Miron Small Antenna Design

1. Auflage 2006
ISBN: 978-0-08-049814-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 304 Seiten

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' emphasisCovers antenna design techniques from very low frequency (below 300 kHz) to microwave (above 1 GHz) rangesSpecial attention is given to antenna design for mobile/portable applications such as cell phones, WiFi, etc

Autoren/Hrsg.

Miron, Douglas B.

Inhaltsverzeichnis

1;Front Cover;1
2;Small Antenna Design;4
3;Copyright Page;5
4;Contents;8
5;Preface;12
6;About the Author;14
7;What’s on the CD-ROM?;16
8;Chapter 1: Introduction;18
8.1;1.1 What Is Small?;18
8.2;1.2 What Are the Problems?;19
8.3;1.3 Some Historical Small Antenna Types and Applications;19
8.4;1.4 Some Present and Future Small Antennas;22
8.5;References;25
9;Chapter 2: Antenna Fundamentals I;26
9.1;2.1 Electromagnetic Waves;26
9.2;2.2 Polarization;37
9.3;2.3 The Short Dipole;40
9.4;2.4 The Small Loop;48
9.5;2.5 Directionality, Efficiency, and Gain;52
9.6;References;54
9.7;Chapter 2 Problems;55
10;Chapter 3: Antenna Fundamentals II;60
10.1;3.1 Bandwidth and Quality Factor, Q;60
10.2;3.2 Impedance Matching and System Efficiency;68
10.3;3.3 Reception;73
10.4;3.4 Ground Effects;76
10.5;3.5 Improvements;85
10.6;References;86
10.7;Chapter 3 Problems;88
11;Chapter 4: Introduction to Numerical Modeling of Wire Antennas;92
11.1;4.1 General Concepts;92
11.2;4.2 The Mathematical Basics of the Numerical Electromagnetic Code (NEC);94
11.3;4.3 Using NEC in the Command Window;109
11.4;4.4 Modeling Guidelines;116
11.5;4.5 NEC in a Graphical User Interface (GUI);120
11.6;4.6 Examples from Chapters 2 and 3;122
11.7;References;126
11.8;Chapter 4 Problems;127
12;Chapter 5: Programmed Modeling;130
12.1;5.0 Introduction;130
12.2;5.1 Using Wire-List Generators in NEC;130
12.3;5.2 Using Code to Generate a Wire List;135
12.4;Chapter 5 Problems;148
13;Chapter 6: Open-Ended Antennas;150
13.1;6.0 Introduction;150
13.2;6.1 Thick Monopoles;152
13.3;6.2 Top Loading;157
13.4;6.3 Coil Loading;169
13.5;6.4 Using Resonance;182
13.6;6.5 Summary;189
13.7;References;191
13.8;Chapter 6 Problems;192
14;Chapter 7: Loops and Other Closed-Wire Antennas;196
14.1;7.0 Introduction;196
14.2;7.1 Thick Loops;197
14.3;7.2 Solenoid Antennas;211
14.4;7.3 The Contrawound Toroidal Helix Antenna (CTHA);215
14.5;7.4 The Folded Spherical Helix Monopole;219
14.6;7.5 Final Comments;222
14.7;References;223
14.8;Chapter 7 Problems;224
15;Chapter 8: Receiving Antennas;228
15.1;8.0 Introduction;228
15.2;8.1 External Noise;229
15.3;8.2 The Ferrite Rod Antenna;230
15.4;8.3 Active Receiving Antennas;240
15.5;References;247
15.6;Chapter 8 Problems;248
16;Chapter 9: Measurements;252
16.1;9.1 What Are You Measuring?;252
16.2;9.2 Measurements Through a Transmission Line;253
16.3;9.3 Ranges and Test Enclosures;257
16.4;9.4 The Wheeler Cap and Variations;259
16.5;References;263
16.6;Chapter 9 Problems;264
17;Appendix A: The Mathematics of Antenna Orientation;266
17.1;A.1 Unit-Vector and Coordinate Variable Relations;266
17.2;A.2 The Horizontal Dipole;268
17.3;A.3 The Vertical Loop;269
17.4;Appendix A Problems;270
18;Appendix B: The Parallel-Ray Approximation;272
18.1;Appendix B Problems;274
19;Appendix C: The Small Loop;276
19.1;Appendix C Problems;279
20;Appendix D: The Proximity Effect;280
20.1;D.1 Current Distribution;280
20.2;D.2 Power and Resistance;286
20.3;References;287
21;Appendix E: What Every EE Student Should Know About Mathematics by the Senior Year;288
21.1;E.1 What Is Mathematics to an Engineer?;288
21.2;E.2 The Process Is as Important as the Result;288
21.3;E.3 Facts and Idioms;289
21.4;E.4 Integrals and Derivatives;290
21.5;E.5 Radians or Degrees?;291
21.6;E.6 Matrix Notation and Operations;291
21.7;E.7 Answers for Section E.3;292
22;Index;294
23;Elsevier Science CD-ROM License Agreement;302

