A programmable metasurface based on liquid crystal is of great significance to versatile dynamic wave manipulation such as beamforming and beam steering. One of the biggest obstacles currently is the lack of a simple way to multi-bit programming scheme for massive electrically addressable arrays, which limits its application and magnifies drawbacks like unavoidable symmetrical beam diffraction. Here, we put forward the concept of a liquid crystal metasurface based on a resonance switching mechanism for quasi-2-bit coding control, which is established upon interdigital structure electrodes. This enables a more elaborate control of meta-unit and a quasi-2-bit modulation based on a universal binary coding system to be realized. Our design could suppress unwanted –1 diffraction order with a maximum single-beam scanning angle of ± 21° under active beam manipulation. This concept paves the way for a feasible and robust multi-bit coding scheme of a liquid crystal metasurface, which is promising for THz applications such as spatial light modulators and wireless communication.

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A novel wideband bandpass filter based on folded substrate integrated waveguide (FSIW) is presented in the article. Five square complementary splitring resonators (CSRRs) are etched in the middle layer of the FSIW. By adjusting the physical size of the CSRR structure, the resonant frequency of the CSRRs can be tuned at the same time and the stopband performance can be changed. As transverse electromagnetic (TEM) mode can be transmitted in the stripline, FSIW excited by stripline shows wider passband than that excited by microstrip line directly. To achieve perfect impedance matching, two microstrip lines to stripline transitions are added in two ports of the filter. The proposed bandpass filter exhibits compact size, high selectivity, good stopband rejection, lower radiation loss, and wideband performances. The measured results show that the fractional bandwidth of the filter is about 35.5%. The measured return loss is better than 15 dB from 4.84 GHz to 6.90 GHz, and the insertion loss is less than 1.2 dB. The comparison between the simulated results and the measured ones validate the possibility of the technology that combines the FSIW and CSRR. K E Y W O R D Sbandpass filter, compact size, complementary split-ring resonators (CSRRs), folded substrate integrated waveguide, radiation loss

An empty substrate integrated waveguide (ESIW) bandpass filter with enhanced electric and magnetic coupling is proposed in this letter. ESIW uses the air as dielectric in order to reduce the dielectric loss caused by usual medium materials. However, the existence of air cavities requires at least three layers of structure, which greatly increases the difficulty of designing complex circuits with high‐performance and multiple requirements. The proposed mixed coupling structure creates the finite transmission zeros based on ESIW technology for the first time, which makes it possible to realize complex high‐performance circuits with ESIW technology. Taking advantage of the multilayer structure of the ESIW, the proposed cross‐coupling is conveniently realized with higher frequency selectivity and lower insertion loss while maintaining the substrate of three layers and the self‐encapsulation characteristics of ESIW. A fourth‐order filter with an H‐shaped slot line on the top metal plane of two adjacent air cavities is produced. A good agreement between the measurements and simulations indicates that the proposed filter achieves an insertion loss as low as 0.7 dB and the 6.1% bandwidth at 9.85‐GHz X band. It demonstrates the improvement of insertion loss and frequency selectivity, which lays a foundation for the design of complex microwave devices.