The ARRL Antenna Book (24th Edition)

The ARRL Antenna Book (24th Edition)

THE ARRL FOR RADIO COMMUNICATIONS >) ARRL misters / : The ARRL Antenna Book FOR RADIO COMMUNICATIONS BOOK" FOR RADIO COMMUNICATIONS _ ) ARR was So Twenty-Fourth Edition Published by: Front Cover ARRL The antenna shown on the cover is a compact 4-element Yagi for 2 meters designed by ARRL Laboratory Engineer Zack Lau, W1VT. It is typical of the types of antenna designs inspired by the wealth of information contained in the the national association for Amateur Radio™ Newington, CT 06111 USA Editor H. Ward Silver, NOAX Contributing Editors Steven R. Ford, WB8IMY Mark J. Wilson, K1RO Editorial Assistant Maty Weinberg, KB1EIB Production Michelle Bloom, WB1ENT Sue Fagan, KB1OKW — Cover Art Jodi Morin, KA1JPA David F. Pingree, NINAS 24th edition of The ARAL Antenna Book. Contributors to the 24th Edition Alan Applegate, KOBG Gary Breed, K9AY Jim Brown, K9YC Glen Brown, W6GJB David Conn, VE3KL Remco den Besten, PASFYM Wyatt Dirks, ACORA James Duffey, KK6MC Darryl Holman, WW7D Justin Johnson, GOKSC Matt Kastigar, N9ES Steve Kostro, N2CEI Sean Kutzko, KX9X Car| Luetzelschwab, K9LA Brian Machesney, K1LI Paul Marsh, MOEYT Tino Pavic, VKSEGN Stu Phillips, K6TU Tom Planer, KJ9P Gilles Renucci, VA2EW Ulrich Rohde, N1iUL Rick Rosen, KiDS Rudy Severns, N6LF Corey Shields, KBQJHU Kai Siwiak, KE4PT Andrea Slack, K2EZ Steve Stearns, KEOIK Steve Sutterer, AKOM John Thompson, K3MD Paul Wade, W1GHZ Mike Willis, GOMJW Ken Wolff, K1EA Grant Zehr, AA9LC Copyright © 2019 by The American Radio Relay League, Inc. Copyright secured under the Pan-American Convention All rights reserved. No part of this work may be reproduced in any form except by written permission of the publisher. All rights of translation are reserved. Printed in the USA Quedan reservados todos los derechos ISBN: 978-1-62595-111-3 Softcover 978-1-62595-114-4 Four-Volume Boxed Set Kindle eBook Editions ISBN: 978-1-62595-115-1 — Volume 1 ISBN: 978-1-62595-116-8 — Volume 2 ISBN: 978-1-62595-118-2 — Volume 3 ISBN: 978-1-62595-119-9 — Volume 4 Twenty-Fourth Edition First Printing We strive to produce books without errors. Sometimes mistakes do occur, however. When we become aware of problems in our books (other than obvious typographical errors), We post corrections on the ARRL website. If you think you have found an error, please check www.arrl.org/arrl-antenna-book-reference for corrections. If you don’t find a correction there, please let us know by sending an email to pubsfdbk @arrlL.org. Foreword Welcome to the 24th edition of the ARRL Antenna Book! Since 1939’s very first edition, the amateur’s ever- expanding affection for and association with the “skyhook” has been chronicled in these pages. That first edi- tion’s 139 pages have grown to more than 1000 and even then, there is more that could be covered. Amateurs use the widest frequency range of any civilian service and being an inventive lot, also use the widest variety of anten- nas. In response, the ARRL Antenna Book provides the amateur with a reference source for a balance of practical designs supplemented with theory and rationale. The inaugural Antenna Book was written by two well-known authors — George Grammer, W1DF, and Byron Goodman, W1DX (then W1JPE). They covered the basics of what amateurs needed to know as our signals spanned ever-greater distances on ever-higher frequencies: propagation, antenna fundamentals, effects of ground, transmission lines, all manner of antennas, special low-frequency and UHF antennas, as well as how to construct, aim, and turn them. Do those topics sound familiar? We have many of the same questions today. As this edition is being developed, amateurs are expanding their reach higher and lower. We’ve been granted our first LF allocation at 2200 meters. (That full-size dipole sure will be impressive!) A second MF allocation at 630 meters brackets the AM broadcast band with 160 meters. Contrary to expectations of noisy, short range contacts, surprising propagation is being discovered daily. To help us “get out” with electrically short antennas, Rudy Severns, N6LF, who participated as a test station before these bands were opened for general use, discusses the basics of effective antennas at long wavelengths. Coupled with some amazing software by the WS/7-X team, hams are more active on the “ultra-highs” than ever before. As consumer electronics pushes far into the microwave region, amateurs are moving right along with them. In response, the entire section of antennas for the shortest of the shortwaves has been rewritten by Paul Wade, W1GHZ. You'll find more about the microwave feed line systems and devices, too. The way we operate is beginning