Modulated Metasurface Antennas Research Articles

Non-linear least squares optimization for the design of low side lobe level bidirectional holographic antennas

In this manuscript, an analytical framework aimed at optimizing the performance of double-sided modulated metasurface antennas featuring independent beams is introduced. To synthesize the aperture field distribution, the proposed framework employs the adiabatic Floquet-wave expansion technique as the analysis strategy and the aperture field estimation method as the synthesis technique. For the modulation index, the nonuniform distribution with unknown coefficients is chosen to improve the side lobe level. The ideal values for each of these unknown coefficients are obtained using a non-linear least squares approach. The obtained results demonstrate the efficiency of the proposed methodology for both vortex and pencil beams. To illustrate its feasibility, a double-sided hologram is designed, radiating a pencil beam and a vortex beam on each side.

Low-Profile Multibeam Antenna Based on Modulated Metasurface

Linearly polarized (LP) multi-port multi-beam antennas (MBAs) based on modulated metasurface (MTS) are presented in this paper. A key challenge for MTS to generate multiple independent beams is the mutual interference of different impedance modulations. This interference can lead to reduced directivities of main beams and high side lobe levels (SLLs). A full-wave based optimization for a large number of beams is significantly time consuming and practically ineffective. The source locations are optimized here by employing the aperture field calculated from a zeroth-order approximation (ZOA) of the currents on the surface. This provides a good preliminary accuracy without any full-wave analysis. The relevant design method is verified by two examples of 3-beam and 7-beam MTS antennas. Furthermore, the 7-beam MTS antenna is fabricated and successfully measured. The measurements show a measured realized gain of 20 dBi at 14.10 GHz with a beam coverage (10-dB overlap) up to ±36°. It is seen that the same performance requires larger areas with conventional summation of apertures.

A Metasurface Radar Monopulse Antenna

This article presents the design, fabrication, and testing of a modulated metasurface (MTS) antenna for monopulse radar tracking at Ka-band. The antenna consists of a circular, thin grounded dielectric layer printed by a texture of metallic patches modulated in shape and size. The patch layer can be modeled as a spatially variable capacitive impedance sheet, which together with the grounded slab contribution provides an overall, modulated, inductive boundary condition (BC). The antenna aperture is divided into four identical angular quadrants each of them radiating independent broadside beams when excited by an individual monopole launcher. Each of the four launchers excites a TM cylindrical surface wave (SW), which is progressively converted into a leaky-wave (LW) by the MTS. The four subapertures are virtually separated by properly designing the MTS modulation. To this end, the LW attenuation constant is calibrated for dumping sufficiently each individual SW, thus preventing the interaction between the adjacent regions. Therefore, the printed structure is not delimited by any physical separation, but only by a continuous change of the equivalent BCs. The monopulse-type linearly polarized <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Sigma $ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \Delta $ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta _{\mathrm{ E}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta _{\mathrm{ H}}$ </tex-math></inline-formula> beams are obtained by combining the source excitation with a simple phasing scheme. Notably, this solution does not affect the overall lightness, low profile, feed simplicity, and low fabrication cost of the structure, which constitutes an inherent advantage with respect to more traditional waveguide-based solutions.

Wide-Scanning Circularly Polarized Reflector-Based Modulated Metasurface Antenna Enabled by a Broadband Polarizer

A circularly polarized leaky-wave antenna capable of frequency scanning is proposed in this article. The main objective is to achieve high gain and polarization purity without the need for a complex feed network. The antenna consists of two independent modules: 1) anisotropic-modulated metasurface antenna (MoMetA) for generating vertically polarized radiation with an operational scanning range of 19°–47° in elevation and 2) wideband polarization converter with high angular stability and capable of converting vertically linear polarization into circular polarization (CP). The aperture field estimation (AFE) method is used to design the MoMetA with high accuracy in predicting far-field pattern without the need for full-wave simulations. The gain of antenna in the bandwidth of 16–21 GHz is obtained well than 18 dBic. Simulation results show that the axial ratio in the maximum gain direction is lower than 3 dB all over its operational frequency bandwidth, demonstrating the proper operation of polarizer. In order to verify the simulation results, one prototype of the antenna is fabricated and its radiation patterns are measured in an anechoic chamber.

