Simulating the fine geometry of Metasurfaces (MTS) structures in a conventional way is a difficult task requiring huge computational resources. On the other hand, the metallization can usually be modeled as a (cascaded) impedance sheet(s) with modulation scale of the order of the operating wavelength. Such modeling, while homogenizing the currents and fields on the surface, is accurate in predicting the far-field as well as the near-field down to a fraction of wavelength from the surface. However, analyzing an impedance sheet lying on a grounded slab with the Method of Moments (MoM) often leads to ill-conditioning; it is certainly the case when the range of impedance spans both the capacitive and inductive domains. Such impedance range is in practice required for Reflective Intelligent Surfaces (RIS), and for some MTS antennas. This paper proposes a preconditioner aiming to solve ill-conditioning issues caused by the wide range of the surface impedance. The preconditioner involves a multiplication by the conjugate of the MoM matrix followed by a block diagonal preconditioner. The preconditioning is implemented in an accelerated scheme relying on FFTs. Given the resulting well-conditioned system of equations matrix, the unknown current distribution is efficiently computed iteratively using the Generalized Minimal RESidual (GMRES) method. A fast convergence is observed independently of the impedance range.This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible  

Observations made using antenna arrays are often limited by the noise coming from the antenna system itself and from the environment. In demanding applications such as radioastronomy, the thermal noise emitted by the multilayered substrate below the antenna array may become significant and needs to be properly characterized. Both the intensity of the noise at each port and the correlation of the noise at pairs of ports need to be quantified. In this paper, we provide a spectral formulation to calculate the noise correlation matrix of antenna arrays. The method is based on the propagative and evanescent plane wave spectrum emitted by each pair of active antenna ports into the lossy layered medium, and can thus be implemented as a post-processing step combined with any full-wave numerical solver. This formulation allows us to account for layered media with different physical temperatures at each layer. Numerical examples are provided using Square Kilometer Array (SKA) stations lying on a finite ground plane, itself lying on a layered semi-infinite soil. The behaviours of noise power and noise correlation coefficients are studied for different positions of the antennas on the ground plane. This is done considering a multilayered soil with different moisture levels. A very good agreement with the commercial software FEKO is observed regarding the evaluation of the noise correlation coefficients. Moreover, the efficiency of the algorithm in terms of computational time is shown by comparison with FEKO. The direction-dependent system noise temperature picked up by a full SKA station in presence of the soil is also given.

This paper addresses the optimization of the radiation pattern of surface-wave (SW) based metasurface (MTS) antennas. Those antennas are considered as a promising alternative to parabolic reflectors, and phased arrays due to their extremely low profile and their ability to provide high gain, shaped beams and multibeams. However, pattern synthesis with MTS antennas is very challenging because of the single active control point and the need to control surface and leaky-waves through the MTS. An accurate optimization of the radiation pattern, along with the sidelobe level requires a full-wave modelling of the feeding structure, including its coupling with the MTS. MTS synthesis methods existing in the literature usually approximate the feeder model, and neglect its coupling with the MTS. Such approximation may lead to more than 1 dB error in the predicted antenna directivity. This paper presents a technique for optimization of the far-field pattern, built on a Method of Moments (MoM) analysis tool in which the MTS coupling with the feeder, a coax probe, is fully considered. The MTS is modeled as an arbitrarily shaped, spatially modulated electric sheet impedance in a layered medium. At each optimization iteration, the complexity of the underlying analysis is O(N log N) owing to the use of a FFT based acceleration. The effectiveness of the method is demonstrated through the optimization of MTSs radiating a pencil beam and a conical beam with orbital angular momentum (OAM).