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Radio Communications I n t h e D i g i t a l A g e

Volume 1 HF TECHNOLOGY

Edition 2

First Edition: September 1996

Library of Congress Catalog Card Number: 96-94476 Harris Corporation, RF Communications Division Radio Communications in the Digital Age Volume One: HF Technology, Edition 2

Printed in USA © 10/05 R.O. 10K B1006A

All Harris RF Communications products and systems included herein are registered trademarks of the Harris Corporation.

TABLE OF CONTENTS

INTRODUCTION...............................................................................1

CHAPTER 1

PRINCIPLES OF RADIO COMMUNICATIONS .....................................6

CHAPTER 2

THE IONOSPHERE AND HF RADIO PROPAGATION..........................16

CHAPTER 3

ELEMENTS IN AN HF RADIO ..........................................................24

CHAPTER 4

NOISE AND INTERFERENCE............................................................36

CHAPTER 5

HF MODEMS .................................................................................40

CHAPTER 6

AUTOMATIC LINK ESTABLISHMENT (ALE) TECHNOLOGY...............48

CHAPTER 7

DIGITAL VOICE ..............................................................................55

CHAPTER 8

DATA SYSTEMS .............................................................................59

CHAPTER 9

SECURING COMMUNICATIONS .....................................................71

CHAPTER 10

FUTURE DIRECTIONS .....................................................................77

APPENDIX A

STANDARDS ..................................................................................79

APPENDIX B

GLOSSARY ....................................................................................81 FURTHER READING........................................................................93

INTRODUCTION

Military organizations have used HF radios for both strategic and tacti- cal communications for more than 80 years; however, with the advent of satellites, HF had been de-emphasized and fell into disuse. As a result of that, many present-day communicators don't have an under- standing of what modern HF communication capabilities are.

This document is a layman's tutorial on HF radio communications. It presents reasons why HF radio should be considered as a communica- tions means and the unique advantages and versatility of the medium. It explains how HF, even at the low power levels of packsets or vehicu- lar radios, can be used for:

Line-of-Sight (LOS): Range, typically less than 30 km, is limited by ter- rain obstructions and/or earth curvature.

+ Range is also a function of operating frequency, power level, and antenna height.

+ Offers possibility of high data rate transmission. + Restricting range reduces adjacent area interference and

eases frequency reuse requirements.

Ground (surface) Wave: Useful range is up to 50 km on land, 300 or more km over the sea.

+ Range depends on operating frequency and terrain obstructions. + Requires vertically polarized antennas. + Historically used for voice communications. Data rates are

generally high, but may have some limitations depending on waveform used.

Note: HF often provides “extended LOS” coverage compared to VHF communications and is often used when operators require greater dis- tance then VHF radios can provide.

Beyond Line-of-Sight (BLOS): Range to about 400 km using Near Vertical Incidence Skywave (NVIS).

+ Can be used where satellite communication is not available. + Terrain obstruction not a limiting factor, HF can communicate

over mountains etc. + Requires horizontally polarized antennas. + Frequency range generally restricted to < 10 MHz. + Voice or low/medium rate data, data rate depends on

waveform. + Operating frequency often dependent on ionospheric conditions

and Solar Cycles. + Automatic Link Establishment (ALE) helps solve the operating

frequency selection problem.

Long Range Communications: Communications to ranges to 4000 km and beyond.

+ Range depends on antenna, power level, atmospheric conditions. + Operating frequency selection is more difficult — ALE is really

useful here. + Often requires directional antennas.

By judicious choice of operating frequency and antenna, the same HF radio can provide communications ranging from short range to long range communications.

We’ve all seen black-and-white wartime film clips of radio operators sending Morse code using bulky radio equipment. After World War II, the communications industry turned its attention to other technolo- gies, leading to a period of slow growth in High-Frequency (HF) radio communications during the 1960s and 1970s. However, HF, also known as short wave, has undergone an exciting revival propelled by an infusion of new technology.

GENESIS

Modern radio technology had its birth with the publication of James Clerk Maxwell’s Treatise on Electricity and Magnetism in 1873, setting forth the basic theory of electromagnetic wave propagation.

