30 dB are obtained. Also, a flat group delay lower than 1.4 ns is achieved. Experimental and simulated results are presented, which are in good agreement.