Wideband-, multiband-, and wireless-communications systems demand compact, low-insertion-loss microwave bandpass filters. But wideband couple-line bandpass filters with 3-dB fractional bandwidths greater than 20 percent tend to be difficult to fabricate. A new coupling structure for filter design is proposed by M.K. Mandal, P. Mondal, S. Sanyal, and A. Chakrabarty from the Indian Institute of Technology (Kharagpur, India). This compact bandpass-filter design uses only one resonator, which is excited by two open microstrip stubs. To obtain wide bandwidth, a ground-plane architecture technique is employed.

The resulting filter can vary 3-dB fractional bandwidth between 1.0 and 90.0 percent with good in-band and out-of-band filter characteristics. A prototype of FBW 45.1 percent and passband center frequency f₀ = 3.795 GHz shows a maximum passband insertion loss of 2.0 dB. See "Novel Compact Bandpass Filters with Wide Controllable Fractional Bandwidth," Microwave and Optical Technology Letters, Jan. 2007, p. 29.