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.

We present an efficient procedure for the synthesis of planar array antennas to radiate contoured patterns in the near zone of array aperture, as desired in RFID applications. It first grids the target area, and then utilizes a global basis set to span the array's excitations. Each basis's excitation of the array radiates a focused spot beam in the near zone and serves as a local basis function for representing the radiation field. The unknown coefficients of the array's basis set are then found via a minimum least square error (MLSE) technique. It is found that only a small number of basis functions are sufficient to synthesize the radiation pattern, and make the present approach very fast.

An Effective Synthesis of Planar Array Antennas for Producing Near-Field Contoured Patterns

A microstrip planar array antenna is presented in this paper, which is designed to have near-field focused beams, and it is capable of steering its focus on the designed focal plane. The array is composed of eight series-fed linear arrays, which are able to scan the focus by frequency around 10 GHz in the H-plane, whereas, E-plane focus scan can be achieved by feeding the linear arrays with different phases. The proposed array can be used in near-field scan systems, e.g., microwave imaging system, with a faster scan rate than mechanically scanned antennas and a lower cost than array antennas with full phase control elements. Theoretical analysis is presented in this paper, and is verified by simulations and experiments.

Microstrip Array Antenna With 2-D Steerable Focus in Near-Field Region

With the aid of specific microstructures in a subwavelength array, time-reversed electromagnetic wave can display spatial super-resolution focusing in far-field zone. A subwavelength array of planar monopoles is proposed in this paper. Each element of the array, the planar monopole, is etched aperiodically with many microstructured complementary split rings. An important property of this antenna is that if multiple such planar monopoles are placed face to face, the proposed array can perform time-reversal far-field focusing with a spatial super-resolution as small as one twentieth of a wavelength. The validity of the proposed array with super-resolution characteristics is verified by the communication experiments with a data rate of 660 Mbit/s. The proposed subwavelength array is easy-to-design and convenient for integration.

Subwavelength Array of Planar Monopoles With Complementary Split Rings Based on Far-Field Time Reversal

We present the design and realization of a microstrip patch array antenna, which is operated at 24 GHz band, and can be used in the applications of vital-sign detection with a higher resolution. The design uses the steepest descent method (SDM) to synthesize the excitation phases of the array such that the radiation has an optimum near-field pattern in the desired area. Both theoretical development of SDM formulations and experimental validations over the antenna prototype are presented.

Design of Periodic Antenna Arrays With the Excitation Phases Synthesized for Optimum Near-Field Patterns via Steepest Descent Method

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 analytically simple and numerically efficient technique for calculating the pattern distortion of aperture antennas radiating in the presence of conducting platforms located in the near or far field of the antenna is presented. The technique presented, based on uniform geometrical theory of diffraction (UGTD), is also applicable for large aperture antennas (aperture area > 15 \lambda^{2} ). An excellent agreement between the calculated and measured results obtained for a typical aperture antenna mounted on a conducting platform confirms the validity of the analytical technique developed.

B. Philips

Papers

Synthesis of near-field patterns of arrays

Pattern distortion of aperture antennas radiating in the presence of conducting platforms