Introduction to RF Propagation - Hardcover

Seybold, John S.

Hardcover

ISBN 10: 0471655961 ISBN 13: 9780471655961

Publisher: Wiley-Interscience, 2005

Synopsis

An introduction to RF propagation that spans all wireless applications

This book provides readers with a solid understanding of the concepts involved in the propagation of electromagnetic waves and of the commonly used modeling techniques. While many books cover RF propagation, most are geared to cellular telephone systems and, therefore, are limited in scope. This title is comprehensive-it treats the growing number of wireless applications that range well beyond the mobile telecommunications industry, including radar and satellite communications.

The author's straightforward, clear style makes it easy for readers to gain the necessary background in electromagnetics, communication theory, and probability, so they can advance to propagation models for near-earth, indoor, and earth-space propagation. Critical topics that readers would otherwise have to search a number of resources to find are included:
* RF safety chapter provides a concise presentation of FCC recommendations, including application examples, and prepares readers to work with real-world propagating systems
* Antenna chapter provides an introduction to a wide variety of antennas and techniques for antenna analysis, including a detailed treatment of antenna polarization and axial ratio; the chapter contains a set of curves that permit readers to estimate polarization loss due to axial ratio mismatch between transmitting and receiving antennas without performing detailed calculations
* Atmospheric effects chapter provides curves of typical atmospheric loss, so that expected loss can be determined easily
* Rain attenuation chapter features a summary of how to apply the ITU and Crane rain models
* Satellite communication chapter provides the details of earth-space propagation analysis including rain attenuation, atmospheric absorption, path length determination and noise temperature determination

Examples of widely used models provide all the details and information needed to allow readers to apply the models with confidence. References, provided throughout the book, enable readers to explore particular topics in greater depth. Additionally, an accompanying Wiley ftp site provides supporting MathCad files for select figures in the book.

With its emphasis on fundamentals, detailed examples, and comprehensive coverage of models and applications, this is an excellent text for upper-level undergraduate or graduate students, or for the practicing engineer who needs to develop an understanding of propagation phenomena.

About this title

About the Author

JOHN S. SEYBOLD, PHD, is a Communication Systems Engineer at the Harris Corporation. Prior to joining Harris, he was an associate professor of electrical engineering at Florida Institute of Technology where he also served as the associate director of the Institute's Wireless Center of Excellence. During his career, Dr. Seybold has worked in radar systems, digital signal processing, and communication systems, including spread spectrum.

From the Back Cover

An introduction to RF propagation that spans all wireless applications

This book provides readers with a solid understanding of the concepts involved in the propagation of electromagnetic waves and of the commonly used modeling techniques. While many books cover RF propagation, most are geared to cellular telephone systems and, therefore, are limited in scope. This title is comprehensive it treats the growing number of wireless applications that range well beyond the mobile telecommunications industry, including radar and satellite communications.

The author's straightforward, clear style makes it easy for readers to gain the necessary background in electromagnetics, communication theory, and probability, so they can advance to propagation models for near-earth, indoor, and earth-space propagation. Critical topics that readers would otherwise have to search a number of resources to find are included:

• RF safety chapter provides a concise presentation of FCC recommendations, including application examples, and prepares readers to work with real-world propagating systems
• Antenna chapter provides an introduction to a wide variety of antennas and techniques for antenna analysis, including a detailed treatment of antenna polarization and axial ratio; the chapter contains a set of curves that permit readers to estimate polarization loss due to axial ratio mismatch between transmitting and receiving antennas without performing detailed calculations
• Atmospheric effects chapter provides curves of typical atmospheric loss, so that expected loss can be determined easily
• Rain attenuation chapter features a summary of how to apply the ITU and Crane rain models
• Satellite communication chapter provides the details of earth-space propagation analysis including rain attenuation, atmospheric absorption, path length determination and noise temperature determination

Examples of widely used models provide all the details and information needed to allow readers to apply the models with confidence. References, provided throughout the book, enable readers to explore particular topics in greater depth. Additionally, an accompanying Wiley ftp site provides supporting MathCad files for select figures in the book.

