Metasurfaces deployed for generating electromagnetic waves that carry orbital angular momentum (OAM) in the transmission mode are generally inefficient in operation. In this paper, we present a method for the design, fabrication, and characterization of ultra-thin metasurface which could be used for generating OAM waves at microwave frequencies with high efficiency. We achieve this objective by proposing a novel bi-layer subwavelength scatterer having π retardation phase between two orthogonal polarizations of an incident circularly polarized (CP) wave that also have high amplitudes. When analyzed our setup using Jones matrices, we find that the scatterer inverts the spin direction of the incident CP wave with high efficiency. Furthermore, the setup provides full phase control with the spatial rotation of the scatterer as per Panchratnum Berry Phase Mechanism. To further illustrate the utility of the proposal, a metasurface was designed based on the proposed scatterer and the generation of OAM waves is validated through both simulations and experimental measurements. To further clarify the points, a rigorous analysis for the transmission and conversion efficiencies is presented.

This paper presents a novel approach to design compact wearable antennas based on metasurfaces. The behavior of compact metasurfaces is modeled with a composite right-left handed transmission line (CRLH TL). By controlling the dispersion curve, the resonant modes of the compact metasurface can be tuned efficiently. A printed coplanar waveguide (CPW) monopole antenna is used as the feed structure to excite the compact metasurface, which will result in a low profile antenna with low backward radiation. Following this approach, two compact antennas are designed for wearable applications. The first antenna is designed to operate at its first negative mode (-1 mode), which can realize miniaturization, but maintain the broadside radiation as for a normal microstrip antenna. The proposed prototype resonates around 2.65 GHz, with a matching bandwidth of 300 MHz. The total dimensions of the antenna are 39.4x33.4 mm2 (0.1 λ02), and its maximum gain is 2.99 dBi. The second antenna targets dual-band operation at 2.45 GHz and 3.65 GHz. A pair of symmetric modes (±1 modes) are used to generate similar radiation patterns in these two bands. The size of the antenna is 55.79x52.25 mm2 (0.2 λ02), and the maximum gains are 4.25 dBi and 7.35 dBi in the two bands, respectively. Furthermore, the performance of the antennas is analyzed on the human body. The results show that the proposed antennas are promising candidates for Wireless Body Area Networks (WBAN).

A planar antenna array based on checkerboard polarization conversion electromagnetic band-gap (EBG) structures with wideband circular polarization and low radar cross section (RCS) is presented. The array is formed by 2×2 circularly polarized (CP) element antennas fed with a sequential feeding network. On the one hand, CP radiation is obtained due to the polarization conversion characteristic of the metamaterials. A sequentially rotated feeding network is employed for wide operating bandwidth, polarization purity and radiation pattern symmetry. Measured results show that the array has a wide usable bandwidth of 18.4%, the common part of an impedance bandwidth of 37.3% (4.80-7.00 GHz), a 3-dB axial ratio (AR) bandwidth of 22.5% (5.00-6.27 GHz) and a 3-dB gain bandwidth of 18.4% (4.94-5.94 GHz). On the other hand, the checkerboard arrangement of the EBG structure generates both an in-band and a partial out-band RCS reduction under normal incidence for both x-and y-polarizations. Besides, the proposed array achieves a significant gain enhancement compared to the traditional slot array. The experimental and simulated results are in good agreement.

A novel metasurface-based method for broadband radar cross section (RCS) reduction of circularly polarized patch antennas is proposed. The RCS reduction is achieved in a much broader band than the operational band of the antenna. The simple technique is based on metasurface (MS) units on a grounded substrate in a checkerboard-like configuration. The broadband RCS reduction is achieved by combining two different destructive interference principles. In the high frequency region, for a normal incident wave, the RCS is reduced by the phase cancellation generated by the MSs and the grounded substrate. In the low frequency region, the RCS is reduced by the polarization conversion property of the MS. In this case the fields reflected by two MS sections are in counter-phase for a normal incident wave. The metasurface is combined with a circularly polarized (CP) patch antenna. It concerns a traditional corner-truncated square patch and four sequentially rotated surrounding MS cells. This dedicated MS allows two things: an additional resonant mode is excited resulting in a 3-dB AR bandwidth extension from 4.2% to 15.9%, and a broadband RCS reduction of over 6 dB from 4.95 to 15.73 GHz (104.25% bandwidth) for both x-and y-polarized incident waves.

Characteristic modes can be used to solve many radiation and scattering problems involving fully conducting structures. The basic concept is to use eigenvalues and eigenvectors of the characteristic mode equations of a certain topology, and to use these eigenmode currents to control the radiating structure’s behavior. This is accomplished by understanding the overall reflection and radiation characteristics across the operating frequency band. More specifically, characteristic modes are effective for example in determining the best location for the excitation of a radiating structure. In this paper, a comprehensive overview is provided to the different studies and their results obtained for different antennas that are based on the characteristic mode analysis method. A future perspective is given regarding the potential of this method in antenna analysis and design.