Title: Antenna System Guide, NIJ Guide 202-00.
Series: Law Enforcement and Corrections Standards and Testing Program
Author: W.A. Kissick, W.J. Ingram, J.M. Vanderau, and R.D. Jennings
Published: National Institute of Justice, April 2001
Subject: Technology in law enforcement
61 pages
134,000 bytes

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U.S. Department of Justice
Office of Justice Programs
National Institute of Justice

National Institute of Justice
Law Enforcement and Corrections Standards and Testing Program

Antenna System Guide
NIJ Guide 202-00

-------------------------------


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 and Technology of the National
Institute of Justice (NIJ), U.S. Department of Justice. The program
responds to the mandate of the Justice System Improvement Act of 1979,
which directed NIJ to encourage research and development to improve the
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 that determines 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 to criminal 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 and sets 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, which develops voluntary national
performance standards for compliance testing to ensure that individual
items of equipment are suitable for use by criminal justice agencies. The
standards are based upon laboratory testing and evaluation of
representative samples of each item of equipment to determine the key
attributes, develop test methods, and establish minimum performance
requirements for each essential attribute. In addition to the highly technical
standards, OLES also produces 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, which supervises a national compliance
testing program conducted by independent laboratories. The standards
developed by OLES 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 Performance
Reports 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 additional information, call 800-248-2742 or 301-519-5060,
or write:

National Law Enforcement and Corrections Technology Center
P.O. Box 1160
Rockville, MD 20849-1160
E-mail: asknlectc@nlectc.org
World Wide Web address: http://www.nlectc.org


The National Institute of Justice is a component of the Office of Justice
Programs, which also includes the Bureau of Justice Assistance, the
Bureau of Justice Statistics, the Office of Juvenile Justice and Delinquency
Prevention, and the Office for Victims of Crime.

-------------------------------

U.S. Department of Justice
Office of Justice Programs
National Institute of Justice

Antenna System Guide
NIJ Guide 202-00

W.A. Kissick, W.J. Ingram, J.M. Vanderau, R.D. Jennings
Institute for Telecommunication Sciences
Boulder, CO 80305-3328

Prepared for:

National Institute of Justice
Office of Science and Technology
Washington, DC 20531

April 2001

NCJ 185030

-------------------------------

National Institute of Justice

The technical effort to develop this guide was conducted under
Interagency Agreement 94-IJ-R-004, Project No. 97-030-CTT.

This guide was prepared by the Office 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 Enforcement Technologies, and Kathleen M. Higgins,
Director of OLES. The work resulting in this guide was sponsored by the
National Institute of Justice, Dr. David G. Boyd, Director, Office of 
Science and Technology.

-------------------------------

FOREWORD

The Office of Law Enforcement Standards (OLES) of the National
Institute of Standards and Technology furnishes technical support to the
National Institute of Justice program to strengthen law enforcement and
criminal justice in the United States. OLES's function is to conduct
research that will assist law enforcement and criminal justice agencies in
the selection and procurement 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, including national standards, user guides, and
technical reports.

This document covers research conducted by OLES under the sponsorship
of the National Institute of Justice. Additional reports as well as other
documents are being issued under the OLES program in the areas of
protective clothing and equipment, communications systems, emergency
equipment, investigative aids, security systems, vehicles, weapons, and
analytical techniques and standard reference materials used by the forensic
community.

Technical comments and suggestions concerning this document are invited
from all interested parties. They may be addressed to the Director, Office
of Law Enforcement Standards, National Institute of Standards and
Technology, Gaithersburg, MD 20899-8102.

Dr. David G. Boyd, Director
Office of Science and Technology
National Institute of Justice

-------------------------------

BACKGROUND

The Office of Law Enforcement Standards (OLES) was established by the
National Institute of Justice (NIJ) to provide focus on two major
objectives: (1) to find existing equipment which can be purchased today,
and (2) to develop new law-enforcement equipment which can be made
available as soon as possible. A part of OLES's mission is to become
thoroughly familiar with existing equipment, to evaluate its performance
by means of objective laboratory tests, to develop and improve these
methods of test, to develop performance standards for selected equipment
items, and to prepare guidelines for the selection and use of this
equipment. All of these activities are directed toward providing law
enforcement agencies with assistance in making 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 OLES projects. The initial emphasis on the development
of standards has decreased, and the emphasis on the development of
guidelines has increased. For the significance of this shift in emphasis to
be appreciated, the precise definitions of the words "standard" and
"guideline" as used in this context must be clearly understood.

A "standard" for a particular item of equipment is understood to be a
formal document, in a conventional format, that details the performance
that the equipment is required to give and describes test methods by which
its actual performance can be measured. These requirements are technical
and are stated in terms directly related to the equipment's use. The basic
purposes of a standard are (1) to be a reference in procurement documents
created by purchasing officers who wish to specify equipment of the
"standard" quality, and (2) to objectively identify equipment of acceptable
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 potential user of the equipment. They
include a general discussion of the equipment, its important performance
attributes, the various models currently on the market, objective test data
where available, and any other information that might help the reader
make a rational selection among the various options or alternatives
available.

Kathleen Higgins
National Institute of Standards and Technology
April 2001

-------------------------------

CONTENTS

FOREWORD

BACKGROUND

FIGURES

COMMONLY USED SYMBOLS AND ABBREVIATIONS

1. INTRODUCTION
o 1.1 Scope
o 1.2 Organization

2. LAND MOBILE RADIO
o 2.1 Users
o 2.2 Frequency Bands
o 2.3 Rules and Regulations

3. ANTENNA FUNDAMENTALS
o 3.1 History of the Antenna
o 3.2 Frequency and Wavelength
o 3.3 Radiation Principles

4. ANTENNA CHARACTERISTICS
o 4.1 Gain and Directivity
o 4.2 Radiation Pattern
o 4.3 Antenna Polarization
o 4.4 Antenna Terminal Impedance
o 4.5 Voltage Standing Wave Ratio
o 4.6 Effective Length and Effective Area
o 4.7 Bandwidth

5. ANTENNA TYPES
o 5.1 Dipoles and Monopoles
o 5.2 Base-Station Applications
o 5.3 Corner Reflector
o 5.4 Yagi
o 5.5 Log-Periodic
o 5.6 Arrays
o 5.7 Unusual Antennas
o 5.8 Active Antennas
o 5.9 Diversity Antennas

6. TRANSMISSION LINES AND OTHER COMPONENTS
o 6.1 Transmission Line Types
o 6.2 Baluns
o 6.3 Duplexers
o 6.4 Combiners
o 6.5 Intermodulation Suppression
o 6.6 Multicouplers

7. RADIO WAVE PROPAGATION
o 7.1 Transmission Loss and the Power Budget

o 7.2 Free-Space Basic Transmission Loss
o 7.3 Terrestrial Propagation
o 7.4 Co-Site Analysis

8. ANTENNA SYSTEM REQUIREMENTS AND DESIGN
o 8.1 Define System Requirements
o 8.2 Design System
o 8.3 Select Appropriate Components

9. INSTALLATION, MAINTENANCE, AND SAFETY
o 9.1 Vehicular Antenna Systems
o 9.2 Fixed-Site Antenna Systems
o 9.3 Lightning Protection
o 9.4 During Installation
o 9.5 Perform Routine Maintenance

10. ANTENNA SYSTEM RESOURCES
o 10.1 Internet Resources
o 10.2 Periodicals
o 10.3 Manufacturers' and Vendors' Catalogs

11. PROFESSIONAL AND STANDARDS ORGANIZATIONS

12. ACRONYMS

13. REFERENCES

14. ACKNOWLEDGMENTS

FIGURES

Figure 1. Elements of a radio system

Figure 2. The propagation of a plane electromagnetic

Figure 3. The horizontal-plane and vertical-plane patterns of a vertical
half-wavelength dipole

Figure 4. Vertical-plane radiation patterns [12] showing increasing
complexity of lobes and nulls

Figure 5. The vertical dipole and its electromagnetic equivalent, the
vertical monopole

Figure 6. Typical monopole antennas for (a) base-station applications and
(b) mobile applications

Figure 7. Omnidirectional base-station antennas

Figure 8. A monopole antenna horizontal-plane pattern, base-station
application. The uniform maximum gain corresponds to the outer line on
the polar plot

Figure 9. A folded-dipole antenna

Figure 10. Typical mobile antennas

Figure 11. A mobile antenna horizontal-plane pattern [13]

Figure 12. Corner reflector antennas

Figure 13. A typical corner-reflector antenna

Figure 14. A corner-reflector antenna horizontal-plane pattern

Figure 15. The Yagi antenna--(a) three elements and (b) multiple elements

Figure 16. A typical Yagi antenna

Figure 17. A Yagi antenna horizontal-plane pattern

Figure 18. A log-periodic antenna

Figure 19. A typical log-periodic antenna

Figure 20. A log-periodic antenna horizontal-plane pattern

Figure 21. A typical vertical array using folded dipoles

Figure 22. Vertical-plane radiation patterns for (a) single half-wave dipole,
(b) two-element array, and (c) three-element array

Figure 23. A coaxial collinear array

Figure 24. A vertical-plane radiation pattern without "tilt"

Figure 25. A vertical-plane radiation pattern with 8/ "tilt"

Figure 26. A slot antenna

Figure 27. A simple active antenna

Figure 28. Common types of transmission lines

Figure 29. Coaxial cables commonly used for LMR--(a) base station
applications and (b) mobile applications

Figure 30. A VHF duplexer

Figure 31. Typical types of combiners--(a) hybrid combiner and
(b) cavity combiner

Figure 32. Intermodulation suppression device

Figure 33. A multicoupler

Figure 34. Gains and losses as described in the power-budget equation

Figure 35. A propagation path illustrating direct and reflected rays

Figure 36. An example of diffraction

TABLE

Table 1. Characteristics of selected coaxial cable

-------------------------------

1. INTRODUCTION

Radio communications are essential to the operations of Federal, State,
and local law enforcement and correction agencies. Effective and reliable
communications systems not only enable personnel to perform their
functions efficiently, but also help ensure their safety. It is, therefore, very
important that all components of a radio communications system be
selected and integrated to produce an effective design. Understanding the
capabilities and limitations of a communications system ensures that it is
used most effectively and that performance expectations are realistic.

This guide focuses on a key portion (subsystem) of the radio
communications system--the antenna system. Although the antenna itself
may be the most visible element of radio communications equipment, it is
often the least understood. This guide defines and describes the
components of the antenna system as well as the fundamentals and
characteristics of the antenna itself.[1]

1.1 Scope

The theory and empiricism upon which antenna technology is based are
very complex. It is not the purpose of this guide to tutor the reader in the
study of electromagnetic theory. Rather, it is to provide the reader with
sufficient understanding of the fundamentals, characteristics, and functions
of antennas to enable him or her to develop requirements and discuss
antennas with vendors, installers, repair shops, etc. To this end, the
number of equations and references to theory are kept to a minimum, and
the information is generally restricted to the kind of antennas used by law
enforcement agencies.

This guide is intended for a wide audience. It is written so that the reader
can study only those sections of interest. Cross-referencing is provided
where needed to direct the reader to related, important information.

1.2 Organization

Section 2 of this guide provides the reader with a brief description of land
mobile radio (LMR), which is the radio service used by nearly all law
enforcement agencies. The frequency bands used by LMR are described.
The information provided on antennas in this guide is generally limited to
LMR applications and the radio frequencies used by LMR.

Section 3 begins with an overview of the major elements of a radio
communications system. The function of the antenna (sub)system is
described. The remainder of the section provides a brief history of the
antenna and an introduction to the fundamentals of antennas and radio
wave propagation. Several of the subsections are important to the
understanding of antennas. These are section 3.2 on frequency and
wavelength and section 3.3 on radiation principles. The reader is
encouraged to review the simple, but important, concepts in these
subsections to fully understand the following sections.

Section 4 is the most important part of this guide, as it presents the
engineering, or functional, characteristics of antennas. It defines gain,
directivity, and radiation pattern and how these are used in a radio
communications system design. These and other characteristics are also
used to specify an antenna. Section 5 contains descriptions of common
antennas including their electrical and radiation characteristics as well as
their physical characteristics. Although there are many different kinds of
antennas, the emphasis is on those antennas commonly used for LMR.

Sections 6 and 7 relate to the use of antennas within a radio
communications system. Section 6 describes the transmission lines and
related components needed to connect an antenna to a transmitter or a
receiver, and section 7 describes the modes of radio wave propagation
commonly used for LMR systems. Section 7 also discusses how an
antenna pattern and propagation effects are used to achieve required
geographical coverage.

The next three sections (8, 9, and 10) cover the practical aspects of
developing and expressing requirements for an antenna; the installation,
maintenance, and safety of antennas; and examples of some of the
products and services available, respectively.

Section 11 provides the reader with a list of the relevant regulatory,
standards, and professional organizations. The last two sections (12 and
13) provide reference material. Section 12 is a list of acronyms used in this
guide or related to antennas. Section 13 contains the references cited in
this guide.

2. LAND MOBILE RADIO

The formal title of the type of radio service used by law enforcement
agencies for their radio communications is "land mobile service," and the
frequency bands reserved for their use are labeled "public safety." Land
mobile service is defined as "a mobile radio service between base stations
and land mobile stations, or between land mobile stations," by the
National Telecommunications and Information Administration (NTIA)
[1]2 and the Federal Communications Commission (FCC) [2]. Another
commonly used phrase for "land mobile service" is "land mobile radio"
(LMR).