With its emphasis on fundamentals, detailed examples, and comprehensive coverage of models and applications, this is an excellent text for upper-level undergraduate or graduate students, or for the practicing engineer who needs to develop an understanding of propagation phenomena.

This book provides readers with a solid understanding of the concepts involved in the propagation of electromagnetic waves and of the commonly used modeling techniques. While many books cover RF propagation, most are geared to cellular telephone systems and, therefore, are limited in scope. This title is comprehensive—it treats the growing number of wireless applications that range well beyond the mobile telecommunications industry, including radar and satellite communications.

The author's straightforward, clear style makes it easy for readers to gain the necessary background in electromagnetics, communication theory, and probability, so they can advance to propagation models for near-earth, indoor, and earth-space propagation. Critical topics that readers would otherwise have to search a number of resources to find are included:

• RF safety chapter provides a concise presentation of FCC recommendations, including application examples, and prepares readers to work with real-world propagating systems
• Antenna chapter provides an introduction to a wide variety of antennas and techniques for antenna analysis, including a detailed treatment of antenna polarization and axial ratio; the chapter contains a set of curves that permit readers to estimate polarization loss due to axial ratio mismatch between transmitting and receiving antennas without performing detailed calculations
• Atmospheric effects chapter provides curves of typical atmospheric loss, so that expected loss can be determined easily
• Rain attenuation chapter features a summary of how to apply the ITU and Crane rain models
• Satellite communication chapter provides the details of earth-space propagation analysis including rain attenuation, atmospheric absorption, path length determination and noise temperature determination

Examples of widely used models provide all the details and information needed to allow readers to apply the models with confidence. References, provided throughout the book, enable readers to explore particular topics in greater depth. Additionally, an accompanying Wiley ftp site provides supporting MathCad files for select figures in the book.

With its emphasis on fundamentals, detailed examples, and comprehensive coverage of models and applications, this is an excellent text for upper-level undergraduate or graduate students, or for the practicing engineer who needs to develop an understanding of propagation phenomena.

Excerpt. © Reprinted by permission. All rights reserved.

Introduction to RF Propagation

By John S. Seybold

John Wiley & Sons

Copyright © 2005 John S. Seybold
All right reserved.
ISBN: 978-0-471-65596-1

Chapter One

Introduction

As wireless systems become more ubiquitous, an understanding of radio-frequency (RF) propagation for the purpose of RF planning becomes increasingly important. Most wireless systems must propagate signals through nonideal environments. Thus it is valuable to be able to provide meaningful characterization of the environmental effects on the signal propagation. Since such environments typically include far too many unknown variables for a deterministic analysis, it is often necessary to use statistical methods for modeling the channel. Such models include computation of a mean or median path loss and then a probabilistic model of the additional attenuation that is likely to occur. What is meant by "likely to occur" varies based on application, and in many instances an availability figure is actually specified.

While the basics of free-space propagation are consistent for all frequencies, the nuances of real-world channels often show considerable sensitivity to frequency. The concerns and models for propagation will therefore be heavily dependent upon the frequency in question. For the purpose of this text, RF is any electromagnetic wave with a frequency between 1MHz and 300GHz. Common industry definitions have RF ranging from 1MHz to about 1GHz, while the range from 1 to about 30GHz is called microwaves and 30-300 GHz is the millimeter-wave (MMW) region. This book covers the HF through EHF bands, so a more appropriate title might have been Introduction to Electromagnetic Wave Propagation, but it was felt that the current title would best convey the content to the majority of potential readers.

