A low-profile broadband dual-circularly-polarized metasurface antenna is proposed. The antenna consists of a modified cross-slot coupling antenna with a lattice of 4 × 4 square metallic patches. A characteristic mode analysis is used to investigate the operating mechanism of the metasurface antenna. Then, the desired modes are chosen and excited by the modified slots. The modified slot introduces a new resonance to achieve broadband characteristic. Two orthogonally placed microstrip lines are used to achieve both right-hand circular polarization and left-hand circular polarization in the same frequency band. The final prototype, with an overall size of 100 mm × 100 mm × 6 mm (0.71 λ ₀ × 0.71 λ ₀ × 0.04 λ ₀ at 2.15 GHz) is measured. The measured results show that the proposed antenna has an axial-ratio bandwidth of 31.3% and an impedance bandwidth of 52.5%.

A broadband metasurface antenna is proposed in this letter. The antenna is based on a nonperiodic metasurface. The modal behavior of the metasurface and the slot is analyzed using characteristic modes analysis (CMA). Three desired modes of the metasurface are found. Furthermore, the slot acts not only as a feeding structure but also as a metasurface-loaded antenna. By combining the three modes of the metasurface and the modes of the slot, the wide bandwidth is achieved under multimode resonance. To verify the design, the proposed metasurface antenna is simulated, fabricated, and tested. The results show that the proposed metasurface antenna achieves a wide impedance bandwidth above 79% and a wide 3 dB realized gain bandwidth of 77%.

A low profile dual-polarized metasurface antenna with high isolation and low cross-polarization is proposed in this paper. The proposed metasurface consists of an array of 4×4 patches. Through characteristic mode analysis (CMA), a pair of degenerate modes are identified to achieve the desired radiation features. By modifying the patches of the metasurface, the strongest mode current distributions of the two modes are moved from the center to the edge of the metasurface. One feed port can efficiently excite only one particular mode, and the other mode cannot be excited simultaneously. Each of the modes are differentially fed using two Y-shaped microstrip lines via two slots. As a result, both high isolation and low cross-polarization can be obtained. The simulation and experimental results show that the proposed antenna achieves not only a low profile of 0.05 λ0 (where λ0 is the free-space wavelength at the center resonant frequency) but also a broad impedance bandwidth of 36% and a high isolation of 53 dB. In particular, the enhanced cross polarization discrimination (XPD) is lower than -40 dB at the broadside, lower than -33 dB within ±30° of the main lobe, and lower than -28 dB within ±60° of the main lobe.

A low-profile circularly polarized (CP) omnidirectional metasurface antenna is developed and integrated with solar cells. The proposed metasurface consists of an array of 4x4 square rings. Square solar cells are mounted inside each square ring. Characteristic mode analysis (CMA) is used to analyze the modal behavior of the metasurface. The results demonstrate that the solar cells inside the square rings have a slight influence on the metasurface antenna. This arrangement not only enables total exposure of the solar cells to sunlight but also exhibits no degradation of performance of the antenna. Then, by using a proper feeding structure to excite the selected modes, the CP omnidirectional radiation pattern is realized. The proposed antenna, with and without solar cells, is fabricated and measured. The results demonstrate that the proposed antenna with and without solar cells realizes a similar impedance bandwidth and a similar CP bandwidth. Furthermore, the radiation efficiencies of the metasurface antennas with and without solar cells are similar.

A single-layer broadband vertically polarized (VP) omnidirectional metasurface antenna is proposed in this communication. To guide the optimization and design of the antenna, characteristic mode (CM) analysis (CMA) is performed to investigate the intrinsic modal behaviors of the metasurface. A mode with VP omnidirectional radiation is first identified by analyzing an array of $4 \times 4$ squared patches. To excite this mode, a complex feeding network is required, which leads to gain loss and increased structural complexity. Furthermore, the mode currents change with the frequency. Consequently, this mode cannot be excited efficiently in a wide bandwidth using a fixed feeding structure. To solve these problems, the modal current is analyzed at different frequencies. The sizes of the four corner patches and the other four edge patches are adjusted to optimize the modal behaviors. After optimization, the mode becomes much more desirable and can be efficiently excited in a wide bandwidth using a single probe. Therefore, the proposed antenna can be designed on a single-layer substrate and excited by a simple feeding structure with broadband characteristics. To verify the design, the proposed metasurface antenna is simulated, fabricated, and evaluated. The results show that the proposed metasurface antenna can achieve a broad bandwidth of 64.2%.

For the application of ambient electromagnetic energy harvesting, it is important to collect as much electromagnetic energy as possible in broadband and with arbitrary polarizations. Meanwhile, the antenna should also have high receiving efficiency in order to effectively convert the collected energy into radio-frequency current. To meet this requirement, a novel broadband dual circularly polarized (CP) dielectric resonator antenna (DRA) with high receiving efficiency is proposed. The characteristic mode analysis (CMA) is applied to analyze the proposed DRA to reveal the intrinsic behavior of the DR, so that proper mode excitations can be guided. It is found that broadband CP radiation is achieved by exciting two pairs of orthogonal degeneration modes. By choosing a broadband feeding structure, a broadband DRA can be obtained. When different ports are excited, left-hand CP (LHCP) or right-hand CP (RHCP) field can be generated. Measurements show that the antenna has a 3-dB axial-ratio (AR) bandwidth of 43.5%. The impedance bandwidth is wide enough to cover the whole AR bandwidth. The receiving efficiency of the proposed antenna is about 85% within most of the operating frequency range. With these appealing features, the proposed DRA is an excellent candidate for ambient electromagnetic energy harvesting.