2.1 Users

There are two general classes of users. One class of users is radio common
carrier (RCC). RCC owners build and operate LMR systems and charge a
fee to third parties that actually use the system. The other class of users
includes groups that meet specific requirements for LMR use. These
groups include: Public Safety Radio Services, Special Emergency Radio
Services, Industrial Radio Services, Land Transportation Radio Services,
Radio Location Services, and Specialized Mobile Radio Services. The first
group listed here, the Public Safety Radio Services, is the category that
includes State and local governments, law enforcement and corrections,
fire, emergency medical services, highway maintenance, and forestry
conservation. Federal law enforcement and corrections agencies also
belong to this category of LMR users. These Federal agencies may use
some of the same frequency bands and channels as State and local
agencies, but, in many cases, they use frequency bands and channel
assignments unique to the Federal Government.

Most LMR user groups maintain formal associations that provide for the
exchange of information within their groups and represent their members
before the FCC and other official or government bodies. Those
associations relevant to the law enforcement community include the
Association of Public-Safety Communications Officials (APCO), the
National Association of State Telecommunications Directors (NASTD),
and the Telecommunications Industry Association (TIA). The first two of
these represent the users, and the last represents the manufacturers of the
equipment used for LMR. More details on these associations and other
relevant organizations are presented in section 11.

2.2 Frequency Bands

There are a number of frequency bands allocated for use by LMR. The
particular frequency ranges for these bands have been established at the
international level, in plenipotentiary conferences, by the International
Telecommunications Union (ITU). Administration and assignment of
frequencies within the LMR bands in the United States are performed by
two different government agencies. The FCC provides frequency
assignments and licenses for all non-Federal users. Federal users obtain
their frequency assignments through NTIA.

The LMR bands described in the following sections are subdivided for
Federal use only, shared Federal and non-Federal use, and non-Federal use
only. Portions of some bands may be shared with other services such as
maritime mobile, mobile satellite, and broadcast television.

2.2.1 VHF Low-Band

This band is located at 25 MHz to 50 MHz and generally uses channels 20
kHz wide. Communications ranges can be the greatest in this band;
however, the radio signals do not reflect effectively off hills, buildings,
and other surfaces, so there may be dead spots within the general area of
coverage of a base station. These frequencies, especially at the lower end
of the band, are subject to ionospheric "skip," which can carry a signal
very long distances (and cause interference to other users). This band also
has the highest level of ambient noise, which reduces the performance of
receivers operating in this band compared to other, higher
frequency-bands.

2.2.2 VHF High-Band

This band is located at 150 MHz to 174 MHz and uses channels 25 kHz or
30 kHz wide. Adjacent channels may be separated by 12.5 kHz to 15 kHz
for geographically separated systems. Radio signals in this band have a
shorter propagation range, experience less noise than those in the VHF
low-band, and are not subject to ionospheric "skip." Diffraction over hills
and around other obstacles reduces dead areas of coverage.

2.2.3 UHF Bands

The first UHF band is located at 406 MHz to 420 MHz and is designated
for Federal usage. The second band is located at 450 MHz to 470 MHz
and is designated for non-Federal usage. The last band, from 470 MHz to
512 MHz, shares spectrum with UHF television channels 14 through 20 in
a few major urban areas. Channels in all three UHF bands are 25 kHz
wide. The propagation range in the UHF band is even less than that found
in the VHF high-band; however, the radio signals easily reflect off hills
and buildings, so dead areas are generally very small.

2.2.4 700 MHz Band

New bands from 764 MHz to 776 MHz and 794 MHz to 806 MHz have
been proposed by the FCC. These new channels would be converted from
TV channels 63-64 and 68-69 to Public Safety use. The channels would be
either narrowband for voice plus low-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 from 806 MHz to 940 MHz is
apportioned among many services, including cellular telephone, paging,
nonpublic safety, and conventional and trunked public safety. Federal
Government users, however, have no authorizations in this spectrum.
Channels for public safety are reserved in blocks that are generally 25 kHz
wide. These blocks reside in several locations within this portion of the
radio spectrum. These radio signals reflect off hills, buildings, vehicles,
etc., well enough that dead areas are nearly nonexistent. The propagation
range for this band is shorter than for the UHF bands.

2.3 Rules and Regulations

2.3.1 General

In the United States, the FCC allocates different bands of the radio-
frequency spectrum to different entities for a wide variety of commercial
and private applications [2]. The NTIA performs this function for the
Federal Government. In recent years, with the proliferation of portable
phones and other mobile communications devices, the FCC and the NTIA
have received an increasing number of requests for an ever-dwindling
amount of spectrum available for allocation.

One way that the FCC and NTIA attempt to maximize spectrum usage is
to assign the right to transmit over the same channel to different
commercial or private groups that are located in different geographic
regions, so that their signal coverage areas will not overlap. Consideration
is also given when assigning adjacent channels, so signals from one
transmission channel will not affect adjacent channels.

Further description of the allocation process and the most recent listing of
allocations can be found in the NTIA Manual of Regulations and
Procedures for Federal Radio Frequency Management [1] and the Code of
Federal Regulations [3] pertaining to telecommunications.

2.3.2 Future Options

At the present time, LMR bands are crowded and channel assignments can
be difficult to obtain in some areas of the country. For this reason, work is
underway to develop new, spectrally efficient modulation and signal-
processing methods and improved techniques to share existing channels.
Historically, this problem has been addressed by making narrower
channels within a band. Frequency-modulation (FM) systems have been
improved to allow them to operate in bandwidths as small as 12.5 kHz.
Further improvement requires different modulation methods, but care must
be taken in choosing and standardizing the modulation method and
channel widths to ensure interoperability among different systems and
equipment from different manufacturers.

New LMR systems conforming to the "Project 25" standard [4] use
narrowband modulation techniques and channel-sharing methods (called
trunking). These new digital modulation methods will be able to use
channels as narrow as 6.25 kHz. Trunking techniques will accommodate
two, three, or more times as many users for a given number of channels.

3. ANTENNA FUNDAMENTALS

The antenna is often the most visible element of a radio system. The sizes
and shapes of the conductors that comprise the antenna determine the
directional characteristics of the electromagnetic (radio) waves it radiates.
However, the antenna cannot be considered independently. Additional
elements, such as the transmission line, duplexers, matching networks,
etc., must be considered as part of the antenna system.

The full description of the interaction of an antenna with its surrounding
environment is based on very complex mathematics, but its function in a
radio system is quite simple. Figure 1 shows the key elements of a radio
communications system. When an antenna is used for transmitting, it
converts electrical signals, delivered by a transmission line, from a
transmitter into propagating electromagnetic waves. When an antenna is
used for receiving, it converts electromagnetic waves back into electrical
signals that are delivered by a transmission line to a receiver for
processing. In fact, the same antenna (used for both transmitting and
receiving) is often attached to a transmitter and a receiver using either a
duplexer or a transmit/receive (XMT/RCV) switch. A duplexer allows one
antenna to be used by both the transmitter and receiver at the same time
(see sec. 6.3), and a transmit/receive switch connects the antenna to either
the transmitter or receiver.

3.1 History of the Antenna

Over a century has elapsed since James Clerk Maxwell [5] formulated his
celebrated equations that provide the foundation of classical
electromagnetism. By means of these equations, Maxwell was able to
predict the existence of electromagnetic waves which, 20 years later in
1887, were confirmed experimentally by Heinrich Hertz [6]. Hertz
constructed a center-driven wire about 60 cm long, terminated at each end
by a 40 cm square metal plate. Driven by a spark-gap generator
(broadband source), this antenna resonated at about 50 MHz and
effectively generated and radiated electromagnetic waves. He detected
those waves using a loop of wire, 35 cm in radius, with a very small gap
where he could observe a spark. Although a number of researchers became
interested in this new phenomenon, it was Gugliermo Marconi who, in
1897, described and then demonstrated a complete system for the
transmission of signals. On December 12, 1901, the first transatlantic
communication was achieved. The transmitting antenna in Cornwall,
England, was a fan-like structure of 50 copper wires supported by a
horizontal wire stretched between two poles about 45 m high and 65 m
apart. The receiving antenna in Newfoundland was comprised of copper
wires supported by kites. By 1907, commercial, transatlantic telegraph
services had been established [7].

It is interesting to note that the original, commonly used term for a
signaling system that uses the phenomenon of electromagnetic waves was
"wireless." Although the term "radioconductor" (a contraction of radiation
conductor) appears as early as 1897, the term "radio" does not emerge
until later. The Radio Ship Act of 1910 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 exists because of the fundamental
interdependence of the electric and magnetic fields that comprise it. An
electric field that changes with time produces a magnetic field whose
strength is determined by the rate of change of the electric field. And, in
complementary fashion, a magnetic field that changes with time produces
an electric field whose strength is determined by the rate of change of the
magnetic field. This means that the energy contained in, and transmitted
by, the radio wave is shared by the two fields.

When transmitting, the electrical current on an antenna produces the
magnetic field, and the voltage on that antenna produces the electric field.
Similarly, when receiving, an electromagnetic wave incident on the
antenna produces electrical current and voltage. If these currents and
voltages have a sinusoidal time dependence (sine or cosine wave), then
several very important, fundamental phenomena occur. The first derivative
(rate of change) of a sine wave is another sine wave shifted in time by one
quarter of the period of the first sine wave. This means that if the changing
electric field in the radio wave is sinusoidal, then so is the magnetic field.
These two sinusoidal time-varying fields, in effect, regenerate each other
as they travel. This is called wave propagation.

The frequency of a sine wave is expressed as the number of cycles per
second or hertz. The period of the wave is the reciprocal of its frequency
and is expressed in seconds. The wavelength is the length of one cycle of
the traveling wave in space, or the distance the wave travels during one
period. The relationship between frequency and wavelength is [Equation
1] where wavelength is the wavelength (m), c is the speed of light (2.9979
10 8 m/s), and f is the frequency (Hz). A practical, approximate version
of this equation is [Equation 2] where wavelength is in meters and f is in
megahertz.

3.3 Radiation Principles

The important characteristics of an antenna are its radiation properties,
such as gain, directionality, and polarization; and the electrical property,
input impedance. These and other characteristics can be determined
theoretically, or they can be obtained by measurement. In practice, both
theory and measurement are used to design or evaluate an antenna.

The remainder of this section is necessary to the understanding and
usefulness of section 4, Antenna Characteristics. Although based in
electromagnetic theory, these fundamental principles are not complicated
and they are essential to an understanding of antennas.

3.3.1 What Is an Antenna?

An antenna is a device that provides suitably localized and oriented paths
for oscillating electric currents. The sizes and shapes of the conductors that
comprise the antenna determine the directional characteristics of the radio
waves it radiates. A transmitting antenna converts electrical currents,
delivered by the transmission line from a transmitter, into propagating
radio waves, and a receiving antenna converts radio waves back into
electrical currents that are delivered by a transmission line to a receiver.

3.3.2 Reciprocity

The theoretical determination of an antenna's characteristics is usually
accomplished by treating the antenna as a transmitting device. However,
under most conditions, the antenna characteristics are exactly the same
when it is used as a receiving device. If the antenna is linear (i.e., it
contains neither active elements nor nonlinear components such as
ferrites), then the principle of reciprocity holds. This principle was first
described by the famous mathematician Lord Rayleigh [9]. The principle
of reciprocity means that an antenna will have exactly the same
characteristics whether it is used for transmitting or receiving. So, if a
particular characteristic of an antenna is obtained by measurement while
using the antenna for reception, then it is known that the same antenna will
have exactly the same characteristic when used for transmission.

3.3.3 Radiated Waves and the Near Field

The electromagnetic field produced by an antenna is quite complex near
the antenna, and it can be described as having several components. Only
one of these actually propagates, or travels though space. This component
is called the radiated field, radiated wave, or radio wave. The propagation
of this radiated field, or radio-wave propagation, is usually treated as a
separate topic. The strength of the radiated field does decrease with
distance, as it must, since the energy must spread as it travels. Some
knowledge of propagation is useful in understanding antennas or in
choosing an antenna to meet certain requirements, such as coverage--the
area over which a radio wave has sufficient strength to be useful. Section 7
presents a brief overview of radio-wave propagation.

The other components of the electro-magnetic field remain near the
antenna and do not propagate. There are generally two other components:
the static field and the induction field. Even though they do not propagate,
their strength decreases very rapidly with distance. The entire field--all of
the components--near the antenna is called the near field. In this region,
approximately one wavelength in extent, the field strength can be
relatively high and pose a hazard to the human body. See section 9.5.4 for
more information on this radiation hazard.

3.3.4 Plane Waves

The radiated wave, as with all electromagnetic waves including light, is
composed of an electric field and a magnetic field. For most cases, the
field lines, and the vectors that are used to illustrate them, are at a right
angle to each other and to the direction of propagation, as shown in figure
2.

At large distances from the antenna, say beyond ten wavelengths, the
radiated field is essentially a plane wave. This means that there is no
curvature of the field lines.

3.3.5 Wave Polarization

The polarization of a wave, by definition, is simply the orientation of the
electric field vector. The plane wave depicted in the illustration of figure 2
is vertically polarized. For nearly all cases encountered in LMR, the
polarization of the wave does not change as it propagates. This is called
linear polarization, and the two most common examples in practice are
vertical or horizontal polarization. Other polarizations include circular,
where the electric field vector (and the magnetic field vector) rotate as the
wave travels.

