Angel Cardama

On the behavior of the Sierpinski multiband fractal antenna

The multiband behavior of the fractal Sierpinski (1915) antenna is described. Due to its mainly triangular shape, the antenna is compared to the well-known single-band bow-tie antenna. Both experimental and numerical results show that the self-similarity properties of the fractal shape are translated into its electromagnetic behavior. A deeper physical insight on such a behavior is achieved by means of the computed current densities over the antenna surface, which also display some similarity properties through the bands.

The Koch monopole: a small fractal antenna

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.

Cylindrical geometry: a further step in active microwave tomography

A prototype imaging system for active microwave tomography using cylindrical geometry has been developed, making it possible to obtain images of the dielectric properties of biological targets at 2.45 GHz. This configuration allows a fast exploration of body slices placed along the array axis, in a way similar to that of present X-ray scanners. The electromagnetic compatibility (EMC) of this approach is critical because the strongly attenuated received fields are measured on the same array which is being used to emit a high-level illuminating signal. Therefore, carefully designed high-frequency architectures and detection techniques are necessary. The system requires no mechanical movements to illuminate the body from multiple directions (views) and measure the scattered fields. In this way, a complete data set consisting of 64 views is acquired in 3 s using low-power illumination. The system is described, and images obtained with biological phantoms and actual bodies are presented. >

Spherical wave near-field imaging and radar cross-section measurement

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.

Integral formulation of the measured equation of invariance: a novel sparse matrix boundary element method

A novel integral formulation of the measured equation of invariance method is derived from the reciprocity theorem and implemented for perfectly conducting (PEC) 2-D scattering problems. This formulation uses the electric and magnetic Greens functions of the environment to obtain a matrix equation for the induced surface current with the same number of unknowns as the conventional boundary element-method of moments (BE-MoM) approach. However, the matrix that must be inverted in the new formulation is sparse and circulant, with only three non-zero elements per row. Sample results for two-dimensional TM and TE problems with perfectly conducting scatterers show enormous CPU time and memory savings over the conventional BEM-MoM approach. The new formulation has important advantages over the original finite difference formulation of MEI, but also shares some of its limitations.

UWB Tomographic Radar Imaging of Penetrable and Impenetrable Objects

In this paper, the capability of ultra-wide-band (UWB) sensor arrays for tomographic radar of electrically large objects is presented. The major concern when imaging is extended to real objects is to achieve a correct reconstruction of the object shape and its electric properties. A general framework based on a UWB bifocusing operator (UWB-BF) with good tomographic imaging capabilities is presented. This general approach provides a comprehensive understanding of the basic tradeoffs with regard to sensing geometry and image quality parameters. Through numerical simulations and measurements applied to canonical as well as to complex objects, basic design criteria are assessed and the potential of UWB tomographic radar imaging is presented.

Near field in the vicinity of wireless base-station antennas: an exposure compliance approach

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.

A near field spherical wave inverse synthetic aperture radar technique

The authors present an imaging near-field spherical wave inverse synthetic aperture radar (SWISAR) algorithm based on a focusing operator of the scattered fields on the target surface. A spherical wave can be easily generated in an anechoic chamber using electrically small and low-cost horn antennas; no reflectors are used, avoiding surface tolerance problems, and diffraction can be minimized with an appropriate horn design. An additional advantage of this algorithm is its ability to account for bistatic measurement geometries and lateral tapers in the spherical wave illumination. A 2-D formulation has been used which is strictly valid for bodies which satisfy far-field conditions in the height dimension, as easily occurs with aircraft. The reconstruction of numerically simulated five point-like objects placed on a radial line for a frequency range of 18 to 26 GHz and an angular scan of 360 degrees is shown.<<ETX>>

Analysis of electrically large concave scatterers with the integral equation MEI

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.

Active microwave sensing of highly contrasted dielectric bodies

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 are 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.