1.1 FREQUENCY DESIGNATIONS

The electromagnetic spectrum is loosely divided into regions as shown in Table 1.1. During World War II, letters were used to designate various frequency bands, particularly those used for radar. These designations were classified at the time, but have found their way into mainstream use. The band identifiers may be used to refer to a nominal frequency range or specific frequency ranges. Table 1.2 shows the nominal band designations and the official radar band designations in Region 2 as determined by international agreement through the International Telecommunications Union (ITU).

RF propagation modeling is still a maturing field as evidenced by the vast number of different models and the continual development of new models. Most propagation models considered in this text, while loosely based on physics, are empirical in nature. Wide variation in environments makes definitive models difficult, if not impossible, to achieve except in the simplest of circumstances, such as free-space propagation.

1.2 MODES OF PROPAGATION

Electromagnetic wave propagation is described by Maxwell's equations, which state that a changing magnetic field produces an electric field and a changing electric field produces a magnetic field. Thus electromagnetic waves are able to self-propagate. There is a well-developed theory on the subtleties of electromagnetic waves that is beyond the requirements of this book. An introduction to the subject and some excellent references are provided in the second chapter. For most RF propagation modeling, it is sufficient to visualize the electromagnetic wave by a ray (the Poynting vector) in the direction of propagation. This technique is used throughout the book and is discussed further in Chapter 2.

1.2.1 Line-of-Sight Propagation and the Radio Horizon

In free space, electromagnetic waves are modeled as propagating outward from the source in all directions, resulting in a spherical wave front. Such a source is called an isotropic radiator and in the strictest sense, does not exist. As the distance from the source increases, the spherical wave (or phase) front converges to a planar wave front over any finite area of interest, which is how the propagation is modeled. The direction of propagation at any given point on the wave front is given by the vector cross product of the electric (E) field and the magnetic (H) field at that point. The polarization of a wave is defined as the orientation of the plane that contains the E field. This will be discussed further in the following chapters, but for now it is sufficient to understand that the polarization of the receiving antenna should ideally be the same as the polarization of the received wave and that the polarization of a transmitted wave is the same as that of the antenna from which it emanated.

P = E x H

This cross product is called the Poynting vector. When the Poynting vector is divided by the characteristic impedance of free space, the resulting vector gives both the direction of propagation and the power density.

The power density on the surface of an imaginary sphere surrounding the RF source can be expressed as

S = P/4[pi][d.sup.2] W/[m.sup.2] (1.1)

where d is the diameter of the imaginary sphere, P is the total power at the source, and S is the power density on the surface of the sphere in watts/[m.sup.2] or equivalent. This equation shows that the power density of the electromagnetic wave is inversely proportional to [d.sup.2]. If a fixed aperture is used to collect the electromagnetic energy at the receive point, then the received power will also be inversely proportional to [d.sup.2].

The velocity of propagation of an electromagnetic wave depends upon the medium. In free space, the velocity of propagation is approximately

c = 3 x [10.sup.8] m/s

The velocity of propagation through air is very close to that of free space, and the same value is generally used. The wavelength of an electromagnetic wave is defined as the distance traversed by the wave over one cycle (period) and is generally denoted by the lowercase Greek letter lambda:

[lambda] = c/f (1.2)

The units of wavelength are meters or another measure of distance.

When considering line-of-sight (LOS) propagation, it may be necessary to consider the curvature of the earth (Figure 1.1). The curvature of the earth is a fundamental geometric limit on LOS propagation. In particular, if the distance between the transmitter and receiver is large compared to the height of the antennas, then an LOS may not exist. The simplest model is to treat the earth as a sphere with a radius equivalent to the equatorial radius of the earth.

From geometry

[d.sup.2] + [r.sup.2] = [(r + h).sup.2]

So

[d.sup.2] = (2r + h)h

and

d [congruent to] [square root of 2rh] (1.3)

since rh [much greater than] [h.sup.2].