to change, as well. Amateur Radio “on the go” is enjoying a renaissance from better equipment, better batteries, better antennas, and exciting award programs that encourage mobile, por- table, and maritime operation. Recognizing the need for more coverage of antennas to suit these circumstances, chapters on antennas for Portable, Space, Mobile and Maritime stations have all expanded. VHF/UHF rover sta- tions get more attention as veterans of this exciting aspect of radiosport describe what makes their stations work. Antenna restricted? We’ve got antennas for you in the Stealth and Limited Space Antennas chapter. Along with the antennas themselves, the transmission line system — from the transmitter to the terminals of the antenna — gets more attention. Jim Brown, K9YC, and Glen Brown, W6GJB have built and tested dozens of ferrite-core choke designs and materials, updating the amateur state of the art. If your antenna tuner won’t tune, Matt Kastigar, N9ES, explains how to troubleshoot and repair it. For deciding where that new antenna should go, you'll find simple instructions for using Stu Philips, K6TU’s online service that generates terrain profiles for use with HFTA. Along with the many new projects in the book itself, there is plenty of new supplementary material to be downloaded. Check the inside cover of your book to find the code that gives you access to many megabytes of articles and papers, as well as propagation prediction tables, antenna modeling files, and some useful software. Many familiar designs and sections from previous editions are here for reference. There are also articles from OST, QEX, and the ARRL’s companion IARU societies around the world. Even with all that new and expanded material, there is still so much more to learn. We are encouraged to ex- periment and develop, one of the only services with that mandate and opportunity. Antennas and propagation are so fundamental to radio and so accessible. Amateurs are hard at work fulfilling the Basis and Purpose by advancing “the state of the radio art.” Thank you to all of our many contributors who enrich every edition. May it always be so! 73, Ward Silver, NOAX Lead Editor June 2019 The ARAL Antenna Book Downloadable Supplemental A wealth of additional material for this edition of The ARRL Antenna Book is available with the downloadable supplemental content. As a purchaser of the print edition, you are entitled to download this material — see the instructions for doing so on the insert at the front of the printed book. Searchable Edition of The ARRL Antenna Book The downloadable content includes a PDF version of this edition of The ARRL Antenna Book, including text, drawings, tables, illustrations, and photographs. Using Adobe Reader, you can view, print, or search the entire book. Supplemental Files for Each Chapter The downloadable content contains supplemental information for most chapters of this book. This includes articles from QS7T; QEX and other sources, material from previous editions of The ARRL Antenna Book, tables and figures in support of the chapter material, and files that contain information to build and test the projects provided in the chapters. The supplemental information is arranged in folders for each chapter. ARRL Antenna Modeling Files A set of EZNEC modeling files representing many of the antennas discussed in The ARRL Antenna Book. The models are grouped in folders named for the type of antenna they represent. Requires EZNEC antenna modeling software (www.eznec.com), not supplied. Propagation Prediction Tables Propagation-prediction tables generated by Dean Straw, N6BV, for more than 240 different trans- mitting locations throughout the world, including 42 locations in the USA. Each file is in PDF format for viewing and printing using Adobe Acrobat Reader. Companion Software The following software is also included with the downloadable supplemental content: e HFTA (HF Terrain Assessment for Windows) — A ray-tracing program designed to evaluate the effect of foreground terrain on the elevation pattern of up to four multi-element HF monoband Yagis in a stack. See the HF Antenna System Design chapter in this book for details about the theory behind ray tracing and diffraction in HFTA. e TLW (Transmission Line for Windows) — A sort of “Swiss Army Knife” for transmission line and antenna tuner calculations. e YW (Yagi for Windows) — A special-purpose program, designed strictly for evaluation of mono- band Yagis. It has the advantage of running many times more quickly than general-purpose programs such as NEC, but it has some attendant limitations. Contents A detailed Table of Contents is included at the beginning of each chapter. Foreword ARRL Antenna