Wideband Bandpass Microstrip Filters with Triple Coupled Lines and Open/Short Stubs

Compact planar wideband bandpass filters are proposed in this paper. They are constructed of only one section of three coupled lines which is a quarter wavelength long at the midband frequency. In addition, two quarter wavelength short-circuited stubs or one open-circuited stub are added to the central line. As a result, the filters exhibit two new symmetric transmission zeros around the passband which increase the selectivity of both edges of the passband significantly. The transmission zeros can be located at any desired frequency by adjusting the width of the additional stubs. Hence, different bandwidths can be obtained where the bandwidth is reversely proportional to the width of the short-circuited stubs and directly proportional to the width of the open-circuited stub. Four filters with fractional bandwidth of about 80% and 50% are successfully realized on microstrip lines. The filters are designed, simulated, fabricated, and measured where excellent agreement is obtained.