But the first radio waves were actually detected 15 years later. In 1888, Heinrich Rudolph Hertz (the scientist for whom the unit of frequency is named) demonstrated that disturbances generated by a spark coil showed the characteristics of Maxwell’s radio waves. His work inspired Guglielmo Marconi’s early experiments with wireless telegraphy using Morse code. By 1896, Marconi had communicated messages over dis- tances of a few kilometers.

It was thought at the time that radio waves in the atmosphere traveled in straight lines and that they; therefore, would not be useful for over- the-horizon communication. That opinion did not discourage Marconi, however, who became the first to demonstrate the transmission of radio waves over long distances. In 1901 in Newfoundland, Canada, he detected a telegraphic signal transmitted from Cornwall, England, 3,000 kilometers away. For an antenna, he used a wire 120 meters long, held aloft by a simple kite.

Marconi’s success stimulated an intensive effort to explain and exploit his discovery. The question of how radio waves could be received around the surface of the earth was eventually answered by Edward Appleton. It was this British physicist who discovered that a blanket of electrically charged, or “ionized,” particles in the earth’s atmosphere (the ionosphere) were capable of reflecting radio waves. By the 1920s, scientists had applied this theory and developed ways to measure and predict the refractive properties of the ionosphere.

GROWTH

In time, the characteristics of HF radio propagation became better understood. Operators learned, for example, that usable frequencies varied considerably with time of day and season. HF technology devel- oped quickly.

By World War II, HF radio was the primary means of long-haul com- munications for military commanders because it provided communica- tions with land, sea, and air forces.

In the hands of a skilled operator armed with years of experience and an understanding of the propagating effects of the ionosphere, HF

radio was routinely providing reliable, effective links over many thou- sands of miles. Today, HF radio also plays an important role in allowing emerging nations to establish a national communications system quick- ly and inexpensively.

HIATUS

The advent of long-range communications by satellite in the 1960s ini- tiated a period of declining interest in HF radio. Satellites carried more channels and could handle data transmission at higher speeds. Additionally, satellite links seemed to eliminate the need for highly trained operators. As long-range communications traffic migrated to satellites, HF was often relegated to a backup role. The result was user preference for wider bandwidth methods of communication, such as satellites, resulting in declining proficiency in HF as the number of expe- rienced radio operators decreased.

It became clear over time, however, that satellites (for all their advan- tages) had significant limitations. Military users became increasingly concerned about the vulnerability of satellites to jamming and physical damage, and questioned the wisdom of depending exclusively on them. Moreover, satellites and their supporting infrastructure are expensive to build and maintain, and there are a limited number of channels available.

REVIVAL

In the last decade, we’ve seen major resurgence in HF radio. Research and development activity has intensified, and a new generation of automated HF equipment has appeared. These systems provide dra- matic improvements in automation, reliability, and throughput. Today’s Automatic Link Establishment (ALE)-based HF radios are as easy to use as wireless telephones.

Nonetheless, the perception that HF radio is an inherently difficult-to- use medium continues to linger. This perception continues because some communicators remember how HF used to be.

As your interest in this book shows, HF is again being recognized as a robust and highly competitive medium for long-haul communications, offering countless capabilities. In this introduction to HF radio commu- nications, we present information that will help you understand mod- ern HF radio technology. We’ll cover the principles of HF radio, talk about specific applications, and then, consider the future of HF radio communication.

CHAPTER 1

PRINCIPLES OF RADIO COMMUNICATIONS

An understanding of radio communications begins with the compre- hension of basic electromagnetic radiation.

Radio waves belong to the electromagnetic radiation family, which includes x-ray, ultraviolet, and visible light. Much like the gentle waves that form when a stone is tossed into a still lake, radio signals radiate outward, or propagate, from a transmitting antenna. However, unlike water waves, radio waves propagate at the speed of light.

We characterize a radio wave in terms of its amplitude, frequency, and wavelength (Figure 1-1).

Radio wave amplitude, or strength, can be visualized as its height being the distance between its peak and its lowest point. Amplitude, which is measured in volts, is usually expressed in terms of an average value called root-mean-square, or RMS.