Book Downloadable Supplemental Content About the ARRL Schematic Symbols used in Circuit Diagrams Basic Antenna Topics 1 Antenna Fundamentals 1.1 Introduction to Electromagnetic Fields and Waves 1.2 Antenna Impedance 1.3 Antenna Directivity and Gain 1.4 Antenna Polarization 1.5 Other Antenna Characteristics 1.6 RF Radiation and Electromagnetic Field Safety 1.7 Bibliography 2 Dipoles and Monopoles 2.1 Dipoles 2.2 Monopoles 2.3 Bibliography 3. =‘ The Effects of Ground 3.1 Effects of Ground in the Reactive Near Field 3.2 Ground Systems for Vertical Monopoles 3.3 The Effect of Ground in the Far Field 3.4 Ground Parameters for Antenna Analysis 3.5 References and Bibliography Appendix A: Optimum Radial System with a Given Amount of Wire 4 Radio Wave Propagation 4.1 The Nature of Radio Waves 4.2 HF Sky-Wave Propagation 4.3 When and Where HF Bands Are Open 4.4 Propagation Prediction Software 4.5 Bibliography Loop Antennas 5.1 Large Loops 5.2 Small Receiving Loops 5.3 Small Transmitting Loops 5.4 Construction Guidelines for Small Transmitting Loops 5.5 Bibliography Multielement Arrays 6.1 Creating Gain and Directivity 6.2 Driven Arrays 6.3 Phased Array Techniques 6.4 Phased Array Design Examples 6.5 Practical Aspects of Phased Array Design 6.6 Bibliography Appendix A — EZNEC Examples Log-Periodic Dipole Arrays 7.1 Basic LPDA Design 7.2 Designing an LPDA 7.3 LPDA HF Projects 7.4 Bibliography Antenna Modeling 8.1 Overview: Antenna Analysis by Computer 8.2 The Basics of Antenna Modeling 8.3 References and Bibliography LF, MF and HF Antennas Single-Band MF and HF Antennas 9.1 Horizontal Antennas 9.2 Vertical Antennas 9.3 Loading Techniques for Short Antennas 9.4 Inverted-L and T Antennas 9.5 Half-Sloper Antennas 9.6 LF and MF Antennas 9.7 Bibliography 10 11 12 13 14 Multiband HF Antennas 10.1 Simple Wire Antennas 10.2 Trap Dipoles 10.3 The Terminated Folded Dipole 10.4 Multiband Vertical Antennas 10.5 The Coupled-Resonator Dipole 10.6 Loop Antennas 10.7 Bibliography HF Yagi and Quad Antennas 11.1 Yagi Antennas 11.2 Yagi Performance Parameters 11.3 Monoband Yagi Performance Optimization 11.4 Monoband Yagi Designs 11.5 Multiband Yagis 11.6 Shortening Yagi Elements 11.7 The Moxon Rectangle 11.8 Quad Antennas 11.9 Two Multiband Quad Designs 11.10 Bibliography Broadside and End-Fire Arrays 12.1 Broadside Arrays 12.2 Parallel Broadside Arrays 12.3 Other Forms of Broadside Arrays 12.4 End-Fire Arrays 12.5 Bibliography Long-Wire and Traveling-Wave Antennas 13.1 Overview 13.2 Combinations of Long Wires 13.3 The Resonant Rhombic Antenna 13.4 Terminated Long-Wire Antennas 13.5 Project: Four-Wire Steerable V Beam for 10 through 40 Meters 13.6 Bibliography HF Antenna System Design 14.1 System Design Basics 14.2 Propagation and Coverage 14.3 Effects of Local Terrain 14.4 Stacking Yagis and Switching Systems 15 16 17 18 19 VHF, UHF, and Microwave Antennas VHF, UHF and Microwave Antennas 15.1 Design Factors at and above VHF 15.2 Basic Antennas for VHF and UHF 15.3 Yagis and Quads at VHF and UHF 15.4. Log-Periodic and Discone Antennas 15.5 Reflector Antennas 15.6 Helical Antennas 15.7 Microwave Antennas 15.8 Bibliography VHF and UHF Mobile and Rover Antennas 16.1 Antennas for VHF-UHF FM 16.2 Mounts for Whip Antennas 16.3 Bicycle Mobile Antennas for VHF and UHF 16.4 Project: Big Wheel for Two Meters 16.5 Project: Halo for Six Meters 16.6 Rover Antenna Systems 16.7 References and Bibliography Antennas for Space Communications 17.1 Space Communication Antenna Systems 17.2 Circularly Polarized Antennas 17.3 Yagi Arrays 17.4 Parabolic Reflector (Dish) Antennas 17.5 Antenna Position Control 17.6 Bibliography VHF, UHF and Microwave Antenna Systems 18.1 Transmission Lines and Devices 18.2 Impedance Matching 18.3 Baluns 18.4 Stacking Yagis 18.5 Weatherproofing Relays and Preamplifiers 18.6 References and Bibliography Special Applications Portable Antennas 19.1 Horizontal Antennas 19.2 Vertical Antennas 19.3 Beam Antennas 19.4 Portable Masts and Supports 19.5 Bibliography 20 21 22 23 24 25 Stealth and Limited Space Antennas 20.1 Installation Safety 20.2 Locations for Antennas 20.3 RF Interference 20.4 Indoor Antennas 20.5 Outdoor Antennas 20.6 Small Transmitting Loops 20.7 Bibliography Mobile and Maritime HF Antennas 21.1 HF Mobile Antenna Fundamentals 21.2 HF Mobile Antenna Types 21.3 Bibliography for HF Mobile Antennas 21.4 HF Antennas for Sail and Power Boats Receiving and Direction-Finding Antennas 22.1 Receiving Antennas 22.2 Direction-Finding Antennas Transmission Lines Transmission Lines 23.1 Basic Theory of Transmission Lines 23.2 Practical Transmission Lines 23.3 Feed Line Construction and Operating Characteristics 23.4 RF Connectors 23.5 Choosing and Installing Feed Lines 23.6 