Ultra Wideband Antennas

Design, Methodologies, and Performance

Giselle M. Galvan-Tejada, Marco Antonio Peyrot-Solis, Hildeberto Jardón Aguilar

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351831406
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Ultra Wideband Antennas: Design, Methodologies, and Performance presents the current state of the art of ultra wideband (UWB) antennas, from theory specific for these radiators to guidelines for the design of omnidirectional and directional UWB antennas. Offering a comprehensive overview of the latest UWB antenna research and development, this book:

• Discusses the developed theory for UWB antennas in frequency and time domains
• Delivers a brief exposition of numerical methods for electromagnetics oriented to antennas
• Describes solid-planar equivalence, which allows flat structures to be implemented instead of volumetric antennas
• Examines the impedance matching, phase linearity, and radiation patterns as design objectives for omnidirectional and directional antennas
• Addresses the time domain signal analysis for UWB antennas, from which the distortion phenomenon can be modeled
• Includes illustrative examples, design equations, CST MICROWAVE STUDIO® simulations, and MATLAB® plot generations
• Compares the performance of different UWB antennas, supplying useful insight into particular tendencies and unresolved problems

Ultra Wideband Antennas: Design, Methodologies, and Performance provides a valuable reference for the scientific community, as UWB antennas have a variety of applications in body area networks, radar, imaging, spectrum monitoring, electronic warfare, wireless sensor networks, and more.

1
Introduction

Contents

1.1 Importance of Antennas in Modern Life

1.2 Ultra Wideband Systems

1.3 UWB Antennas

1.4 Scope of the Book

Bibliography

1.1 Importance of Antennas in Modern Life

Nowadays, wireless applications have become an important part of people’s lives. In the field of telecommunications, for example, it is common throughout the world to find people of all ages carrying at least one wireless device with them on a daily basis. Interest in wireless technology lies in the desire for the freedom to move, while having access to information, communication and the control of different devices. Thus, wireless technology provides a flexible and attractive option for carrying out a range of tasks.

Antennas are a very important component of wireless devices, as they represent the way to receive and transmit certain signals. Basically, the antenna is the mechanism that transforms guided energy from a transmission line into radiated energy traveling through space at distances ranging from a few centimeters to hundred of kilometers. These devices have been studied and designed for a wide range of applications for over 100 years. Each application demands particular signal features, of which the required bandwidth is one of the most significant. Thus, depending on the bandwidth, three types of system can be defined: narrowband, wideband, and ultra wideband.

1.2 Ultra Wideband Systems

The need for increasingly wider bandwidths for many modern radar, imaging and telecommunications applications has propelled the search for new technologies. Several emerging lines of research sought to rise to this new challenge, and discussions took place around the world about the different ways of generating special signals to be radiated at a very large frequency range. The Ultra Wideband (UWB) concept appears to have been adopted by the United States Department of Defense in 1989 to refer to a range of terms, such as: impulse, carrier-free, and large-relative-bandwidth signals [1]. In 1992, the United States Federal Communications Commission (FCC) specified different technical standards and operation restrictions for three types of UWB systems, and marked their frequency range from 3.1 to 10.6 GHz [2]. The FCC also specifies that for an antenna (or more generally, a system) to be considered as UWB-type, it must have a bandwidth greater than 500 MHz [3].

Systems based on UWB technology transmit streams of extremely short pulses (around 10 to 10,000 picoseconds [4]), which can be spread through a very broad range of frequencies. Some interesting features of the UWB systems are obviously the possibility to carry a huge amount of information data and that this signal spread makes them robust to interference. Another important aspect is security, provided that UWB short pulses are harder to jam.

Nevertheless, at system level, the most important drawback of UWB networks is that they are range limited (between 10–20 m approximately [4]) and medium or large scale deployments cannot be implemented. Indeed, the original idea from which UWB technology was formulated was for Personal Area Networks (PANs). This implies that they can only be rolled out for short range networking, either in a centralized or distributed wireless mode (for example PANs, wireless sensor networks, etc.). For in-home purposes, for instance, UWB systems can be competitive with limited range technologies like Bluetooth and can be a complement to WiFi networks. Thus, through UWB access huge quantities of information (videos, photos, music, presentations, etc.) can be shared while avoiding cables. Naturally, UWB can also be implemented in other indoor scenarios e.g. offices, hospital rooms, academic laboratories, and industrial areas, among others.

The extremely large bandwidth of UWB systems introduces some peculiarities into the propagation and therefore in the system performance. A much more diverse multipath phenomenon is presented. Because the power is now distributed over a larger bandwidth (i.e., over many multipath components) the energy on each path could be too low to be distinguished with classical techniques. In addition, these multiple paths could suffer distinct frequency selective distortions, which could affect the pulse shapes, and as a consequence new schemes of synchronization are required. Another problem is related to the estimation of the time of arrival of UWB signals. This topic is crucial of course for applications that require high time resolution like medical imaging or radar.

Finally, the spectrum regulation is certainly another central premise of UWB systems. Basically, the frequency range assigned for these systems over-laps with other licensed and unlicensed systems, under certain spectral masks depending local regulations, which sometimes are not uniform. Generally speaking, the spectral shape of a signal is determined by the type of pulse transmitted and the modulation format [5]. Then for UWB systems, the design of pulses is also a delicate design subject. In this concern, the chapter Ultra wideband pulse sharper design can be consulted [6].