Multi-beam modulated metasurface antenna for 5G backhaul applications at K-band

We explore the use of a new modulated metasurface (MTS) antenna topology as solution for wireless backhaul at K band. The proposed structure is composed of a quasi-optical beamformer, which feeds the modulated MTS radiating aperture. These two elements are vertically stacked in a two-layer pillbox architecture to produce a very compact antenna. Furthermore, our design is able to provide several beams at different pointing angles and, hence, it offers the possibility of discrete beam steering by beam switching. The employment of a modulated MTS and the compactness given by the pillbox approach lead to a high-gain and low-profile antenna that could be an appealing solution for mobile backhaul networks.

Multifrequency Band Synthesis of Modulated Metasurface Antennas

We present and discuss the synthesis of modulated metasurface (MTS) antennas capable of radiating multiple beams at multiple frequencies. The proposed technique is built on a recently developed integral equation formalism. Assuming that the MTS can be homogenized over the whole frequency band, the sheet impedance describing the MTS can be accurately modeled w.r.t. the frequency. This allows us to simultaneously establish multiple systems of equations standing for a design at different frequencies, and possibly different feed points, and to solve the overall problem in the least-squares sense. The method can be used either for improving the antenna bandwidth (if the frequencies of interest are relatively close) or for multiplexing several applications operating at different frequencies. Several synthesis examples are provided and numerically validated.

Design Method for Modulated Metasurface Antennas Composed of Anisotropic Elements Based on Generalized Boundary Conditions

A novel and efficient design method for modulated metasurface antennas composed of anisotropic elements is proposed in this letter, based on generalized boundary conditions. An analytical model of surface impedance is derived with the effective surface susceptibilities of the patch layer, an equivalent circuit model of the element, and the transverse resonance technique. The analysis method of elements is then utilized to design modulated metasurface antennas with desired radiation patterns. A linearly polarized antenna prototype with a circular aperture of 180 mm in diameter is designed and measured to demonstrate the effectiveness of this approach. The prototype shows good radiation characteristics at 9.75 GHz with the measured gain of 21.77 dB and the aperture efficiency of 44.5%.

Method of Moments Simulation of Modulated Metasurface Antennas With a Set of Orthogonal Entire-Domain Basis Functions

A family of orthogonal and entire-domain basis functions (named Fourier-Bessel) is proposed for the analysis of circular modulated metasurface (MTS) antennas. In the structures at hand, the MTS is accounted for in the electric field integral equation (EFIE) as a sheet transition impedance boundary condition on the top of a grounded dielectric slab. The closed-form Hankel transform of the Fourier-Bessel basis functions (FBBFs) allows one to use a spectral domain formulation in the method-of-moments (MoM) solution of the EFIE. Moreover, these basis functions are fully orthogonal, which implies that they are able to represent the global evolution of the current distribution in a compact form. FBBFs also present a better filtering capability of their spectrum compared to other well-known orthogonal families such as the Zernike functions. The obtained MoM matrix is sparse and compact, and it is thus very well-conditioned and can be efficiently computed and inverted. The numerical results based on the proposed decomposition are presented and compared with those based on the use of the Gaussian ring basis functions and with the full-wave analysis of MTS antennas implemented with small printed elements. A very good agreement is observed.

A Quasi-Direct Method for the Surface Impedance Design of Modulated Metasurface Antennas

A new approach is presented for synthesizing modulated metasurface (MTS) antennas (MoMetAs) with arbitrary radiation patterns, assumed to be given in amplitude, phase, and polarization. The MTS is defined on a circular domain and is represented as a continuous sheet transition impedance boundary condition (IBC) on the top of a grounded substrate. The proposed method relies on an entire-domain discretization of the electric field integral equation (EFIE). Via the dyadic Green’s function of the grounded substrate, the desired radiation pattern is translated into the visible part of the surface current spectrum, decomposed into entire-domain and orthogonal basis functions, while the invisible part of the spectrum stems from the solution of the unmodulated sheet problem. The EFIE is then inverted to obtain the sheet impedance, which is constrained to be anti-Hermitian, as required for implementation with lossless patches. The efficiency of the method relies on the precomputation of the reaction integrals between three functions: basis functions for currents and impedances and testing functions for fields. The formulation is presented first for the scalar (isotropic) MTS case and then generalized to the tensorial (anisotropic) MTSs. Several radiation patterns are presented and designed successfully. A full-wave method-of-moment code is used to validate the designed MTSs IBC.