Bibliography Transmission Line System Techniques 24.1 Coupling the Transmitter and Line 24.2 Impedance Matching Networks 24.3 Transmission Line System Design 24.4 Transmission Line Matching Devices 24.5 Matching Impedance at the Antenna 24.6 Common-Mode Transmission Line Current 24.7 Current Baluns, Chokes, and Choke Baluns 24.8 Transmission-Line Baluns and Matching Devices 24.9 Bibliography Building and Maintaining Antenna Systems Antenna Materials and Construction 25.1 Wire for Antenna Systems 25.2 Antenna Insulators 25.3 Antennas of Aluminum Tubing 25.4 Other Materials for Antenna Construction 25.5 Hardware 25.6 Bibliography 26 27 28 Building Antenna Systems and Towers 26.1 Safety and Safety Equipment 26.2 Trees and Masts 26.3 Types of Towers 26.4 Engineering the Tower Project 26.5 Tools and Equipment 26.6 Tower Construction 26.7 Raising and Lowering Antennas 26.8 Notes on Cables and Connectors 26.9 Rotators 26.10 Grounding and Lightning Protection 26.11 Corrosion 26.12 General Maintenance 26.13 Bibliography Appendix A — Determining Antenna Areas and Wind Load Appendix B — Calculating the Required Mast Strength Antenna and Transmission Line Measurements 27.1 Line Current and Voltage 27.2 SWR Measurements 27.3 RF Power Measurement 27.4 Field Strength Meters 27.5 Antenna Analyzer Measurements 27.6 Time-Domain Reflectometry 27.7 Vector Network Analyzer 27.8 Antenna Field Measurements 27.9 Bibliography Antenna System Troubleshooting 28.1 Antenna System Troubleshooting for Beginners 28.2 Guidelines for Antenna System Troubleshooting 28.3 Analyzing an Antenna Problem 28.4 Antenna Tuner Troubleshooting and Repair 28.5 Refurbishing Aluminum Antennas Appendix Glossary of Terms Abbreviations Length Conversions Metric Equivalents Gain Reference Index Project Index Author’s Index ARRL Membership Benefits and Services ARRL membership includes QST magazine, plus dozens of other services and resources to help you get involved and enjoy Amateur Radio to the fullest. 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Public service and emergency communication volunteers enjoy support and training from ARRL. Group Benefits* BARRL Ham Radio Equipment Insurance @ Liberty Mutual Auto and Home Insurance (*US Only) About the ARRL The seed for Amateur Radio was planted in the 1890s, when Guglielmo Marconi began his experiments in wire- less telegraphy. Soon he was joined by dozens, then hundreds, of others who were enthusiastic about sending and. receiving messages through the air — some with a commercial interest, but others solely out of a love for this new com- munications medium. The United States government began licensing Amateur Radio operators in 1912. By 1914, there were thousands of Amateur Radio operators — hams — in the United States. Hiram Perey Maxim, a leading Hartford, Connecticut inventor and industrialist, saw the need for an organization to unify this fledgling group of radio experimenters. In May 1914 he founded the American Radio Relay League (ARRL) to meet that need. ARRL is the national association for Amateur Radio in the US. ARRL numbers within its ranks the vast majority of active radio amateurs in the nation and has a proud history of achievement as the standard-bearer in amateur affairs. ARRL’s underpinnings as Amateur Radio’s witness, partner, and forum are defined by five pillars: Public Service, Ad- vocacy, Education, Technology, and Membership. ARRL is also International Secretariat for the International Amateur Radio Union, which is made up of similar societies in 150 countries around the world. ARRL’s Mission Statement: To advance the art, science, and enjoyment of Amateur Radio. ARRL’s Vision Statement: As the national association for Amateur Radio in the United States, ARRL: e Supports the awareness and growth of Amateur Radio worldwide; e Advocates for meaningful access to radio spectrum; e Strives for every member to get involved, get active, and get on the air; e Encourages radio experimentation and, through its members, advances radio technology and education; and e Organizes and trains volunteers to serve their communities by providing public service and emergency communications. At ARRL headquarters in the Hartford, Connecticut suburb of Newington, the staff helps serve the needs of mem- bers. ARRL publishes the monthly journal OST and an interactive digital version of OST, as well as newsletters and many publications covering all aspects of Amateur Radio. Its headquarters station, W1AW, transmits bulletins of inter- est to radio amateurs and Morse code practice sessions. ARRL also coordinates an extensive field organization, which includes volunteers who provide technical information and other support services for radio