David González-Ovejero

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.

Dual-Frequency Printed Dipole Loaded With Split Ring Resonators

A novel approach to design dual-frequency printed dipoles is presented. This approach is based on an antipodal printed dipole loaded with split ring resonators (SRRs). This technique allows the choice of any pair of working frequencies. Two prototypes, the first one working at 1.32 and 2.83 GHz and the second one working at 1.2 and 2.05 GHz, have been manufactured and measured. The experimental results show reasonable values for the efficiency at both working frequencies. Moreover, the obtained radiation pattern is dipolar at both frequencies with low cross polarization levels.

A review on array mutual coupling analysis

An overview about mutual coupling analysis in antenna arrays is given. The relationships between array impedance matrix and embedded element patterns, including beam coupling factors, are reviewed while considering general-type antennas; approximations resulting from single-mode assumptions are pointed out. For regular arrays, a common Fourier-based formalism is employed, with the array scanning method as a key tool, to explain various phenomena and analysis methods. Relationships between finite and infinite arrays are described at the physical level, as well as from the point of view of numerical analysis, considering mainly the method of moments. Noise coupling is also briefly reviewed.

Interpolatory Macro Basis Functions Analysis of Non-Periodic Arrays

An efficient method-of-moments (MoM) technique for analyzing non-periodic antenna arrays of identical elements with fixed orientation is presented. The proposed method, which uses macro basis functions (MBFs), is based on a compact representation of the interactions between MBFs. These interactions are expressed via a low-order harmonic-polynomial function after an explicit pre-computation of the reaction integrals in a very limited set of relative positions. This is possible for arrays of arbitrary size thanks to three transformations- subtraction of the far-field expression for the interactions, phase correction and modified radial distance- applied successively to the pre-computed interactions. After obtaining the harmonic-polynomial expressions, the computation time for the reaction integral between two MBFs is independent from the complexity of the antenna array element.

Gaussian Ring Basis Functions for the Analysis of Modulated Metasurface Antennas

This paper presents a novel type of basis functions, whose spectral-and space-domain properties can be exploited for the efficient method of moments (MoM) analysis of planar metasurface (MTS) antennas. The effect of the homogenized MTS is introduced in the integral equation as an impedance boundary condition (IBC). The proposed basis functions are shaped as Gaussian-type rings with small width and linear azimuthal phase. The analytical form of the spectrum of the Gaussian ring basis allows for a closed-form evaluation of the MoM impedance matrixs entries. Moreover, these basis functions account for the global evolution of the surface current density in an effective manner, reducing the size of the MoM system of equations with respect to the case of subdomain basis functions. These features allow one to carry out a direct solution for problems with a diameter of up to 15 wavelengths in less than 1 min using a conventional laptop. The applicability on practical antennas has been tested through the full-wave analysis of MTS antennas implemented with small printed elements.

Metasurfing by Transformation Electromagnetics

Transformation electromagnetics has been extended to design modulated anisotropic metasurfaces (MTSs) able to control the propagation path of surface waves (SWs). The proposed methodology consists in simple formulas that link the parameters of the transformation to the local SW wave-vector. The space-dependent wave-vector distribution is eventually implemented by subwavelength patches printed on a grounded slab. New MTS devices are presented based on these formulas.

Surface Waves Supported by Metasurfaces With Self-Complementary Geometries

This paper investigates the fundamental dispersion properties of surface waves (SWs) supported by a class of metasurfaces (MTSs) that consists of a planar layer made of metal patches and apertures with self-complementary geometries. When the MTS is suspended in free space, the supported SW is TM or TE depending on whether the vertexes of the metallic parts are interconnected or not, whereas the phase velocity is equal in the two cases. A simple analytical model, that depends only on the geometry, is derived to predict the dispersion curves for a quite general class of geometries. The proposed model is also extended to cases in which the MTSs are printed on a grounded or ungrounded dielectric slab, by using an equivalent dielectric constant. Comparisons with dispersion curves obtained through full-wave simulations confirm the accuracy of the model all over the Brillouin region. Finally, it is shown that connecting or disconnecting the metal patches along a given path allows for a confinement of the SWs on such a path. An experimental validation of this concept is also presented. This feature provides the possibility of controlling the waves direction of propagation by changing the vertexes status by means of miniaturized switches or optical control.

Metasurface transformation optics

Transformation optics has been recently proposed as a powerful method to manipulate electromagnetic fields by using anisotropic inhomogeneous volumetric media. This method can be extended to design anisotropic modulated metasurfaces (MTSs) able to control the propagation path of surface waves. In this paper, this extension is formalized by defining a systematic procedure that can be applied to design a large number of planar devices, with a significant technological simplification with respect to the realization based on volumetric media. Practical MTS designs are also presented.

A Global-Local Synthesis Approach for Large Non-Regular Arrays

A method is proposed for the synthesis of large planar non-regular arrays assuming constant magnitude for the excitation of the elements. The approach, which combines global and local optimization, is based on distinguishing aperture-type and non-coherent parts of the array factor. The following constraints are considered: minimal separation of the antennas, maximum size of the array and fixed mainbeam width. The level of the sidelobes is reduced via the minimization of a new type of flexible averaging cost function based on an Lp-norm. Possible applications of this model lie in the field of radio astronomy, satellite communications and radar systems. The proposed optimization strategy consists of three successive steps designed to be independent of each other. Starting with an equivalent continuous aperture, the first two steps act as global transformations of the radial and azimuthal positions of the elements in the initial array, while the third step performs a local optimization of individual elements of the array. This last step heavily relies on computations of the gradient of the cost function, which can be done quickly using an FFT-based procedure. The method is illustrated for several large-scale examples by considering as inputs four different types of non-regular arrays.

Accelerated Macro Basis Functions Analysis of Finite Printed Antenna Arrays Through 2D and 3D Multipole Expansions

An efficient technique is presented for the analysis of finite printed antenna arrays made of identical elements. It is based on a closed-form expression for the spatial-domain Greens function (GF) given as a finite sum of cylindrical waves (obtained through rational function fitting) plus one spherical wave. From there, a multipole expansion can be obtained for planar layered medium GFs. The macro basis function (MBF) technique is applied to the method of moments (MoM) solution of a mixed-potential integral equation, this reduces the size of the MoM impedance matrix and allows for a direct solution. However, the evaluation of the entries of this reduced matrix becomes the dominant contribution to the total computation time. The aforementioned multipole expansion is exploited to provide a fast construction of the reduced MoM matrix, whose elements are the reaction integrals between the MBFs considered to characterize the currents on the array element. The complexity of evaluating the interactions between MBFs is found to be dominated by the calculations related to the spherical wave term. Thus, taking into account the layered medium does not increase the order of the complexity with respect to a multipole-accelerated computation of reaction integrals in a homogeneous medium.