This article presents a new generalized direct synthesis method to design multiband bandpass filters (MBPFs). Based on the proposed frequency transformation technique, it can be used to synthesize an arbitrary number of bandpass filters starting from a low-pass prototype filter. This synthesis allows determining analytically all the resonant angular frequencies and slope parameters of all the bandpass filters from the low and high cut-off angular frequencies used as initial specifications whatever the number of bands. To validate our method, triple-, quad-, and quint-band third-order Chebyshev bandpass filters are designed and implemented in microstrip technologies. Very good agreements were achieved between simulation responses and measurements.

This letter introduces two dual-band bandpass filters (BPFs) based on novel metal-integrated suspended line (MISL) technology. These filters are designed to operate simultaneously in both the C -band and Ku -band based on dual-mode MISL resonators with low loss. Unlike traditional coaxial designs, the proposed MISL-based dual-band filters offer a compact footprint and allow for greater flexibility in design and fabrication. To improve the attenuation between the two passbands, the mixed electric and magnetic couplings have been used to achieve controllable between-band transmission zeros (TZs). To validate the concept, two dual-band BPFs are designed, fabricated, and measured. The measured results of the filters align well with the simulated results, confirming the compact size and low-loss characteristics of the proposed designs.

In this brief, a universal coupled topology to design cascaded multiple passbands bandpass filters (BPFs) with numerous in-band poles and out-band transmission zeros (TZs) is presented. Based on the proposed topology, the coupling matrices are constructed to design multiband bandpass filters (MBPFs) with cascaded trisection (CT) BPFs’ filtering responses. To validate the theory and topologies, a quad-band CT BPFs with TZs located in each lower sub-band is successfully synthesized firstly. Then, a dual-band and tri-band CT BPF with TZs generated in the upper side of each passband are theoretically investigated by employing the coupling matrix. Finally, the proposed dual- and tri-band examples are implemented in compact planar multi-coupled lines structures. In all individual passbands, measured insertion losses (ILs) and measured return losses (RLs) are better than 2.82 dB and 15.9 dB, respectively. Good matching level between the simulated and experimental results have been achieved.

This article proposes a novel 360 $^{\circ}$ continuously tunable phase shifter employing a synergy of high-pass and low-pass networks. The analysis of the L-type high-pass and low-pass networks elucidates the correlation between phase shift and phase slope, alongside capacitance and frequency. Exploiting the divergent phase slope of high-pass and low-pass networks at different frequencies and circuit parameters, a 180 $^{\circ}$ continuously tunable phase shifter with low in-band phase error is designed by utilizing the eight orders of high-pass and low-pass networks to compensate for each other. Further innovation yields a 360 $^{\circ}$ phase shifter, achieved by integrating a 180 $^{\circ}$ switch-type phase shifter (STPS) reliant on dual $\pi $ -type pathways. The proposed phase shifter is fabricated and measured, confirming the efficacy of the proposed phase shifter, revealing a maximal in-band phase error of merely 4.98 $^{\circ}$ within the 2.3–2.6 GHz. The incorporation of substrate-integrated suspended line (SISL) double-side inductors and capacitors consolidates its advantages, this phase shifter achieving a compact size which is only 0.335 $\lambda_{{\text{g}}}$ $\times$ 0.085 $\lambda_{{\text{g}}}$ .

Multimode resonators (MMRs) can be utilized to build multiband bandpass filters (MBPFs). Each individual mode serves as the operating mode for one of the passbands in a well-known coupling topology. However, several difficulties are involved with the classical design method. In this article, we present a novel star-like topology to design MBPFs based on MMRs. Differently from the classical use, in the proposed novel topology, they are so regarded as bandstop resonators and provide multiple controllable transmission zeros. To explain the proposed concept and the implementation method, two bandstop MMRs – a classical dual-mode stub loaded resonator (DSLR) and a new tri-mode dual-stub loaded resonator (TDSR) – are investigated with detailed design formulas and diagrams. Then, a tri-band MBPF with equal individual sub-bands, a quad-band and two quint-band MBPFs with unequal individual sub-bands are synthesized and designed using the proposed bandstop MMRs. Additionally, simulations and experiments are used to verify the efficiency of all prototypes. Good agreements with the theoretical calculations are observed for all the MBPFs.

This article presents a similarity transformation method for multiband bandpass filters (MBPFs) to convert the star-like topology into the in-line topology. A general theoretical technique is introduced, which replaces the traditional step-by-step extraction of $LC$ circuit through filter synthesis with the similarity transformation rotation of coupling matrix, addresses the round-off errors in the parameter extraction process, and improves the accuracy of the theoretical synthesis results. The application of in-line topology significantly enhances the flexibility of filter design, reduces circuit complexity, and simplifies the fabrication of high-order MBPFs. Based on the substrate integrated waveguide (SIW) technology, a series of examples, including tri-band, quad-band, and particularly the first reported quint-band third-order Chebyshev SIW bandpass filters, is designed and implemented. Good agreement is achieved between simulated responses and measured results, validating the designed filter models and the proposed theoretical method.