Rafael Florencio

Reflectarray Antennas for Dual Polarization and Broadband Telecom Satellite Applications

A reflectarray antenna with improved performance is proposed to operate in dual-polarization and transmit-receive frequencies in Ku-band for broadcast satellite applications. The reflectarray element contains two orthogonal sets of four coplanar parallel dipoles printed on two surfaces, each set combining lateral and broadside coupling. A 40-cm prototype has been designed, manufactured, and tested. The lengths of the coupled dipoles in the reflectarray cells have been optimized to produce a collimated beam in dual polarization in the transmit and receive bands. The measured radiation patterns confirm the high performance of the antenna in terms of bandwidth (27%), low losses, and low levels of cross polarization. Some preliminary simulations at 11.95 GHz for a 1.2-m antenna with South American coverage are presented to show the potential of the proposed antenna for spaceborne antennas in Ku-band.

Enhanced MoM Analysis of the Scattering by Periodic Strip Gratings in Multilayered Substrates

The method of moments (MoM) is applied to the analysis of the scattering of a multilayered periodic strip grating by a plane wave with oblique incidence and arbitrary polarization. Although this problem has been traditionally solved by means of the MoM in the spectral domain, this is an approach which leads to the computation of slowly convergent infinite summations. In this paper, the problem is solved by means of the mixed potential integral equation (MPIE) formulation of the MoM in the spatial domain. While applying the MoM in the spatial domain, two improvements are introduced which lead to important CPU time savings. First, the multilayered periodic Greens functions are accurately interpolated in terms of Chebyshev polynomials. Second, half the integrals involved in the computation of the MoM matrix entries are obtained in closed form. As a consequence of these two improvements, the spatial domain version of the MoM presented in this paper turns out to be between one and two orders of magnitude faster than the conventional spectral domain version when basis functions that account for edge singularities are used in the modeling of the current density on the metallizations.

Accurate and Efficient Modeling to Calculate the Voltage Dependence of Liquid Crystal-Based Reflectarray Cells

Two models that can predict the voltage-dependent scattering from liquid crystal (LC)-based reflectarray cells are presented. The validity of both numerical techniques is demonstrated using measured results in the frequency range 94-110 GHz. The most rigorous approach models, for each voltage, the inhomogeneous and anisotropic permittivity of the LC as a stratified media in the direction of the biasing field. This accounts for the different tilt angles of the LC molecules inside the cell calculated from the solution of the elastic problem. The other model is based on an effective homogeneous permittivity tensor that corresponds to the average tilt angle along the longitudinal direction for each biasing voltage. In this model, convergence problems associated with the longitudinal inhomogeneity are avoided, and the computation efficiency is improved. Both models provide a correspondence between the reflection coefficient (losses and phase-shift) of the LC-based reflectarray cell and the value of biasing voltage, which can be used to design beam scanning reflectarrays. The accuracy and the efficiency of both models are also analyzed and discussed.

Fast and Accurate MoM Analysis of Periodic Arrays of Multilayered Stacked Rectangular Patches With Application to the Design of Reflectarray Antennas

The scattering of plane waves by periodic arrays of stacked rectangular patches in multilayered substrates is a problem that has to be solved many times when designing reflectarray antennas made of those patches under the local periodicity assumption. The solution to the periodic multilayered problem has been traditionally carried out by means of the Galerkins version of the method of moments (MoM) in the spectral domain. This approach involves the computation of double infinite summations, and whereas some of these summations converge very fast, some other converge very slowly. In this paper, the slowly convergent summations are computed by making use of an enhanced mixed potential integral equation (MPIE) formulation of the MoM in the spatial domain. This enhanced formulation is based on the interpolation of the multilayered periodic Greens functions, and on the efficient computation of the four-dimensional (4-D) integrals leading to the MoM matrix entries. Both the novel hybrid spectral-spatial MoM code and the standard spectral domain MoM code have been used for the design of a contoured beam reflectarray antenna. It has been verified that the spectral-spatial MoM code requires CPU times that are typically 30 times smaller than those required by the pure spectral domain MoM code.

Efficient Crosspolar Optimization of Shaped-Beam Dual-Polarized Reflectarrays Using Full-Wave Analysis for the Antenna Element Characterization

A method for the optimization of the crosspolar component of dual-polarized reflectarrays using full-wave analysis at the element level is described and demonstrated. The reflectarray full-wave analysis is based on local periodicity (LP) and integrated within the optimization process in order to accurately characterize the crosspolar far field. The proposed method is based on the generalized Intersection Approach framework using the Levenberg–Marquardt Algorithm as backward projector, and the employed full-wave analysis is based on the Method of Moments assuming Local Periodicity (MoM-LP). Several strategies to accelerate the computations are exploited, such as the parallelization of all the algorithm building blocks. To minimize the impact of MoM-LP in the optimization process, a strategy to reduce the number of MoM-LP calls is described, further accelerating the algorithm. Moreover, the convergence is improved by working with the squared field amplitude, alleviating the trap problem of local optimizers. This method allows to optimize the crosspolar component in the whole visible region or only in the coverage zone to facilitate the convergence, reducing computing time and memory usage. Two test cases are provided to validate the technique, one with an isoflux pattern for global Earth coverage and another with European coverage for direct broadcast satellite application.

