Marcos R. Pino

Reconstruction of Equivalent Currents Distribution Over Arbitrary Three-Dimensional Surfaces Based on Integral Equation Algorithms

A technique for the determination of the equivalent currents distribution from a known radiated field is described. This Inverse Radiation Problem is solved through an Integral Equation algorithm that allows the characterization of antennas of complex geometry both for near field to far field (NF-FF) transformation purposes as well as for diagnostic tasks. The algorithm is based on the representation of the radiating structure by means of a set of equivalent currents over a three-dimensional (3D) surface that can be fitted to the arbitrary geometry of the antenna. The innovative formulation uses an integral equation involving the electric field due to the currents tangential components to the represented antenna 3D surface. For that purpose, both the magnetic and electric equivalent currents are considered in the integral equations. Regularization techniques are also introduced to improve the convergence of the proposed iterative solution. The paper concludes with several results related to the practical verification of the Equivalence Principle and the characterization of a horn antenna.

Echo identification and cancellation techniques for antenna measurement in non-anechoic test sites

This paper focuses on the measurement of the antenna radiation patterns when fully anechoic conditions are not available and, as consequence, some undesirable echoes are initially present in the measure. Two techniques are analyzed and compared, which - starting from data measured in the frequency domain - allow the echo contributions to be identified and the antenna radiation pattern to be retrieved. The accuracy of both techniques is evaluated at the frequency of 22 GHz by comparison with measurements obtained in an anechoic chamber.

Solution of Electrically Large Problems With Multilevel Characteristic Basis Functions

We present a multilevel version to the characteristic basis function method (CBFM). This technique extends the range of applicability of the conventional CBFM and allows a direct solution of problems even when they exceed 250,000 degrees of freedom. In this paper, it is shown how we can use the multilevel CBFM, which is a natural extension of the conventional approach and which saves both time and memory, and yet retains excellent accuracy in the computed far fields. Also, the adaptive cross algorithm is used for fast calculation of the interaction between the blocks in the process of generating the reduced matrix.

An Improved Super-Resolution Source Reconstruction Method

The source reconstruction method (SRM) is a technique developed for antenna diagnostics and for carrying out near-field (NF) to far-field (FF) transformation. The SRM is based on the application of the electromagnetic equivalence principle, in which one establishes an equivalent current distribution that radiates the same fields as the actual currents induced in the antenna under test (AUT). The knowledge of the equivalent currents allows the determination of the antenna radiating elements, as well as the prediction of the AUT-radiated fields outside the equivalent currents domain. The unique feature of the novel methodology presented in this paper is that it can resolve equivalent currents that are smaller than half a wavelength in size, thus providing super-resolution. Furthermore, the measurement field samples can be taken at field spacings greater than half a wavelength, thus going beyond the classical sampling criteria. These two distinctive features are possible due to the choice of a model-based parameter estimation methodology where the unknowns are approximated by a continuous basis and, secondly, through the use of the analytic Greens function. The latter condition also guarantees the invertibility of the electric field operator and provides a stable solution for the currents even when evanescent waves are present in the measurements. In addition, the use of the singular value decomposition in the solution of the matrix equations provides the user with a quantitative tool to assess the quality and the quantity of the measured data. Alternatively, the use of the iterative conjugate gradient (CG) method in solving the ill-conditioned matrix equations can also be implemented. Two examples of an antenna diagnostics method are presented to illustrate the applicability and accuracy of the proposed methodology.

