Aperture Antennas for Millimeter and Sub-Millimeter Wave Applications

This chapter presents the Integrated Lens Antenna (ILA) technology as it evolved since its introduction aiming to respond to the needs of emerging applications such as high-data-rate communication, intelligent transport, and mm-wave imaging. The topics covered include the ILA design concepts as well as the electromagnetic phenomena intrinsic to dielectric lenses that may affect ILA performance. The aspects of the ILA technology related to selection of the primary feeds, lens materials, and fabrication methods are also revised. A few practical examples are provided to illustrate the current and future trends of this technology.

In this chapter, we review several aspects of the analysis and the design of multi-shell spherical and circular cylindrical lens antennas. Such lens antennas are attractive for implementation in communication and radar systems, in particular in the millimeter-wave frequency band, due to their broadband behavior, excellent focusing properties, possibility of beam scanning, and the ability to form multiple beams. In order to develop an efficient analysis tool, needed for successful design of multi-shell lens antennas, we first demonstrate the principles of the analysis algorithm for calculating the EM field distribution in general multilayer structures (i.e., inside a structure with an arbitrary number of layers). This algorithm is designed for spherical and circular-cylindrical geometries with elementary excitation. To model realistic lens antennas, we introduce additional flexibility that allows the analysis of actual feed antennas that usually do not follow the symmetry properties of the lens. Finally, by connecting the algorithm with an optimization subroutine, a powerful analysis and design tool is created. All the aspects of the proposed analysis approach are explained and illustrated with examples. Furthermore, some practical problems which are encountered in the design of these types of lens antennas are highlighted and common solutions are presented and compared to the ideal situations.

Millimeter-wave antenna systems have traditionally required high performance feeds in order to fulfill its stringent requirements. Therefore, this goal has been achieved by corrugated horns. However, in the last years new applications mainly in the communication systems have driven the use of other types of antenna feed with slightly reduced performance but simpler manufacturing at mm-wave and submm-wave frequencies with improvements in cost reduction. These advanced profiles are usually based in smooth waveguide advanced profiles. Besides, the chapter describes different feed configurations based on metamaterial structures and several examples of metamaterial based or inspired antennas are considered. Then, this chapter covers the different alternatives currently used for mm-wave antenna feed: corrugated horns, spline horns, and metamaterial horns. At the end of each section, it includes some research successful results.

This chapter is focused on a family of antennas recently proposed, whose common denominator is that they consist in a flat metallic plane with a central narrow aperture surrounded by corrugations. This new family of antennas has occupied a very important place in the development of communication technologies and solutions for several communication applications since, compared to higher volume structures, as horn or parabolic antennas, they present equal or even higher radiation characteristics. An introduction, which also serves as an historical overview, is presented in the first place, emphasizing the relation with both extraordinary transmission structures demonstrated initially at optical wavelengths as well as with leaky-wave antennas, developed mainly in microwaves. Afterwards, the physical mechanism for radiation and the main guidelines for the design of these antennas are discussed. Next, an overview of some of the most appealing designs and results related with this technology is presented, putting special emphasis in terahertz-band applications. Finally, tips on foreseen future trends are summarized to conclude the chapter.

High-gain antennas are a crucial component of microwave, millimetre, and sub-millimetre wave communication and sensing systems. Many systems, such as satellite communication links, point-to-point terrestrial links, deep-space communication links, RADAR systems, and remote-sensing systems depend on high-gain antennas that can be realized in low-profile form at low cost. Combining the best features of reflector antennas and antenna arrays, reflectarrays are promising planar antenna solution for these applications, providing good efficiency and a high degree of flexibility in synthesizing arbitrary antenna patterns. Furthermore, they can be engineered to provide reconfigurable beam-forming capabilities using a variety of approaches. This chapter describes the fundamental operating and design principles of reflectarrays, both in fixed and reconfigurable form. Several practical design examples are included to help readers build confidence with designing this type of space-fed array. Important developments in the field, particularly in the active research area of reconfigurable reflectarrays, are summarized and important future challenges enumerated. Finally, the state of the art in reflectarray applications is reviewed, illustrating the promising future of this emergent antenna architecture.

Transmitarray antennas have attracted a lot of interest for the last 10 years to address the challenges of high-directivity reconfigurable antenna designs for emerging applications at millimeter-wave frequencies. This chapter presents the main principles of operation of transmitarray antennas, their main advantages and challenges, and typical envisioned applications. The main achievements and research orientations are summarized. Next, a theoretical model is presented enabling an accurate and efficient simulation and optimization; generic theoretical results are included to illustrate the main optimization trade-offs. In the final section, practical examples taken from the most recent state of the art are presented covering passive, beam-steering, and reconfigurable antennas in different frequency ranges as well as innovative illumination schemes to reduce the total height of these antennas. Finally, the conclusion highlights some important perspectives and research topics toward future developments.

Fabry-Perot Cavity Antennas (FPAs) are a type of highly directive planar antennas that offer a promising alternative to standard planar microstrip patch arrays or waveguide slot array antennas. They offer significant advantages in terms of low fabrication complexity, particularly at mm wave frequencies, high radiation efficiency, and good radiation pattern performance. These advantages, in conjunction with a renewed interest in periodic surfaces and meta-surfaces, led to a reinvigoration of international research on this antenna type. This chapter reports recent advances on the design and implementation of FPAs at mm-wave bands. The main concept of FPAs, their operating principles and analysis approaches are briefly introduced. The basic types of FPAs are summarized following a historical account of various implementations until recent years. The main body of this chapter provides an overview of recent designs with a main focus on mm-wave bands and the advantages of the reported antennas for high-frequency implementations.

