Analysis Of Electrically Small Antenna Designs And Limitations

Author's abstract: Electronic sizes are constantly decreasing. The need for smaller communication systems is at an all-time high. The antenna is a major part of wireless communication systems, so the need for smaller antennas is also paramount. Electrically-small antennas are the solution to this problem. Electrically-small antennas have many inherent limitations. In this thesis, a comprehensive background on electrically-small antennas is conducted to illustrate the common design limitations that face electrically-small antennas. Three novel, size-reduced antennas are presented in this thesis. A 15-element size-reduced Yagi-Uda antenna, a 6-element size-reduced antenna, and a spherical helix electrically-small antenna are all introduced in this thesis. The antennas are all designed, simulated, fabricated, and measured for verification of results.

Now in an completely revised, updated, and enlarged Second Edition, Small Antennas in Portable Devices reviews recent significant theoretical and practical developments in the electrically small antenna area. Examining antenna designs that work as well as those that have limitations, this new edition provides practicing engineers and upper level and graduate students with new information on: work on improving bandwidth using spherical helix dipoles; work on electromagnetically coupled structures; exact derivation of the Q for electrically small antennas for both the TE and TM modes; and a new simplified Q formula.

As the demand for compact, portable communication electronics increases, the technology of miniaturization has made great progress. A beneficiary of that progress has been research into new concepts for the antenna, one of the essential components in wireless communications. As the size of an antenna becomes smaller, however, the antenna suffers from high Q and low radiation resistance. The results are narrow bandwidth, poor matching, low efficiency, and, more generally, poor performance throughout the communication system. First, the design of a small antenna for HF/VHF communications is described. As the operating frequency of an antenna decreases, for example, into the HF and low VHF regions, the physical size of the antenna becomes a critical issue. It is desirable to design a truly electrically small antenna by reducing the ground plane size. Moreover, when the antenna size is very small, the bandwidth of the antenna is extremely narrow, which is critical to various deployment variances and propagation effects such as multi-path fading. The new design, which is an inductively coupled, top-loaded, monopole structure optimized by a genetic algorithm (GA), maximizes transmission of HF/VHF waves. Electrically small, spiral ground planes for the monopole and the electrically small antenna are designed for HF ground-wave transmission. In addition, a tunable small antenna is investigated that overcomes the narrow-bandwidth limitation of electrically small antennas. Second, new design methodologies for electrically small antennas are discussed. Use of an inductively coupled feed is one of the well-known methods for boosting input resistance. As the antenna size becomes smaller, however, it is found that the efficiency of an antenna using an inductively coupled feed is lower than an antenna using multiple folds. After a comparison of the two methods, the design of a thin, multiply folded, electrically small antenna is proposed for achieving high efficiency in a physically compact size. The GA is used to assess the effect of geometry on the performance (in terms of efficiency and bandwidth) of the electrically small antennas, including the folded conical helix and folded spherical helix. Finally, the prospects of using the new Yagi antennas to achieve small size are explored. Yagi antennas are used widely to obtain high gain in a simple structures. The antenna is composed of the driven element and the parasitic elements, which include a reflector and one or more directors. Typically, sufficient spacing on the order of 0.15[lambda] to 0.4[lambda] between the driven element and the parasitic elements is needed for the Yagi antenna to operate well. For some applications, however, it is desirable to reduce the spacing and the length of the elements to achieve a physically more compact size. In this dissertation, closely spaced, folded Yagi antennas in both three dimensions and two dimensions are investigated, and a design for an electrically small Yagi antenna is suggested.

Next-generation small antenna design techniques This authoritative text provides the most up-to-date methods on the theory and design of small antennas, including an extensive survey of small antenna literature published over the past several years. Written by experts at the forefront of antenna research, Small Antennas: Miniaturization Techniques & Applications begins with a detailed presentation of small antenna theory--narrowband and wideband--and progresses to small antenna design methods, such as materials and shaping approaches for multiband and wideband antennas. Generic miniaturization techniques are presented for narrowband, multiband, and wideband antennas. Two chapters devoted to metamaterials antennas and methods to achieve optimal small antennas, as well as a chapter on RFID technologies and related antennas, are included in this comprehensive volume. Coverage includes: Small antenna theory and optimal parameters Theory and limits of wideband electrically small antennas Extensive literature survey of small antenna designs Practical antenna miniaturization approaches Conformal wideband antennas based on spirals Negative refractive index (NRI) metamaterial and electromagnetic band gap (EBG) based antennas Small antennas based on magnetic photonic and degenerate band edge crystals Impedance matching for small antennas using passive and active circuits RFID antennas and technology

As wireless devices and systems get both smaller and more ubiquitous, the demand for effective but small antennas is rapidly increasing. Small Antenna Design describes the theory behind effective small antenna design and give design techniques and examples for small antennas for different operating frequencies. Design techniques are given for the entire radio spectrum, from a very hundred kilohertz to the gigahertz range. Unlike other antenna books which are heavily mathematical and theoretical, Douglas Miron keeps mathematics to the absolute minimum required to explain design techniques. Ground planes, essential for operation of many antenna designs, are extensively discussed. Author's extensive experience as a practicing antenna design engineer gives book a strong "hands-on" emphasis Covers antenna design techniques from very low frequency (below 300 kHz) to microwave (above 1 GHz) ranges Special attention is given to antenna design for mobile/portable applications such as cell phones, WiFi, etc