amateurs as well as commu- nications for public service activities. In addition, ARRL represents US radio amateurs to the Federal Communications Commission and other government agencies in the US and abroad. Membership in ARRL means much more than receiving OST each month. In addition to the services already described, ARRL offers membership services on a personal level, such as the Technical Information Service, where members can get answers — by phone, e-mail, or the ARRL website — to all their technical and operating questions. A bona fide interest in Amateur Radio is the only essential qualification of membership; an Amateur Radio license is not a prerequisite, although full voting membership is granted only to licensed radio amateurs in the US. Full ARRL membership gives you a voice in how the affairs of the organization are governed. ARRL policy is set by a Board of Directors (one from each of 15 Divisions). Each year, one-third of the ARRL Board of Directors stands for election by the full members they represent. The day-to-day operation of ARRL HQ is managed by a Chief Executive Officer and his/her staff. Join ARRL Today! No matter what aspect of Amateur Radio attracts you, ARRL membership is relevant and im- portant. There would be no Amateur Radio as we know it today were it not for ARRL. We would be happy to welcome you as a member! Join online at www.arrl.org/join. For more information about ARRL and answers to any questions you may have about Amateur Radio, write or call: ARRL — The national association for Amateur Radio® 225 Main Street Newington CT 06111-1400 USA Tel: 860-594-0200 FAX: 860-594-0259 e-mail: hq@arrl.org www.arrlorg Prospective new radio amateurs call (toll-free): 800-32-NEW HAM (800-326-3942) You can also contact ARRL via e-mail at newham@arrl.org or check out the ARRL website at www.arrLorg Common Schematic Symbols Used in Circuit Diagrams Fixed Variable \ Photo - Non- Split-stator Air-core Adjustable © Phasing Tube A\- ANA— Ne Polarized rorn === | Anode Heater Or SE Eh Ot = 1) Flame Adjustable Tapped = Thermistor Feed- (TTY) mn Triode Pentode Heated Cath, OT © Gas Filled “~y- “- WN meee. Variable Through Ferrt hietie cou > —o—o— Cathod 0 as TT —=}- rrr one ° Cathode Resistors Capacitors Inductors er >— Deflection Plates Toggle pT Normally Open 5 re Twin Tetrode oe 12) oO General 3 + soe alae oo; aS 5 Oo “| 2 Ampitier op Amp Spst Spat Dpat Terminal Om (e) ps” > Held Conductors Shielded wie or O ome) Coed Lam 7-seg o— ov ov Joined Limit =) a Multi- Switch Momentary Thermal a a a Point Grounds Line-break fone me) oeen OS fe) alo L 74 Integrated T] ° Address or Data Multiple Conductor / ? 7 = Circuits a iri Bus Cable : O=yJO Chassis A-analog Wiring Switches Normally Closed —— Earth D-digital (U#) Other # Thermal Relays 7 vonage Adjustable Adjustable | y —>>- Capacitor Air Core with Link Inductance Coupling a LED (DS#) wt > oe WOO WU Ese ————Phone Jacks (J#) ————— Phone Plug (P#) Thyristor (YYY) (TN morn i -pin . VES — P| (SCR) Bridge _i~o- Key Ceramic A 5: U#) Resonator Diode/Rectifier Rectifier ; ot rer with Ferrite Core Meter Contacts Phono Jack MIC Jack (P#) QU UJ Adjustable Antenna —-> << — “ab Core a wb am am cute CG) | we? Promo TES) Triac Crystal Zener Schottky Tunnel Diodes (om) Transformers *=V, mV Multiol z A, mA, pA Movable (A) (Cv) P-channel P-channel P-channel P-channel P-channel Assembly OF (wer) Mutiple [AT] [YO] D D G2 D D Module Fixed s & Ss G1 8s G s (other than 0) Motor Coaxial Connectors Miscellaneous N-channel N-channel N-channel N-channel N-channel O- Ce ©) -?. AND OR XOR Female Male Female Male D D D Gc G2 D {D- ->- jbD- 220 V Male Female s G s G1 s G s NAND Invert Terminal Strip Bipolar Junction FET Single-gate Dual-gate Single-gate [> _>o —Do— fojololjo| u Depletion Made 1 Er Mode Transistors Mosfet Mosfet Schmitt Ground Ground Darlingtons in | other _| Logic (U#) Female Male Male ot ee mount Neut Neut Neut Batteries Ss 120 i20V aris e see EB Mosfet with Single Multi Shoe Se PNP Protection Diode "opto: -isolators Cell Cell Cell Connectors SYMBOLSMM TABLE OF CONTENTS 1.1 Introduction to Electromagnetic Fields and Waves 1.1.1 Eand H Fields 1.1.2 Conduction and Displacement Currents 1.1.3. Electromagnetic Waves 1.2 Antenna Impedance 1.2.1 Radiation Resistance and Efficiency 1.2.2 Current and Voltage Distribution 1.2.3. Feed Point Impedance 1.3 Antenna Directivity and Gain 1.3.1 The Isotropic Radiator 1.3.2 Directivity and the Radiation Pattern 1.3.3. Near and Far Fields 1.3.4 Types of Radiation Patterns 1.3.5 Directivity and Gain 1.3.6 Radiation Pattern Measurements 1.4 Antenna Polarization 1.5 Other Antenna Characteristics 1.5.1 Reciprocity in Receiving and Transmitting 1.5.2. Antenna Bandwidth 