Focusing the electromagnetic field radiated by an antenna at a point in the antenna near-field (NF) region is a wellknown technique to increase the electromagnetic power density in a size-limited spot region close to the antenna aperture. This article encompasses the basic working principles and the applications of the NF-focused (NFF) microwave antennas as well as the synthesis procedures suggested for the NF shaping around the focal point and the technologies currently used for their implementation.

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Near-Field-Focused Microwave Antennas: Near-field shaping and implementation.

A spherical mode expansion with Cartesian vector coefficients is presented for the study of the radiation patterns of arbitrary wire antennas. Coefficients are obtained by using a spherical harmonic series for the free space Green's function in conjunction with the current distribution obtained with the method of moments. Transverse components of the far field are ultimately obtained yielding a compact pattern representation suitable for rapid retrieval and display. The length of the expansion is shown to be related to the precision specified for the pattern; i.e., more precision requires more terms. A useful truncation criterion is proposed and verified by comparing results obtained for several typical wire antennas. >

Radiation pattern analysis of arbitrary wire antennas using spherical mode expansions with vector coefficients

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Advanced spherical antenna measurements

A technique of synthesizing or reconstructing the excitation currents of a planar array of aperture-type antennas from the known near-field patterns of the radiating source is presented. This technique uses an exact solution to the fields radiated by the aperture antenna without disregarding the source currents. Typical numerical computations have been carried out to validate the analytical technique developed. Sensitivity and stability of the numerical computations performed have been studied. The available iterative bandlimited signal extrapolation technique is used to reconstruct the aperture excitation currents only if the far-field patterns of the radiating source are known. Far-field patterns of aperture antennas measured in the laboratory were also used to reconstruct the aperture electric field distribution in the principal plane. >

A technique of synthesizing the excitation currents of planar arrays or apertures

A new technique of synthesis of near-field (NF) amplitude and phase patterns of linear, planar, of volume arrays of finite size or arrays located on a planar contour of finite size is presented The array could consist of point dipoles or directive elements The criterion for prescribing the NF (amplitude and phase) pattern information in the synthesis problem for unique determination of array excitation currents is also stated The proposed near-field synthesis technique is based on the potential integral solution of source currents, Nyquist sampling of the near-field data and the technique of linear least square approximation (LLSA) The NF pattern synthesis technique is illustrated to synthesize a variety of NF patterns with a number of array configurations Application of the proposed NF pattern synthesis technique to minimize distortion in far-field patterns of arrays mounted on a conducting platform and to realize array antennas with low sidelobes in the near and far field is also presented

Synthesis of near-field patterns of arrays

An exact analysis for deriving closed-form expressions for the coefficients of free-space spherical modes radiated by typical linear, array, and aperture type of antennas is presented. This analysis is employed to determine the number of spherical modes required to reconstruct the near/ far fields (NF/FF) with a prescribed accuracy. The analysis developed together with the theory of probe-compensated spherical scanning is also employed to arrive at a criterion for the minimum distance of separation between the probe and test antenna which will enable accurate nonprobe corrected spherical scanning possible.

Modal behavior of spherical waves from a source of EM radiation with application to spherical scanning

A unified approach for synthesizing the near- or far-field radiation patterns of an array of uniformly or nonuniformly spaced point sources or directive elements located on a planar contour of arbitrary shape is presented. The synthesis problem is formulated employing the free-space Green's dyadic as well as the translational and rotational addition theorems for spherical vector wave functions. A similar unified approach for synthesizing the near- or far-field radiation patterns of a uniformly or nonuniformly spaced linear array of point dipoles is also presented. The validity of the technique presented is established with several typical numerical illustrations.

K. Varadarangan

Papers

Modal behavior of spherical waves from a source of EM radiation with application to spherical scanning

A new technique of synthesis of the near- or far-field patterns of arrays