4. ANTENNA CHARACTERISTICS

In a radio communications system, an antenna has two basic functions.
The primary function is to radiate, as radio waves, the RF signals from the
transmitter, or to convert radio waves into RF signals for processing by a
receiver. The other function is to direct the radiated energy in the desired
direction or directions, or to be "sensitive" to reception from the desired
direction or directions. Another, often overlooked, aspect of an antenna's
directional properties is the suppression of radiation in undesired
directions, or the rejection of reception from undesired directions.

The directional characteristics of an antenna are fundamental to an
understanding of the antenna and how it is used in a radio communications
system. These interrelated characteristics include gain, directivity,
radiation (antenna) pattern, and polarization. Other characteristics such as
beamwidth, effective length, and effective aperture are derived from the
four listed above. Terminal (input) impedance is one other characteristic
that is of fundamental importance. It is necessary to know the impedance
of an antenna in order to efficiently couple the transmitter's output power
into it, or to efficiently couple the power from it into the receiver. All of
these antenna characteristics are a function of frequency.

4.1 Gain and Directivity

The gain of an antenna is the radiation intensity 3 in a given direction
divided by the radiation intensity that would be obtained if the antenna
radiated all of the RF power delivered to it equally in all directions [10,
11]. Note that this definition of gain requires the concept of an isotropic
radiator; that is, one that radiates the same power in all directions.
Examples of nondirectional sources can be achieved (at least
approximately) with sound and light; these are sometimes called point
sources.

An isotropic antenna, however, is just a concept, because all practical
radio antennas must have some directional properties. Nevertheless, the
isotropic antenna is very important as a reference. It has a gain of unity (g
= 1 or G = 0 dB) in all directions, since all of the power delivered to it is
radiated equally well in all directions.

Although the isotrope is a fundamental reference for antenna gain, another
commonly used reference is the dipole. In this case the gain of an ideal
(lossless) half-wavelength dipole is used. Its gain is 1.64 (G = 2.15 dB)
relative to an isotropic radiator.

The gain of an antenna is usually expressed in decibels (dB). When the
gain is referenced to the isotropic radiator, the units are expressed as dBi;
but when referenced to the half-wave dipole, the units are expressed as
dBd. The relationship between these units is [Equation 3].

Directivity is the same as gain, but with one difference. It does not include
the effects of power lost (inefficiency) in the antenna itself. Recall that the
definition of gain is based on the power delivered to (and accepted by) the
antenna. In practice, some of that power is lost in the antenna due to ohmic
losses (heating) in the elements, leakage across insulators, etc. If an
antenna were lossless (100 % efficient), then the gain and directivity (in a
given direction) would be the same.

4.2 Radiation Pattern

The radiation pattern (also called antenna pattern) is a representation of the
gain of an antenna for all directions. Since this is a three-dimensional
description of the power density, it is difficult to display or use. It is
common to display or plot cross-sections of it. Figure 3 shows the
radiation pattern of a vertical half-wavelength dipole in the horizontal
plane and a vertical plane. As one can see in this figure, the patten in the
horizontal plane has no structure. This antenna has constant gain versus
azimuth. On the other hand, the pattern in a vertical plane shows that the
antenna has maximum gain in the horizontal plane and no radiation in the
directions coincident with the axis of the antenna. Therefore, one can now
visualize the three-dimensional pattern as a torus (doughnut shaped).

Often, the direction is not specified when referring to an antenna's gain. In
this case, it is assumed that the gain's direction is the direction of
maximum radiation--the maximum gain for the antenna. An associated
pattern then will present values relative to that maximum gain.

4.2.1 Lobes and Nulls

The regions of a pattern where the gain has local maxima are called lobes,
and those places where the gain has local minima are called nulls. The
vertical plane "cut" for the half-wave dipole (fig. 3b) has two lobes and
two nulls. Figure 4 shows several other examples. A complex antenna
pattern may have many lobes and nulls in both the horizontal-plane and
vertical-plane patterns. The lobe with the greatest gain is called the main
lobe or main beam of the antenna. If a single value of gain is given for an
antenna, it is assumed to be the main lobe or main beam gain.

4.2.2 Beamwidth

Beamwidth is an angular measure of the main lobe (or main beam) in
either (or both) the horizontal-plane or vertical-plane pattern [11]. There
are several definitions for beamwidth, including: half-power or 3 dB
beamwidth, 10 dB beamwidth, and first-null beamwidth. The 3 dB
beamwidth is the angular extent about the maximum value of gain for
which the gain is 3 dB below the maximum. First-null beamwidth is the
angular extent about the maximum value of gain of the first occurring
local minima in the 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 the electric field vector E at some point in space. If the E-
field vector retains its orientation at each point in space, then the
polarization is linear; if it rotates as the wave travels in space, then the
polarization is circular or elliptical. In most cases, the radiated-wave
polarization is linear and either vertical or horizontal. At sufficiently large
distances from an antenna, e.g., well beyond 10 wavelengths, the radiated,
far-field wave is a plane wave (see sec. 3.3.4).

The term polarization is often applied to the antenna itself. In this sense,
the polarization of the antenna is the polarization of the plane wave it
radiates. Based on the principle of reciprocity (see sec. 3.3.2), this is true
for a receiving antenna, as well. For example, if a receiving antenna is
vertically polarized, this means that a vertically polarized, incoming wave
will produce maximum output from that antenna. If the incoming wave
were polarized at some other angle, only the vertical component would be
detected by the antenna. Ideally, a horizontally polarized incoming wave
would not be detected at all by a vertically polarized antenna. Vertical
polarization is used for most LMR applications.

4.4 Antenna Terminal Impedance

There are three different kinds of impedance relevant to antennas. One is
the terminal impedance of the antenna, another is the characteristic
impedance of a transmission line, and the third is wave impedance.
Antenna terminal impedance is discussed in this section. Transmission line
characteristic impedance is discussed in section 6, and wave impedance is
the ratio of the electric field strength to the magnetic field strength of a
propagating wave (see sec. 3.2 and sec. 3.3.4).

Terminal impedance is defined as the ratio of voltage to current at the
connections of the antenna (i.e., the point where the transmission line is
connected). The complex form of Ohm's law defines impedance as the
ratio of voltage across a device to the current flowing through it. The
terminal impedance is expressed mathematically as [Equation 4] where Z
is the impedance, in ohms, V is the voltage, in volts, and I is the current, in
amperes, at the antenna terminals for a given frequency. Each of these
variables can be expressed as a complex number, each with real and
imaginary parts. Such complex numbers can also be expressed by using a
magnitude and phase angle--this is called phasor notation.

The real part of the impedance is called the resistive component, and the
imaginary part is called the reactive component. This is often expressed as
[Equation 5] where R is the resistive (real) component, X is the reactive
(imaginary) component, and j = the square root of -1.

The most efficient coupling of energy between an antenna and its
transmission line occurs when the characteristic impedance of the
transmission line and the terminal impedance of the antenna are the same
and have no reactive component. When this is the case, the antenna is
considered to be matched to the line.

Matching usually requires that the antenna be designed so that it has a
terminal impedance of about 50 ohm or 75 ohm to match the common
values of available coaxial cable. A half-wave dipole can be shortened
slightly to achieve this. For other antennas, it can be difficult to remove
(reduce to zero) the reactive component. In these cases, a matching
network is often made part of the antenna to change its complex terminal
impedance into something that better matches a transmission line.

The resistive part R of the terminal impedance is the sum of two
components and is expressed in ohms, [Equation 6].

The radiation resistance Rr is the "effective load" that represents the power
radiated by that antenna as radio waves, and the dissipative resistance Rd
is the "load" into which power is lost. The efficiency of an antenna is the
ratio of the power radiated to the total power delivered to the antenna. It
can be expressed as [Equation 7].

As discussed in section 4.1, the dissipative losses are due to ohmic losses
(heating) in the antenna elements, leakage across insulators, and similar
effects. Furthermore, it should be noted that the efficiency of an antenna
can also be expressed as the ratio of the gain to the directivity (for a given
direction).

4.5 Voltage Standing Wave Ratio

The standing wave ratio (SWR), also known as the voltage standing wave
ratio (VSWR), is not strictly an antenna characteristic, but is used to
describe the performance of an antenna when attached to a transmission
line. It is a measure of how well the antenna terminal impedance is
matched to the characteristic impedance of the transmission line.
Specifically, the VSWR is the ratio of the maximum to the minimum RF
voltage along the transmission line. The maxima and minima along the
lines are caused by partial reinforcement and cancellation of a forward-
moving RF signal on the transmission line and its reflection from the
antenna terminals.

If the antenna terminal impedance exhibits no reactive (imaginary) part
and if the resistive (real) part is equal to the characteristic impedance of
the transmission line, then the antenna and transmission line are said to be
matched. If this is true, then none of the RF signal sent to the antenna will
be reflected at its terminals. There is no standing wave on the transmission
line and the VSWR has a value of one. However, if the antenna and
transmission line are not matched, then some fraction of the RF signal sent
to the antenna is reflected back along the transmission line. This causes a
standing wave, characterized by maxima and minima, to exist on the line.
In this case, the VSWR has a value greater than one.

The VSWR is easily measured with a device called an SWR meter. It is
inserted in the transmission line and directly gives a value for the VSWR.
At a VSWR value of 1.5, approximately 4 % of the power incident at the
antenna terminals is reflected. At a value of 2.0, approximately 11 % of
the incident power is reflected. VSWR values of 1.1 to 1.5 are considered
excellent, values of 1.5 to 2.0 are considered good, and values higher than
2.0 may be unacceptable.

As stated above and elsewhere, an ideal match between the antenna and
transmission line is desired; but this can often be achieved only for a
single frequency. In practice, an antenna may be used for an entire
frequency band, and its terminal impedance will vary across the band. In
an antenna specification, either the impedance versus frequency across a
band is given or the VSWR versus frequency is given.

4.6 Effective Length and Effective Area

The effective length and the effective area (also called effective aperture)
are alternative ways of expressing the gain of an antenna. These
characteristics are most useful and meaningful when the antenna is used
for receiving. Of course, due to the principle of reciprocity, these
characteristics are the same if the antenna is used for transmitting.

The effective length defines the ability of an antenna to produce a voltage
at its terminals from an incident electric field. It is defined as [Equation 8]
where effective length is expressed in meters, V is the open circuit voltage
in volts, and E is the electric field strength in volts/meter. This definition
assumes that the polarization of the incident field and the antenna are the
same. The effective length can also be computed from the gain and the
radiation resistance.

Effective area, or aperture, is more commonly used than effective length.
It is defined as [Equation 9] where Pr is the power available at the
terminals of the antenna in watts, and p is the power density of the incident
wave in watts per square meter. The relationship between effective area
and gain is [Equation 10].

4.7 Bandwidth

Bandwidth is the difference between two frequencies, or the frequency
range, within which the performance of an antenna is acceptable. In other
words, one or more characteristics (e.g., gain, pattern, terminal impedance)
have acceptable values between the bandwidth limits. For most antennas,
gain and pattern do not change as rapidly with frequency as the terminal
impedance does, so the latter is often used to describe the bandwidth of an
antenna.

VSWR (see sec. 4.5) is a measure of the effect of mismatch between an
antenna's terminal impedance and the transmission line characteristic
impedance. Since the transmission line characteristic impedance hardly
changes with frequency, VSWR is a useful, practical way to describe the
effects of terminal impedance and to specify an antenna's bandwidth. For
example, an antenna specification may give a plot of the VSWR across
some frequency band. It will likely have a minimum value at about the
middle of the band.

Another way of specifying the bandwidth is a statement of the maximum
VSWR within a band. Half-wave dipoles, and similar antennas, have a
narrow bandwidth. Other antennas, like the log-periodic, are designed
specifically to be broadband.

5. ANTENNA TYPES

Antennas can be classified in several ways. One way is the frequency band
of operation. Others include physical structure and
electrical/electromagnetic design. The antennas commonly used for LMR
--both at base stations and mobile units--represent only a very small
portion of all the antenna types.

Most simple, nondirectional antennas are basic dipoles or monopoles.
More complex, directional antennas consist of arrays of elements, such as
dipoles, or use one active and several passive elements, as in the Yagi
antenna.

New antenna technologies are being developed that allow an antenna to
rapidly change its pattern in response to changes in direction of arrival of
the received signal. These antennas and the supporting technology are
called adaptive or "smart" antennas and may be used for the higher-
frequency LMR bands in the future.

5.1 Dipoles and Monopoles

The vertical dipole--or its electromagnetic equivalent, the monopole--
could be considered one of the best antennas for LMR applications. It is
omnidirectional (in azimuth) and, if it is a half-wavelength long, has a gain
of 1.64 (or G = 2.15 dBi) in the horizontal plane. A center-fed, vertical
dipole is illustrated in figure 5(a). Although this is a simple antenna, it can
be difficult to mount on a mast or vehicle. The ideal vertical monopole is
illustrated in figure 5(b). It is half a dipole placed in half-space, with a
perfectly conducting, infinite surface at the boundary.

A monopole over an infinite ground plane is theoretically the same
(identical gain, pattern, etc., in the half-space above the ground plane) as
the dipole in free space. In practice, a ground plane cannot be infinite, but
a ground plane with a radius approximately the same as the length of the
active element, is an effective, practical solution. The flat surface of a
vehicle's trunk or roof can act as an adequate ground plane. Figure 6 shows
typical monopole antennas for base-station and mobile applications.