The radius of the earth is approximately 3960 miles at the equator. The atmosphere typically bends horizontal RF waves downward due to the variation in atmospheric density with height. While this is discussed in detail later on, for now it is sufficient to note that an accepted means of correcting for this curvature is to use the "4/3 earth approximation," which consists of scaling the earth's radius by 4/3. Thus

r = 5280 miles

and

d [congruent to] [square root of 2 4/3 3960] h/5280

or

d [congruent to] [square root of 2h] (1.4)

where d is the distance to the "radio horizon" in miles and h is in feet (5280 ft = 1 mi). This approximation provides a quick method of determining the distance to the radio horizon for each antenna, the sum of which is the maximum LOS propagation distance between the two antennas.

Example 1.1. Given a point-to-point link with one end mounted on a 100-ft tower and the other on a 50-ft tower, what is the maximum possible (LOS) link distance?

[d.sub.1] = [square root of 2 x 100] [congruent to] 14.1 miles

[d.sub.2] = [square root of 2 x 50] = 10 miles

So the maximum link distance is approximately 24 miles.

1.2.2 Non-LOS Propagation

There are several means of electromagnetic wave propagation beyond LOS propagation. The mechanisms of non-LOS propagation vary considerably, based on the operating frequency. At VHF and UHF frequencies, indirect propagation is often used. Examples of indirect propagation are cell phones, pagers, and some military communications. An LOS may or may not exist for these systems. In the absence of an LOS path, diffraction, refraction, and/or multipath reflections are the dominant propagation modes. Diffraction is the phenomenon of electromagnetic waves bending at the edge of a blockage, resulting in the shadow of the blockage being partially filled-in. Refraction is the bending of electromagnetic waves due to inhomogeniety in the medium. Multipath is the effect of reflections from multiple objects in the field of view, which can result in many different copies of the wave arriving at the receiver.

The over-the-horizon propagation effects are loosely categorized as sky waves, tropospheric waves, and ground waves. Sky waves are based on ionospheric reflection/refraction and are discussed presently. Tropospheric waves are those electromagnetic waves that propagate through and remain in the lower atmosphere. Ground waves include surface waves, which follow the earth's contour and space waves, which include direct, LOS propagation as well as ground-bounce propagation.

1.2.2.1 Indirect or Obstructed Propagation While not a literal definition, indirect propagation aptly describes terrestrial propagation where the LOS is obstructed. In such cases, reflection from and diffraction around buildings and foliage may provide enough signal strength for meaningful communication to take place. The efficacy of indirect propagation depends upon the amount of margin in the communication link and the strength of the diffracted or reflected signals. The operating frequency has a significant impact on the viability of indirect propagation, with lower frequencies working the best. HF frequencies can penetrate buildings and heavy foliage quite easily. VHF and UHF can penetrate building and foliage also, but to a lesser extent. At the same time, VHF and UHF will have a greater tendency to diffract around or reflect/scatter off of objects in the path. Above UHF, indirect propagation becomes very inefficient and is seldom used. When the features of the obstruction are large compared to the wavelength, the obstruction will tend to reflect or diffract the wave rather than scatter it.

1.2.2.2 Tropospheric Propagation The troposphere is the first (lowest) 10 km of the atmosphere, where weather effects exist. Tropospheric propagation consists of reflection (refraction) of RF from temperature and moisture layers in the atmosphere. Tropospheric propagation is less reliable than ionospheric propagation, but the phenomenon occurs often enough to be a concern in frequency planning. This effect is sometimes called ducting, although technically ducting consists of an elevated channel or duct in the atmosphere. Tropospheric propagation and ducting are discussed in detail in Chapter 6 when atmospheric effects are considered.

1.2.2.3 Ionospheric Propagation The ionosphere is an ionized plasma around the earth that is essential to sky-wave propagation and provides the basis for nearly all HF communications beyond the horizon. It is also important in the study of satellite communications at higher frequencies since the signals must transverse the ionosphere, resulting in refraction, attenuation, depolarization, and dispersion due to frequency dependent group delay and scattering.

