Near and far field

About: Near and far field is a research topic. Over the lifetime, 15922 publications have been published within this topic receiving 220571 citations.

The influence of complex waves on the radiation field of a slot-excited plasma layer

Abstract: A study of the near field of a magnetic line source located inside an isotropic plasma layer reveals, in addition to a weakly-excited space wave, strong E mode complex waves corresponding to poles in the integral representation of the field solution. Under these conditions, the radiation field may be calculated by means of a Kirchhoff-Huygens integration over the pole contributions in the near field. This result is in excellent agreement with the accurate steepest-descent evaluation whenever the complex waves are dominant. Depending on its location in the steepest-descent plane, each pole contributes either a broad peak at broadside or a sharp peak at an oblique angle. In the frequency range in which the plasma is opaque, the complex waves are spectral and yield a single radiation peak at broadside, and with small power. Also, surface waves may be present and contribute to the far field located near the plasma-air interface. When the plasma is transparent only leaky waves exist, the strongest of which accounts for a sharp major peak that closely corresponds to the critical angle obtained by geometrical optics. The weaker leaky waves may yield additional but minor peaks, or contribute at broadside only. The more practical case of a distributed source is also treated. Its solution is shown to be obtainable directly from that for the line source; the expression for the radiation pattern is then given by the product of the line source solution and that for the source distribution in the absence of the plasma layer.

UHF near-field RFID reader antenna with capacitive couplers

Abstract: A segmented loop antenna with capacitive couplers is proposed for ultra-high frequency (UHF) near-field radio frequency identification (RFID) applications, which is capable of generating a strong and uniform magnetic field in a near-field zone even though the perimeter of the loop is comparable to the operating wavelength. As an example, an antenna printed onto an FR4 printed circuit board with an interrogation zone of 154 × 154 mm exhibits good impedance matching and uniform magnetic field distribution over an operating bandwidth of 790-1000 MHz, which is desirable for UHF near-field RFID reader applications.

Chiral Light Design and Detection Inspired by Optical Antenna Theory

Abstract: Chiral metallic nanostructures can generate evanescent fields which are more highly twisted than circularly polarized light. However, it remains unclear how best to exploit this phenomenon, hindering the optimal utilization of chiral electromagnetic fields. Here, inspired by optical antenna theory, we address this challenge by introducing chiral antenna parameters: the chirality flux efficiency and the chiral antenna aperture. These quantities, which are based on chirality conservation, quantify the generation and dissipation of chiral light. We then present a label-free experimental technique, chirality flux spectroscopy, which measures the chirality flux efficiency, providing valuable information on chiral near fields in the far field. This principle is verified theoretically and experimentally with two-dimensionally chiral coupled nanorod antennas, for which we show that chiral near and far fields are linearly dependent on the magnetoelectric polarizability. This elementary system confirms our concept ...

Nonactive antenna compensation for fixed-array microwave imaging. I. Model development

Abstract: Fixed-array microwave imaging with multisensor data acquisition can suffer from nonactive antenna element interactions which cause distortions in the measurements. In Part I of a two-part paper, the authors develop a nonactive antenna compensation model for incorporation in model-based near-field microwave image reconstruction methods. The model treats the nonactive members of the antenna array as impedance boundary conditions applied over a cylindrical surface of finite radius providing two parameters, the effective antenna radius and impedance factor, which can be determined empirically from measured data. Results show that the effective radius and impedance factor provide improved fits to experimental data in homogeneous phantoms where measurements are obtained with and without the presence of the nonactive antenna elements. Once deduced, these parameters are incorporated into the nonactive antenna compensation model and lead to systematic data-model match improvements in heterogeneous phantoms. While the improvements afforded by the nonactive antenna model are small on a per measurement basis, they are not insignificant. As shown in Part II (see ibid., vol. 18, no. 6, p. 508, 1999), inclusion of this new model for nonactive antenna compensation produces significantly higher quality image reconstructions from measurements obtained with a fixed-array data acquisition system over the frequency band 500-900 MHz.

A reciprocity theorem for the electromagnetic field scattered by an obstacle

Abstract: When a time-harmonic plane electromagnetic wave is incident upon a scattering obstacle of finite dimensions, the far-zone scattered field satisfies a reciprocity relation. This reciprocity relation is derived with the aid of H. A. Lorentz’s theorem. The result is valid under rather general assumptions as far as the electromagnetic properties of the obstacle are concerned. As a special case, the result for a perfectly conducting obstacle is obtained.