N this unit we'll examine discuss specific aspects of radio communications across the radio-frequency band of the electromagnetic spectrum. Since HF is the primary mode of our communications we'll look at it first. Specifically, we'll cover examine HF wave propagation, the ionosphere, and its effects on propagation. We'll also examine HF engineering ideas, limitations on the use of HF, and satellites and their effect on HF communications. Next, we'll look at the lower frequency bands and the upper frequency bands. Again, we'll examine wave propagation and limitations on the use of these frequency bands.

We've been using high-frequency (HF) communications for many years. Throughout the 1950's and 1960's HF sky-wave systems were was the backbone of our long-distance communication. The advent of communications satellites in the late 1960's and early 1970's brought a change of philosophy about long-distance communications. The feeling was, "HF is dead." People were eager to switch to satellites because of the simplicity of operations. No longer did we have to study the ionosphere or calculate what frequency would work. In satellite communications all you had to know was which direction to point your antenna and what channel to use.

As more reliance was placed on satellite communications, we discovered it wasn't a perfect system. Military planners realized the vulnerability of communications satellites as they orbit the earth and saw increasing congestion on the few satellites we have. With this realization, philosophies again changed. HF was "reborn" as communicators realized the reliability and versatility of HF communications.

In your AF career you will will learn more about the art of HF communications. To achieve this goal you must be knowledgeable in many areas. You must understand what a radio wave does when it leaves an antenna and what controls the wave's behavior. You must understand how the atmosphere, solar activity, and the influence of the ground affect radio waves.

The key to successful HF radio communications lies in a workable understanding of ionospheric phenomena and its effect on radio communications. In the early days of radio communications, it was thought that all radio waves propagate in a straight horizontal line from the transmitting antenna to the receiving antenna. This reasoning was based on the fact that, in the LF and MF bands, radio signals could only be received a relatively short distance away (100 to 200 miles). After much experimentation and theorizing, however, it was concluded that radio waves actually have specific propagating components and these components have predictable characteristics. Early reasoning on radio-wave propagation was faulty because transmitter power and the frequency band being used have a direct bearing on how far the signal will propagates.

During the development of HF radio, we found that radio waves in this band could be received at much greater distances than was previously thought possible. Investigating this new phenomenon proved that the earth's atmosphere has properties that affect radio waves in different ways.

Wave propagation deals with the properties and the nature of the atmosphere through which radio waves must travel. As you already know, the atmosphere isn't uniform, but varies with the altitude, geographic location, time, season, and year. Knowing atmospheric characteristics help solve problems that arise in planning or using radio communications paths and in predicting path reliability. For radio operations purposes, the atmosphere is made up of three basic regions: the troposphere, the stratosphere, and the ionosphere.TEST

The troposphere. The troposphere is the part of the atmosphere extending from the surface of the earth to about 11 kilometers (km) or 7 miles. Within the troposphere, bending of radio waves by refraction makes the distance to the radio horizon exceed the distance to the optical horizon. Tropospheric refraction (bending caused by sudden changes in the characteristics of air in a lower atmosphere) affects the received signal at distances beyond the radio horizon.

The stratosphere. The stratosphere lies between the troposphere and the ionosphere, about 11 to 48km (7 to 31 miles) above the earth. The temperature in this region is more nearly constant than in the troposphere.

The ionosphere. The ionosphere is the portion of the earth's upper atmosphere where ions and electrons are present in quantities sufficient to affect the propagation of radio waves. Normally the ionosphere extends from about 48 to 1,000km (31 to 600 miles) above the earth. At certain times and locations however, it may reach even lower. Long-distance, HF communication is made possible by reflections of radio waves from ionized layers in this portion of the earth's atmosphere.

Formation. The ionosphere is the portion of the earth's atmosphere that's sufficiently ionized by the sun's ultraviolet (UV) and X-ray radiation to affect radio-wave propagation significantly. Before we go further into the structure of the ionosphere, let's get a clearer idea of what ionization is.

The sun radiates energy that enters the earth's atmosphere at different wavelengths. The atmosphere is composed of neutral atoms (neutrons) and negative atoms (electrons). When UV rays from the sun reach the atmosphere, the neutrons split into positively-charged atoms called ions. This process is called ionization.

In the upper levels of the ionosphere, ions are thinly spread and remain highly charged. In its the lower levels, of the ionosphere, ions are more densely concentrated and tend to turn into neutrons more easily. This relationship of electron density with height leads to the subdivision of the ionosphere. The ionosphere is divided into layers. These layers are called the D, E, and F-regions or layers.

The different layers in the atmosphere are caused by the different wavelengths of the UV rays expending their energy at different heights. Radiation and particles from outside the solar system also contribute to the formation of the different ionospheric layers. A highly variable source of ionizing agents is solar wind. Solar wind consists of ions thrown out from the surface of the sun as a result of its turbulent processes. there. As particles travel out through space, some come close enough to the earth to be trapped by the earth's magnetic fields.