5.2 Base-Station Applications

For base-station installations (where an omnidirectional pattern is desired),
there are two practical implementations of the vertical dipole. The first
type is the sleeve antenna, as illustrated in figure 7(a). The sleeve antenna
is a vertical dipole with the feed (transmission line) entering from one end
of a hollow element. The second type is a monopole over a ground plane,
as illustrated in figure 7(b). The monopole in this illustration uses a set of
four wire elements to provide the ground plane. Figure 8 shows a typical
pattern for a base-station monopole.

A variation of the dipole antenna is the folded dipole as shown in figure 9.
Its radiation pattern is very similar to the simple dipole, but its impedance
is higher and it has a wider bandwidth.

5.2.1 Mobile Applications

Nearly all vehicular antennas are monopoles mounted over a (relatively)
flat body surface (as described above). In this application, the monopole is
often called a "whip" antenna. At VHF low-band, a quarter-wave
monopole can be 2.5 m (approximately 8 ft) long. However, an inductor
(coil) at the base of a monopole adds electrical length, so the physical
length of the antenna can be shorter. Although this kind of "loaded"
antenna will appear to be a quarter-wave antenna, it will have a gain value
somewhat less than a true quarter-wave monopole. This disadvantage can
be somewhat offset, however, by the ability to mount the (shorter) antenna
in the center of a surface that will act as an acceptable ground plane (e.g.,
the roof or trunk of the vehicle). Figure 10(a) shows an illustration of this
kind of antenna.

Many of the vehicular antennas at VHF high-band are quarter-wave
monopoles. At 150 MHz, this would mean that a whip antenna,
approximately 0.5 m (1.5 ft) long, is needed. Half-wave and 5/8 wave
monopoles also are used, but they require some sort of matching network
(i.e., inductors and/or capacitors) in order to match the antenna impedance
to that of the transmission line. These longer antennas have a gain of 
approximately 3 dBi.

At UHF, a quarter-wave whip is approximately 15 cm (6 in) long. Since
this length is physically small, some design considerations can be used to
increase the gain. For example, as shown in figure 10(b), two 5/8 wave
monopoles can be "stacked" with a phasing coil between them. This is,
effectively, an antenna array (see sec. 5.5) that provides a gain of
approximately 5 dBi.

At 800 MHz, a quarter-wave monopole does not perform well, so the
approach of stacking two monopoles, with a phasing coil between, is used.
Such an antenna, illustrated in figure 10(c), looks much like a mobile
cellular phone antenna and has a gain of approximately 3 dBi.

The azimuthal pattern of all monopoles is ideally a circle. In other words,
the gain versus azimuth angle in the horizontal plane is constant. In
practice, the pattern in the horizontal plane generally is not
omnidirectional, since the portion of the vehicle used as a ground plane is
not symmetric, and usually there are other obstructions. Figure 11 shows
the horizontal plane pattern for an 840 MHz whip located in the center of
the roof of a vehicle [13]. The dotted line in the figure shows the effects,
on the pattern, of a law-enforcement light bar mounted on the roof ahead
of the antenna.

5.3 Corner Reflector

An antenna comprised of one or more dipole elements in front of a corner
reflector, called the corner-reflector antenna, is illustrated in figure 12. A
photograph of a typical corner reflector is shown in figure 13.

This antenna has moderately high gain, but its most important pattern
feature is that the forward (main beam) gain is much higher than the gain
in the opposite direction. This is called the front-to-back ratio and is
evident in the pattern shown in figure 14.

5.4 Yagi

Another antenna design that uses passive elements is the Yagi antenna.
This antenna, illustrated in figure 15, is inexpensive and effective. It can
be constructed with one or more (usually one or two) reflector elements
and one or more (usually two or more) director elements. Figure 16 shows
a Yagi antenna with one reflector, a folded-dipole active element, and
seven directors, mounted for horizontal polarization.

Figure 17 is a typical pattern for a three-element (one reflector, one active
element, and one director) Yagi antenna. Generally, the more elements a
Yagi has, the higher the gain, and the narrower the beamwidth. This
antenna can be mounted to support either horizontal or vertical
polarization and is often used for point-to-point applications, as between a
base station and repeater-station sites.

5.5 Log-Periodic

A somewhat novel, but very useful, design is the log-periodic antenna.
This antenna is based on the dipole element. As shown in the illustration
of figure 18, it is in fact comprised of a set of dipoles, all active, that vary
in size from smallest at the front to largest at the rear. Usually, this
antenna is constructed so the antenna terminals are located at the front (on
the shortest dipole). Figure 19 shows a typical installation. The key
features of this antenna are, first of all, its broadband nature, and second,
its relatively high front-to-back gain ratio. The latter feature is evident in
the typical radiation pattern shown in figure 20.

5.6 Arrays

An antenna array (or array antenna) is, much like it sounds, several
elements interconnected and arranged in a regular structure to form an
individual antenna. The purpose of an array is to produce radiation
patterns that have certain desirable characteristics that a single element
would not. A stacked dipole array, as shown in figure 21, is comprised of
vertical dipole elements.

This dipole array has an omnidirectional pattern like the element dipole
does; but has higher gain and a narrower main lobe beamwidth in the
vertical plane. Figure 22 shows how the vertical-plane gain of the dipole
element can be "enhanced" by making an array of them. Figure 22(a)
represents the radiation pattern of one element. Figure 22(b) is the pattern
of two elements, and figure 22(c) is for three elements.

This is called a binomial or collinear array [14]. As the number of
elements is increased, the gain increases and the beamwidth decreases.

The omnidirectional coaxial collinear antenna (often referred to as an
"omni") is a very popular array design for base stations. It is comprised of
quarter-wave coaxial sections with inner and outer conductors transposed
at each junction.

A conceptual illustration is shown in figure 23. Although more complex
than the illustration, this antenna array behaves like a series of vertical
dipoles stacked one above the other. The more stacked sections, the
greater the gain and the narrower the vertical beamwidth. A vertical-plane
pattern for this type of antenna is shown in figure 24. Variations in
electrical design can produce a downward tilt of the vertical-plane pattern
as shown in figure 25. This antenna often is enclosed in a fiberglass
sheath, called a radome, and appears as a simple pole that can be mounted
off the side or on top of a mast or tower.

As with all antennas, the array is frequency-dependent. The gain,
directivity, and radiation pattern are each a function of frequency. Some
antennas will work well only for the design frequency, and their
performance will degrade as the operating frequency is separated from the
design frequency.

5.7 Unusual Antennas

There are many other antenna types. Most of these are beyond the scope of
this report, but knowledge about some may be useful for LMR users.

While not as commonplace as wire or rod antennas, aperture antennas are
by no means unusual. These antennas are implemented as an opening in a
relatively large, conductive (metal) surface.

The simplest aperture antenna is the slot antenna, which is equivalent to a
dipole. As shown in figure 26, it is a long, narrow opening with terminals
located at the middle of the long sides of the slot. This simple slot and
more complex versions are well-suited to covert operations. They can be
located on a vehicle surface and concealed behind a cover of thin
insulating material. Slot antennas are common on aircraft and missiles.

Not so much antenna types as antenna features, broadband and multiband
antennas are the result of design efforts to make an antenna perform well
over a wide band of channels. There may be a trade-off in making an
antenna broadband, such as a reduction in gain or an increase in physical
size. The usual design goals for this type of antenna are to make the gain
and radiation pattern, as well as the terminal impedance, relatively
constant over the frequency range of operation. The log-periodic array is
an example of a broadband antenna.

Multiband antennas are designed to operate on several bands, for example,
at both VHF high-band and UHF. These antennas often involve clever
designs where one part of the antenna is active for one band, and another
part for a different band. Again, there will be compromises. The antenna
may have lower average gain or may be physically larger than an
equivalent single-band antenna.

5.8 Active Antennas

An active antenna is one that contains some electronic circuitry that can
amplify a received signal at the antenna and thus avoid interference that
may enter the system at the transmission line. Figure 27 shows this
concept. The antenna "element" is connected to the input of an amplifier.
The output terminals of the amplifier are the antenna terminals for this
active antenna. The antenna element and the amplifier are included in the
"active antenna," shown as a dashed box in the figure.

Another purpose of an active antenna is to transform an unusual antenna
terminal impedance to a constant value that matches the characteristic
impedance of the transmission line. This function is useful for some
antenna designs in which a specific pattern feature is desired, but cannot
be achieved without causing the antenna to have an unusual terminal
impedance. An active antenna is nonreciprocal and cannot be used for
transmitting.

5.9 Diversity Antennas

Diversity is a technique that improves reception of radio waves by taking
advantage of the fact that signals that vary with time (e.g., fading) are not
the same at separated locations. In other words, the fading of a signal may
be quite different for two locations separated by as little as one
wavelength. To take advantage of this, two antennas, separated by some
distance, are used to receive the same signal. Of the two signals, the one
with the highest signal level, at any given time, is automatically sent to the
receiver. This process is only useful for reception. The electronics required
for this kind of signal processing are sometimes part of the antenna
system.

Adaptive antennas extend the concept of diversity another step further.
These antennas usually incorporate more than just two elements (i.e.,
individual antennas) in the array. An adaptive antenna can modify its
radiation pattern (within limits) in real time to ensure that the main lobe
points in the direction of greatest signal level. Alternatively (or, possibly,
simultaneously), the same technique can be used to point a null in the
direction of an unwanted, interfering signal.

6. TRANSMISSION LINES AND OTHER COMPONENTS

Transmission lines are conduits for transporting RF signals (and the
energy contained in those signals) between elements of a communications
system (fig. 1).

Transmission lines are not simply conductors that carry electrical current
like the power cord for an electrical appliance does. Transmission line
principles, derived from electromagnetic theory, must be used when the
line exceeds a few tenths of a wavelength.

On a transmission line, not only do currents flow within and on the
surfaces of the conductors, but traveling electromagnetic fields are also
"guided" by the conductors. Therefore, the geometry of the transmission
line is fundamental to its electrical characteristics.

6.1 Transmission Line Types

Figure 28 shows the arrangement of conductors for several common types
of transmission lines. The open, two-wire line, shown in figure 28(a), is
easy to construct and its characteristics are readily adjusted by changing
the diameter and spacing of the wires. However, the electromagnetic field
created between and around the conductor extends far beyond the line, so
radiation losses become excessive at high frequencies. For this reason, it is
only practical for use below several hundred megahertz. This line is also
called "ribbon," "parallel," or "twin-lead" cable and was used extensively,
from the 1950s to the 1970s, to connect a home television set to its
antenna.

The most common transmission line in use today is coaxial cable, which
gets its name from the coaxial arrangement of its conductors, as shown in
figure 28(b). The center conductor may be held in position with
periodically-spaced dielectric (insulating) beads or with a continuous,
solid dielectric that fills the space between the conductors. The outer
conductor can be either solid or braided; the latter making a more flexible
cable. The coaxial cable is effectively self-shielded and has no external
fields. For this reason, its losses are low and it is useful for frequencies as
high as 3 GHz. Since the outer conductor of a coaxial line is usually
grounded, it is not a balanced line. In other words, both conductors are not
equally isolated from ground as is the case with the open, two-wire line.
This distinction is an important factor at the connection to the antenna. A
balun (see sec. 6.2) may be needed at that point to "match" the unbalanced
line to an antenna.

A very wide variety of coaxial cable (often called "coax") is available to
meet different requirements. Generally available in only two values of
characteristic impedance--50 ohm and 75 ohm--coaxial cable is made in
many sizes of various materials.

Figure 29 shows two common coax types. The cable shown in figure 29(a)
has a semi-rigid, helical outer conductor, so it is less flexible. This cable is
commonly used for base-station applications. The cable shown in figure
29(b) has a braided outer conductor. This type of cable is commonly used
in mobile applications and for less-demanding base-station applications.

Coaxial cable is described by nomenclature derived from military
standards [15]. The "JAN type" or "RG-" number assigned to a coaxial
cable defines specific values for a wide range of physical and electrical
characteristics. Selected characteristics for a few of the several hundred
types of cables are given in table 1. Not shown in this table are other
physical and electrical characteristics such as weight, tensile strength,
materials used for the dielectric and sheath, maximum voltage, and
attenuation.

There are many other transmission line geometries. The shielded pair,
shown in figure 28(c), is not uncommon. It offers the fully shielded
characteristic of a coaxial line and the balanced nature of a two-wire,
parallel line.

6.2 Baluns

Since the outer conductor of a coaxial line is grounded, it is not a balanced
line. In other words, the voltage potential difference between the inner
conductor and ground is different from the voltage potential difference
between the outer conductor and ground. This distinction is an important
factor at the connection to the antenna. Antennas such as dipoles are
balanced. A balun will be needed at the antenna terminals to connect the
unbalanced coaxial transmission line to the antenna.

A balun is a device for transforming a load on an unbalanced transmission
line (coaxial) or system to a balanced line or system. The name balun is a
contraction of the terms balanced and unbalanced.

6.3 Duplexers

In order for transmitters and receivers to share a single antenna, a duplexer
must be used. The duplexer acts as two parallel, frequency selective filters,
directing transmitted signals to the antenna, preventing those signals from
reaching and overloading collocated receivers, and routing all of the
received signals from the antenna to the receivers. Duplexers used in the
VHF and UHF bands are typically constructed from mechanically tunable,
highly selective cavity filters, as depicted in figure 30.

6.4 Combiners

In order to combine more than one transmitter onto one antenna system,
combiners are used. Two typical configurations, shown in figure 31, use
either hybrid combiners (commonly called hybrids), or mechanically
tuned, highly selective cavity combiners that use cavity filters quite similar
to those used in duplexers. Depending on a number of factors, such as how
close in frequency the transmitters are to one another and how many
transmitters must be combined, one approach will offer superior
performance over the others. Vendors and manufacturers should be able to
explain why they selected one combining method as opposed to the other
for their design.

