Fernando Las-Heras

A direct optimization approach for source reconstruction and NF-FF transformation using amplitude-only data

In this paper, we present a direct optimization procedure that utilizes phaseless electric field data over arbitrary shaped surfaces for the reconstruction of an equivalent magnetic current density that represents the radiating structure of an antenna under test. Once the equivalent magnetic current density is determined, the electric field at any point can be calculated. Numerical results (both simulated and experimental) are presented to illustrate the applicability of this approach for nonplanar near-field to far-field (NF-FF) transformation as well as to antenna diagnostics. The results are presented using both theoretical and experimental phaseless data over one and two planes.

Evaluating near-field radiation patterns of commercial antennas

A source reconstruction technique from the measured near fields is proposed to obtain a set of equivalent currents that will characterize the forward and backward radiation patterns of an antenna. Once the equivalent sources are determined, the electromagnetic field at any aspect angle and distance from the antenna can be calculated. In this paper, the method is applied to the evaluation of the radiation from commercial antennas at any observation point. The electric field patterns of a DCS base station antenna at 1800 MHz and a horn antenna at 2500 MHz have been calculated and plotted at several distances from the antenna. This method can be used in characterizing the reference volumes or exclusion zones for transmitting antennas dealing with the maximum levels of electromagnetic radiation safe for human exposure, as stated in many national and international regulations

Radial field retrieval in spherical scanning for current reconstruction and NF-FF transformation

A near-field to far-field (NF-FF) transformation is addressed for the case of spherical scanning using equivalent magnetic currents (EMCs) and matrix methods. It is based on the decoupling of the field components and the iterative retrieval of the radial component of the electric field. The technique is applied for far-field calculation as well as for the estimation of the current distribution of the antenna under test (AUT) using spherical near-field facilities. Results from measured near-field data of several antennas are presented and compared to those of the analytical solution via a spherical wave mode expansion method.

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.

Far-field performance of linear antennas determined from near-field data

The main plane far-field radiation pattern of an antenna under test from the corresponding main plane near-field data, using a circular-line acquisition, is presented. The method is based on the reconstruction of equivalent magnetic currents (EMCs) using decoupled integral equations and one-dimensional source components. The resultant fast procedure is applicable to linear and quasilinear array antennas. Experimental data results and comparison with complete spherical acquisition and center-line acquisition are presented.

Equivalent source modelling and reconstruction for antenna measurement and synthesis

In this paper we present an algorithm that from a given 2D radiation pattern (in near, Fresnel or far field region) identifies a linear distribution of equivalent magnetic current. The equivalent current distribution radiates in an unbounded medium a field that best fits (in the sense of rms error) the given radiated field. The purpose of this paper is to discuss different alternatives of modeling the equivalent currents in terms of choosing the appropriate basis functions for each application. The first application is the obtention far field patterns of antennas, with separable distribution, from measurements in the near or Fresnel field regions. If we associate the given 2D radiation pattern to the field data measured in an anechoic chamber facility and we associate the equivalent currents to the tangential fields in the plane of the antenna under test (AUT) then we can obtain the field at any distance and thus the far field radiation pattern. The second application is the synthesis of linear arrays by associating the given 2D radiation pattern to the desired far field pattern and admitting a tolerance error in terms of the typical deviation at each aspect angle. The reconstructed equivalent current distribution can be used to synthesize an antenna by means of an array of slots or conducting patches (sampling of the reconstructed current distribution).

Improvement of the sources reconstruction techniques: analysis of the SVD algorithm and the RWG basis functions

The objective of this paper is to present a formulation for the Inverse Scattering Problem [1] based on the expansion of the equivalent currents by using the Rao-Wilton-Glisson (RWG) basis functions [2]. The purpose is to reconstruct the sources from measured near-field data. The goal is to improve the resolution for the equivalent sources characterization, by avoiding the currents discontinuity along the edges when using pulse basis function. Moreover, in order to improve the accuracy of the reconstructed currents, the resulting matrix system has been solved by using the singular value decomposition (SVD) method. A comparison between the formulation with the pulse basis functions and the RWG basis for the inverse problem is presented.

Efficient calculation of the reduced matrix in the characteristic basis functions method

A new efficient technique for calculating the reaction between characteristics basis functions has been presented. The proposed method enables us to bypass the computation of the off-diagonal of the MoM matrix. For the case of contiguous blocks, an efficient hybrid approach has been developed that combines the conventional MoM matrix approach with the interpolation technique. Numerical results show that the technique is able to speed up the reduced matrix filling time without sacrificing the accuracy of the RCS results.

Full-wave-based location system method evaluation

Evaluation of received signal strength (RSS)-based indoor location system is presented. The proposed method starts from the field amplitude decay law in free-space conditions, but also taking into account the near-field terms for those cases where the distance between transmitter and receiver does not meet far-field conditions. The proposed method is tested in a real indoor scenario, being the field level measured at different positions. The recorded field data are used to evaluate the method accuracy when determining the position of the RF transmitter. Practical considerations of a location system implementation using a ZigBee-based sensor network are finally discussed.

Integral equation algorithms for equivalent currents distribution retrieval over arbitrary three-dimensional surfaces

In the following article a new algorithm for retrieval of electromagnetic currents from known radiated fields is presented. The method, based on matrix formulation, solves inverse radiation problem over perfect electric conductor (PEC) surfaces from near field measurements. The final goal is the diagnosis and characterization of any kind of antennas, independently of the complexity of the antenna geometry. Code has been optimized in time and memory consumption, using matrix decomposition and iterative techniques