Author's abstract: A growing need for efficient wireless communication is prevalent in the world in which we live. From cell phones to television to GPS applications, wireless communications are vital in consumer electronics and military applications. In these applications, a miniaturized antenna is sometimes necessary for reducing overall size of the communication system. For many satellite based communication applications, circular polarization in antennas is needed for efficient communication. In this thesis, the miniaturization technique known as T-top loading is utilized on two novel antenna designs. One design is an electrically small, circularly polarized planar cross dipole and the other design is a compact circularly polarized log-periodic dipole array. Both antennas are designed in simulation software with the intent for prototype fabrication for measurement verification of simulation results.

Advances in Electrically Small Antennas provides a detailed discussion on the theory, challenges and performance trade-offs associated with the design of electrically small antennas. The author presents many electrically small antenna designs and describes them with a particular focus on recent advances in the field. The text begins with an overview of the basic theory and concepts associated with electrically small antennas to provide you with an understanding of performance limitations in terms of impedance, radiation patterns, bandwidth, efficiency, and Quality-factor. After presenting detailed discussions of recent advances made in the field of electrically small antennas, the book concludes with a discussion of recent advances made in the design of low profile, conformal and integrated device antennas.

Abstract: Emerging broadband applications with market pressures for miniaturized communication devices have encouraged the use of electrically small antennas (ESA) and highly integrated RF circuitry for high volume low cost mobile devices. This research work focuses on developing a novel scheme to design wideband electrical small antennas that incorporates active and passive loading as well as passive matching networks. Several antennas designed using the proposed design technique and built and measured to assess their performance and to validate the design methodology. Previously, the theory of Characteristic Modes (CM) has been used mostly for antennas analysis. However; in this chapter a design procedure is proposed for designing wide band (both the input impedance bandwidth and the far field pattern bandwidth) electrically small to mid size antennas using the CM in conjunction with the theory of matching networks developed by Carlin. In order to increase the antenna gain, the antenna input impedance mismatch loss needs to be minimized by carefully exciting the antenna either at one port or at multiple ports and/or load the antenna at different ports along the antenna body such that the Q factor in the desired frequency range is suitable for wideband matching network design. The excitation (feeding structure), the loading of the antenna and/or even small modifications to the antenna structure can be modeled and understood by studying the eigenvalues and their corresponding eigencurrents obtained from the CM of the antenna structure. A brief discussion of the theory of Characteristic Modes (CM) will be presented and reviewed before the proposed design scheme is introduced. The design method will be used to demonstrate CM applications to widen the frequency bandwidth of the input impedance of an electrically small Vee shape Antenna and to obtain vertically polarized Omni-directional patterns for such antenna over a wide bandwidth. A loading technique based on the CM to either design frequency reconfigurable antennas or broaden their bandwidth by Non-Foster loading will also be discussed as part of the design methodology. In the Appendix, a brief discussion of the fundamental limits of electrical small antennas is presented and then followed by a discussion of the fundamental limits of the impedance bandwidth of the ESA when a passive matching network is used. Matching network implemented using Non-Foster matching is also discussed in the appendix.

Antennas smaller than a quarter wavelength are fundamentally constrained in a variety of ways. One of the more problematic limitations is that the antenna's bandwidth declines sharply as the size of the antenna decreases. Myriad studies have sought antennas that perform close to the fundamental limits, and they use a patchwork of good and bad design approaches. Our primary goal is to describe a new, complete framework to model the fundamental behavior of small antennas. We base our analysis in characteristic mode theory which allows us to decompose the antenna behavior into the behavior of a few well-defined modes. Using this decomposition we can better understand, design, and analyze small antennas. First, we explain a unified approach to model the antenna input impedance, rather than the haphazard array of approaches that are currently used. Using our model for the input impedance, we are then able to establish the conditions under which a small antenna can be effectively impedance matched, and analyze some simple methods for matching an antenna without using an external matching network. Through this study, we find that near-optimum modes actually exist in nearly every geometry but are often masked by higher order modes. From this result, a new design paradigm is proposed in which designs seek to couple into these existing modes and match using the simple methods described herein, rather than creating ever more complex and impractical structures. We also design and fabricate two novel, spherical, electrically small antennas, the TM10 antenna and the spherical meanderline antenna. Both of these antennas exhibit quality factor close to the lower limit, and hence, a near-optimum bandwidth. The spherical meanderline antenna is particularly well-suited for automated fabrication and can achieve bandwidth comparable to the best known values. In collaboration with materials scientists, we demonstrate the spherical meanderline antenna, which is one of the first microwave structures printed on a curved surface using a direct-ink write process. Finally, to circumvent some of the bandwidth limitations imposed on small antennas, we propose an approach to design multimode antennas. Estimates are derived for the bandwidth increases that can be achieved with this approach to antenna broadbanding, and a simple figure of merit is suggested. A case study in broadbanding the TM10 antenna provides some idea of what types of modal combinations are practical. Finally, a multimode spherical meanderline antenna matched with the simple techniques described herein is designed and fabricated.