International Journal of Antennas and Propagation

Volume 2017 (2017), Article ID 7694281, 17 pages

https://doi.org/10.1155/2017/7694281

## Technologies for Near-Field Focused Microwave Antennas

Department of Information Engineering, University of Pisa, Via G. Caruso 16, 56122 Pisa, Italy

Correspondence should be addressed to Giuliano Manara

Received 7 September 2016; Revised 20 November 2016; Accepted 13 December 2016; Published 19 March 2017

Academic Editor: Shiwen Yang

Copyright © 2017 Paolo Nepa et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper provides a review spanning different technologies used to implement near-field focused antennas at the microwave frequency band up to a few tens of GHz: arrays of microstrip patches and printed dipoles, arrays of dielectric resonator antennas, reflectarrays, transmitarrays, Fresnel zone plate lenses, leaky-wave antennas, and waveguide arrays.

#### 1. Introduction

In the past, near-field focusing techniques based on lens antennas and reflectors have been extensively used at optical and mm-wave bands. Nonetheless, over the last decades, several near-field focused (NFF) antennas have been designed and characterized for applications at lower frequencies. Indeed, NFF antennas are receiving considerable attention in several applications such as RFID (Radio Frequency Identification) systems, gate access control systems, industrial microwave applications, local hyperthermia, and wireless power transfer systems [1].

NFF microwave antennas can be implemented by a number of different technologies and layouts, which can be seen as proper modifications of those that are conventionally used to design and realize far-field (FF) focused antennas. Available solutions include ellipsoidal reflector antennas and pyramidal/conical horns with a dielectric lens in front of the antenna aperture, but they are quite bulky and heavy antennas at the microwave band. Therefore, other technologies that allow for the implementation of planar NFF microwave antennas are preferred. Most of them are array antennas, where the phase of each element current is adjusted to get constructive interference of all field contributions at the focal point.

In this paper, technologies and layouts proposed for the implementation of NFF antennas, at the microwave frequency band up to a few tens of GHz, are revised. The paper is organized as follows. Main parameters and general design criteria for NFF array antennas are concisely introduced in Section 2, while the reader is referred to [1, 2] for a more detailed analysis. Then, a large set of NFF arrays are reviewed in Sections 3–6, which have been classified as follows:(i)Rectangular (Section 3.1) and circular (Section 3.2) arrays of printed antennas (patches or dipoles)(ii)Arrays of dielectric resonator antennas (Section 3.3)(iii)Reflectarrays (Section 4.1)(iv)Transmitarrays (Section 4.2)(v)Fresnel zone plate lens antennas (Section 4.3)(vi)Linear (Section 5.1) and planar (Section 5.2) leaky-wave antennas(vii)Waveguide arrays (Section 6)It is worth noting that the review is limited to those technologies suitable to implement NFF antennas for short-range wireless links at the microwave frequency band (up to a few tens of GHz) and does not include the optical devices, such as lenses, dielectrically loaded horns, and reflector mirrors, which are the most valuable technologies at mm-wave frequencies and beyond. Moreover, attention has been mostly devoted to NFF antennas for which the antenna size, , and the distance between the focal point and the radiation sources on the array/antenna aperture are both greater than the free-space wavelength .

#### 2. Main Features of NFF Array Antennas

NFF arrays essentially exploit the extreme flexibility of array antennas to control the side lobe level, shape the −3 dB focal spot, implement multifocus antennas, and electronically scan the focal point. Important features of the NFF antennas are the focus depth (or depth of focus, ), the focus width at the focal plane, and the level of the secondary lobes around the focal spot region, namely, the axial lobes and the side lobes [1]. Typical parameters of conventional FF-focused antennas (radiation pattern, antenna gain) are also of interest in most of the applications, to quantify the capability of the NFF antenna to minimize the field radiated in the FF region. Indeed, in the context of short-range applications, reducing FF radiation helps to limit the interference with adjacent wireless systems, the effects of unwanted multipath phenomena, and the personnel radiation hazards.

In the conjugate-phase approach [1], the phase of the excitation of each array element is set to compensate for the phase delay introduced by the path between the array element and the assigned focal point, to achieve constructive interference of all the contributions at the focal point. If the focal distance is larger enough than the antenna size , then the above phase tapering can be approximated by a quadratic phase profile (Fresnel approximation). Actually, due to the field spreading factor, the peak of the radiated power density does not occur at the focal point where all field contributions sum in phase (focal shift) [1]. The field peak is always located at a point between the antenna aperture and the focal point [1]. Additionally, proper tapering of the amplitude of the excitation may be added to control the level of the secondary lobes around the focal spot region. Indeed, a relatively high level of the secondary lobes around the focal spot may degrade measurement accuracy in noncontact sensing applications, or heat healthy tissues in microwave hyperthermia systems. Also, high secondary lobe level may reduce transmission efficiency in wireless power transfer systems, increase the interference with nearby wireless systems, raise the personnel exposure to radiation hazards, and enlarge the number of false positive readings in RFID systems. Although the conjugate-phase approach is the most used design criteria for NFF antennas, a number of multiobjective optimization techniques have been proposed for reducing the level of the side lobes around the focal spot region, shaping the antenna near field, getting multifocus antennas, or achieving a simultaneous control of the NF radiation and FF pattern [1]. The NFF antennas illustrated in the present review have been designed by using the conjugate-phase approach, unless otherwise stated.

In [2], Buffi et al. summarized the basic design criteria for NFF planar square arrays, also giving design and performance curves. The effects of the array geometry on NFF planar array performance have been numerically analyzed in [3] by considering elementary sources and a number of different arrangements for the array elements: ring, ring with cross, four orthogonal arrows, and so forth. There, a quadratic phase profile and a uniform amplitude excitation are assumed for the array excitations. The distance between the elements is kept equal to half a wavelength along the two orthogonal array directions, for any array geometry. The different geometries have been compared in terms of the focal spot size, the side lobe level, and the amplitude of the field at the focal point. This latter has been normalized by dividing the total electric field by the number of array elements, which is between 32 and 52 for the different array arrangements. All the considered arrays occupy an area of , and the focal point is at 5*λ* from the array surface. A similar numerical analysis can be found in [4] for additional array geometries.

As an example, the −3 dB and −6 dB focused beams of an NFF array of 2.4 GHz circularly polarized patches (as those used in [5]) are shown in Figure 1. The interelement spacing is and the array size is . The focal distance is m. The phase of the array excitation has been calculated by using the conjugate-phase approach and no amplitude tapering is applied. The converging feature of the radiated beam close to focal plane is apparent from the figure, as well as the expected diverging behavior when moving far from the array surface toward the array FF region. The achieved −3 dB focus width at the focal plane is cm, the depth of focus is cm, the side lobe level at the focal plane is less than −15 dB, and the focal shift is 22.5 cm.