6.5 Intermodulation Suppression

Cavities and hybrids will likely not provide sufficient attenuation to RF
signals (from other collocated transmitters) that could cause
intermodulation interference. Isolators are used to provide additional
protection against intermodulation interference. These devices provide
very low attenuation to signals passing through them in one direction,
while providing a high degree of attenuation to signals passing through
them in the other direction. They are the principal components of
intermodulation suppression devices (fig. 32).

6.6 Multicouplers

Just as multiple transmitters can share an antenna, so can multiple
receivers. Inbound RF signals are "split" and routed to the appropriate
receivers. Because RF splitters attenuate the signals passed through them,
a preamplifier precedes the splitter. Its amplifier gain is carefully adjusted
to compensate for the splitter loss, but is not set too high, as the resultant
intermodulation interference signals would be detrimental to proper
receiver performance. These preamplifiers and signal splitters are
combined into a single assembly known as a multicoupler (fig. 33).

7. RADIO WAVE PROPAGATION

The propagation of radio waves through space (and the atmosphere) is the
essential phenomenon exploited by a radio communications system. As
described earlier (sec. 3), this phenomenon has been studied extensively
using theoretical and empirical methods. The simplest mode of
propagation occurs between two point-sources in free space--the ideal
situation.

Radio-wave propagation, in realistic situations, is affected by reflections
from the earth, scattering by particles, diffraction over hills, and bending
due to atmospheric refractivity. The study of propagation has led to
models that can be used to predict the field strength (and/or power density)
expected at a specific receiver location of a radio wave radiated from a 
distant transmitter location.

7.1 Transmission Loss and the Power Budget

The concept of transmission loss is used to quantify the effects of radio
wave propagation in the analysis and engineering of radio communications
systems. It is defined as the ratio of power delivered to the terminals of the
transmitter antenna to the power available at the terminals of the receiver
antenna. The transmitter and receiver antenna gains are implicitly included
in this definition. In practice, this is not useful, so the concept of basic
transmission loss is used. It does not include the antenna gains. Basic
transmission loss Lb is defined as the ratio of the power delivered to a
lossless isotropic antenna at the transmit location to the power available at
the terminals of a lossless isotropic antenna at the receive location.

With this definition of basic transmission loss, a power budget can be
developed. This is shown graphically in figure 34, which shows the effects
of the major elements in the radio link.

It begins with the power output of the transmitter Pt in decibels relative to
some reference (if referenced to 1 W, this is expressed as dBW). The loss,
in decibels, due to the transmitter transmission line Lt is subtracted from
Pt . Then the transmitter antenna gain Gt is added. The basic transmission
loss Lb is subtracted. Then at the receive end of the link, the receive gain
Gr is added; and the receiver transmission line loss Lr is subtracted to
arrive at the power delivered to the receiver input Pr .

Expressed as an equation, the power budget is [Equation 11].

The received power is in the same units as those used to express the
transmitted power.

For example, a transmit power of 25 W, expressed in decibels, would be
10 log10 (25) or 14 dBW. The result is that the power available at the
receiver input Pr is expressed in the same units (dBW in this example).

7.2 Free-Space Basic Transmission Loss

The transmission loss between two lossless isotropic antennas in free
space 4 is a hypothetical, but very useful, propagation model. It can be
used as a "first estimate" in radio link design or a "best case" value for
transmission loss over any real, terrestrial path.

The free-space basic transmission loss Lbf is very easy to calculate. Since
the transmit antenna is considered to be a lossless isotrope (and the
transmission line is considered to be lossless as well), all of the transmitter
power is radiated equally in all directions. At a distance d selected to be
much greater than the wavelength 8, the radiated power density (expressed
in watts per square meter) is simply the transmitter power divided by the
area of a sphere with radius d, as follows: [Equation 12].

Now, looking at the receive side of the link, the signal power output Pr in
watts, of a lossless, isotropic receive antenna can be computed using
equation 9. It is the product of that antenna's effective area Ae times the
power density p of the incident wave as follows, [Equation 13].

Using equation 10, the effective area, in square meters, of a lossless
isotropic antenna (g = 1) is [Equation 14].

Substituting equations 12 and 14 into equation 13, the result is [Equation
15].

Rearranging this equation to form the definition of Lbf as the ratio of Pt to
Pr (see sec. 7.1), [Equation 16].

In decibels, this equation becomes, [Equation 17].

By converting wavelength to frequency using equation 1 and taking the
logarithm of the terms in parentheses, a very practical version of this
equation results in [Equation 18], where d is expressed in kilometers and f
is expressed in megahertz.

This last equation is the propagation model for free-space, basic
transmission loss. It predicts a value of basic transmission loss under a set
of assumptions (i.e., lossless isotropic antennas located in free space). As
an example of how to use this model and the power-budget equation,
consider a radio link 10 km long operating at 400 MHz. For this link,
assume that the transmit antenna has a gain of 10 dB, the receive antenna
has a gain of 3 dB, and that both transmit and receive transmission line
losses are 1 dB. The transmitter power is assumed to be 20 W (13 dBW).

The basic free-space transmission loss is first computed as [Equation 19].

Then, using the power budget equation 11, we find that [Equation 20].

This result shows a receive power of about one one-hundredth of a
microwatt, or about nine orders of magnitude less than the transmit power.
At this level, it is a strong receive signal. An actual radio link, over varied
terrain, may have a larger measured transmission loss by 5 dB to 20 dB.
The transmission loss over a realistic link will also vary with time.

7.3 Terrestrial Propagation

A variety of natural and man-made objects and phenomena will affect the
radio signal as it propagates from the transmitter to the receiver. Some of
these effects are described in the following subsections. Although it is
important for the reader to be familiar with these concepts, it is
unnecessary for the reader to determine the exact extent that each effect
affects the antenna system. Several computer models are available that
provide relatively accurate propagation predictions. Such models are
described in section 7.4.

7.3.1 Effects of Earth

The Earth acts as a reflecting surface for those waves that radiate from an
antenna at angles lower than the horizon. Waves that strike the Earth are
reflected along the same direction of travel, at the same angle as the angle
of incidence. The illustration in figure 35 shows how waves propagate
over both direct and reflected paths to reach a distant location.

The reflected waves combine with the direct waves with a variety of
results (i.e., they enhance or cancel each other to some degree). Some of
the factors that cause this variety are the height of the antenna, the
orientation of the antenna, the length of the antenna, and the characteristics
of the ground reflecting the wave. As part of the wave strikes the ground
and reflects forward, that part of the wave will take a slightly longer time
to arrive at the receiver. At some reflection angles, the direct wave and
reflected wave will arrive almost in phase (i.e., the amplitude of each wave
will be at its maximum at the same time). When this happens, the power of
the received wave is approximately twice that of the direct wave. At some
angles, the reflected wave is exactly out of phase with the direct wave,
essentially nullifying the wave. This is known as cancellation. At other
angles, the resultant wave will be somewhere in between.

As radio waves strike a radio-opaque object, some of the signal will be
reflected in directions away from the receiver. Some of the signal will be
absorbed by the object.

The waves that strike the edges of the object, however, will be diffracted
into the shadow of the object. To a receiver positioned within such a
shadow, the object's edge will seem like another source. This can cause
interference with the original signal or provide signals to areas that should
not receive them. This phenomenon is shown in figure 36.

7.3.2 Coverage

The area over which the signal can be detected is called the coverage area
for the antenna. The coverage area is often displayed as contours on a two-
dimensional drawing or on a map.

A radiation pattern is not the same thing as a coverage area, although they
are related. The radiation pattern for an antenna is a gain factor, in every
direction away from the antenna, and is a function only of the antenna
design.

The coverage area for an antenna is the area over which a signal, of
predetermined strength (or greater), can be received. The coverage area is
a function of the transmit power, antenna gain, radiation pattern, noise,
and propagation factors related to the environment.

7.3.3 Noise and Interference

All things emit some radiation at all frequencies. In most cases, for most
objects, the level of this continuous radiation, at any given frequency, is
small and of little concern. This radiation is called noise. The most
common sources of noise for a radio receiver are:

o Atmospheric and galactic noise.
o Noise from the first amplifier in the receiver.
o Man-made noise (motors, flourescent lights, etc.).

When a radio receiver is far enough away from a transmitter (or in an
antenna pattern null where reception is difficult), the strength of the
transmitted signal is low enough that ambient radiation noise from other
objects or transmissions in the area can obscure the desired signal. When
the strength of a received signal is less than the strength of the ambient
noise for that frequency, the signal is said to be "lost in the noise."

Ambient noise for a specific transmission is usually measured at the
transmission frequency.

Interference is the term for unwanted signals, generated from other
transmitters, that interfere with clear reception of the intended signal.
Interference is not technically included under the definition of noise,
although slang usage of the term "noise" includes any unwanted signals.

7.3.4 Terrestrial Propagation Models

Radio-wave propagation in the terrestrial environment is an enigmatic
phenomenon whose properties are difficult to predict. This is particularly
true for LMR applications where terrain features (hills, trees, buildings,
etc.) and the ever-changing atmosphere provide scattering, reflection,
refraction, and diffraction obstacles with dimensions of the same order of
magnitude as the wavelengths.

Some models are general and some are more specialized. An example of
the former is a model that would predict radio coverage areas in "generic"
urban areas or "generic" rural areas, without regard to specific terrain
profiles. One of these generalized models is the Okumura-Hata model
[16]. Models such as the Okumura-Hata model are based on extensive
collections of empirical measurements.

Other more sophisticated computer programs predict transmission loss,
and account for the time and location variability of that loss, over defined
terrain profiles. These terrain profiles are compiled from terrain-elevation
data tabulated by agencies such as the Defense Mapping Agency and the
U.S. Geological Survey. One such computer program, written and
maintained by the U.S. Department of Commerce's Institute for
Telecommunication Sciences (ITS), is the Communication Systems
Performance Model (CSPM) [17]. This program is based on the ITS
Irregular Terrain Model [18].

Usually, manufacturers and vendors of radio and antenna systems and
components have computer programs similar to CSPM to assist customers
in defining radio-coverage areas. Private-industry radio-engineering
consultants also have computer programs like CSPM to perform radio-
coverage analysis for customers. Alternatively, agencies can access CSPM
on a fee-reimbursable basis through the ITS Internet site.[5]

7.4 Co-Site Analysis

Intermodulation (IM) interference (i.e., "intermod") and receiver
desensitization are detrimental to the performance of co-sited repeaters and
base stations. There are several different ways intermod interference can
be generated. One way is when sufficiently large power from a transmitter
enters into the final output power stage of another transmitter. This may
occur when, for example, the transmitters are connected to a combiner
junction and the combining cavity/isolators of the affected transmitter do
not provide sufficient rejection between transmitters.

Intermodulation may also arise when several transmitting antennas are
located in very close proximity to each other, such as multiple omni-
directional antennas on a building rooftop. In these situations, mixing
of signals may occur between the offending transmitter(s) and the desired
signal, thereby generating new signals in the victim transmitter, at
frequencies determined by the intermod products. These intermod signals
will be emitted by the victim transmitter.

Other ways that intermod can occur are by the mixing of signals from two
or more transmitters in the front-end of a receiver. Off-frequency signals
strong enough to overcome the suppression of bandpass cavities and
preselector filters may saturate the nonlinear first or second intermediate
frequency (IF) mixers of the victim receiver, creating intermod products
which adversely affect receiver performance.

External intermod can also be created in elements such as corroded
antenna guy wires, anchor rods, and even chain link fences. As strong
signals impinge upon these items, the corroded objects act as diodes and
detect the signals, mix them, and passively reradiate the energy at the
intermod product frequencies.

8. ANTENNA SYSTEM REQUIREMENTS AND DESIGN

The selection of a particular antenna system for use in a radio
communications system is one of many interrelated decisions that must be
made to meet the system-level requirements for a complete LMR system.
Other system-level decisions include the number and locations of base and
repeater stations, the antenna heights, and the transmitter powers.

Selecting the appropriate antennas for vehicular and hand-held units is a
much simpler process than selecting the appropriate antennas for fixed
stations. Although there are exceptions, the whip antenna is, essentially,
the only practical antenna design for vehicular and hand-held radios. The
most important aspect for vehicular antennas has to do with where it is
mounted on the vehicle. The best location for vehicular antennas is in the
middle of the roof. This is the highest point and presents the flattest and
most symmetric ground plane to the antenna, both important factors to
optimize communications range and performance. Vehicular obstructions,
such as the light bar on law enforcement vehicles, will, however, distort
the radiation pattern and/or alter the antenna's terminal impedance slightly.

Bumper mount installations for low-band VHF, for example, may be
selected based more on the antenna installation structural rigidity
requirements than on ground plane symmetry, radiation pattern distortion,
and other RF performance parameters.

Choices for hand-held radio antennas are, in practice, limited to short whip
antennas. These antennas are physically very short, are typically
inductively or resistively loaded (to simulate, electrically, a longer antenna 
and/or to provide a better impedance match). These features and attributes
are accomplished at the cost of decreased antenna gain.

Selecting appropriate antennas for fixed stations is more critical than
selecting the antennas for mobile and hand-held units. This is because
antennas for fixed stations must be chosen to adequately receive signals
from the least-capable mobile/hand-held units (those with low transmitter
power, low antenna gain, low antenna height, etc.).

