Law Enforcement and Corrections Standards and Testing Program

U.S. Department of JusticeOffice of Justice ProgramsNational Institute of Justice

National Institute of Justice

Antenna System Guide

NIJ Guide 202–00

The National Institute of Justice is a component of the Office of JusticePrograms, which also includes the Bureau of Justice Assistance, theBureau of Justice Statistics, the Office of Juvenile Justice andDelinquency Prevention, and the Office for Victims of Crime.

ABOUT THE LAW ENFORCEMENT AND CORRECTIONS STANDARDS AND TESTING PROGRAM

The Law Enforcement and Corrections Standards and Testing Program is sponsored by the Office of Science andTechnology of the National Institute of Justice (NIJ), U.S. Department of Justice. The program responds to the mandateof the Justice System Improvement Act of 1979, which directed NIJ to encourage research and development to improvethe criminal justice system and to disseminate the results to Federal, State, and local agencies.

The Law Enforcement and Corrections Standards and Testing Program is an applied research effort thatdetermines the technological needs of justice system agencies, sets minimum performance standards for specific devices,tests commercially available equipment against those standards, and disseminates the standards and the test results tocriminal justice agencies nationally and internationally.

The program operates through:The Law Enforcement and Corrections Technology Advisory Council (LECTAC), consisting of nationally

recognized criminal justice practitioners from Federal, State, and local agencies, which assesses technological needs andsets priorities for research programs and items to be evaluated and tested.

The Office of Law Enforcement Standards (OLES) at the National Institute of Standards and Technology, whichdevelops voluntary national performance standards for compliance testing to ensure that individual items of equipmentare suitable for use by criminal justice agencies. The standards are based upon laboratory testing and evaluation ofrepresentative samples of each item of equipment to determine the key attributes, develop test methods, and establishminimum performance requirements for each essential attribute. In addition to the highly technical standards, OLES alsoproduces technical reports and user guidelines that explain in nontechnical terms the capabilities of available equipment.

The National Law Enforcement and Corrections Technology Center (NLECTC), operated by a grantee, whichsupervises a national compliance testing program conducted by independent laboratories. The standards developed byOLES serve as performance benchmarks against which commercial equipment is measured. The facilities, personnel,and testing capabilities of the independent laboratories are evaluated by OLES prior to testing each item of equipment,and OLES helps the NLECTC staff review and analyze data. Test results are published in Equipment PerformanceReports designed to help justice system procurement officials make informed purchasing decisions.

Publications are available at no charge from the National Law Enforcement and Corrections Technology Center.Some documents are also available online through the Internet/World Wide Web. To request a document or additionalinformation, call 800–248–2742 or 301–519–5060, or write:

National Law Enforcement and Corrections Technology CenterP.O. Box 1160Rockville, MD 20849–1160E-mail: asknlectc@nlectc.orgWorld Wide Web address: http://www.nlectc.org

U.S. Department of JusticeOffice of Justice ProgramsNational Institute of Justice

Antenna System Guide

NIJ Guide 202–00

W.A. Kissick, W.J. Ingram, J.M. Vanderau, R.D. JenningsInstitute for Telecommunication SciencesBoulder, CO 80305–3328

Prepared for:National Institute of JusticeOffice of Science and TechnologyWashington, DC 20531

April 2001

NCJ 185030

National Institute of Justice

The technical effort to develop this guide was conductedunder Interagency Agreement 94–IJ–R–004,

Project No. 97–030–CTT.

This guide was prepared by theOffice of Law Enforcement Standards (OLES)

of the National Institute of Standards and Technology (NIST)under the direction of A. George Lieberman, Program

Manager, Detection, Inspection, and EnforcementTechnologies, and Kathleen M. Higgins, Director of OLES.

The work resulting in this guide was sponsored by theNational Institute of Justice, Dr. David G. Boyd,

Director, Office of Science and Technology.

iii

FOREWORD

The Office of Law Enforcement Standards (OLES) of the National Institute of Standards andTechnology furnishes technical support to the National Institute of Justice program to strengthenlaw enforcement and criminal justice in the United States. OLES’s function is to conductresearch that will assist law enforcement and criminal justice agencies in the selection andprocurement of quality equipment.

OLES is: (1) subjecting existing equipment to laboratory testing and evaluation, and(2) conducting research leading to the development of several series of documents, includingnational standards, user guides, and technical reports.

This document covers research conducted by OLES under the sponsorship of the NationalInstitute of Justice. Additional reports as well as other documents are being issued under theOLES program in the areas of protective clothing and equipment, communications systems,emergency equipment, investigative aids, security systems, vehicles, weapons, and analyticaltechniques and standard reference materials used by the forensic community.

Technical comments and suggestions concerning this document are invited from all interestedparties. They may be addressed to the Director, Office of Law Enforcement Standards, NationalInstitute of Standards and Technology, Gaithersburg, MD 20899–8102.

Dr. David G. Boyd, DirectorOffice of Science and Technology

National Institute of Justice

v

BACKGROUND

The Office of Law Enforcement Standards (OLES) was established by the National Institute ofJustice (NIJ) to provide focus on two major objectives: (1) to find existing equipment which canbe purchased today, and (2) to develop new law-enforcement equipment which can be madeavailable as soon as possible. A part of OLES’s mission is to become thoroughly familiar withexisting equipment, to evaluate its performance by means of objective laboratory tests, todevelop and improve these methods of test, to develop performance standards for selectedequipment items, and to prepare guidelines for the selection and use of this equipment. All ofthese activities are directed toward providing law enforcement agencies with assistance inmaking good equipment selections and acquisitions in accordance with their own requirements.

