One of the main concerns when imaging extended real objects is the capacity of the system to correctly reconstruct the object cross section electric properties. In this paper, the imaging capabilities of an UWB short range system are analyzed, based on the Multifrequency Bi-Focusing Imaging (MFBF) and Time Difference of Arrival (TDOA) techniques, and its capability to reconstruct high contrast objects are studied. Based on an analytical formulation for a dielectric cylinder the different spatial and frequencial sampling criteria have been verified and its behaviour for highly contrasted objects has been obtained. An experimental validation has been finally performed using two collinear arrays of antennas working in transmission and reception respectively.

Metamaterial slabs made of magnetic resonators (artificial magnetic conductor, AMC surfaces) can provide unusual electromagnetic properties, such as the dual PMC/PEC behavior, when used as reflectors. This property has been assessed and tested with the design and fabrication of a one‐layer spiral AMC slab. Moreover, two bidirectional AMC surfaces have also been designed and measured. These structures, composed of spiral (SR) or capacitive loaded loop (CLL) resonators printed on dielectric strips, can provide 0° reflection coefficient phase when the incident electric field wave impinges on any side. A very low electrical thickness is achieved. Compact antenna systems with two (or more) close isolated antennas can be designed with such structures. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1949–1953, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22567

The radiation pattern of the large parabolic reflectors of the Transportable Atmospheric RAdar system (TARA), developed at Delft University of Technology, has been accurately simulated. The electric field integral equation (EFIE) formulation has been applied to a model of the reflectors including the feed housing and supporting struts, discretised using the method of moments. Because the problem is electrically large (the reflector has a diameter of 33/spl lambda/) and nonsymmetrical, this lead to a badly conditioned linear system of approximately half a million unknowns. In order to solve this system, an iterative solver (generalized minimum residual method) was used, in combination with the multilevel fast multipole method. Because of the bad conditioning, the system could only be solved by using a huge preconditioner. A new block-incomplete LU preconditioner (ILU) algorithm has been employed to allow for efficient out-of-computer core memory preconditioning.

Introduction: One of the main problems regarding the efficiency of industrial heating systems using high power microwaves is related to uncontrolled changes in the load. These changes can be caused both by the different kinds of materials to be processed in the same oven and by the temperature rise which modifies the material’s physical properties, particularly its dielectric permittivity. Usually, the microwave generator is matched to the oven by means of a waveguide matching system, typically a two, three or four capacitive-screw tuner empirically adjusted for a given load [l]. Once the heating process is started, the temperature dependence of the complex permittivity usually changes the impedance seen by the generator, worsening the mismatch factor and reducing the efficiency of the heating process. A similar problem affects an oven with inhomogeneous and discontinuous loads; it is not feasible to manually readjust the matching network for every different sample processed. Until now the solution has been simple but highly inefficient: to increase the incident power in order to overcome the mismatching effects. In this Jitter we describe an autonomous system which is able to detect load mismatches and reduce them automatically.

After two years of basic research on fractal electrodynamics, the FractalComs consortium presents the conclusions of the FractalComs Project (http://www.tsc.upc.es/fractalcoms/). They state the performance limits (fundamental and technological) of pre-fractal shaped miniature devices compared with conventional ones. Many miniature antennas and devices have been designed, analyzed and/or measured, the most outstanding being the two-arm square spiral antennas and the Hilbert superconductor resonators for miniature filters.

Great social concern has risen about the potential health hazard of living near a cellular telephony base-station antenna, and certain technical questions have been posed on the appropriate way to measure exposure in its vicinity. In this paper, a standard spherical near-near field transformation is proposed to obtain the electromagnetic field close to the antenna in free space conditions. The field obtained in this way allows us to define an exclusion zone from the exposure compliance point of view, but also makes it possible to bound the error committed by standard field measurement procedures. Furthermore, the visualization of the electromagnetic field in the proximity of the antenna in free space conditions, allows us to define clearance templates that have to be met in the siting of the antenna in complex environments in order not to obstruct its main beam.

Aperture synthesis interferometric radiometers can overcome mass, weight and mechanical scan limitations of present-day radiometers. A full antenna array is "thinned" by eliminating selected antennas, while keeping all possible antenna separations. This paper describes a new technique for the direct synthesis of low-redundancy large arrays, consisting in growing small arrays by inserting a seed repeatedly. This technique has achieved the largest thinned arrays with the least redundancy reported in the literature.

A waveguide reflectometer, in combination with a fibre optic temperature probe, is used to characterise the permittivity changes of small rubber samples heated with high power microwaves. The molecular changes of rubber during vulcanisation can be clearly seen in the evolution of the measured complex dielectric constant against time and temperature.

Fractal objects have some unique geometrical properties. One of them is the possibility to enclose in a finite area an infinitely long curve. The resulting curve is highly convoluted being nowhere differentiable. One such curve is the Koch curve. In this paper, the behavior the Koch monopole is numerically and experimentally analyzed. The results show that as the number of iterations on the small fractal Koch monopole are increased, the Q of the antenna approaches the fundamental limit for small antennas.