Leseproben

CHAPTER 1 Introduction 1.1 What Is Small?
“Small” is obviously a comparative term, and so one must ask what is the reference standard. For ordinary usage, we may use as reference the middle of the size range of objects in the class being discussed. For example, among dogs a dachshund is small and a St. Bernard is large. But the medium dog is small compared to the medium human, whether we’re talking about height or weight. For antennas, it’s hard to say what the middle size is, so people talk ordinarily in reference to the human scale. I call this physical size. That is, an antenna that fits in your hand is physically small, while one that is 20 m tall is physically large. Physical size, along with the environment in which the antenna will be used, is very important in the mechanical design of an antenna, but it is only secondary to the electrical design process. The scale of interest for electrical design is the free-space wavelength at the operating frequency. From physics, we know (1.1) where c = speed of light, f = frequency, and ?= wavelength. Generally speaking, an antenna is considered electrically small if its largest dimension is at or under ?/10. If f is in MHz, it is convenient to use c = 300 Mm/s, because then ? is in m. At the middle of the AM broadcast band, f = 1 MHz, so ?= 300 m. An antenna that is ?/20 = 15 m is electrically small but physically large. At the middle of the FM broadcast band, f = 100 MHz, so ?= 3 m. A ?/20 antenna will be 0.15 m long, which is physically small. At f = 2.4 GHz, a cell phone band, ?= 0.125 m and a ?/20 antenna would be 6.25 mm long, physically tiny. For all three of these applications, the electrical design considerations for a given antenna type are the same. A kind of hybrid size category is low profile. This usually means the antenna is short compared to the object on which it is mounted, and usually turns out to be electrically short as well, but not necessarily electrically small because its width dimensions can be ?/4 or larger. Typical applications include vehicles, especially military ones, and handheld radios and phones. Since I am concerned only with the electrical design aspect, I present material on electrically small and low-profile antennas in this text. Larger antennas are extensively covered in numerous books on general electromagnetics and antennas. In sections 1.3 and 1.4, I describe briefly some of the past, present, and possible future types you might see. They, along with others, will be described and analyzed in detail in later chapters. 1.2 What Are the Problems?
The nature of antennas, definitions of performance, and effects of size on performance are described in detail in the next two chapters, but I think a few introductory words here will help the reader to understand the next two sections a little better. An antenna is a device whose purpose is to convert between circuit power, voltage and current at the radio terminals, and radiated power carried in an electromagnetic wave. Without a size constraint, most antennas would be built close to size multiples of ?/4. This is because the terminal impedance of the antenna for this condition is real and easily made compatible with the radio or transmission line to which it is connected. Antennas sized this way are called resonant-length or resonant-size. When traditional antennas are operated at frequencies for which they are electrically small, their input impedances become more and more reactive and this makes it harder to transfer power between them and the radio. Also, the coupling the antenna provides between the circuit terminals and the wave becomes less, whether we’re talking about transmitting (generating a wave) or receiving (extracting power from a wave). For a series model of the antenna impedance, the reduction in coupling is manifested in a reduction in radiation resistance. This makes circuit loss and antenna copper loss relatively more important in degrading system efficiency. 1.3 Some Historical Small Antenna Types and Applications