This chapter illustrates the capabilities of non-diffractive Bessel beams for near-field focusing. After a brief introduction of the non-diffractive phenomenon and its origin, the generation of non-diffractive Bessel beams by inward cylindrical traveling waves is analyzed in detail. A ray interpretation is proposed for such beams for infinite and finite radiating apertures. Their main radiation capabilities and limitations in terms of focusing, bandwidth, and operating range are discussed. In particular, it is shown that inward cylindrical traveling wave aperture field distributions can generate non-diffractive Bessel beams over a large bandwidth. As a practical implementation of the discussed theory, a class of launchers based on radial waveguides loaded by metallic gratings or slots is proposed, for which an efficient design and optimization technique is described. In addition, a different and unconventional leaky-wave approach is also adopted for the design of radial waveguides loaded by metasurfaces. The chapter ends by outlining the future research activities and possible applications of non-diffractive beams.

This chapter reports design and analysis methods for planar antennas based on modulated metasurfaces (MTSs). These antennas transform a surface wave (SW) into a leaky wave by means of the interaction with a MTS having a spatially modulated equivalent impedance. The basic concept is that the MTS imposes the impedance boundary conditions (BCs) seen by the SW, and therefore the MTS controls amplitude, phase, and polarization of the aperture field. Thus, MTS antennas are highly customizable in terms of their performances, by simply changing the MTS and without affecting the overall structure. Several technological solutions can be adopted to implement the MTS, from sub-wavelength patches printed on a grounded slab at microwave frequencies, to a bed of nails structure in the millimetre and sub-millimetre wave range: in any case, the resulting device has light weight and a low profile. The design of the MTS is based on a generalized form of the Floquet wave theorem adiabatically applied to curvilinear locally periodic BCs. The design defines the continuous BCs required for reproducing a desired aperture field, and it is verified by a fast full-wave solver for impedance BCs. Next, the continuous BCs are discretized and implemented by a distribution of electrically small printed metallic elements in a regular lattice, like pixels in an image. The final layout is composed of tens of thousands of pixels and it is analyzed by a full-wave solver which makes use of entire domain basis functions combined with a fast-multipole algorithm. Examples of design and realizations of MTS antennas are shown, proving the effectiveness of the concept.

Terahertz antennas present a different set of challenges to the antenna designer typically striving for very high performance while at the very limit of the chosen fabrication process. Many of the same design techniques used at lower frequencies are still applied, but fabrication constraints impose significant limitations on the type of structure that can be used, forcing the designer to consider unique fabrication processes or completely new antenna structures. Through advances in fabrication and computational techniques, the variety of terahertz antennas is growing. This chapter presents a range of antennas applied at these frequencies from 1.9 THz horn antennas to superconducting planar arrays. The chapter will cover different antenna technologies and feeds such as corrugated horn antennas, smooth-walled profiled horn antennas, multi-flare angle horn antennas, lens antennas, microlens leaky wave antennas, metasurface antennas, antenna arrays, off-chip antennas, and others. It will detail theory, simulation, fabrication techniques, and state-of-the-art antenna results in all these different technologies at millimeter and terahertz frequencies. The chapter will also provide details for terahertz antennas in the context of terahertz systems.

This chapter describes the way in which transformation optics can be used to design lens antennas. It begins with a brief introduction to the analytical method and describes the difficulties faced by practical applications, in terms of anisotropic material requirements and bandwidth restrictions. Two further variants of transformation optics are provided to circumvent these issues, which are the quasiconformal transformation technique, and non-Euclidean transformations for surface devices. The use of both these methods results in material property requirements that can be easily implemented with dielectrics. These two techniques are thoroughly described, with the benefits and drawbacks they present detailed. The discussion is supported throughout with a number of application examples in each section, which include planar lenses, bespoke lenses, curved surface wave lenses, and a surface wave cloak. Examples are chosen to illustrate the variety of ways in which transformation optics can be applied to antenna systems, and are all supplemented by simulation or measurement data.

Testing of electrically large, high-gain antennas as well as that of small integrated antennas at millimeter and submillimeter wavelengths is extremely challenging. Basically, there are three methods for measuring radiation properties of an antenna: the far-field method, the near-field method, and the compact antenna test range (CATR). In case of large antennas, the classical far-field method has two major obstacles at mm and submm wavelengths: impractically large measurement distance and high atmospheric loss. The planar near-field scanning method has been used up to 1 THz. However, the applied near-field methods often give useful information only on the main beam and its vicinity, because the field-sampling is typically very sparse. Reflector-based and hologram-based compact antenna test range (CATR) measurements have been demonstrated up to 500 GHz and 650 GHz, respectively. In the case of small integrated antennas, various techniques for on-wafer measurements have been developed. This chapter discusses the theory, techniques and limitations of the various test methods—the far-field method, planar near-field scanning and CATR as well as on-wafer measurements. Also, antenna pattern correction techniques are discussed.

This chapter provides an overview over common categories of microwave holography systems. Terms like resolution or speckle are defined and design rules are presented. Performance and cost of the various microwave holography systems are discussed as well as their applications. A special focus is devoted to indirect holographic systems at millimeter wave frequencies. A setup using arrays of planar antenna coupled zero bias Schottky diodes as detectors is presented in detail.