Structure. Figure 3-1 shows typical day and night profiles of electron density in the ionosphere. The figure shows several regions in which the electron density increases with height (the D, E, Es, F, F1, and F2 layers). The existence of layers is caused by the fact that the atmosphere is a mixture of gases that differ in their susceptibility to ionizing radiation, and thus produce maximum ionization at different altitudes. The degree of ionization and height of these layers is affected by time of day, the season, and variations in solar activity. Figure 3-2 shows the daytime ionospheric layers. The Es (sporadic E) layer is omitted in this figure because of its irregular occurrence and limited geographic extent.

Ionization is less in the lower layer (D layer) and increases in the higher layers. TESTWhen a radio wave enters the ionosphere, the free electrons are set into motion by the alternating electric field on the wave. The energy that is transferred from the wave to the free electrons is lost when the electrons collide with a molecule. Therefore the greatest energy loss is in the D layer. TESTThis loss of energy is called absorption loss (attenuation). Experimentation revealed that absorption loss is inversely proportional to the transmitted frequency. In other words, the higher the frequency, the less the attenuation by absorption.

D layer. The D layer extends from about 48 to 90km (30 to 55 miles) above the earth. Normal D-layer ionization is produced by solar UV light and X-rays during daylight hours. This ionization effect is very low and there is little or no refraction of radio waves. As a result, the D layer accounts for the majority of ionospheric noise and radio-wave absorption. Because this layer disappears at night, variations in absorption and noise of the transmitted signals from day to day are apparent. Additional D-layer ionization may be produced at any time of day or night by high-energy electrons and protons originating from the sun. This type of increase in D-layer ionization is likely to

be associated with geomagnetic disturbances. HF radio waves aren't reflected by the D layer. TESTThis layer is important because of its adverse effect in absorbing energy from waves traversing it. The absorption is small at night and greatest about midday. It's quite variable, generally increasing with sunspot activity.

Ionization is less in the lower layer (D layer) and increases in the higher layers. TESTWhen a radio wave enters the ionosphere, the free electrons are set into motion by the alternating electric field on the wave. The energy that is transferred from the wave to the free electrons is lost when the electrons collide with a molecule. Therefore the greatest energy loss is in the D layer. TESTThis loss of energy is called absorption loss (attenuation). Experimentation revealed that absorption loss is inversely proportional to the transmitted frequency. In other words, the higher the frequency, the less the attenuation by absorption.

D layer. The D layer extends from about 48 to 90km (30 to 55 miles) above the earth. Normal D-layer ionization is produced by solar UV light and X-rays during daylight hours. This ionization effect is very low and there is little or no refraction of radio waves. As a result, the D layer accounts for the majority of ionospheric noise and radio-wave absorption. Because this layer disappears at night, variations in absorption and noise of the transmitted signals from day to day are apparent. Additional D-layer ionization may be produced at any time of day or night by high-energy electrons and protons originating from the sun. This type of increase in D-layer ionization is likely to be associated with geomagnetic disturbances. HF radio waves aren't reflected by the D layer. TESTThis layer is important because of its adverse effect in absorbing energy from waves traversing it. The absorption is small at night and greatest about midday. It's quite variable, generally increasing with sunspot activity.

E layer. The E layer extends from about 90 to 140km (55 to 85 miles) above the earth. Since ionization is produced by solar UV and X-ray radiation, ionization drops to low values at night. As a result, the E-layer critical frequency is practically nonexistent and little, if any, nighttime reflection will occurs. E-layer effects are irregular. During the day, reflections from the E layer are useful for communicating at distances up to about 2,000km (1,250 miles).

Sporadic E areas. Within and somewhat above the region normally occupied by the E layer, a thin patchy area of high electron density sometimes occurs. These sporadic E areas are thought to be caused by sudden increases in solar activity such as solar flares. When bursts of high-intensity solar energy enter the ionosphere, areas of high ionization may occur in the E layer. We refer to these areas as sporadic E (Es). Es are spotty in geographical extent and in time. When they're formed, these areas are so highly charged that frequencies that normally use F-layer refraction never reach the F layer. Instead, frequencies are reflected to earth from the sporadic E areas.

Notice, we used the term "reflection" and not the term "refraction." This is because signals returned from sporadic E areas are almost the mirror image of the signals that enter them. TESTVirtually no penetration or absorption takes place. In the arctic regions, sporadic E is often associated with auroral activity. The existence of Es may be detrimental or beneficial to communications. Although very temporary, the Es area may offer a useful reflection medium that lets us use higher critical frequencies, thus improving communications during daylight hours. The Es area may also interfere with use of higher ionospheric layers, thus degrading the communications path.

F region. As a factor in HF communications, the F region is the most important part of the ionosphere. TESTMost sky-wave transmissions involve one or more refraction from the F region. During the day there are two separate layers in the F region, the F1 and F2 layers. At night these two layers combine to form a single F layer.

The F1 layer is the lower part of the daytime F layer. It extends from about 140 to 240km (85 to 150 miles) above the earth, and it exists only during daylight hours, disappearing at night. Maximum density of the F1 layer occurs shortly after noon, local time, when the sun is directly overhead.