HF communication relying on ionospheric propagation was once the backbone of all long-distance communication. Over the last few decades, ionospheric propagation has become primarily the domain of shortwave broadcasters and radio amateurs. In general, ionospheric effects are considered to be more of a communication impediment rather than facilitator, since most commercial long-distance communication is handled by cable, fiber, or satellite. Ionospheric effects can impede satellite communication since the signals must pass through the ionosphere in each direction. Ionospheric propagation can sometimes create interference between terrestrial communications systems operating at HF and even VHF frequencies, when signals from one geographic area are scattered or refracted by the ionosphere into another area. This is sometimes referred to as skip.

The ionosphere consists of several layers of ionized plasma trapped in the earth's magnetic field (Figure 1.2) [9, 10]. It typically extends from 50 to 2000 km above the earth's surface and is roughly divided into bands (apparent reflective heights) as follows:

D 45-55 miles E 65-75 miles F1 90-120 miles F2 200 miles (50-95 miles thick)

The properties of the ionosphere are a function of the free electron density, which in turn depends upon altitude, latitude, season, and primarily solar conditions.

Typically, the D and E bands disappear (or reduce) at night and F1 and F2 combine. For sky-wave communication over any given path at any given time there exists a maximum usable frequency (MUF) above which signals are no longer refracted, but pass through the F layer. There is also a lowest usable frequency (LUF) for any given path, below which the D layer attenuates too much signal to permit meaningful communication.

The D layer absorbs and attenuates RF from 0.3 to 4MHz. Below 300kHz, it will bend or refract RF waves, whereas RF above 4 MHz will be passed unaffected. The D layer is present during daylight and dissipates rapidly after dark. The E layer will either reflect or refract most RF and also disappears after sunset. The F layer is responsible for most sky-wave propagation (reflection and refraction) after dark.

Faraday rotation is the random rotation of a wave's polarization vector as it passes through the ionosphere. The effect is most pronounced below about 10GHz. Faraday rotation makes a certain amount of polarization loss on satellite links unavoidable. Most satellite communication systems use circular polarization since alignment of a linear polarization on a satellite is difficult and of limited value in the presence of Faraday rotation.

Group delay occurs when the velocity of propagation is not equal to c for a wave passing through the ionosphere. This can be a concern for ranging systems and systems that reply on wide bandwidths, since the group delay does vary with frequency. In fact the group delay is typically modeled as being proportional to 1/[f.sup.2]. This distortion of wideband signals is called dispersion. Scintillation is a form of very rapid fading, which occurs when the signal attenuation varies over time, resulting in signal strength variations at the receiver.

When a radio wave reaches the ionosphere, it can be refracted such that it radiates back toward the earth at a point well beyond the horizon. While the effect is due to refraction, it is often thought of as being a reflection, since that is the apparent effect. As shown in Figure 1.3, the point of apparent reflection is at a greater height than the area where the refraction occurs.

1.2.3 Propagation Effects as a Function of Frequency

As stated earlier, RF propagation effects vary considerably with the frequency of the wave. It is interesting to consider the relevant effects and typical applications for various frequency ranges.

The very low frequency (VLF) band covers 3-30kHz. The low frequency dictates that large antennas are required to achieve a reasonable efficiency. A good rule of thumb is that the antenna must be on the order of one-tenth of a wavelength or more in size to provide efficient performance. The VLF band only permits narrow bandwidths to be used (the entire band is only 27kHz wide). The primarily mode of propagation in the VLF range is ground-wave propagation. VLF has been successfully used with underground antennas for submarine communication.

(Continues...)

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Edition details

• Publisher: Wiley-Interscience
• Publication date: 2005
• Language: English
• ISBN 10: 0471655961
• ISBN 13: 9780471655961
• Binding: Hardcover
• Edition number: 1
• Number of pages: 352