As the OLES program has matured, there has been a gradual shift in the objectives of the OLESprojects. The initial emphasis on the development of standards has decreased, and the emphasison the development of guidelines has increased. For the significance of this shift in emphasis tobe appreciated, the precise definitions of the words “standard” and “guideline” as used in thiscontext must be clearly understood.

A “standard” for a particular item of equipment is understood to be a formal document, in aconventional format, that details the performance that the equipment is required to give anddescribes test methods by which its actual performance can be measured. These requirements aretechnical and are stated in terms directly related to the equipment’s use. The basic purposes of astandard are (1) to be a reference in procurement documents created by purchasing officers whowish to specify equipment of the “standard” quality, and (2) to objectively identify equipment ofacceptable performance.

Note that a standard is not intended to inform and guide the reader; that is the function of a“guideline.” Guidelines are written in nontechnical language and are addressed to the potentialuser of the equipment. They include a general discussion of the equipment, its importantperformance attributes, the various models currently on the market, objective test data whereavailable, and any other information that might help the reader make a rational selection amongthe various options or alternatives available.

Kathleen HigginsNational Institute of Standards and Technology

April 2001

vii

CONTENTS

FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiBACKGROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vFIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiiCOMMONLY USED SYMBOLS AND ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . x1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. LAND MOBILE RADIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1 Users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Frequency Bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Rules and Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. ANTENNA FUNDAMENTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1 History of the Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 Frequency and Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3 Radiation Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4. ANTENNA CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.1 Gain and Directivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2 Radiation Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.3 Antenna Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.4 Antenna Terminal Impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.5 Voltage Standing Wave Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.6 Effective Length and Effective Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.7 Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5. ANTENNA TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175.1 Dipoles and Monopoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175.2 Base-Station Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185.3 Corner Reflector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.4 Yagi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.5 Log-Periodic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215.6 Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.7 Unusual Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.8 Active Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.9 Diversity Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

6. TRANSMISSION LINES AND OTHER COMPONENTS . . . . . . . . . . . . . . . . . . . . . . . . . . 276.1 Transmission Line Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276.2 Baluns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286.3 Duplexers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.4 Combiners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306.5 Intermodulation Suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306.6 Multicouplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

viii

7. RADIO WAVE PROPAGATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337.1 Transmission Loss and the Power Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337.2 Free-Space Basic Transmission Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347.3 Terrestrial Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357.4 Co-Site Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

8. ANTENNA SYSTEM REQUIREMENTS AND DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . 398.1 Define System Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398.2 Design System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418.3 Select Appropriate Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

9. INSTALLATION, MAINTENANCE, AND SAFETY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439.1 Vehicular Antenna Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439.2 Fixed-Site Antenna Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439.3 Lightning Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449.4 During Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449.5 Perform Routine Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

10. ANTENNA SYSTEM RESOURCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4910.1 Internet Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4910.2 Periodicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4910.3 Manufacturers’ and Vendors’ Catalogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

11. PROFESSIONAL AND STANDARDS ORGANIZATIONS . . . . . . . . . . . . . . . . . . . . . . . 5112. ACRONYMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5713. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5914. ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

FIGURES

Figure 1. Elements of a radio system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Figure 2. The propagation of a plane electromagnetic wave . . . . . . . . . . . . . . . . . . . . . . . . . . 10Figure 3. The horizontal-plane and vertical-plane patterns of a vertical

half-wavelength dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Figure 4. Vertical-plane radiation patterns [12] showing increasing complexity of

lobes and nulls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Figure 5. The vertical dipole and its electromagnetic equivalent, the vertical

monopole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Figure 6. Typical monopole antennas for (a) base-station applications and (b) mobile

applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 7. Omnidirectional base-station antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 8. A monopole antenna horizontal-plane pattern, base-station application. The

uniform maximum gain corresponds to the outer line on the polar plot . . . . . . . . . . 18Figure 9. A folded-dipole antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Figure 10. Typical mobile antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Figure 11. A mobile antenna horizontal-plane pattern [13] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

ix

Figure 12. Corner reflector antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Figure 13. A typical corner-reflector antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Figure 14. A corner-reflector antenna horizontal-plane pattern . . . . . . . . . . . . . . . . . . . . . . . . . 20Figure 15. The Yagi antenna — (a) three elements and (b) multiple elements . . . . . . . . . . . . . 21Figure 16. A typical Yagi antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Figure 17. A Yagi antenna horizontal-plane pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Figure 18. A log-periodic antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Figure 19. A typical log-periodic antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Figure 20. A log-periodic antenna horizontal-plane pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Figure 21. A typical vertical array using folded dipoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Figure 22. Vertical-plane radiation patterns for (a) single half-wave dipole,

(b) two-element array, and (c) three-element array . . . . . . . . . . . . . . . . . . . . . . . . . . 23Figure 23. A coaxial collinear array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Figure 24. A vertical-plane radiation pattern without “tilt” . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Figure 25. A vertical-plane radiation pattern with 8/ “tilt” . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Figure 26. A slot antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Figure 27. A simple active antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Figure 28. Common types of transmission lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Figure 29. Coaxial cables commonly used for LMR — (a) base station applications

and (b) mobile applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Figure 30. A VHF duplexer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Figure 31. Typical types of combiners — (a) hybrid combiner and

(b) cavity combiner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Figure 32. Intermodulation suppression device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Figure 33. A multicoupler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Figure 34. Gains and losses as described in the power-budget equation . . . . . . . . . . . . . . . . . . 33Figure 35. A propagation path illustrating direct and reflected rays . . . . . . . . . . . . . . . . . . . . . 35Figure 36. An example of diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