1.5.3. Frequency Scaling 1.5.4 Effective Radiated Power (ERP) 1.6 RF Radiation and Electromagnetic Field Safety 1.6.1 Thermal Effects of RF Energy 1.6.2 Athermal Effects of EMR 1.7 Bibliography Chapter 1 — Downloadable Supplemental Content Supplemental Articles e “Radio Mathematics” — supplemental information about math used in radio and a list of online resources and tutorials about common mathematics e “Why an Antenna Radiates” by Kenneth MacLeish, W7TX Antenna Fundamentals Where does the word “antenna” come from? As related by Dr. Ulrich Rohde, NIUL, the term originated with Guglielmo Marconi during early radio tests in 1895 during which he used wire “aerials” attached to a vertical tent pole. The aerial wire then ran down the pole to the transmitter. In Italian, a tent pole is known as “antenna central” and so the pole with the wire became simply, “l’ antenna.” In the beginning of radio, anten- nas were attached directly to generators and transmitters and were considered part of a common assembly. It wasn’t until after 1900 that antennas began to be regarded as separate ele- ments of the system, independent of the transmitter or receiver. While there are an enormous variety of antennas, they share basic characteristics and all are designed to radiate and receive electromagnetic waves. In this chapter, we begin by defining what an electromagnetic wave is and how it is de- scribed. We then define the most important characteristics of an antenna — impedance, directivity and polarization — and show how those characteristics are measured and displayed. Finally, a section reviews how exposure to those waves af- fects the human body and the measures necessary for all amateurs to use antennas and electromagnetic waves safely. 1.1 INTRODUCTION TO ELECTROMAGNETIC FIELDS AND WAVES 1.1.1 E AND H FIELDS In 1820 Hans Oerstad discovered that a current flowing in a wire would deflect the needle of a nearby compass. We attribute this effect to a magnetic or H-field, which at any given location is denoted by the letter H. The magnetic field’s amplitude is expressed in A/m (Amperes/meter) along with a direction. (Direction can also be expressed as some value of phase with respect to a reference.) Because a magnetic Iron Filings ANTOO39 Figure 1.1 — Visualization of a magnetic field. The magnetic lines of force that surround a conductor with an electric cur- rent flowing in it are shown by iron filings and small com- pass needles. The needles point in the direction of the mag- netic or H-field. The filings give a general view of the field distribution in the plane perpendicular to the conductor. field has both amplitude and direction, it is a vector. Symbols representing a vector are printed in bold-face. Figure 1.1 shows a typical experimental arrangement that demonstrates the presence of a magnetic field. The shape of the magnetic field is roughly shown by the distribution of the iron filings. This field distribution is very similar to that around a vertical antenna. A compass needle (a small magnet itself) will try to align itself parallel to H. As the compass is moved around the conductor, the orientation of the needle changes accord- ingly. The orientation of the needle gives the direction of H. If you attempt to turn the needle away from alignment you will discover a torque trying to restore the needle to its original Ic —~ 7 | PEPE EEE EEE EEE | Vde Vac! Ly |" +L d — E = rit titi ti tit tit - | | l | _EFed) | ANT0040 Io Figure 1.2 — Visualization of an electric field, E=V,,/d. When the dc source is replaced with an ac source there will be a displacement current (I,) flowing between the ca- pacitor plates. Antenna Fundamentals 1.1 Math Tutorials You will encounter a fair amount of intermediate- level mathematics in this book. If you would like to brush up on your math skills or learn about an unfamiliar topic, a list of free online math tutorials is included with the downloadable supplemental infor- mation for this book and on the ARRL website under “Math Tutorials” at www.arrl.org/tech-prep-resource- library. position. The torque is proportional to the strength of the magnetic field at that point. This strength is called the field intensity or amplitude of H at that point. If a larger current flows in the conductor the amplitude of H will increase in proportion. Currents flowing in an antenna also generate an H-field. An antenna will also have an electric or E-field, which can be visualized using a parallel-plate capacitor, as shown in Figure 1.2. If we connect a battery with a dc potential across the capacitor plates there will be an electric field E established between the plates, as indicated by the lines and directional arrows between the plates. (Like H, the electric field also has an amplitude and direction and so is a vector as well.) The magnitude of vector E is expressed in V/m (volts per meter), so for a potential of V volts and a spacing of d meters, E = V/d V/m. The amplitude of E will increase with voltage and/or a smaller separation distance (d). In an an- tenna, there will be ac potential differences between different parts of the antenna and from the antenna to ground. These ac potential differences establish the electric field associated with the antenna. 