When designing a radio system for LMR use, a systematic development
plan must be created and followed:

o Define System Requirements.
o Design System.
o Select Appropriate Components.
o Procure and Install Components.
o Perform Routine Maintenance.

Each of these steps is dependent on the previous step in the chain.

8.1 Define System Requirements

The design and deployment of an LMR communications system, or the
upgrade or expansion of an existing system, generally begins with some
knowledge of what geographic regions need to be "covered" by the radio
communications system. This is a fundamental requirement that is
determined by the nature of an agency's jurisdiction, where population and
transportation routes are located, and so on. These requirements must be
developed by the agency itself.

Several other system-level requirements can also be developed by the
agency before obtaining the services of a radio system vendor or
consultant. Larger law enforcement agencies will have a communications
department that will perform these services. The number of mobile and
handheld units, for example, is usually determined by the number of users
that must be supported.

Next, some decisions must be made regarding how many channels will be
needed. For example, one or more channels may be needed for dispatch
functions. Additional channels may be needed for mobile-to-mobile
communications. Other channels that might be needed could include:

o A local command channel for operations at an event site.
o A channel to interoperate with other agencies in mutual-aid situations.
o A channel dedicated to tactical forces.
o Other channels to support the operational and administrative activities of
the agency.

Other aspects that may affect system design include:

o The typical length of messages for the various channels and talk groups.
o Maximum level of activity for the dispatch channel.
o The nature of operations that use the local command channel.
o Geographic coverage requirements.
o The specific level of performance (i.e., speech intelligibility and/or data-
transmission throughput) required.

Having a working knowledge of these system-level requirements will help
ensure efficiency in the system design process that follows, and will help
ensure the reliable performance of the system when it is fully deployed.

Begin by developing some initial system requirements. Consider, for
example, the following:

o At what frequencies will you be transmitting? Lower frequencies require
larger antennas. Higher frequencies have more limited range. A wide
range of frequencies will require multiple antennas, multiband antennas, or
broadband antennas.
o What is the maximum distance over which your users must
communicate? Systems to support distant itinerant users will require a
combination of higher-power transmitters and antennas with greater gain.
o What is your coverage area? Required antenna radiation patterns will be
dictated by the location of the fixed site relative to the required coverage
area.
o Are there other nearby radio systems operating in neighboring frequency
bands or on the same or adjacent frequency channels? Your system might
cause interference to these other users, or conversely, these systems might
cause interference to your new system.
o What physical limits are there on your design? Is there sufficient
property to construct a site, or is there sufficient space on shared antenna
towers for your system? Will building code and zoning ordinances impact
the construction of your system?
o What about standby/back-up systems at this site or at another antenna
site?
o What level of speech intelligibility is required? How much noise and
distortion can be tolerated? For data transmissions, what data-throughput
rates are required (how large is the data file and how much time is
available for transmission)?

8.2 Design System

The decisions that must be made during the design of a new system or the 
expansion of an existing system include:

o The number of base and repeater stations and their locations.
o Some initial choices of antennas and antenna heights for those fixed
sites.

The design process will require the identification of:

o The availability of channels in the various LMR frequency bands.
o Potential/alternative base and repeater station locations.
o The potential for sharing fixed-station infrastructure among multiple
agencies.
o The estimated cost of the system.

8.3 Select Appropriate Components

Once the system performance and system design requirements have been
ascertained, the antenna characteristics that must be considered are the
antenna system gain and radiation pattern. The process of identifying the
needed gain and pattern is usually an iterative one. The process might
proceed as follows:

First, coverage predictions are made. Values for transmitter power,
operating frequency, antenna system losses, antenna gain, antenna height,
antenna pattern, and minimum acceptable received signal strength for
some specified level of speech intelligibility are required in order to make
these coverage predictions.

Intermodulation interference analysis must also be considered during the
design phase, particularly where multiple transmitters, receivers, and/or
repeaters are collocated. Antenna system manufacturers and vendors can
assist agencies in predicting the likelihood of IM interference or receiver
desensitization, either caused by or inflicted upon the proposed new
system. To mitigate IM interference at repeater and base station antenna
sites, their designs will include isolators, cavity filters, duplexers,
combiners, and multicouplers. The number of collocated systems and the
frequency separations between them also influences the choice of
combiner and duplexer components.

System design and component selection continue by interactively trying
different antenna gains, patterns, and heights at each potential site location
until the desired coverage is attained. Then, antenna system installation at
each fixed site needs to be considered. Points to consider include:

o Antenna tower height--Will the proposed antenna tower conform to local
building codes and zoning ordinances?
o Environmental considerations--Will the antenna system and supporting
structure survive expected wind loading, ice loading, and other anticipated
environmental performance factors? EIA/TIA Standard 329-B [19],
EIA/TIA Standard 329-B(1) [20], NIJ Standard-0204.02 [21], and NIJ
Standard-0205.02 [22] all provide guidance regarding the minimum
environmental, as well as RF, performance criteria required of all antennas
used in the law enforcement and corrections arenas.
o Security--Is the site secure against unauthorized intrusions, yet
accessible to maintenance personnel?
o Accessibility--Could inclement weather (i.e., deep snow drifts, washed-
out dirt access roads) prevent maintenance personnel from reaching the
site?
o Co-site analysis and IM interference--Will retrofitting new systems into
existing infrastructures introduce adjacent-channel interference, co-
channel interference, IM interference, or receiver desensitization upon
other existing systems or upon the new system? Will other nearby users
and systems detrimentally affect the performance of the new system
because of these problems? How will the vendor mitigate predicted
interference problems? Does the vendor have a plan to mitigate unforeseen
interference problems?
o Power source--Is commercial electrical power available? Will
solar/battery power be required as primary/backup power?
o Wireline/wireless link--Is telephone service available? Is fiber optics
service available? Will microwave radio be required?

9. INSTALLATION, MAINTENANCE, AND SAFETY

9.1 Vehicular Antenna Systems

The procurement and installation of vehicular antenna systems are
relatively straightforward. If the design requirements have been well
thought out and adequately described in the procurement documents, a
competitive procurement will deliver an acceptable product.

When installing a vehicular antenna system, care must be given to routing
the coaxial cable between the radio and antenna. The cable should not be
exposed to the elements (wind, road salt and sand, rain, intense sunlight,
extreme heat, etc.) nor should it be in a location where it could be severed
or pinched (by opening and closing vehicle doors, for example) or where a
vehicle's occupants might become entangled with it. One preferred routing
for a roof-mounted antenna might be between the roof and interior
headliner of the vehicle, down through a windshield pillar, and behind the
dashboard panel to the radio. Other equipment, such as duplexers (to
combine multiple radios operating in different frequency bands onto one
multiband antenna), should likewise be installed in locations not readily
accessible to vehicle occupants. For example, they should be installed
under seats or in trunks where they are "out of the way," yet reasonably
accessible to maintenance personnel.

RF cable connections should be torqued to the proper force recommended
for the particular connectors used, and the outer grounded conductor of the
antenna base mount must be RF-bonded to the metal of the roof or trunk
deck. Trunk deck installations also require excellent RF bonding from the
trunk lid to the main vehicle body. One way to accomplish this is by using
a short length of low-impedance copper grounding strap affixed to bare
areas of (interior) sheet metal on the underside of the trunk deck and the
main vehicle body, using noncorrosive bolts, star washers, and lock nuts.
A poor ground connection will detrimentally affect antenna operation,
resulting in erratic or unacceptable performance.

9.2 Fixed-Site Antenna Systems

Most new installations of fixed-repeater and base-station antenna systems
will likely be performed by contracted installers or equipment suppliers. In
many cases, retrofitting new or upgraded components into existing
facilities will similarly be performed by contracted installers or equipment
suppliers. Procuring agencies should, nevertheless, ensure that the installer
observes sound installation practices. As with vehicular grounding, proper
ground protection of fixed station antenna facilities is important.

9.2.1 Fixed-Site Antenna System Grounding and Bonding Practices

An effective grounding system is necessary for every antenna tower. In
addition to the protection a grounding system offers from lightning strikes,
grounding also:

o Reduces the hazards of electrical shock resulting from ground/neutral
power faults.
o Protects wiring and circuitry by limiting extraneous over-voltages.
o Facilitates rapid discharge of faulted power circuits.
o Reduces noise voltages.
o Provides a path to dissipate any stray RF current present inside the
transmitter station; ungrounded RF currents can contribute to equipment
malfunction, or create interference with other receivers.

9.2.2 Fixed-Station RF Bonding

RF bonding is another important aspect of fixed-station antenna systems.
Simply connecting each element of a transmitter facility to a metal pipe
stuck in the ground is barely adequate to act as a grounding system. The
components of such a grounding system are not perfect conductors and
each will have different, finite values for resistance. The resistance and
physical design of the grounding system adds to the overall resistance and
reactance of the antenna system and transmission line, causing the system
to have different voltage potentials at different points within the system,
inducing stray currents to flow between equipment chassis. These stray
currents can affect internal circuits of the equipment and cause erratic
operation and unpredictable behavior.

A bonding system ensures that all equipment grounding points are at the
same electrical potential. A good dc and RF bonding system will use high-
quality, low-impedance copper strap or braid and attach all equipment
chassis to a low-impedance copper bus strap installed on the walls
throughout the station facilities. The copper bus leaves the station and is
attached to the Earth using a copper ground rod approximately 3 m long.
The point of egress for the facility ground should be at the same entry
point as RF cable, telephone, and power connections.

9.3 Lightning Protection

The National Fire Protection Association (NFPA) publishes a guideline
related to lightning protection [23]. This guideline details many additional
practices for protecting radio equipment from lightning strikes.

Metal antennas and towers should be connected to the building's lightning
protection system. Wires and metallic elements comprising an antenna
tower's lightning protection system should be electrically attached to the
Earth. Towers and guy wires anchored to concrete forms in the ground are
often assumed to be well grounded, but concrete is a poor electrical
conductor. Tower legs should be electrically attached to the Earth with a
copper ground stake approximately 3 m long. Lightning-ground
connecting leads connecting the tower to the ground stake should be at
least AWG #10 copper, AWG #8 aluminum, or 3/4 in copper braid.

The transmission lines must be protected by lightning arresters, protectors,
and discharge units. Arresters can be placed at both ends of the
transmission line for added protection.

9.4 During Installation

Be sure the installer knows exactly where on the vehicle or on the antenna
tower the antenna components are to be installed. Make sure they are
installed in the correct orientation and positioned correctly.

Make sure that the ends of the transmission line cable have been prepared
properly before affixing RF connectors to the transmission line. Make sure
that the RF connectors are properly installed on the transmission line.
Loose connector assemblies will result in poor ground connections
between the outer connector shell and the outer shield of the transmission
line. Make sure that the center pin is securely affixed to the center
conductor of the coaxial transmission line and that the center pin's depth,
relative to the connector shell, is maintained at the correct distance, or an
impedance mismatch or connector damage will result. Make sure that the
antenna and transmission line connectors will mate properly before
connection to other equipment is attempted. Connectors can be easily
misaligned or over- or under-torqued, resulting in degraded or erratic
overall RF performance. Make sure that the transmission line has not been
damaged in any way, such as crushed, severed, or pinched.

Check the VSWR as soon as possible after installation, and, if possible,
before the installer leaves the job site. In addition to VSWR
measurements, time-domain reflectometry (TDR), line-fault measurements
are helpful. Use a portable transmitting unit if the radio transmitter is not
yet installed. Record the VSWR values and TDR data for future reference.

Measure and record the ambient noise power levels for future reference.

If possible, the installer should conduct "over-the-air" RF power
sensitivity measurements immediately after installation, and document the
test configuration and measurement results for future reference. For
example, a portable antenna mast, 7 m to 10 m high, could be placed at a
geological survey marker that has unobstructed, clear, visual line-of-sight
to the repeater or base station-antenna tower. A portable transmitter could
serve as the signal source. A similar method could be used to measure the
power received by a portable radio service monitor (such as an IFR1500 or
Motorola R-2670) located at the survey-marker point.

Make a physical inspection of the installation. Be sure all connectors and
transmission lines are secured properly.

9.5 Perform Routine Maintenance

After a vehicular or repeater/base-station antenna system has been
installed, the system will require periodic maintenance to ensure optimum
performance.

Agencies should practice three tiers of maintenance. The first is performed
by the radio operators and consists of simple, "common-sense" inspection
of the equipment. The second level of maintenance is performed by site
technicians and requires the use of land mobile radio test equipment. The 
third level of maintenance is performed by factory-authorized technicians.

9.5.1 Local Inspection

Radio operators themselves can perform a wide variety of simple aural and
visual inspections of their radio equipment and antennas. For instance:

o Isolated problems noted with reception or transmission in the field.
Direct comparison between two radios ("I cannot hear the base station
when I am in this location." "Oh, really? I can hear the base station okay.")
gives an excellent indication of a problem with a subscriber unit.
o Loose or missing connectors--Over time, temperature variations, shock,
vibration, exposure to the elements, and handling can cause connections
and connector flanges to become loose or missing altogether. Rubber O-
ring grommet seals may deteriorate, allowing moisture to penetrate into
connectors or the transmission line, altering their performance.
o Cracked or broken whip antenna base-loading coils--Cracks in the
plastic housing of vehicular antenna base-loading coils can permit
moisture and corrosive materials to penetrate into the loading coil, altering
the antenna's electrical performance. Weathering of rubber grommet O-
ring seals (where the loading coil is affixed to the roof or trunk deck of the
vehicle) may similarly permit moisture and corrosive materials to
penetrate into the connector between the loading coil and its attachment to
the coaxial cable connector, altering the antenna's electrical performance.