TABLE

Table 1. Characteristics of selected coaxial cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

x

COMMONLY USED SYMBOLS AND ABBREVIATIONS

A ampere H henry nm nanometerac alternating current h hour No. numberAM amplitude modulation hf high frequency o.d. outside diametercd candela Hz hertz S ohmcm centimeter i.d. inside diameter p. pageCP chemically pure in inch Pa pascalc/s cycle per second IR infrared pe probable errord day J joule pp. pagesdB decibel L lambert ppm parts per milliondc direct current L liter qt quart/C degree Celsius lb pound rad radian/F degree Fahrenheit lbf pound-force rf radio frequencydia diameter lbf•in pound-force inch rh relative humidityemf electromotive force lm lumen s secondeq equation ln logarithm (base e) SD standard deviationF farad log logarithm (base 10) sec. sectionfc footcandle M molar SWR standing wave ratiofig. figure m meter uhf ultrahigh frequencyFM frequency modulation min minute UV ultravioletft foot mm millimeter V voltft/s foot per second mph miles per hour vhf very high frequencyg acceleration m/s meter per second W wattg gram N newton 8 wavelengthgr grain N•m newton meter wt weight

area=unit2 (e.g., ft2, in2, etc.); volume=unit3 (e.g., ft3, m3, etc.)

PREFIXES

d deci (10-1) da deka (10)c centi (10-2) h hecto (102)m milli (10-3) k kilo (103): micro (10-6) M mega (106)n nano (10-9) G giga (109)p pico (10-12) T tera (1012)

COMMON CONVERSIONS (See ASTM E380)

0.30480 m = 1 ft 4.448222 N = 1 lbf2.54 cm = 1 in 1.355818 J = 1 ft•lbf0.4535924 kg = 1 lb 0.1129848 N m = 1 lbf•in0.06479891g = 1gr 14.59390 N/m = 1 lbf/ft0.9463529 L = 1 qt 6894.757 Pa = 1 lbf/in2

3600000 J = 1 kW•hr 1.609344 km/h = 1 mph

Temperature: T°C = (T°F-32)×5/9Temperature: T°F = (T°C×9/5)+32

1

1. INTRODUCTION

Radio communications are essential to theoperations of Federal, State, and local lawenforcement and correction agencies.Effective and reliable communicationssystems not only enable personnel toperform their functions efficiently, but alsohelp ensure their safety. It is, therefore, veryimportant that all components of a radiocommunications system be selected andintegrated to produce an effective design.Understanding the capabilities andlimitations of a communications systemensures that it is used most effectively andthat performance expectations are realistic.

This guide focuses on a key portion(subsystem) of the radio communicationssystem—the antenna system. Although theantenna itself may be the most visibleelement of radio communicationsequipment, it is often the least understood.This guide defines and describes thecomponents of the antenna system as well asthe fundamentals and characteristics of theantenna itself.1

1.1 Scope

The theory and empiricism upon whichantenna technology is based are verycomplex. It is not the purpose of this guideto tutor the reader in the study ofelectromagnetic theory. Rather, it is toprovide the reader with sufficientunderstanding of the fundamentals,

characteristics, and functions of antennas toenable him or her to develop requirementsand discuss antennas with vendors,installers, repair shops, etc. To this end, thenumber of equations and references totheory are kept to a minimum, and theinformation is generally restricted to thekind of antennas used by law enforcementagencies.

This guide is intended for a wide audience.It is written so that the reader can study onlythose sections of interest. Cross-referencingis provided where needed to direct the readerto related, important information.

1.2 Organization

Section 2 of this guide provides the readerwith a brief description of land mobile radio(LMR), which is the radio service used bynearly all law enforcement agencies. Thefrequency bands used by LMR aredescribed. The information provided onantennas in this guide is generally limited toLMR applications and the radio frequenciesused by LMR.

Section 3 begins with an overview of themajor elements of a radio communicationssystem. The function of the antenna(sub)system is described. The remainder ofthe section provides a brief history of theantenna and an introduction to thefundamentals of antennas and radio wavepropagation. Several of the subsections areimportant to the understanding of antennas.These are section 3.2 on frequency andwavelength and section 3.3 on radiationprinciples. The reader is encouraged toreview the simple, but important, concepts

1 Certain commercial companies and their products andinformation are identified in this report to specify adequately thetechnical concepts and principles being presented. In no case doessuch identification imply recommendation or endorsement by theNational Institute of Justice, or any other U.S. Governmentdepartment or agency, nor does it imply that the sources, products,and information identified are necessarily the best available for thepurpose.

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in these subsections to fully understand thefollowing sections.

Section 4 is the most important part of thisguide, as it presents the engineering, orfunctional, characteristics of antennas. Itdefines gain, directivity, and radiationpattern and how these are used in a radiocommunications system design. These andother characteristics are also used to specifyan antenna. Section 5 contains descriptionsof common antennas including theirelectrical and radiation characteristics aswell as their physical characteristics.Although there are many different kinds ofantennas, the emphasis is on those antennascommonly used for LMR.

Sections 6 and 7 relate to the use of antennaswithin a radio communications system.Section 6 describes the transmission linesand related components needed to connectan antenna to a transmitter or a receiver, andsection 7 describes the modes of radio wavepropagation commonly used for LMRsystems. Section 7 also discusses how anantenna pattern and propagation effects areused to achieve required geographicalcoverage.

The next three sections (8, 9, and 10) coverthe practical aspects of developing andexpressing requirements for an antenna; theinstallation, maintenance, and safety ofantennas; and examples of some of theproducts and services available, respectively.