A genetic algorithm (GA) has been developed for designing single-shaped reflector antennas for the synthesis of shaped contour beams. The graphical processing technique is used in order to obtain the antenna radiation patterns very efficiently. Results comparing with the classical conjugate gradient are included to provide validation. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 27: 358–361, 2000.

One of the main problems regarding the efficiency in a heating industrial system using high power microwaves is related with the uncontrolled changes on the load. These changes can be caused both by the different kinds of materials to be processed in the same oven and by the temperature rise which modifies the material's physical properties, particularly its dielectric permittivity. Usually, the microwave generator is matched to the antenna feeding the oven by means of a waveguide matching system, typically a two, three or four capacitive-screw tuner empirically adjusted for a given load. Once the heating process is started, the temperature dependence of the complex permittivity usually changes the impedance seen by the generator, worsening the mismatch factor and reducing the efficiency of the heating process. A similar process happens when the load is inhomogeneous or discontinuous; it is not feasible to manually re-adjust the matching network for every different sample processed. Until now the solution has been simple but highly inefficient: to increase the incident power in order to overcome the mismatching effects.

An automatic matching network has been implemented using a waveguide magic T with two sliding short circuit stubs which are adjusted by a microcontroller running a conjugate gradient algorithm. The inclusion of this device in an industrial heating system allows high energetic efficiency of the oven to be maintained despite changes in the load.

This paper is devoted to the description of a transmission line laboratory demonstrator for intermediate level communication engineering students, within a course on radiation and guided waves. Student activities have been organized according to the needs of intermediate level students and the objectives of an introductory laboratory, which are substantially different from those of specialized laboratory activities aimed at higher level students and placed just before graduation. Since a large number of students attend these laboratory sessions before they choose their specialty, low cost is an unavoidable requirement to be considered. For this reason, the measurement set-up makes use of standard instrumentation, usually available in a basic electronic laboratory, which allows multiplication of work places at a reasonable cost. Both time domain reflectometry and sinusoidal steady state measurements are presented to characterize the main parameters of a coaxial line. However, discussions are mainly focused on sinusoidal measurements, which are scarce in the literature and exclusively devoted to slotted line measurements. The experimental work undertaken by the students gives them insight into main transmission line parameter and features: line impedance, propagation speed, its behavior as an impedance transformer, the complex nature of the measured magnitudes and the standing wave behavior of voltage and attenuation.

The paper presents a new inverse synthetic aperture radar (ISAR) algorithm intended for radar cross-section (RCS) imaging and measurement from scattered fields. The method, based on a spherical-wave near-field illumination of the target, overcomes the requirement for an expensive compact range facility to produce a plane wave illumination. The formulation and the implementation of the algorithm are described. Some experimental results obtained in an anechoic chamber are presented to show RCS results similar to the conventional plane wave methods.

Antennas and antenna arrays are essentially narrowband devices. Their characteristic size determines their operating wavelength. Fractal structures, having no characteristic size and a multiscale self-similar shape, are suggested for the design of multifrequency antennas and arrays. The multiband properties of some fractal arrays and array factors are presented here. Experimental results on multiband fractal antennas are described as well. Fractal antennas are shown to display a multiband behavior from both the return-loss and radiation pattern points of view.

The integral equation MEI (IE-MEI) is a new, sparse matrix formulation of the boundary-element method, which is particularly well suited for large convex scatterers. In this Letter we explain why the original implementation of IE-MEI fails for large concave objects and propose a remedy. (C) 1997 John Wiley & Sons, Inc.

An algorithm to extrapolate in frequency the matrix coefficients of the IE-MEI method is presented. This results in a total operation count to compute the induced current in electromagnetic scattering problems proportional to the number of unknowns, which in 2D is proportional to the working frequency.

-A novel integral formulation of the Measured Equation of Invariance method is derived from t h e reciprocity theorem. This formulation uses t h e electric and magnetic Green’s functions of the environment t o obtain a matrix equation for the induced surface current with the same number of unknowns as t h e conventional Boundary Element Method of Moments (BE-MOM) approach. However, t h e matrix tha t must be inverted in t h e new formulation is sparse and circulant, with only three non-zero elements per row. Sample results for two-dimensional T M and TE problems with perfectly conducting scatterers show enormous CPU t ime and memory savings over t h e conventional BEM-MOM approach. T h e new formulation has important advantages over the original finite difference formulation of MEI, bu t also shares some of its limitations.

The theoretical, or non-numerical, error in the integral equation MEI (IE-MEI) is analysed and compared to the numerical error in the method of moments (MoM). It is found that, although this error does not decrease with the discretisation step, it is smaller for the usual discretisation than the error in the MoM.

In this work the active tomographic imaging of biological bodies which exhibits a high dielectric contrast is studied. Two approaches based on the Born approximation axe presented: a differential reconstruction method which allows to visualize dielectric alterations and a matched synthesis method intended to minimize the distortion suffered by the illuminating field within the body. The presented techniques have been investigated by numerical simulations and experimentally in a cylindrical tomographic microwave system recently developed, considering the possible applications like Hyperthermia treatment monitoring in cancer.