1.3 UWB Antennas

The study of UWB antennas is important because, as mentioned in the previous section, UWB systems transmit and receive ultra-short electromagnetic pulses, which means that they use a very wide bandwidth with low levels of average power, creating difficulties with signal detection; to overcome this, UWB antennas need to receive all the components of the signal spectrum with the same efficiency, and without introducing significant distortion in the phase of those frequency components. Although it is possible to implement a type of compensation as a countermeasure to distortion effects [7], it is far preferable to seek antenna designs that introduce the lowest possible distortion in the phase characteristic, such that the whole system design is simpler.

Therefore, the behavior and performance of an ultra wideband antenna UWB antenna must be coherent and predictable throughout the operational bandwidth, which means: variations must not be introduced into the antenna radiation pattern (failing this, there should be as few variations as possible), there must be good matching conditions (evaluated by the reflection coefficient), and there must be no distortion of the signal waveform. Moreover, besides traditional parameters to describe narrowband antennas (e.g., gain, impedance matching, polarization, etc.), other parameters must be considered, such as phase linearity and radiation pattern variations across the frequency range, which are important for the satisfactory integration of antennas into modern UWB systems.

UWB antenna theory presents an additional challenge compared to classical antenna theory. In the latter, theoretical developments are based on knowledge of the wavelength, as determined by its main resonance frequency; however, the main resonance frequency of UWB antennas is non-determinable, since more than one resonance can be identified in their very broad bandwidths. This characteristic leads, therefore, to uncertainty as to the use of the lower cut-off, the upper cut-off, or any “central” frequency.

In the open literature today there are more than 6000 articles related to UWB antennas, a fact that demonstrates the relevance of the topic. In particular, significant efforts have been focused on the development of omnidirectional antennas, due to their implementation in mobile applications. However, UWB antennas with directional radiation features have also attracted the attention of researchers and manufacturers, since these antennas are particularly attractive for a military environment.

1.4 Scope of the Book

The present text was designed to be reference material for the interested reader, and it is important here to outline the scope of the book. In Chapter 2, general concepts relating to classical antenna theory and design are presented, with a brief explanation of the main parameters of “conventional” antennas that have been used for many years.

Chapter 3 is a compendium of several recent developments in ultra wideband antennas reported in different forums. These developments are grouped according to the identification of their structures. In general terms, two types of structures are defined: planar and planarized. (The ‘planarized’ term is not usually used in the literature, but we introduce it in order to differentiate between ground plane embedded and non-embedded flat radiators). Some early work that intended to achieve a wider operational frequency range is also covered in this chapter, providing many guidelines on the fundamentals of UWB antennas.

Theory developed for UWB antennas is addressed in Chapter 4, where the first parameter to define is bandwidth. As pointed out in this chapter, although it is possible to directly formulate a single definition for bandwidth, there are many founding concepts behind this. A fundamental concept for ultra wideband antennas is the quality factor, which was studied by Wheeler in the context of small antenna [8, 9] and was also explored by Schantz, who analyzed the dependence of this factor on bandwidth [10]. This factor has an inverse relation with bandwidth, which means that UWB antennas must have low quality factors in order to store the least possible energy, thus achieving wide bandwidths. Another important concept is solid-planar equivalence, which allows planar and planarized structures to be implemented instead of volumetric antennas. This principle is vital for the development of UWB antennas in portable devices, and for the recent body area networks where small, low profile antennas are intended to be integrated in wearable devices.

A particular aspect of UWB antennas relates to time domain signal analysis. Due to the relative short duration of UWB pulses, the transient response must not be neglected since it provides a measure of the dispersion (and in general, the distortion phenomenon) that they suffer. The importance of having a measure of this phenomenon is that it imposes certain constraints on the data rate to be transmitted. These issues, together with performance measurements such as group delay and phase linearity, are addressed in Chapter 5. Examples of dispersive and non-dispersive antennas are also given in this chapter.

Chapter 6 presents design guides for UWB omnidirectional antennas, where the challenge is to conserve the radiation pattern shape over the whole bandwidth. A design methodology is described, which considers three objectives: impedance matching, evaluated through the reflection coefficient magnitude; phase linearity, determined through the phase of the reflection coefficient; and variations in the shape of the radiation pattern. In essence, four parameters related to radiator dimensions are the variables within the methodology. The chapter includes different omnidirectional UWB designs as reported in the open literature, and a performance comparison among them.

Directional UWB antennas are covered in Chapter 7. This chapter also presents a specific design methodology, which is based on the methodology of omnidirectional antennas explained in Chapter 6. The difference here is a new variable for the inclination angle of the radiator. This radiator tilt is precisely the mechanism that allows directive radiation patterns to be achieved. A different design methodology based on the solid-planar correspondence principle is also addressed in this chapter, which allows to generate a structure that can be designed for any lower cut-off frequency. Simulation results of UWB directive antennas reported by some authors are compared, in order to evaluate their performance.

It is worth noting that all simulations presented in this book have been conducted using the CST Microwave Studio. The measurement results shown in Chapters 6 and 7 were carried out with an Agilent network analyzer model E8362B.

Some current trends and unresolved problems in the field of UWB antennas are addressed in Chapter 8. Of particular interest are the relatively recent body area networks, where a vast variety of...