Section 11 provides the reader with a list ofthe relevant regulatory, standards, andprofessional organizations. The last twosections (12 and 13) provide referencematerial. Section 12 is a list of acronymsused in this guide or related to antennas.Section 13 contains the references cited inthis guide.

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2. LAND MOBILE RADIO

The formal title of the type of radio serviceused by law enforcement agencies for theirradio communications is “land mobileservice,” and the frequency bands reservedfor their use are labeled “public safety.”Land mobile service is defined as “a mobileradio service between base stations and landmobile stations, or between land mobilestations,” by the National Telecommuni-cations and Information Administration(NTIA) [1]2 and the Federal Communi-cations Commission (FCC) [2]. Anothercommonly used phrase for “land mobileservice” is “land mobile radio” (LMR).

2.1 Users

There are two general classes of users. Oneclass of users is radio common carrier(RCC). RCC owners build and operate LMRsystems and charge a fee to third parties thatactually use the system. The other class ofusers includes groups that meet specificrequirements for LMR use. These groupsinclude: Public Safety Radio Services,Special Emergency Radio Services,Industrial Radio Services, Land Transpor-tation Radio Services, Radio LocationServices, and Specialized Mobile RadioServices. The first group listed here, thePublic Safety Radio Services, is the categorythat includes State and local governments,law enforcement and corrections, fire,emergency medical services, highwaymaintenance, and forestry conservation.Federal law enforcement and correctionsagencies also belong to this category ofLMR users. These Federal agencies may usesome of the same frequency bands and

channels as State and local agencies, but, inmany cases, they use frequency bands andchannel assignments unique to the FederalGovernment.

Most LMR user groups maintain formalassociations that provide for the exchange ofinformation within their groups andrepresent their members before the FCC andother official or government bodies. Thoseassociations relevant to the law enforcementcommunity include the Association ofPublic-Safety Communications Officials(APCO), the National Association of StateTelecommunications Directors (NASTD),and the Telecommunications IndustryAssociation (TIA). The first two of theserepresent the users, and the last representsthe manufacturers of the equipment used forLMR. More details on these associations andother relevant organizations are presented insection 11.

2.2 Frequency Bands

There are a number of frequency bandsallocated for use by LMR. The particularfrequency ranges for these bands have beenestablished at the international level, inplenipotentiary conferences, by theInternational Telecommunications Union(ITU). Administration and assignment offrequencies within the LMR bands in theUnited States are performed by two differentgovernment agencies. The FCC providesfrequency assignments and licenses for allnon-Federal users. Federal users obtain theirfrequency assignments through NTIA.

The LMR bands described in the followingsections are subdivided for Federal use only,shared Federal and non-Federal use, and

2 Numerals in square brackets refer to references identified insection 13.

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non-Federal use only. Portions of somebands may be shared with other servicessuch as maritime mobile, mobile satellite,and broadcast television.

2.2.1 VHF Low-Band

This band is located at 25 MHz to 50 MHzand generally uses channels 20 kHz wide.Communications ranges can be the greatestin this band; however, the radio signals donot reflect effectively off hills, buildings,and other surfaces, so there may be deadspots within the general area of coverage ofa base station. These frequencies, especiallyat the lower end of the band, are subject toionospheric “skip,” which can carry a signalvery long distances (and cause interferenceto other users). This band also has thehighest level of ambient noise, whichreduces the performance of receiversoperating in this band compared to other,higher frequency-bands.

2.2.2 VHF High-Band

This band is located at 150 MHz to174 MHz and uses channels 25 kHz or30 kHz wide. Adjacent channels may beseparated by 12.5 kHz to 15 kHz forgeographically separated systems. Radiosignals in this band have a shorterpropagation range, experience less noisethan those in the VHF low-band, and are notsubject to ionospheric “skip.” Diffractionover hills and around other obstacles reducesdead areas of coverage.

2.2.3 UHF Bands

The first UHF band is located at 406 MHz to420 MHz and is designated for Federalusage. The second band is located at450 MHz to 470 MHz and is designated fornon-Federal usage. The last band, from

470 MHz to 512 MHz, shares spectrum withUHF television channels 14 through 20 in afew major urban areas. Channels in all threeUHF bands are 25 kHz wide. Thepropagation range in the UHF band is evenless than that found in the VHF high-band;however, the radio signals easily reflect offhills and buildings, so dead areas aregenerally very small.

2.2.4 700 MHz Band

New bands from 764 MHz to 776 MHz and794 MHz to 806 MHz have been proposedby the FCC. These new channels would beconverted from TV channels 63–64 and68–69 to Public Safety use. The channelswould be either narrowband for voice pluslow-speed data or wideband (up to 150 kHz)for high-speed data.

2.2.5 800/900 MHz Band

The portion of the radio spectrum from806 MHz to 940 MHz is apportioned amongmany services, including cellular telephone,paging, nonpublic safety, and conventionaland trunked public safety. FederalGovernment users, however, have noauthorizations in this spectrum. Channels forpublic safety are reserved in blocks that aregenerally 25 kHz wide. These blocks residein several locations within this portion of theradio spectrum. These radio signals reflectoff hills, buildings, vehicles, etc., wellenough that dead areas are nearlynonexistent. The propagation range for thisband is shorter than for the UHF bands.

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2.3 Rules and Regulations

2.3.1 General

In the United States, the FCC allocatesdifferent bands of the radio-frequencyspectrum to different entities for a widevariety of commercial and privateapplications [2]. The NTIA performs thisfunction for the Federal Government. Inrecent years, with the proliferation ofportable phones and other mobilecommunications devices, the FCC and theNTIA have received an increasing numberof requests for an ever-dwindling amount ofspectrum available for allocation.