The frequency of a radio wave is the number of repetitions or cycles it completes in a given period of time. Frequency is measured in Hertz (Hz); one Hertz equals one cycle per second. Thousands of Hertz are expressed as kilohertz (kHz), and millions of Hertz as megahertz (MHz). You would typically see a frequency of 2,345,000 Hertz, for example, written as 2,345 kHz or 2.345 MHz.

Radio wavelength is the distance between crests of a wave. The prod- uct of wavelength and frequency is a constant that is equal to the speed of propagation. Thus, as the frequency increases, wavelength decreases, and vice versa. Radio waves propagate at the speed of light (300 million meters per second). To determine the wavelength in meters for any frequency, divide 300 by the frequency in megahertz. So, the wavelength of a 10 MHz wave is 30 meters, determined by dividing 300 by 10.

FIGURE 1-1 | Radio Wave Properties

FIGURE 1-2 | Radio Frequency Spectrum

THE RADIO FREQUENCY SPECTRUM

In the radio frequency spectrum (Figure 1-2), the usable frequency range for radio waves extends from about 20 kHz (just above sound waves) to above 30,000 MHz. A wavelength at 20 kHz is 15 kilometers long. At 30,000 MHz, the wavelength is only 1 centimeter.

The HF band is defined as the frequency range of 3 to 30 MHz. In prac- tice, most HF radios use the spectrum from 1.6 to 30 MHz. Most long- haul communications in this band take place between 4 and 18 MHz. Higher frequencies (18 to 30 MHz) may also be available from time to time, depending on ionospheric conditions and the time of day. (See Chapter 2.)

In the early days of radio, HF frequencies were called short wave because their wavelengths (10 to 100 meters) were shorter than those of commercial broadcast stations. The term is still applied to long- distance radio communications.

FREQUENCY ALLOCATIONS AND MODULATION

Within the HF spectrum, groups of frequencies are allocated to specif- ic radio services — aviation, maritime, military, government, broadcast, or amateur (Figure 1-3). Frequencies are further regulated according to transmission type: emergency, broadcast, voice, Morse code, facsimile, and data. Frequency allocations are governed by international treaty and national licensing authorities. The allocation of a frequency is just the beginning of radio communications. By itself, a radio wave conveys no information. It’s simply a rhythmic stream of continuous waves (CW).

When we modulate radio waves to carry information, we refer to them as carriers. To convey information, a carrier must be varied so that its properties — its amplitude, frequency, or phase (the measurement of a complete wave cycle) — are changed, or modulated, by the informa- tion signal.

The simplest method of modulating a carrier is by turning it on and off by means of a telegraph key. On-off keying (using Morse code) was the only method of conveying wireless messages in the early days of radio.

FIGURE 1-3 | Principles: Frequency Allocations

FIGURE 1-4 | Amplitude Modulation

Today’s common methods for radio communications include amplitude modulation (AM), which varies the strength of the carrier in direct pro- portion to changes in the intensity of a source such as the human voice (Figure 1-4). In other words, information is contained in amplitude variations.

The AM process creates a carrier and a pair of duplicate sidebands — nearby frequencies above and below the carrier (Figure 1-5). AM is a relatively inefficient form of modulation, since the carrier must be con- tinually generated. The majority of the power in an AM signal is con- sumed by the carrier that carries no information, with the rest going to the information-carrying sidebands.

In a more efficient technique, single sideband (SSB), the carrier and one of the sidebands are suppressed (Figure 1-6). Only the remaining side- band — upper (USB) or lower (LSB) — is transmitted. An SSB signal needs only half the bandwidth of an AM signal and is produced only when a modulating signal is present. Thus, SSB systems are more effi- cient both in the use of the spectrum, which must accommodate many users, and of transmitter power. All the transmitted power goes into the information-carrying sideband.

One variation on this scheme, often used by military and commercial communicators, is amplitude modulation equivalent (AME), in which a carrier at a reduced level is transmitted with the sideband. AME lets one use a relatively simple receiver to detect the signal. Another impor- tant variation is independent sideband (ISB), in which both an upper and lower sideband, each capable of carrying different information, are transmitted. This way, for example, one sideband can carry a data sig- nal and the other can carry a voice signal (Figure 1-7).

Frequency modulation (FM) is a technique in which the carrier’s fre- quency is varied to convey the signal. For a variety of technical reasons, conventional FM generally produces a cleaner signal than AM, but uses much more bandwidth than AM. Narrowband FM, which is sometimes used in HF radio, uses less bandwidth, but only at the cost of signal quality.