9.5.2 Site Technician

The site technician will often have an array of RF test equipment at his or
her disposal. For example, a communications service monitor can
determine whether a radio is transmitting on the proper frequency, at the
proper power level, with the proper frequency bandwidth. A VSWR meter
gives an indication of how well RF energy is coupled from the radio
system into the antenna. A time-domain reflectometer can determine
where faults or other discontinuities exist along the length of a
transmission line. Portable field strength meters can give an indication as
to whether the antenna radiates electromagnetic energy as expected.

Vehicular antenna systems should be periodically checked according to an
established maintenance schedule, typically concurrent with the
maintenance schedule of the mobile radio (perhaps once or twice per
year). Performance values such as VSWR should be recorded, compared
to the performance values measured just after installation, and tracked
over time in order to assist in keeping the antenna system functioning at an
optimal performance level.

The above statements apply equally to fixed-site repeaters and base
stations. Unfortunately, whereas problems with mobile and portable radio
systems can be readily identified by direct performance comparison at the
operator level, such is not necessarily the case for fixed sites. Because
performance degradation at the fixed site affects all subscriber units
equally, slowly occurring degradation in performance, caused by corrosion
effects, weathering, etc., may go unnoticed for years until catastrophic
failure finally occurs. Therefore, regularly scheduled site visits to conduct
maintenance performance inspections must be a part of the site
technician's routine. Recording a time history of the RF performance, and
comparing it to the RF performance measured immediately following the
repeater or base-station installation, will be an invaluable maintenance aid.

Remote automated in-line diagnostic test equipment can also provide
indications of needed maintenance. Several vendors of antenna system
equipment, such as Bird Electronic Corp., Decibel Products, Sinclair
Technologies, Telewave, and many others, manufacture remote in-line
diagnostic measurement equipment. This equipment can report the health
and status of a fixed repeater/base-station site's RF performance, up to the
point where the radio wave is launched into space. For example,
conditions of low transmitter power and/or excessive VSWR, which might
result from detuned combiner cavities or duplexers, failing radios,
weathering of components, etc., can be sensed by the automated diagnostic
equipment. Alarm conditions can automatically be reported to a computer
by preconfigured telephone dial-up to the centralized maintenance facility.

9.5.3 Factory-Authorized Technicians

If an antenna system or radio problem is too complicated for the local site
technician to repair, the manufacturer should be contacted. Many large
public safety organizations have service contracts with factory-authorized
repair facilities to maintain, repair, and replace equipment. Even without a
contract, contacting the manufacturer about specific problems is advised if
the problems are beyond the abilities of the agency to repair.

9.5.4 Antenna Tower Safety

Working on antenna towers can be dangerous and potentially fatal. Serious
personal injury and equipment damage can result from personnel falling,
improperly installed equipment, and RF radiation exposure.

When maintenance personnel must work on antenna towers, safety
equipment should be selected, used, and cared for as if their lives depend
on it--because they do! A list of safety equipment should include:

o Safety belt.
o Safety glasses.
o Work boots with firm, nonslip soles and well-defined heels.
o Hard hat.

It is recommended that antenna tower installations have a personnel fall-
arrester system. These systems permit maintenance personnel to attach
their safety belts at ground level, and remain attached during tower ascent
and descent, as well as while working at height. Antenna tower
manufacturers, such as Rohn Tower, offer fall arrester systems, which
comply with OSHA regulations.

In addition to personnel safety while ascending, working on, or
descending antenna towers, maintenance personnel must be cognizant to
the danger of RF electrical burns arising from direct contact with
energized antenna elements, and to exposure to high levels of RF
radiation. Industry standards have been developed that specify the levels
of electromagnetic exposure to which personnel can be "safely" exposed
[24]. Personnel working on transmitting antenna towers must ensure that
all antennas that they are working on or near are disconnected from their
associated transmitters, and that the transmitters are routed into dummy
loads (to prevent damage to the transmitter final amplifier in the event of
inadvertent transmissions), or that some sort of fail-safe mechanism for
disconnecting power to those transmitters has been engaged.

Lastly, maintenance personnel should be alert to weather conditions. It is
ill-advised to work on an antenna tower during an electrical storm or in
high winds. Remember--safety first!

9.5.5 Vehicular Antenna Systems

The same precautions regarding RF exposure on antenna towers should be
observed when operating or performing maintenance on vehicular antenna
systems. For example, personnel should not stand outside and next to their
vehicle (where they do not have the benefit of RF shielding afforded by
the vehicle's roof that they would have if they were inside the passenger
compartment of the vehicle) while operating a 45 W (or greater) mobile
radio transmitter, nor should they touch the antenna when transmitting.

9.5.6 The Importance of Maintaining Your Radio System

It must be stressed that regularly scheduled, periodic testing and
preventive maintenance of the entire radio system is of paramount
importance. Motor vehicles used by law enforcement and corrections, fire,
emergency medical services, and other public-safety agencies are
maintained by complying with strict maintenance schedules. Weapons are
maintained by following regular preventive maintenance schedules of
cleaning, lubrication, and inspection for worn or damaged parts. The same
must hold true for all parts of a radio communications system--the lives of
fire, emergency medical services, and law enforcement and corrections
personnel may depend on it.

10. ANTENNA SYSTEM RESOURCES

This section describes some of the resources available to agencies to
research information about and identify pertinent products, antenna
systems and related components. There are many qualified and
conscientious manufacturers and products available; only a few are cited
herein in order to provide a sampling of what is available.

10.1 Internet Resources

As the World Wide Web (WWW or Web) has grown in recent years,
many antenna manufacturers and suppliers have created Web pages
devoted to their products.

Most Web-page authors will register their page with one or more of the
major "search engines" on the Web. A search engine is a program that will
use a search phrase provided by a user (usually a single word or simple
phrase) to search hundreds of thousands of Web pages looking for
occurrences of the phrase. The engine will then return a list of those Web
pages that most closely match the search criteria (i.e., those that contain
the search phrase). A few search engines, such as Metacrawler
(http://www.metacrawler.com) conduct searches by simultaneously
querying more than one search engine.

10.2 Periodicals

Several periodicals are written for the land mobile radio industry. Most are
free to "qualified" subscribers. These periodicals contain articles of
technical interest related to land mobile radio, and advertisements for land
mobile radio systems, components, and services. These periodicals
publish, as special issues, buyers' guides on an annual or biannual basis.
One such periodical that does this is Mobile Radio Technology (URL:
http://mrtmag.com, telephone 1-913-341-1300). Another is APCO's
monthly APCO Bulletin (URL: http://www.apcointl.org/bulletin/,
telephone 1-888-APCO-911). Public libraries may maintain subscriptions
to this and other LMR-related periodicals, or they may be able to obtain
issues under interlibrary loan agreements.

10.3 Manufacturers' and Vendors' Catalogs

Most antenna manufacturers and vendors distribute free catalogs that
describe antennas, transmission lines, and related components that they
offer for sale. Most catalogs provide useful technical information such as
antenna patterns, operating frequencies, physical dimensions, and costs.
Frequently, these catalogs will provide basic explanations about various
aspects of antenna systems, such as collocated transmitter-combining
techniques, intermodulation interference, transmission line theory, etc.

Many manufacturers have more than one product line, e.g., antennas and
duplexers. Information about manufacturers' offerings can be identified
and researched via the Internet or through the periodicals and buyers'
guides discussed in the previous two subsections.

11. PROFESSIONAL AND STANDARDS ORGANIZATIONS

The following subsections describe some of the various organizations that
create and maintain antenna and radio standards or are otherwise of
interest to the law enforcement or corrections officials interested in
learning about antennas.

Most of the descriptive text in these subsections is taken directly from the
Web site listed for each subsection and edited slightly for format or clarity.

JUSTNET
1-800-248-2742
http://www.nlectc.org

The Justice Technology Information Network (JUSTNET) was created in
1995 at the National Law Enforcement and Corrections Technology
Center (NLECTC) National Center in Rockville, MD, and serves as a
gateway to the products and services of the NLECTC System as well as
other technology information and services of interest to the law
enforcement and corrections communities. JUSTNET is a central element
of the National Institute of Justice Office of Science and Technology's
information collection and dissemination mission. Through JUSTNET,
users have access to interactive bulletin boards on a variety of topics, a
comprehensive database of law enforcement products and technologies,
and NLECTC publications.

NASTD
1-606-244-8182
http://www.nastd.org

The National Association of State Telecommunications Directors
(NASTD) is a member-driven organization whose purpose is to advance
and promote the effective use of telecommunications technology and
services to improve the operation of State government.

NASTD members represent telecommunications professionals from the 50
States, the District of Columbia, the U.S. territories, and the private sector.
State members are responsible for the provision and management of State
government communications facilities and systems for State agencies and
other public entities including hospitals, prisons, colleges, and universities.
These members also play a strategic role in planning and shaping their
States' telecommunications infrastructures and policies. Corporate
members represent companies that provide telecommunications
technology services and equipment to State government.

NASTD was founded in 1978 and has been an affiliate of the Council of
State Governments (CSG) since 1980, with its headquarters in Lexington,
KY.

APCO
1-888-APCO-911
http://www.apcointl.org

The Association of Public-Safety Communications Officials International,
Inc. (APCO International), is the world's oldest and largest not-for-profit
professional organization dedicated to the enhancement of public safety
communications.

With more than 13 000 members around the world, APCO International
exists to serve the people who manage, operate, maintain, and supply the
communications systems used to safeguard the lives and property of
citizens everywhere.

APCO members come from many public safety organizations, including:

o Law Enforcement Agencies.
o Emergency Medical Services.
o Fire Departments.
o Public Safety Departments.
o Colleges and Universities.
o Military Units.
o Manufacturers.

APCO's mission is to:

o Foster the development and progress of the art of public safety
communications by means of research, planning, training, and education.
o Promote cooperation between towns, cities, counties, States, and Federal
public safety agencies in the area of communications.
o Represent its members before communications regulatory agencies and
policy making bodies as may be appropriate.
o Through its efforts strive toward the end that the safety of human life,
the protection of property, and the civic welfare are benefitted to the
utmost degree.
o Aid and assist in the rapid and accurate collection, exchange, and
dissemination of information relating to emergencies and other vital public
safety functions.

ANSI
1-212-642-4900
http://web.ansi.org

The American National Standards Institute (ANSI) has served in its
capacity as administrator and coordinator of the United States private
sector voluntary standardization system for 80 years.

Founded in 1918 by five engineering societies and three government
agencies, the Institute remains a private, nonprofit membership
organization supported by a diverse constituency of private and public
sector organizations.

Throughout its history, the ANSI Federation has maintained as its primary
goal the enhancement of global competitiveness of U.S. business and the
American quality of life by promoting and facilitating voluntary consensus
standards and conformity assessment systems and promoting their
integrity. The Institute represents the interests of its nearly 1 000 company,
organization, government agency, institutional, and international members
through its office in New York City, and its headquarters in Washington,
DC.

ANSI does not develop American National Standards (ANSs); rather, it
facilitates development by establishing consensus among qualified groups.
The Institute ensures that its guiding principles--consensus, due process,
and openness--are followed by the more than 175 distinct entities currently
accredited under one of the Federation's three methods of accreditation
(organization, committee, or canvass). In 1999 alone, the number of
American National Standards increased by nearly 5.5 % to a new total of
14 650. ANSI-accredited developers are committed to supporting the
development of national and, in many cases, international standards,
addressing the critical trends of technological innovation, marketplace
globalization, and regulatory reform.

IEEE
1-800-678-IEEE
http://www.ieee.org

The Institute of Electrical and Electronics Engineers (IEEE) is the world's
largest technical professional society. Founded in 1884 by a handful of
practitioners of the new electrical engineering discipline, today's Institute
is comprised of more than 320 000 members who conduct and participate
in its activities in 152 countries. The men and women of the IEEE are the
technical and scientific professionals making the revolutionary
engineering advances that are reshaping our world today.

The technical objectives of the IEEE focus on advancing the theory and
practice of electrical, electronics, and computer engineering, and computer
science. To realize these objectives, the IEEE sponsors technical
conferences, symposia and local meetings worldwide: publishes nearly
25 % of the world's technical papers in electrical, electronics, and
computer engineering; provides educational programs to keep its
members' knowledge and expertise state-of-the-art. The purpose of all
these activities is twofold: (1) to enhance the quality of life for all peoples
through improved public awareness of the influences and applications of
its technologies; and (2) to advance the standing of the engineering
profession and its members.

The IEEE, through its members, provides leadership in areas ranging from
aerospace, computers, and communications to biomedical technology,
electric power, and consumer electronics. For the latest research and
innovations in the many diverse fields of electrical and electronics
engineering, industry and individuals look to the IEEE.

EIA
1-703-907-7500
http://www.eia.org

For more than 70 years, the Electronics Industry Alliance (EIA) has been
the primary trade organization representing the U.S. high technology
community. EIA has created a host of activities to enhance the
competitiveness of the American producer including such valuable
services as technical standards development, market analysis, government
relations, trade shows, and seminar programs.

TIA
1-703-907-7700
http://www.tiaonline.org

The Telecommunications Industry Association (TIA) is a full-service
national trade organization with membership of 900 large and small
companies that provide communications and information technology
products, materials, systems, distribution services, and professional
services in the United States and around the world. The association's
member companies manufacture or supply virtually all of the products
used in the modern communications network. TIA represents the
telecommunications industry with its subsidiary, the MultiMedia
Telecommunications Association (MMTA), in conjunction with the
Electronic Industries Alliance (EIA).