One way that the FCC and NTIA attempt tomaximize spectrum usage is to assign theright to transmit over the same channel todifferent commercial or private groups thatare located in different geographic regions,so that their signal coverage areas will notoverlap. Consideration is also given whenassigning adjacent channels, so signals fromone transmission channel will not affectadjacent channels.

Further description of the allocation processand the most recent listing of allocations canbe found in the NTIA Manual of Regulationsand Procedures for Federal RadioFrequency Management [1] and the Code ofFederal Regulations [3] pertaining totelecommunications.

2.3.2 Future Options

At the present time, LMR bands are crowdedand channel assignments can be difficult toobtain in some areas of the country. For thisreason, work is underway to develop new,spectrally efficient modulation and signal-processing methods and improvedtechniques to share existing channels.Historically, this problem has beenaddressed by making narrower channelswithin a band. Frequency-modulation (FM)systems have been improved to allow themto operate in bandwidths as small as12.5 kHz. Further improvement requiresdifferent modulation methods, but care mustbe taken in choosing and standardizing themodulation method and channel widths toensure interoperability among differentsystems and equipment from differentmanufacturers.

New LMR systems conforming to the“Project 25" standard [4] use narrowbandmodulation techniques and channel-sharingmethods (called trunking). These new digitalmodulation methods will be able to usechannels as narrow as 6.25 kHz. Trunkingtechniques will accommodate two, three, ormore times as many users for a givennumber of channels.

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3. ANTENNA FUNDAMENTALS

The antenna is often the most visibleelement of a radio system. The sizes andshapes of the conductors that comprise theantenna determine the directionalcharacteristics of the electromagnetic (radio)waves it radiates. However, the antennacannot be considered independently.Additional elements, such as thetransmission line, duplexers, matchingnetworks, etc., must be considered as part ofthe antenna system.

The full description of the interaction of anantenna with its surrounding environment isbased on very complex mathematics, but itsfunction in a radio system is quite simple.Figure 1 shows the key elements of a radiocommunications system. When an antenna isused for transmitting, it converts electricalsignals, delivered by a transmission line,from a transmitter into propagatingelectromagnetic waves. When an antenna isused for receiving, it convertselectromagnetic waves back into electricalsignals that are delivered by a transmissionline to a receiver for processing. In fact, the

same antenna (used for both transmittingand receiving) is often attached to atransmitter and a receiver using either aduplexer or a transmit/receive (XMT/RCV)switch. A duplexer allows one antenna to beused by both the transmitter and receiver atthe same time (see sec. 6.3), and atransmit/receive switch connects the antennato either the transmitter or receiver.

3.1 History of the Antenna

Over a century has elapsed since JamesClerk Maxwell [5] formulated his celebratedequations that provide the foundation ofclassical electromagnetism. By means ofthese equations, Maxwell was able to predictthe existence of electromagnetic waveswhich, 20 years later in 1887, wereconfirmed experimentally by Heinrich Hertz[6]. Hertz constructed a center-driven wireabout 60 cm long, terminated at each end bya 40 cm square metal plate. Driven by aspark-gap generator (broadband source), thisantenna resonated at about 50 MHz andeffectively generated and radiated

Figure 1. Elements of a radio system

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electromagnetic waves. He detected thosewaves using a loop of wire, 35 cm in radius,with a very small gap where he couldobserve a spark. Although a number ofresearchers became interested in this newphenomenon, it was Gugliermo Marconiwho, in 1897, described and thendemonstrated a complete system for thetransmission of signals. On December 12,1901, the first transatlantic communicationwas achieved. The transmitting antenna inCornwall, England, was a fan-like structureof 50 copper wires supported by a horizontalwire stretched between two poles about45 m high and 65 m apart. The receivingantenna in Newfoundland was comprised ofcopper wires supported by kites. By 1907,commercial, transatlantic telegraph serviceshad been established [7].

It is interesting to note that the original,commonly used term for a signaling systemthat uses the phenomenon ofelectromagnetic waves was “wireless.”Although the term “radioconductor” (acontraction of radiation conductor) appearsas early as 1897, the term “radio” does notemerge until later. The Radio Ship Act of1910 contains the terms “radio” and “radio-communication” but not “wireless.” In 1912,the U.S. Navy directed the use of the term“radio” in place of “wireless” [8].

3.2 Frequency and Wavelength

A propagating electromagnetic wave existsbecause of the fundamental interdependenceof the electric and magnetic fields thatcomprise it. An electric field that changeswith time produces a magnetic field whosestrength is determined by the rate of changeof the electric field. And, in complementaryfashion, a magnetic field that changes withtime produces an electric field whose

strength is determined by the rate of changeof the magnetic field. This means that theenergy contained in, and transmitted by, theradio wave is shared by the two fields.

When transmitting, the electrical current onan antenna produces the magnetic field, andthe voltage on that antenna produces theelectric field. Similarly, when receiving, anelectromagnetic wave incident on theantenna produces electrical current andvoltage. If these currents and voltages have asinusoidal time dependence (sine or cosinewave), then several very important,fundamental phenomena occur. The firstderivative (rate of change) of a sine wave isanother sine wave shifted in time by onequarter of the period of the first sine wave.This means that if the changing electric fieldin the radio wave is sinusoidal, then so is themagnetic field. These two sinusoidal time-varying fields, in effect, regenerate eachother as they travel. This is called wavepropagation.