FIGURE 1-5 | Amplitude Modulation Sidebands

FIGURE 1-6 | Modulation, Single Sideband

FIGURE 1-7 | Modulation: Independent Sideband

FIGURE 1-8 | Propagation Paths

Other schemes support the transmission of data over HF channels, including shifting the frequency or phase of the signal. We will cover these techniques in Chapter 5.

Radio Wave Propagation Propagation is defined as how radio signals radiate outward from a transmitting source. Radio waves are often believed to travel in a straight line like a stone tossed into a still lake. The true path radio waves take, however, is often more complex.

There are two basic modes of propagation: ground waves and sky waves. As their names imply, ground waves travel along the surface of the earth, while sky waves “bounce” back to earth. Figure 1-8 shows the different propagation paths for HF radio waves.

Ground waves consist of three components: surface waves, direct waves, and ground-reflected waves. Surface waves travel along the surface of the earth, reaching beyond the horizon. Eventually, surface wave energy is absorbed by the earth. The effective range of surface waves is largely determined by the frequency and conductivity of the surface over which the waves travel. Absorption increases with fre- quency.

Transmitted radio signals, which use a carrier traveling as a surface wave, are dependent on transmitter power, receiver sensitivity, anten- na characteristics, and the type of path traveled. For a given comple- ment of equipment, the range may extend from 200 to 300 km over a conductive, all-sea-water path. Over arid, rocky, non-conductive ter- rain, however, the range may drop to less than 30 km, even with the same equipment.

Direct waves travel in a straight line, becoming weaker as distance increases. They may be bent, or refracted, by the atmosphere, which extends their useful range slightly beyond the horizon. Transmitting and receiving antennas must be able to “see” each other for commu- nications to take place, so antenna height is critical in determining range. Because of this, direct waves are sometimes known as line-of- sight (LOS) waves.

Ground-reflected waves are the portion of the propagated wave that is reflected from the surface of the earth between the transmitter and receiver.

Sky waves make beyond line-of-sight (BLOS) communications possible. At certain frequencies, radio waves are refracted (or bent), returning to earth hundreds or thousands of miles away. Depending on frequency, time of day, and atmospheric conditions, a signal can bounce several times before reaching a receiver.

Using sky waves can be tricky, since the ionosphere is constantly chang- ing. In the next chapter, we’ll discuss sky waves in greater detail.

SUMMARY

Radio signals propagate from a transmitting antenna as waves through space at the speed of light.

Radio frequency is expressed in terms of hertz (cycles per second), kilo- hertz (thousands of Hertz), or megahertz (millions of Hertz).

Frequency determines the length of a radio wave; lower frequencies have longer wavelengths and higher frequencies have shorter wave- lengths.

Long-range radio communications take place in the high-frequency (HF) range of 1.6 to 30 MHz. Different portions of this band are allo- cated to specific radio services under international agreement.

Modulation is the process whereby the phase, amplitude, or frequency of a carrier signal is modified to convey intelligence.

HF radio waves can propagate as sky waves, which are refracted from the earth’s ionosphere, permitting communications over long dis- tances.

CHAPTER 2

THE IONOSPHERE AND HF RADIO PROPAGATION

To understand sky wave propagation, you need to consider the effects of the ionosphere and solar activity on HF radio propagation. You must also be familiar with the techniques used to predict propagation and select the best frequencies for a particular link at a given time. Let’s start with some definitions.

THE IONOSPHERE, NATURE’S SATELLITE

The ionosphere is a region of electrically charged particles or gases in the earth’s atmosphere, extending from approximately 50 to 600 km above the earth’s surface. Ionization, the process in which electrons are stripped from atoms and produces electrically charged particles, results from solar radiation. When the ionosphere becomes heavily ionized, the gases may even glow and be visible. This phenomenon is known as Northern and Southern Lights.

Why is the ionosphere important in HF radio? Well, this blanket of gases is like nature’s satellite, making HF BLOS radio communications possible. When radio waves strike these ionized layers, depending on frequency, some are completely absorbed, others are refracted so that they return to the earth, and still others pass through the ionosphere into outer space. Absorption tends to be greater at lower frequencies, and increases as the degree of ionization increases.