The TIA seeks to provide its members a forum for the examination of
industry issues and information. The association serves as the voice of the
manufacturers and suppliers of communications and information
technology products on public policy and international issues affecting its
membership. TIA supports and strives to further the growth of our
economy, the progress of technology, and the betterment of humankind
through improved communications. In 1924, a small group of suppliers to
the independent telephone industry organized to plan an industry trade
show. Later, that group became a committee of the United States
Independent Telephone Association. In 1979, the groups split off as a
separate affiliated association, the United States Telecommunications
Suppliers Association (USTSA), and became one of the world's premier
organizers of telecom exhibitions and seminars. TIA was formed in April
1988 after a merger of USTSA and the Information and
Telecommunications Technologies Group of EIA. EIA began as the Radio
Manufacturers Association (RMA) in 1924.

TIA is a member-driven organization. Thirty-one board members are
selected from member companies to formulate policy, which is carried out
by a staff of more than 50 in the Washington, DC area. There are six issue-
oriented standing committees:

o Membership Scope and Development.
o International.
o Marketing and Trade Shows.
o Public Policy and Government Relations.
o Small Company.
o Technical.

Each committee addresses the subject areas of major concern to TIA
members. Each committee is chaired by a board member.

TIA's five product-oriented divisions are:

o User Premises Equipment.
o Network Equipment.
o Wireless Communications.
o Fiber Optics.
o Satellite Communications.

Each division is concerned with legislative and regulatory issues of
product manufacturers and prepares standards dealing with performance
testing and compatibility.

PSWN
1-800-565-PSWN
http://www.pswn.gov

Everyone living within the United States expects government entities to
respond, mitigate damage, and provide emergency assistance during
disasters. Emergency workers are trained to respond to a variety of events,
such as natural and technological disasters, terrorist actions, and criminal
activities, as well as to conduct other life-saving activities such as search
and rescue operations. To be effective before, during, and after their
response, public safety officials, throughout all levels of government, must
be able to communicate with each other. Currently, Federal, State, and
local public safety entities compete for limited radio spectrum, have
limited public safety budgets, and face challenges in keeping pace with
advances in technology. Moreover, public safety officials operate separate
tactical communications networks.

Upon these premises, PSWN's mission is to provide seamless,
coordinated, and integrated public safety communications for the safe,
effective, and efficient protection of life and property. PSWN's vision of
improved communications is shared with local, State, and Federal
agencies whose missions encompass the protection of life and property.

12. ACRONYMS

AM--Amplitude Modulation
ANSI--American National Standards Institute
APCO-Association of Public-Safety Communications Officials
CSG--Council of State Governments
EIA--Electronics Industry Alliance
FCC--Federal Communications Commission
FM--Frequency Modulation
IRAC-Interdepartmental Radio Advisory Committee
IEEE--Institute of Electrical and Electronics Engineers
IF--Intermediate Frequency
IM--Intermodulation
ITS--Institute for Telecommunication Sciences
ITU--International Telecommunications Union
JAN--Joint Army-Navy
JUSTNET--Justice Technology Information Network
LMR--Land Mobile Radio
MMTA--MultiMedia Telecommunications Association
NASTD-National Association of State Telecommunications Directors
NFPA-National Fire Protection Association
NIJ-National Institute of Justice
NIST-National Institute of Standards and Technology
NLECTC-National Law Enforcement and Corrections Technology Center
NTIA--National Telecommunications and Information Administration
OLES-Office of Law Enforcement Standards
OSHA--Occupational Safety and Health Administration
PSWN--Public Safety Wireless Network
RCC--Radio Common Carrier
RF--Radio Frequency
RG-Radio Grade (very old term)
RMA--Radio Manufacturers Association
SMR-Specialized Mobile Radio
SWR-Standing Wave Ratio
TDR-Time-Domain Reflectometry
TIA-Telecommunications Industry Association
UHF--Ultra-High Frequency
USTSA-United States Telecommunications Suppliers Association
VHF--Very-High Frequency
VSWR-Voltage Standing Wave Ratio
WWW-World Wide Web

13. REFERENCES

[1] NTIA, Manual of Regulations and Procedures for Federal Frequency
Management, U.S. Government Printing Office, Superintendent of
Documents, Washington, DC.

[2] FCC, "Private Land Mobile Services," Title 47, Code of Federal
Regulations, Part 90, Oct. 1, 1997, U.S. Government Printing Office,
Superintendent of Documents, Washington, DC.

[3] FCC, "Private Radio Services Applications and Proceedings," Title 47,
Code of Federal Regulations, Part 1, Subpart F, Oct. 1, 1997, U.S.
Government Printing Office, Superintendent of Documents, Washington,
DC.

[4] Telecommunications Industry Association, "Project 25, the TIA-
Published 102-Series Documents," Jun. 1998, Arlington, VA.

[5] J.C. Maxwell, "A Dynamical Theory of the Electromagnetic Field,"
Proc. Royal Soc. (London), Vol. 13, p. 531, 1864.

[6] H. Hertz, "Uber Sehr Schnelle Electrische Schwingungen," Wied.
Ann., Vol. 31, p. 421, 1887.

[7] W.L. Weeks, Antenna Engineering, McGraw-Hill, 1968.

[8] W.F. Snyder and C.L. Bragaw, "Achievement in Radio," NBS Sp. Pub.
555, Oct. 1986.

[9] V.H. Rumsey, "Reaction Concept in Electromagnetic Theory," Phys.
Rev., Vol. 94, No. 6, pp. 1483-1491, Jun. 1954.

[10] IEEE, "Standard Dictionary of Electrical and Electronics Terms,"
ANSI/IEEE Std. 100-1996.

[11] IEEE, "Standard Definitions of Terms for Antennas," IEEE Std. 145-
1993.

[12] J.D. Kraus, Antennas, M c Graw-Hill, 1950, pp. 69, 81, and 83.

[13] W.A. Kissick and J.M. Harman, Communications Range Predictions
for Mobile Radio Systems, NIJ Rept. 201-88, Jan. 1988.

[14] E.C. Jordan, "Acoustic Models of Radio Antennas," Ohio State
Experiment Station, Bulletin 108, 1938.

[15] Dept. of Defense, "RF Transmission Lines and Fittings," MIL-
HDBK-216, Jan. 1962.

[16] M. Hata, "Empirical Formula for Propagation Loss in Land Mobile
Radio Services," IEEE Trans. on Veh. Tech., Vol. VT-29, No. 3, Aug.
1980, pp. 317-325.

[17] R.D. Jennings and S.J. Paulson, Communication System Performance
Model for VHF and Higher Frequencies, OT Rept. 77-128, Oct. 1977.

[18] G.A. Hufford, A.G. Longley, and W.A. Kissick, A Guide to the Use
of the ITS Irregular Terrain Model in the Area Prediction Mode, NTIA
Rept. 82-100, Apr. 1982.

[19] Telecommunications Industry Association, "Minimum Standards for
Communication Antennas, Part I - Base Station Antennas," EIA/TIA-329-
B, Sep. 1989.

[20] National Institute of Justice, Mobile Antennas, NIJ Standard-0205.02,
Oct. 1997.

[21] National Institute of Justice, Fixed and Base Station Antennas, NIJ
Standard-0204.02, Jun. 1998.

[22] Telecommunications Industry Association, "Minimum Standards for
Communication Antennas, Part II - Vehicular Antennas," EIA/TIA-329-B-
1, Sep. 1989.

[23] National Fire Protection Association, Lightning Protection Code,
NFPA No. 78-1983, Quincy, MA.

[24] IEEE, "IEEE Standard Safety Levels with Respect to Human
Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz,"
ANSI/IEEE Std. C95.1-1991, revised 1997.

14. ACKNOWLEDGMENTS

Numerous manufacturers have kindly granted permission for photographs
and measured data for certain of their products to be used in this report. As
stated in Footnote 1 (sec. 1), these products and data have been referenced
as typical examples or to explain more clearly the technical concepts and
principles being presented. In no case does such reference to these sources,
their products, and their data imply recommendation or endorsement by
the National Institute of Justice, or any other U.S. Government department
or agency, nor does it imply that the sources, products, or data identified
are necessarily the best available for the selected purpose.

Those manufacturers who have granted permission for photographs of
their products or plots of their measured data to be included in this report
and the World Wide Web references for the information are the following: 

o [copyright] Andrew Corporation
Orlando Park, IL
Figure 29(a)--coaxial cable commonly used for LMR base-station
applications.
Accessed at 
http://www.andrew.com/products/basestation/heliax/lowd.asp on
03/17/99.

o [copyright] Antenna Specialists
Cleveland, OH
Figure 6(b)--a typical monopole antenna for mobile applications.
Accessed at
http://www.decibelproducts.com/marketing/catalog/asp-701/pasp-701.
html on 03/12/99.

Figure 29(b)--coaxial cable commonly used for LMR mobile applications.
Accessed at
http://allentele.com/antenna/lm_cat/lmrpg35.html on 03/09/99.

o [copyright] Bluewave Antenna Systems Ltd.
Calgary, Alberta, Canada
Figure 9--a typical folded-dipole antenna.
Accessed at
http://www.bluewave.ab.ca/bw421e.html on 03/16/99.

o [copyright] Decibel Products
Dallas, TX
Figure 6(a)--a typical monopole antenna for base-station applications.
Accessed at
http://www.allentele.com/antenna/lm_cat/lmrpg09.html on 03/15/99.

Figure 8--a typical monopole antenna horizontal-plane pattern, base-
station application.
Accessed at
http://www.decibelproducts.com/patterns/patterns.cfm?path=patterns/ASP
682_Series on 03/12/99. 

Figure 13--a typical corner-reflector antenna.
Accessed at
http://www.decibelproducts.com/marketing/catalog/db252/pdb252.html on
03/13/99.

Figure 14--a typical corner-reflector antenna horizontal-plane pattern.
Accessed at
http://www.decibelproducts.com/patterns/patterns.cfm?path=patterns/ASP
960_Series on 03/09/99.

Figure 16--a typical Yagi antenna.
Accessed at 
http://www.decibelproducts.com/marketing/catalog/asp-960/pasp-962.
html on 03/12/99.

Figure 17--a typical Yagi antenna horizontal-plane pattern.
Accessed at
http://www.decibelproducts.com/patterns/patterns.cfm?path=patterns/ASP
816_Series on 03/12/99.

Figure 20--a typical log-periodic antenna horizontal-plane pattern.
Accessed at
http://www.decibelproducts.com/patterns/patterns.cfm?path=patterns/ASP
2894_Series on 03/15/99.

Figure 21--a typical vertical array using folded dipoles.
Accessed at
http://www.decibelproducts.com/marketing/catalog/db404/pdb404.html on
03/12/99.

Figure 24--a typical vertical-plane radiation pattern without "tilt."
Accessed at
http://www.decibelproducts.com/patterns/patterns.cfm?path=patterns/DB4
20_Series/420C on 03/15/99.

Figure 25--a typical vertical-plane radiation pattern with 8 o "tilt."
Accessed at
http://www.decibelproducts.com/patterns/patterns.cfm?path=patterns/ASP
975_Series/ASPD975 on 03/15/99.

Figure 30--a VHF duplexer.
Accessed at
http://www.decibelproducts.com/marketing/catalog/db4060/pdb4060.html
on 03/17/99.

Figure 31(a)--a hybrid combiner.
Accessed at
http://www.decibelproducts.com/marketing/catalog/db43516/pdb4351-
2.html on 03/17/99.

Figure 31(b)--a cavity combiner.
Accessed at
http://www.decibleproducts.com/marketing/catalog/db4360/pdb4360.html
on 03/17/99.

Figure 32--an intermodulation suppression device.
Accessed at
http://www.decibelproducts.com/marketing/catalog/db47104/pdb4713ht.
html on 03/17/99.

Figure 33--a multicoupler.
Accessed at
http://www.decibelproducts.com/marketing/catalog/db8100/pdb8100.html
on 03/17/99.

o [copyright] R.N. Electronics Ltd.
Mountnessing, Essex, UK
Figure 19--a typical log-periodic antenna.
Accessed at http://www.rfdesign.co.uk/mlpa30121.htm on 03/18/99.

Endnotes:

1. Certain commercial companies and their products and information are
identified in this report to specify adequately the technical concepts and
principles being presented. In no case does such identification imply
recommendation or endorsement by the National Institute of Justice, or
any other U.S. Government department or agency, nor does it imply that
the sources, products, and information identified are necessarily the best
available for the purpose.

2. Numerals in square brackets refer to references identified in section 13.

3. Radiation intensity is defined as the power density in terms of power
per unit solid angle.

4. Free space is a theoretical concept of space devoid of all matter. In
practice, free space implies remoteness from material objects that could
influence the propagation of electromagnetic waves.

5. http://flattop.its.bldrdoc.gov/tas.html.
For additional information, contact the Institute for
Telecommunication Sciences at 325 Broadway (ITS.E), Boulder,
Colorado 80305-3328, Telephone 303-497-5301.

-------------------------------

U.S. Department of Justice
Office of Justice Programs
810 Seventh Street N.W.
Washington, DC 20531

Office of Justice Programs
World Wide Web Site:
http://www.ojp.usdoj.gov

National Institute of Justice
World Wide Web Site:
http://www.ojp.usdoj.gov/nij

-------------------------------

NCJ 185030