Guided Waves on Scalar and Tensorial Reactance Surfaces Modulated by Periodic Functions: A Circuital Approach

A technique to derive the propagation characteristics and field distributions of waves guided by a scalar and tensorial reactance surfaces modulated by periodic or discrete Fourier spectrum functions in the propagation direction is presented. The method based on an equivalent lumped circuits approach can be seen as a generalization of Oliner's method for the TM propagation on scalar sinusoidally modulated reactance surfaces. The numerical results are obtained for both surface wave and leaky wave solutions. The relevance of these studies to modulated metasurface antennas is discussed.

Additive Manufactured Metal-Only Modulated Metasurface Antennas

In metasurface (MTS) antennas, a surface wave (SW) is gradually transformed into a leaky wave, owing to the SW interaction with a reactance tensor periodically modulated in the aperture plane. In such antennas, the modulated reactance is typically obtained by subwavelength patches printed on a dielectric substrate. As an alternative technology, we propose the use of a class of metal-only MTSs in the synthesis of the reactance tensor. The absence of dielectric is useful to withstand harsh environments in space applications. The proposed element consists of a metallic cylinder with elliptical cross section, placed on a ground plane and arranged in a square lattice with subwavelength unit cells. This structure is well suited for fabrication by metal additive manufacturing. We have applied the metal cylinders in the design of a right-handed circularly polarized antenna, with broadside pencil beam at Ka -band. The obtained performance has been verified by full-wave simulations. A prototype has been successfully manufactured and measured, showing a good performance.

Dual Circularly Polarized Broadside Beam Metasurface Antenna

This paper presents the design of a modulated metasurface (MTS) antenna capable to provide both right-hand (RH) and left-hand (LH) circularly polarized (CP) boresight radiation at Ku-band (13.5 GHz). This antenna is based on the interaction of two cylindrical-wavefront surface wave (SW) modes of transverse electric (TE) and transverse magnetic (TM) types with a rotationally symmetric, anisotropic-modulated MTS placed on top of a grounded slab. A properly designed centered circular waveguide feed excites the two orthogonal (decoupled) SW modes and guarantees the balance of the power associated with each of them. By a proper selection of the anisotropy and modulation of the MTS pattern, the phase velocities of the two modes are synchronized, and leakage is generated in broadside direction with two orthogonal linear polarizations. When the circular waveguide is excited with two mutually orthogonal TE11 modes in phase–quadrature, an LHCP or RHCP antenna is obtained. This paper explains the feeding system and the MTS requirements that guarantee the balanced conditions of the TM/TE SWs and consequent generation of dual CP boresight radiation.

Realization and Measurement of Broadside Beam Modulated Metasurface Antennas

Realizations and measurements of broadside-beam circularly polarized anisotropic metasurface antennas are presented. The validity of the design methodology proposed in a previous paper is here experimentally demonstrated by developing a 28-dBi transmitting antenna and a 22-dBi receiving antenna for space-to-ground links at X-band. High polarization purity is obtained by using a simple vertical (i.e., linearly polarized) monopole as feeder, thus providing experimental evidence of the fact that anisotropic surface increases aperture efficiency with respect to isotropic surface implementation. Both the realized antennas are 1.5 mm thick and do not require a beamforming network as they only incorporate the feeding monopole in the same substrate.

Modulated Metasurface Antennas for Space: Synthesis, Analysis and Realizations

This paper presents design and analysis methods for planar antennas based on modulated metasurfaces (MTSs). These antennas operate on an interaction between a cylindrical surface-wave (SW) excited by an isotropic TM radiator, and an MTS having a spatially modulated equivalent impedance. The MTS is realized by using sub-wavelength patches printed on a grounded slab, thus resulting in a structure with light weight and compact volume. Both features are appealing characteristics for space applications. This paper introduces for the first time an impedance-based amplitude synthesis of the aperture field distribution and shows several new examples of antennas for space applications obtained in recent research projects financed by the European Space Agency.