The angle at which sky waves enter the ionosphere is known as the incident angle (Figure 2-1). This is determined by wavelength and the type of transmitting antenna. Like a billiard ball bouncing off a rail, a radio wave reflects from the ionosphere at the same angle it hits it. Thus, the incident angle is an important factor in determining commu- nications range. If you need to reach a station that is relatively far from you, you would want the incident angle to be relatively large. To communicate with a nearby station, the incident angle should be relatively small.

FIGURE 2-1 | The Ionosphere: Incident Angle

FIGURE 2-2 | The Ionosphere: Layers

The incident angle of a radio wave is critical because if it is too nearly vertical, it will pass through the ionosphere without being refracted back to earth. If the angle is too great, the waves will be absorbed by the lower layers before reaching the more densely ionized upper layers. So, incident angle must be sufficient for bringing the radio wave back to earth, yet not so great that it will lead to absorption.

LAYERS OF THE IONOSPHERE

Within the ionosphere, there are four layers of varying ionization (Figure 2-2). Since ionization is caused by solar radiation, the higher layers of the ionosphere tend to be more highly ionized, while the lower layers, protected by the outer layers, experience less ionization. Of these layers, the first, discovered in the early 1920s by Appleton, was designated E for electric waves. Later, D and F were discovered and noted by these letters. Additional ionospheric phenomena were dis- covered through the 1930s and 1940s, such as sporadic E and aurora.

In the ionosphere, the D layer is the lowest region affecting HF radio waves. Ionized during the day, the D layer reaches maximum ionization when the sun is at its zenith and dissipates quickly toward sunset.

The E layer reaches maximum ionization at noon. It begins dissipating toward sunset and reaches minimum activity at midnight. Irregular cloud-like formations of ionized gases occasionally occur in the E layer. These regions, known as sporadic E, can support propagation of sky waves at the upper end of the HF band and beyond.

The most heavily ionized region of the ionosphere, and therefore the most important for long-haul communications, is the F layer. At this altitude, the air is thin enough that the ions and electrons recombine very slowly, so the layer retains its ionized properties even after sunset.

In the daytime, the F layer consists of two distinct layers, F1 and F2. The F1 layer, which exists only in the daytime and is negligible in winter, is not important to HF communications. The F2 layer reaches maximum ionization at noon and remains charged at night, gradually decreasing to a minimum just before sunrise.

During the day, sky wave reflection from the F2 layer requires wave- lengths short enough to penetrate the ionized D and E layers, but not so short as to pass through the F layer. Generally, frequencies from 10 to 20 MHz will accomplish this, but the same frequencies used at night would penetrate the F layer and pass into outer space. The most effec- tive frequencies for long-haul nighttime communications are normally between 3 and 8 MHz.

FACTORS AFFECTING ATMOSPHERIC IONIZATION

The intensity of solar radiation, and therefore ionization, varies period- ically. Hence, we can predict solar radiation intensity based on time of day and the season, and make adjustments in equipment to limit or optimize ionization effects.

Ionization is higher during spring and summer because the hours of daylight are longer. Sky waves are absorbed or weakened as they pass through the highly charged D and E layers, reducing, in effect, the communication range of most HF bands.

Because there are fewer hours of daylight during autumn and winter, less radiation reaches the D and E layers. Lower frequencies pass easily through these weakly ionized layers. Therefore, signals arriving at the F layer are stronger and are reflected over greater distances.

Another longer term periodic variation results from the 11-year sunspot cycle (Figure 2-3). Sunspots generate bursts of radiation that cause higher levels of ionization. The more sunspots, the greater the ioniza- tion. During periods of low sunspot activity, frequencies above 20 MHz tend to be unusable because the E and F layers are too weakly ionized to reflect signals back to earth. At the peak of the sunspot cycle, how- ever, it is not unusual to have worldwide propagation on frequencies above 30 MHz.

