Miniaturized Wide-Stopband FSIW-LC Diplexer With High Selectivity and Narrow Channel Spacing

In this brief, a compact diplexer on folded substrate integrated waveguide (FSIW)-LC resonator with high selectivity and narrow channel spacing is proposed. For Channel I/II, two LC modes are embedded in the dual-mode FSIW (TE101 and TE201) to achieve the miniaturized quadruple-mode resonator, and these two LC modes are cross-coupled with TE101 and TE201 to generate two transmission zeros near the passband. Therefore, the high selective fourth-order filtering response can be designed. In addition, the embedded stepped-impedance-resonator (SIR) isolation network is applied to improve input/output matching and output isolation, while further miniaturizing the diplexer. In final, the proposed diplexer with 3-dB fractional bandwidths of 14% (Channel I) and 12.6% (Channel II) is designed at 4.84 GHz (Channel I @ <inline-formula> <tex-math notation="LaTeX">$f_{1})$ </tex-math></inline-formula> and 5.86 GHz (Channel II @ <inline-formula> <tex-math notation="LaTeX">$f_{2})$ </tex-math></inline-formula>. The shape factors, upper-stopband rejection and isolation are 1.49/1.28 (Channel I/Channel II), 24 dB extended to <inline-formula> <tex-math notation="LaTeX">$2.49f_{1}$ </tex-math></inline-formula> and 33.4 dB, respectively. It occupies only <inline-formula> <tex-math notation="LaTeX">$0.28~{\lambda }_{\mathrm{ g}}^{2}$ </tex-math></inline-formula>, where <inline-formula> <tex-math notation="LaTeX">${\lambda }_{\mathrm{ g}}$ </tex-math></inline-formula> is the guided wavelength at <inline-formula> <tex-math notation="LaTeX">$f_{1}$ </tex-math></inline-formula>.