This paper presents new designs, implementation and experiments of metasurface (MTS) antennas constituted by subwavelength elements printed on a grounded dielectric slab. These antennas exploit the interaction between a cylindrical surface wave (SW) wavefront and an anisotropic impedance boundary condition (BC) to produce an almost arbitrary aperture field. They are extremely thin and excited by a simple in-plane monopole. By tailoring the BC through the shaping of the printed elements, these antennas can be largely customized in terms of beam shape, bandwidth and polarization. In this paper, we describe new designs and their implementation and measurements. It is experimentally shown for the first time that these antennas can have aperture efficiency up to 70%, a bandwidth up to 30%, they can produce two different direction beams of high-gain and similar beams at two different frequencies, showing performances never reached before.
Modulated metasurface (MTS) antennas with broadside beam rely on the interaction between a radially modulated equivalent impedance and a surface wave (SW) with cylindrical wave-front, launched by a point source. At the frequency where the SW wavelength matches the period of the impedance modulation, the-1 indexed (leaky) mode of the Floquet-mode expansion in cylindrical-coordinates provides a broadside beam. The mismatch between the SW wavelength and the period of the modulation imposes a limitation on the product bandwidth-gain. Here, we overcome this limitation by exponentially stretching the radial period of the impedance modulation. Doing so, an annular active region is generated on the antenna aperture, which moves from the antenna center to the circular rim as the frequency decreases. This mechanism enables a broadside beam over an extreme large bandwidth. We therefore extend significantly the applicability of these antennas, e.g., to requirements of 30 1.5 dB gain over 30% bandwidths. Here, an analytical formulation is proposed to treat the active region migration and edge outgoing by a Fresnel-type transition function. This function predicts in closed form the antenna bandwidth and average gain. A more accurate gain versus frequency response is also introduced by an integral formula that accounts for the frequency dependent amplitude distribution of the aperture fields. The theory is validated by full-wave simulations and by measurements of a prototype realized by subwavelength elliptical patches. The presented results show that these antennas can provide a performance difficult to reach by any other flat antennas based on printed technology.
This paper investigates the conditions for a perfect anomalous reflection through a modulated metasurface consisting of a metallic cladding printed over a grounded slab. Differently to what has been previously published, the problem is rigorously addressed by modeling the metallic cladding through an equivalent penetrable impedance and accounting for the grounded slab through the problem's Green's function. It is shown that without polarization transformation, the exact solution exists only for the special case of retroreflection, and, in that case, it can be done simultaneously for the two orthogonal polarizations, with an arbitrary phase shift among the two. On the other hand, changing the polarization of the reflected wave allows one to find an exact solution for arbitrary combinations of incidence and reflection angles. The exact solution is found by imposing that the induced currents radiating with the Green's function of the background problem simultaneously create the desired reflected beam and cancel the specular reflection from the grounded slab. This approach leads to the derivation of a closed-form expression for the homogenized penetrable impedance profile providing perfect anomalous reflection, i.e., ensuring the vanishing of all the coefficients of the waves associated with unwanted diffraction orders, including the specular reflected wave and the evanescent waves. This result is of great practical interest, since the derived penetrable impedance profile can be readily implemented through a simple distribution of metallic patches. The feasibility of this approach is verified through full wave simulations of both the ideal impedance and the patch-based structure, which confirm the effectiveness of the proposed solution.
Modulated metasurface (MTS) antennas can provide a broadside pencil beam at the frequency where the cylindrical surface wave (SW) wavelength matches the period of the impedance modulation. For modulations with constant period, the mismatch between the SW wavelength and the period imposes a limitation on the gain-bandwidth product. However, this limitation can be overcome by shaping the local period as a function of the radial distance. Doing so, we generate an annular active region on the antenna aperture, where the SW-toimpedance interaction mainly occurs. Such active region moves from the antenna center to the circular rim as the frequency decreases. This paper shows that one can optimize the profile of the local periodicity function to obtain broadside pencil beams over large bandwidths, while preserving the flatness of the gain versus frequency response and a good stability of the phase center. The antenna performances so obtained are really unique for flat antennas based on printed technology. Finally, we present a simple formula for the product between average gain and bandwidth, which gradually blends into the already known expression for modulations with constant period. This formula establishes an absolute limit of the gain-bandwidth product, which only depends on the wavelength-normalized antenna radius at the central frequency.
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