The frequency of a sine wave is expressed asthe number of cycles per second or hertz.The period of the wave is the reciprocal ofits frequency and is expressed in seconds.The wavelength is the length of one cycle ofthe traveling wave in space, or the distancethe wave travels during one period. Therelationship between frequency andwavelength is

where 8 is the wavelength (m), c is thespeed of light (2.9979 × 108 m/s), and f isthe frequency (Hz). A practical, approximateversion of this equation is

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where 8 is in meters and f is in megahertz.

3.3 Radiation Principles

The important characteristics of an antennaare its radiation properties, such as gain,directionality, and polarization; and theelectrical property, input impedance. Theseand other characteristics can be determinedtheoretically, or they can be obtained bymeasurement. In practice, both theory andmeasurement are used to design or evaluatean antenna.

The remainder of this section is necessary tothe understanding and usefulness ofsection 4, Antenna Characteristics. Althoughbased in electromagnetic theory, thesefundamental principles are not complicatedand they are essential to an understanding ofantennas.

3.3.1 What Is an Antenna?

An antenna is a device that provides suitablylocalized and oriented paths for oscillatingelectric currents. The sizes and shapes of theconductors that comprise the antennadetermine the directional characteristics ofthe radio waves it radiates. A transmittingantenna converts electrical currents,delivered by the transmission line from atransmitter, into propagating radio waves,and a receiving antenna converts radiowaves back into electrical currents that aredelivered by a transmission line to areceiver.

3.3.2 Reciprocity

The theoretical determination of anantenna’s characteristics is usuallyaccomplished by treating the antenna as atransmitting device. However, under mostconditions, the antenna characteristics areexactly the same when it is used as areceiving device. If the antenna is linear(i.e., it contains neither active elements nornonlinear components such as ferrites), thenthe principle of reciprocity holds. Thisprinciple was first described by the famousmathematician Lord Rayleigh [9]. Theprinciple of reciprocity means that anantenna will have exactly the samecharacteristics whether it is used fortransmitting or receiving. So, if a particularcharacteristic of an antenna is obtained bymeasurement while using the antenna forreception, then it is known that the sameantenna will have exactly the samecharacteristic when used for transmission.

3.3.3 Radiated Waves and the NearField

The electromagnetic field produced by anantenna is quite complex near the antenna,and it can be described as having severalcomponents. Only one of these actuallypropagates, or travels though space. Thiscomponent is called the radiated field,radiated wave, or radio wave. Thepropagation of this radiated field, or radio-wave propagation, is usually treated as aseparate topic. The strength of the radiatedfield does decrease with distance, as it must,since the energy must spread as it travels.Some knowledge of propagation is useful inunderstanding antennas or in choosing anantenna to meet certain requirements, suchas coverage—the area over which a radiowave has sufficient strength to be useful.

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Section 7 presents a brief overview of radio-wave propagation.

The other components of the electro-magnetic field remain near the antenna anddo not propagate. There are generally twoother components: the static field and theinduction field. Even though they do notpropagate, their strength decreases veryrapidly with distance. The entire field—allof the components—near the antenna iscalled the near field. In this region,approximately one wavelength in extent, thefield strength can be relatively high and posea hazard to the human body. Seesection 9.5.4 for more information on thisradiation hazard.

3.3.4 Plane Waves

The radiated wave, as with allelectromagnetic waves including light, iscomposed of an electric field and a magneticfield. For most cases, the field lines, and thevectors that are used to illustrate them, are ata right angle to each other and to thedirection of propagation, as shown infigure 2.

At large distances from the antenna, saybeyond ten wavelengths, the radiated field isessentially a plane wave. This means thatthere is no curvature of the field lines.

3.3.5 Wave Polarization

The polarization of a wave, by definition, issimply the orientation of the electric fieldvector. The plane wave depicted in theillustration of figure 2 is vertically polarized.For nearly all cases encountered in LMR, thepolarization of the wave does not change asit propagates. This is called linearpolarization, and the two most common

examples in practice are vertical orhorizontal polarization. Other polarizationsinclude circular, where the electric fieldvector (and the magnetic field vector) rotateas the wave travels.

Figure 2. The propagation of a planeelectromagnetic wave

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4. ANTENNA CHARACTERISTICS

In a radio communications system, anantenna has two basic functions. Theprimary function is to radiate, as radiowaves, the RF signals from the transmitter,or to convert radio waves into RF signals forprocessing by a receiver. The other functionis to direct the radiated energy in the desireddirection or directions, or to be “sensitive”to reception from the desired direction ordirections. Another, often overlooked, aspectof an antenna’s directional properties is thesuppression of radiation in undesireddirections, or the rejection of reception fromundesired directions.

The directional characteristics of an antennaare fundamental to an understanding of theantenna and how it is used in a radiocommunications system. These interrelatedcharacteristics include gain, directivity,radiation (antenna) pattern, andpolarization. Other characteristics such as beamwidth, effective length, and effectiveaperture are derived from the four listedabove. Terminal (input) impedance is oneother characteristic that is of fundamentalimportance. It is necessary to know theimpedance of an antenna in order toefficiently couple the transmitter’s outputpower into it, or to efficiently couple thepower from it into the receiver. All of theseantenna characteristics are a function offrequency.

4.1 Gain and Directivity

The gain of an antenna is the radiationintensity3 in a given direction divided by theradiation intensity that would be obtained if

the antenna radiated all of the RF powerdelivered to it equally in all directions [10,11]. Note that this definition of gain requiresthe concept of an isotropic radiator; that is,one that radiates the same power in alldirections. Examples of nondirectionalsources can be achieved (at leastapproximately) with sound and light; theseare sometimes called point sources.

