Ultracompact Single- and Dual-Band FSIW Filters With Wide Stopband Based on Multiple Embedded Hybrid Resonant Modes

In this article, three ultracompact single-/dual-band folded substrate integrated waveguide (FSIW) filters using multiple embedded hybrid resonant modes are proposed. As a novel approach, multiple planar hybrid modes are embedded in the closed FSIW to construct multiple-hybrid-resonator units in the single cavity, which exhibit significant advantages, including ultrasmall size, high unloaded quality factor, and controllable modes. For the single-band fourth-order filter, two LC modes are embedded in dual-mode FSIW featuring uncoupled striplines (SLs) to achieve the ultraminiaturized hybrid-mode resonator. Moreover, embedded LC modes are cross-coupled with FSIW modes to generate a wide stopband and two transmission zeros near the passband. Furthermore, two SL resonators with magnetic coupling and three SL resonators with cross-coupling are integrated into the aforementioned four-hybrid-resonator unit to realize ultracompact, large frequency radio, and wide stopband dual-band fourth-/second-order and fourth-/third-order FSIW filters. Finally, one single-band and two dual-band filters are designed at 4.52 GHz ( ${f} _{0}$ ), 4.51/8.18 GHz ( ${f} _{1}$ / ${f} _{2}$ ), and 4.56/8.89 GHz ( ${f} _{1}$ / ${f} _{2}$ ), and their upper stopband suppressions can reach 20 dB up to $2.82{f} _{0}$ , 18 dB up to $3.01{f} _{1}$ , and 20 dB up to $3.05{f} _{1}$ . They occupy only $0.095\lambda _{\mathrm {g}}^{2}$ , $0.097\lambda _{\mathrm {g}}^{2}$ , and $0.103\lambda _{\mathrm {g}}^{2}$ , where $\lambda _{\mathrm {g}}$ is the guided wavelength at ${f} _{0}$ or ${f} _{1}$ .