Characteristic Analysis of Parallel and Anti-Parallel Coupled Line Structures and Their Integrated Design in Filtering Power Dividers

In this article, two filtering power dividers (FPDs) with wideband and high in-band isolation, sharp bandpass selectivity, and deep out-of-band rejections based on parallel coupled lines (PCLs) and anti-parallel coupled lines (APCLs), respectively, are proposed. The open-stub loaded PCL (OSL-PCL) and open-stub loaded APCL (OSL-APCL) are, respectively, proposed, and their transmission characteristics are investigated theoretically and numerically. Based on Wilkinson power divider (WPD) topology, when a pair of OSL-PCLs are used in place of the quarter-wavelength transmission lines, the filtering characteristic is realized. The frequency selectivity is substantially enhanced by achieving transmission zeros (TZs) close to each side of the passband. To broaden the bandwidth and improve frequency selectivity and out-of-band rejection level, OSL-APCLs instead of OSL-PCLs are employed to introduce an additional TZ in the upper stopband and achieve a better out-of-band rejection. To validate these design concepts, two FPDs with center frequencies at 3.54 and 2.77 GHz are fabricated and measured, achieving good in-band isolations of more than 19 dB. The proposed OSL-APCLs-based FPD also indicates that the minimum in-band insertion loss is 3.5 dB, the 3-dB fractional bandwidth (FBW) is approximately 54.6%, and the out-of-band rejections are greater than 23 dB. The simulated and measured results are in good agreement, proving the feasibility of the proposed design strategies.