An isotropic antenna, however, is just aconcept, because all practical radio antennasmust have some directional properties.Nevertheless, the isotropic antenna is veryimportant as a reference. It has a gain ofunity (g = 1 or G = 0 dB) in all directions,since all of the power delivered to it isradiated equally well in all directions.

Although the isotrope is a fundamentalreference for antenna gain, anothercommonly used reference is the dipole. Inthis case the gain of an ideal (lossless) half-wavelength dipole is used. Its gain is 1.64(G = 2.15 dB) relative to an isotropicradiator.

The gain of an antenna is usually expressedin decibels (dB). When the gain isreferenced to the isotropic radiator, the unitsare expressed as dBi; but when referenced tothe half-wave dipole, the units are expressedas dBd. The relationship between these unitsis

Directivity is the same as gain, but with onedifference. It does not include the effects ofpower lost (inefficiency) in the antenna3 Radiation intensity is defined as the power density in terms of

power per unit solid angle.

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itself. Recall that the definition of gain isbased on the power delivered to (andaccepted by) the antenna. In practice, someof that power is lost in the antenna due toohmic losses (heating) in the elements,leakage across insulators, etc. If an antennawere lossless (100 % efficient), then the gainand directivity (in a given direction) wouldbe the same.

4.2 Radiation Pattern

The radiation pattern (also called antennapattern) is a representation of the gain of anantenna for all directions. Since this is athree-dimensional description of the powerdensity, it is difficult to display or use. It iscommon to display or plot cross-sections ofit. Figure 3 shows the radiation pattern of avertical half-wavelength dipole in thehorizontal plane and a vertical plane. As onecan see in this figure, the patten in thehorizontal plane has no structure. Thisantenna has constant gain versus azimuth.On the other hand, the pattern in a verticalplane shows that the antenna has maximumgain in the horizontal plane and no radiationin the directions coincident with the axis ofthe antenna. Therefore, one can nowvisualize the three-dimensional pattern as atorus (doughnut shaped).

Often, the direction is not specified whenreferring to an antenna’s gain. In this case, itis assumed that the gain’s direction is thedirection of maximum radiation—themaximum gain for the antenna. Anassociated pattern then will present valuesrelative to that maximum gain.

4.2.1 Lobes and Nulls

The regions of a pattern where the gain haslocal maxima are called lobes, and thoseplaces where the gain has local minima arecalled nulls. The vertical plane “cut” for thehalf-wave dipole (fig. 3b) has two lobes andtwo nulls. Figure 4 shows several otherexamples. A complex antenna pattern mayhave many lobes and nulls in both thehorizontal-plane and vertical-plane patterns.The lobe with the greatest gain is called themain lobe or main beam of the antenna. If asingle value of gain is given for an antenna,it is assumed to be the main lobe or mainbeam gain.

Figure 3. The horizontal-plane andvertical-plane patterns of a vertical half-

wavelength dipole

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Figure 4. Vertical-plane radiation patterns [12] showing increasing complexity of lobes and nulls

4.2.2 Beamwidth

Beamwidth is an angular measure of themain lobe (or main beam) in either (or both)the horizontal-plane or vertical-plane pattern[11]. There are several definitions forbeamwidth, including: half-power or 3 dBbeamwidth, 10 dB beamwidth, and first-nullbeamwidth. The 3 dB beamwidth is theangular extent about the maximum value ofgain for which the gain is 3 dB below themaximum. First-null beamwidth is theangular extent about the maximum value ofgain of the first occurring local minima inthe pattern. The half-power, or 3 dB,beamwidth is most commonly used.

4.3 Antenna Polarization

The term polarization has several meanings.In a strict sense, it is the orientation of theelectric field vector E at some point in space.If the E-field vector retains its orientation ateach point in space, then the polarization islinear; if it rotates as the wave travels inspace, then the polarization is circular orelliptical. In most cases, the radiated-wavepolarization is linear and either vertical orhorizontal. At sufficiently large distancesfrom an antenna, e.g., well beyond10 wavelengths, the radiated, far-field waveis a plane wave (see sec. 3.3.4).

The term polarization is often applied to theantenna itself. In this sense, the polarizationof the antenna is the polarization of theplane wave it radiates. Based on theprinciple of reciprocity (see sec. 3.3.2), thisis true for a receiving antenna, as well. Forexample, if a receiving antenna is verticallypolarized, this means that a verticallypolarized, incoming wave will producemaximum output from that antenna. If theincoming wave were polarized at some otherangle, only the vertical component would bedetected by the antenna. Ideally, ahorizontally polarized incoming wave wouldnot be detected at all by a verticallypolarized antenna. Vertical polarization isused for most LMR applications.

4.4 Antenna Terminal Impedance

There are three different kinds of impedancerelevant to antennas. One is the terminalimpedance of the antenna, another is thecharacteristic impedance of a transmissionline, and the third is wave impedance.Antenna terminal impedance is discussed inthis section. Transmission line characteristicimpedance is discussed in section 6, andwave impedance is the ratio of the electricfield strength to the magnetic field strengthof a propagating wave (see sec. 3.2 andsec. 3.3.4).

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Terminal impedance is defined as the ratioof voltage to current at the connections ofthe antenna (i.e., the point where thetransmission line is connected). Thecomplex form of Ohm’s law definesimpedance as the ratio of voltage across adevice to the current flowing through it. Theterminal impedance is expressedmathematically as

where Z is the impedance, in ohms, V is thevoltage, in volts, and I is the current, inamperes, at the antenna terminals for a givenfrequency. Each of these variables can beexpressed as a complex number, each withreal and imaginary parts. Such complexnumbers can also be expressed by using amagnitude and phase angle—this is calledphasor notation.