A Wideband Slot-Coupled Stacked-Patch Array for Wireless Communications

In this letter, a novel slot-coupling feeding technique is used to realize a dual-polarized 2 × 1 microstrip stacked patch array for mobile wireless communication systems. The array is intended as a basic module for base-station linear arrays, whose final size depending on beamwidth and gain requirements. Each array element is fed through two microstrip lines arranged on the basis of a sequential rotation technique. Each stacked square patch is excited through a square ring slot realized in the feeding network ground plane. Design procedure, simulation results and measurement data are presented for a 2 × 1 array working in the GSM 1800-1900 band (1710-1910 MHz), UMTS band (1920-2170 MHz), ISM band (2400-2484 MHz), and UMTS 3G expansion band (2500-2690 MHz) or, alternatively, WiMAX band (2300-2700 MHz), with a resulting 45% percentage bandwidth (reflection coefficient <; -10 dB). Due to both the symmetry properties of the novel slot-coupling feeding configuration and the implementation of a sequential rotation technique, good results have been obtained in terms of port isolation and cross-polar radiation patterns.

A Novel Dual-Feed Slot-Coupling Feeding Technique for Circularly Polarized Patch Arrays

In this letter, a novel slot-coupling feeding technique has been adopted to realize a circularly polarized (CP) 2 × 2 microstrip array. Each array element is fed through two microstrip lines that are excited 90° out of phase (dual-feed technique) and coupled to a square patch by means of a square-ring slot realized in the feeding network ground plane. Design procedure, simulation results, and measurement data are presented for a 2 × 2 array working in the WiMax 3.3-3.8 GHz frequency band (14% percentage bandwidth). Due to both the symmetry properties of the novel slot-coupling feeding configuration and the implementation of a sequential rotation technique, excellent axial ratio (AR) performance is achieved in the WiMax band (AR < 1.35 dB at broadside) and for any direction in the antenna main beam (AR < 2.25 dB at 3.55 GHz). Actually, the 3-dB AR bandwidth is larger than the WiMax frequency band, as it goes up to about 30%.

The Sources Reconstruction Method for Amplitude-Only Field Measurements

An extension of the sources reconstruction method (SRM) for antenna diagnostics using amplitude-only field measurements is presented. While previous works limit the application to canonical domain cases, the combination of the SRM capabilities for handling with arbitrary-geometry domains jointly with phase retrieval techniques, allows antenna diagnostics extension to complex geometry antenna and field acquisition domains. The consideration of the radiation inverse problem with a general integral equation formulation using arbitrary-geometry field and currents domains, and field phaseless information, supposes a challenging ill-posed problem that is solved using iterative minimization techniques for non-linear problems. Different examples are presented, from simple to general antenna diagnostics cases.

Acceleration of the sources reconstruction method via the fast multipole method

From the results it can be concluded that the CGNR-FMM allows a significant computational time reduction with respect to the previous CGNR-MST, completing the SRM algorithm optimization both in memory and time requirements. One of the key points of this work is the fact that the improvement on time requirements can be achieved at the expense of a slightly increase on memory consumption. Further improvements of the CGNR-FMM performance will lie on the optimization of the source and observation groups size, and on the reduction of the number of multipoles.

On the Comparison Between the Spherical Wave Expansion and the Sources Reconstruction Method

A comparison between two well-known near field to far field (NF-FF) transformation methods is presented: the spherical wave expansion (SWE) and the sources reconstruction method (SRM). The goal of this paper is to show the methods performance for NF-FF transformation pointing out the advantages and limitations of each one. First, the computational cost is studied, showing its dependence with the antenna under test (AUT) electric size. Second, the NF-FF transformation accuracy of each method is compared for a fixed AUT electric size, analyzing also the influence of the main parameters that affect the SRM and SWE accuracy. Third, the SRM capabilities are presented, with focus on the NF-NF transformation inside the minimum sphere. Practical applications of the SWE and SRM are illustrated by means of an example of NF-FF transformation and antenna diagnostics problem using measured NF data.

Synthesis of Phased Arrays in Complex Environments with the Multilevel Characteristic Basis Function Method

The aim of this paper is to present a method to carry out the synthesis of large phased arrays when they are afiected by complex environments which can in∞uence the radiation pattern. The synthesis is performed with the help of the Multilevel Characteristic Basis Function Method to calculate a matrix relating input voltages and the far fleld pattern samples. The method is illustrated with the synthesis of a Secondary Surveillance Radar antenna on a turret containing multiple obstacles.