Design of a reflectarray antenna at 300 GHz based on cells with three coplanar dipoles

Simulated results are presented for a reflectarray antenna designed to produce a collimated beam at 300 GHz within a 13% bandwidth. The reflectarray cells are made of three parallel dipoles printed on one side of a 110-μm Quartz wafer coated with a conductive ground plane on the back side, where the phase is adjusted by varying the length of the dipoles. A practically linear phase variation is achieved in a range greater than 360° and frequencies from 280 GHz to 320 GHz. A reflectarray antenna was designed taking into account the angle of incidence and the polarization of the incident field. The simulated radiation patterns show a fixed collimated beam with variations in gain lower than 2.6 dB within a 13% bandwidth.

Reflectarray in K and Ka bands with independent beams in each polarization

This contribution describes the design of a printed reflectarray antenna to operate at 19.7 and 29.5 GHz with independent beams in H and V polarizations. A dual-frequency dual-polarization reflectarray cell, made of two stacked sets of parallel dipoles for each polarization, is proposed. The dipole lengths are adjusted to produce the required phase-shift at each frequency and each polarization. A 1.6-m reflectarray has been designed to produce two closely spaced beams in H and V polarizations. The simulated radiation patterns show a gain of 48.3 dBi at 19.7 GHz and 50.7 dBi at 29.5 GHz, with side-lobe levels close to -25 dB and low cross-polar radiation.

Pole-zero matching technique for multilayered periodic structures with application to the design of reflectarray antennas

In order to design reflectarray antennas within reasonable CPU times, fast and accurate numerical tools for the analysis of periodic multilayered structures are required. In this paper, a pole-zero matching technique is applied to the determination of the scattering matrix of periodic arrays of stacked rectangular microstrip patches as a function of the dimensions of the patches. The pole-zero matching technique makes it possible to obtain closed-form equations for the scattering matrix of the periodic structures from a reduced set of full-wave data. These closed-form equations are very useful because they lead to important CPU time savings when they are applied to the design of reflectarray antennas. In fact, in this paper we present a comparison between a reflectarray design based on a full-wave analysis of each element and a reflectarray design based on the derived closed-form equations. It is shown that the CPU time required by the full-wave approach is nearly one order of magnitude larger than that required by the approach based on closed-form equations 1.

Dual Polarized Reflectarray Transmit Antenna for Operation in Ku- and Ka-Bands With Independent Feeds

A reflectarray antenna capable of operating independently in the transmit frequencies (from a satellite) in Ku-band (11–13 GHz) and Ka-band (19–20 GHz) has been proposed and demonstrated. To prove that independent beams can be optimized in each frequency band using separate feeds, a 25-cm demonstrator that generates a focused beam in dual polarization (linear or circular) has been designed, manufactured, and tested. The reflectarray cells comprise two stacked sets of coupled parallel dipoles for each polarization, which permits an independent optimization of the phase for each frequency and polarization. The simulated and measured radiation patterns for both copolar and cross-polar components are in good agreement in Ku- and Ka-bands.

Optimized Periodic MoM for the Analysis and Design of Dual Polarization Multilayered Reflectarray Antennas Made of Dipoles

A hybrid version of the Method of Moments (MoM) is applied to the analysis of the scattering of plane waves by periodic multilayered structures containing dipoles at two metallization levels. The MoM matrix entries involving basis functions (BFs) at different metallization levels are computed in the spectral domain as double infinite summations with fast exponential convergence. The MoM matrix entries involving BFs at the same metallization level are computed in the spatial domain as double integrals, which require low-order quadrature rules. The integrands are cross correlations between BFs times multilayered periodic Green’s functions (MPGFs). The cross correlations between BFs are obtained in terms of elliptic integrals of first and second kind. Also, the MPGFs are accurately interpolated in 4-D in terms of both the spatial variables and the angles of incidence. The hybrid MoM proposed is used in the design of dual polarization reflectarray antennas under the local periodicity assumption. Thanks to the 4-D interpolation of the MPGFs, which minimizes the total number of MPGFs that have to be computed per reflectarray element, the proposed hybrid MoM is shown to be around 15 times faster than the standard spectral domain MoM in the design of the antennas.