1.1.2 CONDUCTION AND DISPLACEMENT CURRENTS If we replace the de voltage source in Figure 1.2 with an ac source, an ac current will flow in the circuit. In the conduc- tors between the ac source and the capacitor plates, current (I,) flows, because of the movement of charge, usually elec- trons. But in the space between the capacitor plates (particu- larly in a vacuum) there are no charge carriers available to carry a conduction current. Nonetheless, current still flows in the complete circuit, and we attribute this to a displacement current (Iq) flowing between the capacitor plates to account for the continuity of current in the circuit. Displacement and conduction currents are two different phenomena but they both represent current, just two different kinds. Some observ- ers prefer to call conduction currents “currents” and displace- ment currents “imaginary currents.” That terminology is OK, but to account for the current flow in a closed circuit with capacitance you have to keep track of both kinds of current, whatever you call them. The accepted convention is to use the term “displacement current.” 1.1.3 ELECTROMAGNETIC WAVES An electromagnetic wave, as the name implies, is com- posed of both an electric field and a magnetic field that vary 1.2 Chapter 1 ee Magnetic Lines Y of Force ARRLO257 Figure 1.3 — Representation of electric and magnetic lines of force in an electromagnetic wavefront. Arrows indicate the instantaneous directions of the fields for a wavefront in a wave traveling toward you, out of the page. Reversing the direction of one of the fields would also reverse the direc- tion of the wave. with time. Electric and magnetic fields that do not change with time, such as those created by a dc current or voltage, are called electrostatic fields. The fields of a radio wave are cre- ated by an ac current in an antenna, usually having the form of a sine wave. As a result, the fields in a radio wave vary in the same sinusoidal pattern, increasing and decreasing in strength and reversing direction with the same frequency, f, as the ac current. It is the movement of electrons — specifically the acceleration and deceleration as the ac current moves back and forth — that creates the electromagnetic wave. The two fields of the electromagnetic wave are oriented at right angles to each other as shown by Figure 1.3. The term “lines of force” in the figure means the direction in which a force would be felt by an electron (from the electric field) or by a magnet (from the magnetic field). The direction of the right angle from the electric field to the magnetic field, clock- wise or counterclockwise, determines the direction the wave travels, as illustrated in the figure. This is called a propagating wave. To an observer staying in one place, such as a stationary receiving antenna, the electric and magnetic fields of the wave appear to oscillate as the wave passes. That is, the fields create forces on electrons in the antenna that increase and decrease in a sine wave pattern. Some of the energy in the propagat- ing wave is transferred to the electrons as the forces from the changing fields cause them to move. This creates a sine wave current in the antenna with a frequency determined by the rate at which the field strength changes as the wave passes. If the observer is moving in the same direction as the wave and at the same speed, however, the strength of the fields will not change. To that observer, the electric and mag- netic field strengths are fixed, as in a photograph. This is a wavefront of the electromagnetic wave; a flat surface or plane moving through space on which the electric and magnetic fields have a constant value as illustrated in Figure 1.3. Just as an ac voltage is made up of an infinite sequence of instantaneous voltages, each slightly larger or smaller than the next, an infinite number of wavefronts make up a propa- gating electromagnetic wave, one behind another like a deck of cards. The direction of the wave is the direction in which the wavefronts move. The fields on each successive wavefront have a slightly different strength so as they pass a fixed loca- tion, the detected field strength changes as well. The