In this paper, the spoof surface plasmon polaritons (SSPPs) transmission line (TL) of periodical grooved bow-tie cells is proposed. The complex propagation constant and characteristic impedance of the SSPPs TLs and microstrip lines (MLs) are extracted using the analytical method of generalized lossy TL theory. The properties of the SSPPs TLs with different substrates and the same geometrical configuration are experimented. Then, for comparison, two ML counterparts are also experimented, which shows that the SSPPs TL is less sensitive to the thickness, dielectric constant and loss tangent of the chosen substrate below the cutoff frequency, compared with the ML ones. The single-conductor co-planar quasi-symmetry unequal power divider based on this SSPPs TL is presented in microwave frequencies. For experimental validation, the 0-dB, 2-dB, and 5-dB power dividers are designed, fabricated, and measured. Both simulated and measured results verify that the unequal power divider is a flexible option, which offers massive advantages including single-conductor co-planar quasi-symmetry structures, wide-band operation, and convenient implementations of different power-dividing ratios. Hence, it can be expected that the proposed unequal power dividers will inspire further researches on SSPPs for future design of novel planar passive and active microwave components, circuits and systems.

A multi-band vehicle-mounted garden-themed antenna with isolation improvement for 2 G/3 G/LTE/5 G sub-6 GHz/WiFi/Bluetooth is proposed in this paper. This antenna is mainly comprised of three printed circuit board (PCB) substrates. For good isolation level, two pavilion-shaped 4G-band (including the 2 G/3 G/LTE frequency bands) patches and two tree-shaped 5G-band (including the 5 G sub-6 GHz bands) patches are diagonally placed at the four corners of the top-layer substrate, whereas the other two tree-shaped 5G-band patches and a peach-shaped WiFi-band patch, are arranged in a line on the bottom substrate. To further improve the isolation, two fence posts are arranged between the neighboring antenna elements on the bottom substrate. Finally, a flower-shaped patch array printed on the middle substrate is employed to reduce the mutual coupling factor, broaden the frequency bandwidth, improve the peak gain and the radiation efficiency, especially for the 5G-band antenna element printed on the bottom-layer substrate. Characteristic mode analysis (CMA) method is introduced to the antenna radiation pattern and frequency bandwidth. The measured results showed good impedance bandwidths of 20.69 $\%$ (0.78-0.96 GHz) and 106.8 $\%$ (1.37-4.51 GHz) for 2 G/3 G/LTE bands, 71.81 $\%$ (2.41-5.11 GHz) for 5 G bands, and 84.34 $\%$ (2.4-5.9 GHz) for WiFi bands are achieved with a compact size of 0.338 $\times 0.130\times$ 0.094 $\lambda _{0}^{3}$ (free-space wavelength at the starting frequency). Furthermore, across the above bands of interest, an isolation level of better than 16 dB can be achieved.

A dual-polarized (DP) antenna with conical radiation pattern and high gain characteristics is proposed. It is mainly comprised of a horizontally polarized (HP) array, a vertically polarized (VP) element, a fence, and a feeding network. In the HP direction, a rotatable stacked SIW-to-coaxial-to-SIW transition (SCST) is meticulously designed to yield omni-directional radiation with low gain variations and low transmission loss. As for the VP direction, two orthogonal substrates that are arranged above the VP radiating element act as a holder to fix the director, resulting in gain enhancement. Here, the VP element is placed in the center of the HP array to share the aperture for size reduction. Moreover, the top-hat-shaped metal fence collaborates with the large-size ground plane to yield dual conical beams with high gain in DP directions. From the measured results, the HP direction has exhibited desirable bandwidth of 11.4% (24.8-27.8 GHz) with a peak gain of 11.6 dBi, and the VP direction has demonstrated wider bandwidth of 13.7% (24.5-28.1 GHz) with a corresponding gain up to 8.1 dBi. Notably, low gain variations of ±0.65 dBi and ±0.8 dBi are also realized for the HP and VP directions, respectively. Therefore, a DP antenna with high gain conical beams and low gain variations can be obtained for the fifth generation (5G) millimeter-wave (MMW) ceiling communications.

A dual-polarized wideband omnidirectional ceiling antenna with low gain variations (GVs) for 2G/3G/LTE/5G sub-6 GHz indoor applications is presented. The vertical-polarized (VP) antenna element that is arranged in the center is composed of a pair of orthogonal semicircular plates and a top-loaded circular brass disk, whereas the horizontal-polarized (HP) antenna element is comprised of four printed dipoles with truncated corners, four arc-shaped parasitic patches around the dipoles, and a 1-to-4 power divider feeding network on the bottom. The feeding network with the “hook”-shaped stubs cooperates with the slot between the adjacent printed dipoles to improve the isolation between the VP and HP antenna elements. Furthermore, the top-loaded circular brass disk, the four arc-shaped parasitic patches, and two pairs of extended strips printed along the ±45° direction are used to reduce the GVs but broaden the operating bandwidths. Experimental results demonstrate that a wide overlapped frequency bandwidth of 74.9% (1.66–3.65 GHz) with an excellent port isolation (≥40 dB) is achieved for the aforesaid application scenarios.

Driven by the continuous demand for system integration and device miniaturization, integrating multiple diversified functions into a single metasurface hybridized with the tunable metaparticle is highly demanding at terahertz (THz) range. However, up to now, because of the limitation of the tunable metaparticle at terahertz range, most of the metasurfaces feature a single function only or process similar functionalities at a single frequency. A reconfigurable multifunctional metasurface which can realize the switch of transmission and reflection and manipulate the linearized polarization state of electromagnetic waves simultaneously over a controllable terahertz frequency range based on the vanadium dioxide was designed for the first time in the paper. The numerical result demonstrates the validity of the appropriately designed metasurface. Simulation results show that the reconfigurable and multifunctional performance of this metasurface can be acquired over 1.59 THz to 1.74 THz without re-optimizing or re-fabricating structures, which effectively extends the operating frequencies. The proposed metasurface holds potential for electromagnetic wave manipulation and this study can motivate the realization of the wideband multifunctional metasurface and the software-driven reconfigurable metasurface at terahertz frequencies.