The antennas we see around us every day show a variety of shapes. Most people are familiar with the large curved reflectors used in radar and satellite receiving systems. An observant person will notice that often there is a short rectangular or circular pipe with a flared opening, called a horn, in front of the big dish. Both the dish and the horn are electrically large. At the 4-GHz satellite TV frequency, a 3-m dish is 40? in diameter. Its purpose is to scoop energy out of the passing electromagnetic wave and focus it into the horn. I mention this because it provides a picture of the idea of an antenna having an effective area. Almost all the other antennas you are likely to see are wire or wire-equivalent structures. The typical set-top TV broadcast antenna is a two-wire “rabbit-ears” which should be adjusted to be ?/2 long for the channel being watched. This antenna is for the VHF channels. Sometimes the set-top antenna also includes a small loop to be used for UHF reception. This loop is typically electrically small. AM radios in the ’40s and ’50s commonly had a flat coil of many turns attached to the inside of the back cover. As radios became smaller in the ’60s, these coils were replaced with solenoidal coils wound on ferrite cores. Figure 1.1 illustrates these antennas. From the discussion in section 1.1, you can estimate how extremely electrically small these antennas are. They function by coupling to the magnetic field component of the passing wave, which induces enough voltage in the coil to produce a signal larger than the electrical noise generated by the receiver itself. A tuning capacitor is usually placed in parallel with the coil. The two together form a resonant circuit to limit the bandwidth at the input of the first stage of the receiver. If an open-wire antenna were used, a separate coil would be needed for this purpose. Figure 1.1 AM receiving antennas. (a) Flat coil, typically 25 by 20 cm, with many turns of insulated thin wire. (b) Ferrite loopstick. Anywhere from 3 to 30 cm long, with many turns of enameled fine wire. Until the ’70s, all small antennas were variations on coils or open-wire designs. Formally, open-wire antennas are called dipoles if they have two wires like the rabbit-ears, or monopoles if they have one wire. When transmitting, dipoles are driven by a voltage applied across a small gap between the two wires. Monopoles are driven by a voltage applied between the wire and a ground system. In the case of AM transmitters, the wire is actually a steel tower whose height is ?/4 at the operating frequency, which is why they have aircraft warning lights on top. At VLF, 30–300 kHz, and ELF, below 30 kHz, ?/4 is an impractical height. There are transmitters operating in these frequency ranges for navigation beacons and submarine communications. For these applications, the vertical wire is an electrically short tower with a layer of horizontal cables going from the top of the vertical wire (tower) to supporting poles. This arrangement makes the antenna somewhat like a capacitor that happens to radiate. This method of improving the performance of a short vertical is called top-loading. Since it increases the capacitance over what it would be for the short tower alone, it decreases the terminal reactance, which makes the power transfer problem easier. It also raises the radiation resistance. Most people will never see a VLF transmitter, but the same principle is used by radio amateurs, especially in the 150, 75, and 40 m bands. So you have a reasonable probability of seeing a top-loaded monopole in someone’s back yard. The ground system for monopole applications mentioned above is usually an artificial ground made by laying either a metal mesh or a number of radial wires over as much area as possible. Another monopole that one sees more frequently than the top-loaded version is the whip mounted on a vehicle or a handheld radio or radio-telephone. In the case of the vehicle, the ground is the metal body. In the case of the handheld radio, the ground is the metal enclosure, inside the plastic case, of the radio box. The performance of electrically short whips can be improved by including a series coil, either at the antenna base or part-way up. This is called coil-loading when the coil is part-way up, and has better aerodynamics than top loading. Coil-loading is not as efficient as top-loading, but it does provide a real input impedance and improves the radiation resistance. Sometimes top-loading and coil-loading are combined to get an electrically short antenna with a real input impedance [1]. This arrangement is sketched in Figure 1.2. Figure 1.2 Sketch of a multiply-loaded and tuned monopole for the HF band. The ground wires are laid on the earth or buried. Four top radials are shown, but performance can be significantly improved by using more. 1.4 Some Present and Future Small Antennas
Numerically, the most common present and future potential application of small antennas is in handheld radio devices such as...