In addition to these regular variations, there is a class of unpredictable phenomena known as sudden ionospheric disturbances (SID), which can affect HF communications as well. SIDs are random events due to solar flares that can disrupt sky wave communication for hours or days at a time. Solar flares produce intense ionization of the D layer, causing

FIGURE 2-3 | Sunspot Cycle: Ionization Factors

FIGURE 2-4 | The Ionosphere: FOT

it to absorb most HF signals on the side of the earth facing the sun. Magnetic storms often follow the eruption of solar flares within 20 to 40 hours. Charged particles from the storms have a scattering effect on the F layer, temporarily neutralizing its reflective properties.

FREQUENCY AND PATH OPTIMIZATION

Because ionospheric conditions affect radio wave propagation, com- municators must determine the best way to optimize radio frequencies at a particular time. The highest possible frequency that can be used to transmit over a particular path under given ionospheric conditions is called the Maximum Usable Frequency (MUF). Frequencies higher than the MUF penetrate the ionosphere and continue into space. Frequencies lower than the MUF tend to refract back to earth.

As frequency is reduced, the amount of absorption of the signal by the D layer increases. Eventually, the signal is completely absorbed by the ionosphere. The frequency at which this occurs is called the Lowest Usable Frequency (LUF). The “window” of usable frequencies, there- fore, lies between the MUF and LUF.

The Frequency of Optimum Transmission (FOT) is typically 85 percent of the MUF. Generally, the FOT is lower at night and higher during the day. These frequencies are illustrated in Figure 2-4.

In addition to frequency, the route the radio signal travels must also be considered in optimizing communications. A received signal may be comprised of components arriving via several routes, including one or more sky wave paths and a ground wave path. The arrival times of these components vary because of differences in path length; the range of time differences is called the multipath spread. The effects of multipath spread can be minimized by selecting a frequency as close as possible to the MUF.

PROPAGATION PREDICTION TECHNIQUES

Since many of the variables affecting propagation follow repetitive cycles and can be predicted, techniques for effectively determining FOT have been developed.

A number of propagation prediction computer programs are available. Two widely used and effective programs are Ionospheric Communications Analysis and Prediction (IONCAP) and Voice of America Coverage Analysis Program (VOACAP), which predict system performance at given times of day as a function of frequency for a given HF path and a specified complement of equipment.

Of course, since computerized prediction methods are based on his- toric data, they cannot account for present conditions affecting com- munications, like ionospheric changes caused by random phenomena such as interference and noise (more about these later).

A more immediate automated prediction method involves ionospheric sounding. One system, the Chirpsounder®, uses remote stations to transmit test signals (chirps) that sweep through all frequencies from 2 to 30 MHz. The receiver tracks the signal, analyzes its reception on assigned operating frequencies, and displays frequency ranges for opti- mum propagation.

In addition, modern HF communications systems make use of Link Quality Analysis (LQA) techniques. In these systems, transmitting and receiving stations cooperate to automatically assess the quality of the channels available to them. When the need to communicate arises, the LQA data is used to select the best frequency. We’ll take a closer look at this technique in Chapter 6.

SUMMARY

The ionosphere is a region of electrically charged particles or gases in the earth’s atmosphere, extending from 50 to 600 km (approximately 30 to 375 miles) above the earth’s surface.

There are layers of varying electron density in the ionosphere that absorb, pass, or reflect radio waves, depending on the density of the layer, the angle with which the radio waves strike it, and the frequen- cy of the signal.

Ionization, caused by solar radiation, strips electrons from atoms, pro- ducing electrically charged particles.

The density of ionospheric layers varies with the intensity of solar radi- ation, which changes according to time of day, season, and sunspot cycle.

The angle of radiation is determined by the wavelength of a signal and the type of antenna used.

Radio waves are absorbed as they pass through the ionosphere. The absorption rate increases as frequency decreases.

Communication is best at the frequency of optimum transmission (FOT), typically 85 percent of the maximum usable frequency (MUF).

Sunspots increase and decrease in 11-year cycles. Higher sunspot num- bers increase ionization and therefore improve propagation. Lower sunspot numbers cause less ionization.

Solar flares cause sudden ionospheric disturbances (SIDs), which can disrupt HF communications for short periods of time.

Propagation prediction techniques, such as IONCAP and VOACAP, predict the MUF, LUF, and FOT for a given transmission path and time of day. Other methods include ionospheric sounding and Link Quality Analysis (LQA).