The real part of the impedance is called theresistive component, and the imaginary partis called the reactive component. This isoften expressed as

where R is the resistive (real) component, Xis the reactive (imaginary) component, and j = .−1

The most efficient coupling of energybetween an antenna and its transmission lineoccurs when the characteristic impedance ofthe transmission line and the terminalimpedance of the antenna are the same andhave no reactive component. When this isthe case, the antenna is considered to bematched to the line.

Matching usually requires that the antennabe designed so that it has a terminalimpedance of about 50 S or 75 S to matchthe common values of available coaxialcable. A half-wave dipole can be shortenedslightly to achieve this. For other antennas, itcan be difficult to remove (reduce to zero)the reactive component. In these cases, amatching network is often made part of theantenna to change its complex terminalimpedance into something that bettermatches a transmission line.

The resistive part R of the terminalimpedance is the sum of two componentsand is expressed in ohms,

The radiation resistance Rr is the “effectiveload” that represents the power radiated bythat antenna as radio waves, and thedissipative resistance Rd is the “load” intowhich power is lost. The efficiency of anantenna is the ratio of the power radiated tothe total power delivered to the antenna. Itcan be expressed as

As discussed in section 4.1, the dissipativelosses are due to ohmic losses (heating) inthe antenna elements, leakage acrossinsulators, and similar effects. Furthermore,it should be noted that the efficiency of anantenna can also be expressed as the ratio ofthe gain to the directivity (for a givendirection).

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4.5 Voltage Standing Wave Ratio

The standing wave ratio (SWR), also knownas the voltage standing wave ratio (VSWR),is not strictly an antenna characteristic, but isused to describe the performance of anantenna when attached to a transmissionline. It is a measure of how well the antennaterminal impedance is matched to thecharacteristic impedance of the transmissionline. Specifically, the VSWR is the ratio ofthe maximum to the minimum RF voltagealong the transmission line. The maxima andminima along the lines are caused by partialreinforcement and cancellation of a forward-moving RF signal on the transmission lineand its reflection from the antenna terminals.

If the antenna terminal impedance exhibitsno reactive (imaginary) part and if theresistive (real) part is equal to thecharacteristic impedance of the transmissionline, then the antenna and transmission lineare said to be matched. If this is true, thennone of the RF signal sent to the antennawill be reflected at its terminals. There is nostanding wave on the transmission line andthe VSWR has a value of one. However, ifthe antenna and transmission line are notmatched, then some fraction of the RF signalsent to the antenna is reflected back alongthe transmission line. This causes a standingwave, characterized by maxima and minima,to exist on the line. In this case, the VSWRhas a value greater than one.

The VSWR is easily measured with a devicecalled an SWR meter. It is inserted in thetransmission line and directly gives a valuefor the VSWR. At a VSWR value of 1.5,approximately 4 % of the power incident atthe antenna terminals is reflected. At a valueof 2.0, approximately 11 % of the incidentpower is reflected. VSWR values of 1.1 to

1.5 are considered excellent, values of 1.5 to2.0 are considered good, and values higherthan 2.0 may be unacceptable.

As stated above and elsewhere, an idealmatch between the antenna and transmissionline is desired; but this can often be achievedonly for a single frequency. In practice, anantenna may be used for an entire frequencyband, and its terminal impedance will varyacross the band. In an antenna specification,either the impedance versus frequencyacross a band is given or the VSWR versusfrequency is given.

4.6 Effective Length and Effective Area

The effective length and the effective area(also called effective aperture) arealternative ways of expressing the gain of anantenna. These characteristics are mostuseful and meaningful when the antenna isused for receiving. Of course, due to theprinciple of reciprocity, these characteristicsare the same if the antenna is used fortransmitting.

The effective length defines the ability of anantenna to produce a voltage at its terminalsfrom an incident electric field. It is definedas

where Re is expressed in meters, V is theopen circuit voltage in volts, and E is theelectric field strength in volts/meter. Thisdefinition assumes that the polarization ofthe incident field and the antenna are thesame. The effective length can also becomputed from the gain and the radiationresistance.

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Effective area, or aperture, is morecommonly used than effective length. It isdefined as

where Pr is the power available at theterminals of the antenna in watts, and p isthe power density of the incident wave inwatts per square meter. The relationshipbetween effective area and gain is

4.7 Bandwidth

Bandwidth is the difference between twofrequencies, or the frequency range, withinwhich the performance of an antenna isacceptable. In other words, one or morecharacteristics (e.g., gain, pattern, terminalimpedance) have acceptable values betweenthe bandwidth limits. For most antennas,gain and pattern do not change as rapidlywith frequency as the terminal impedancedoes, so the latter is often used to describethe bandwidth of an antenna.

VSWR (see sec. 4.5) is a measure of theeffect of mismatch between an antenna’sterminal impedance and the transmissionline characteristic impedance. Since thetransmission line characteristic impedancehardly changes with frequency, VSWR is auseful, practical way to describe the effectsof terminal impedance and to specify anantenna’s bandwidth. For example, anantenna specification may give a plot of theVSWR across some frequency band. It willlikely have a minimum value at about themiddle of the band. Another way ofspecifying the bandwidth is a statement ofthe maximum VSWR within a band.

Half-wave dipoles, and similar antennas,have a narrow bandwidth. Other antennas,like the log-periodic, are designedspecifically to be broadband.

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