fixed observer “sees” fields with strengths that vary as a sine wave. Figure 1.4 is a drawing of what would happen if we could suddenly freeze all of the wave-fronts in the wave and take measurements of the electric and magnetic field strengths in each. In this example, the electric field is ori- ented vertically and the magnetic field horizontally. (Each of the vertical lines in the electric field can be thought of as representing an individual wavefront.) All of the wavefronts are moving in the direction indicated — the whole set of them moves together at the same speed. As the wave — the set of wavefronts — moves past the receiving antenna, the varying field strengths of the different wavefronts is perceived as a continuously changing wave. What we call a “wave” is really this entire group of wavefronts moving through space. One more important note about electromagnetic waves: The electric and magnetic fields are coupled, that is they are both aspects of the same entity, the electromagnetic wave. They are not perpendicular electric and magnetic fields that simply happen to be in the same place at the same time! The fields cannot be separated, although the energy in the wave can be detected as electric or magnetic force. The fields are created as a single entity — an electromagnetic wave — by the motion of electrons in the transmitting antenna. Speed of Propagation and Wavelength Because the velocity of wave propagation is so great, we tend to ignore it. Only 4 of a second is needed for a radio wave to travel around the world — but in working with anten- nas the time factor is extremely important. The wave concept evolved because an alternating current flowing in a wire (antenna) creates propagating electric and magnetic fields. We can hardly discuss antenna theory or performance at all without involving travel time, consciously or otherwise. The speed at which electromagnetic waves travel is called the velocity of propagation. It is determined by the permittivity (¢) and permeability (u) of the medium in which the wave is traveling. This is commonly referred to as the “speed of light’ and represented by c. oT () The speed of light is highest in the vacuum of free space, approximately 300 million or 3 x 108 meters per second and the special symbols ¢9 and [Up are used. It is often more con- venient to remember this value as 300 m/us (the actual value is 299.7925 m/us). It is also useful to know a radio wave’s wavelength — the distance traveled during one complete cycle of a wave. Since one complete cycle takes 1/f the velocity of a wave is the speed of light, c, the wavelength, A, is thus: A=c/f (2a) In free-space A = 299.7925 x 10°/ f where A is the free-space wavelength in meters. More convenient approximate formulas for use at radio frequencies are: A in meters = 300 / f in MHz, and (2b) i in feet = 983.6 / f in MHz (2c) Electric Field Magnetic Field Direction of Wave Travel The ratio between the wave’s veloc- ity in a specific medium and that of free space is called the medium’s velocity fac- tor (VF) and is a value between 0 and 1. If the medium is air, the reduction in veloc- ity of propagation can be ignored in most discussions of propagation at frequencies below 30 MHz. In the VHF range and higher, temperature and moisture content of the medium have increasing effects on the communication range, as will be discussed later in the Radio Wave Propa- gation chapter. In materials such as glass ARRLO258 : ; . : or plastic the wave’s velocity can be quite Figure 1.4 — Representation of the magnetic and electric field strengths of an electromagnetic wave. In the diagram, the electric field is oriented vertically and the magnetic field horizontally. a bit lower than that of free space. For ex- ample, in polyethylene (commonly used as a center insulator in coaxial cable), the velocity of propagation is about % that in Antenna Fundamentals 1.3 free space. In distilled water (a good insulator) the speed is about % that of free space. Phase of Waves There will be few pages in this book where phase, wave- length and frequency do not enter the discussion. It is es- sential to have a clear understanding of their meaning in order to understand the design, installation, adjustment or use of antennas, matching systems or transmission lines in detail. In essence, phase means time. When something goes through periodic variations as an alternating current does, corresponding instants in succeeding periods are in phase. It is important to distinguish between phase and polar- ity. Polarity is simply a convention that assigns a positive and negative direction or convention. Reversing the leads on a feed line reverses a signal’s polarity but does not change