The F2 layer has a range of from about 245 to 400km (155 to 250 miles) above the earth. It is present 24 hours a day but varies in altitude with geographical location, solar activity, and local time. TESTThe critical frequency for this layer will peaks after local noon and decreases gradually throughout the night. It is most useful for communication at night.

Long-term variables. Since radiation from the sun is the principal cause of ionization in the earth's atmosphere, solar activity and conditions in the ionosphere are closely related. This correlation seems to hinge on certain cooler, darkened areas on the sun's surface that we call sunspots. TESTAstronomers have developed an index of sunspot activity, called the sunspot number. Records of sunspot numbers date extend back about 200 years. Sunspot numbers have a well-documented 11-year cycle.

Figure 3-3 shows how noon and midnight critical frequencies vary in phase with variations in the sunspot number. The curves in the figure are based on smoothed data. Knowledge of each phase of the sunspot cycle is useful for predicting average sunspot numbers a few months in the future. Attempts at forecasting years in advance have not been successful because of variations in the 11-year cycle (fig.ure 3-4).

Another type of solar phenomena affecting ionospheric communications is solar flux. Flux is the amount of something (X-rays, radio energy, etc.) passing through a specified area in a given time period. Solar flux is the amount of electromagnetic radiation being emitted from the sun. It is measured by a solar flux unit that is the standard unit for reporting solar radio background flux and bursts. Normally, a higher flux indicates high ionization levels in the atmosphere. Measurements of solar flux are taken daily at the Dominion Astrophysical Radio Observatory (DARO) in Penticton, Canada. A solar radio flux summary is transmitted four times daily near 0015Z, 0640Z, 1240Z, and 1840Z by the Automated Weather Network (AWN). This summary contains a list of current and previous day's background solar flux measurements.

Short-term variables. An important characteristic of the ionosphere is its daily variation in structure. The D layer is ionized during the day but weakens and is unimportant at night. The E layer is useful for short-range communications during the day, but not at night. The single F layer that exists at night becomes the F1 and F2 layers during the day. These regular variations dictate the need for at least two frequencies for all around-the-clock sky-wave links—a day frequency and a night frequency.

Besides the predictable changes in the ionosphere, there are unpredictable variations related to disturbances on the sun. The effect of these disturbances on communications can range from a decrease in circuit quality on a few circuits for a few hours to an almost complete blackout of all HF sky-wave circuits for a day or two. Generally, high latitudes are more affected than low latitudes.

Since ionization in the ionosphere is caused mostly by the sun's radiation, any variation in solar activity affects the ionosphere, thus affecting sky-wave propagation. The actual number of layers, their height, and the intensity of ionization all vary from hour to hour, day to day, month to month, season to season, and year to year. There are four regular variations in the ionosphere: diurnal (daily), seasonal, 27-day variations, and 11-year sunspot cycles (fig. 3-5).TESTAnother type of solar phenomena affecting ionospheric communications is solar flux. Flux is the amount of something (X-rays, radio energy, etc.) passing through a specified area in a given time period. Solar flux is the amount of electromagnetic radiation being emitted from the sun. It Solar flux is measured by a solar flux unit that is the standard unit for reporting solar radio background flux and bursts. Normally, a higher flux indicates high ionization levels in the atmosphere. Measurements of solar flux are taken daily at the Dominion Astrophysical Radio Observatory (DARO) in Penticton, Canada. A solar radio flux summary is transmitted four times daily near 0015Z, 0640Z, 1240Z, and 1840Z by the Automated Weather Network (AWN). This summary contains a list of current and previous day's background solar flux measurements.

Short-term variables. An important characteristic of the ionosphere is its daily variation in structure. The D layer is ionized during the day but weakens and is unimportant at night. The E layer is useful for short-range communications during the day, but not at night. The single F layer that exists at night becomes the F1 and F2 layers during the day. These regular variations dictate the need for at least two frequencies for all around-the-clock sky-wave links—a day frequency and a night frequency.

Besides the predictable changes in the ionosphere, there are unpredictable variations related to disturbances on the sun. The effect of these disturbances on communications can range from a decrease in circuit quality on a few circuits for a few hours to an almost complete blackout of all HF sky-wave circuits for a day or two. Generally, high latitudes are more affected than low latitudes.

Since ionization in the ionosphere is caused mostly by the sun's radiation, any variation in solar activity affects the ionosphere, thus affecting sky-wave propagation. The actual number of layers, their height, and the intensity of ionization all vary from hour to hour, day to day, month to month, season to season, and year to year. There are four regular variations in the ionosphere: diurnal (daily), seasonal, 27-day variations, and 11-year sunspot cycles (figure 3-5).TEST

Diurnal effects. Obviously, the sun's radiation is not present at night. When solar activity is no longer present, the D, E, and F1 layers disappear, leaving only the F2 layer. The F2 layer decreases in altitude with the setting of the sun and combines with the remnants of the F1 layer to form a single nighttime F layer. (These variations were illustrated in fig.ure 3-1.)

Seasonal effects. The seasons affect atmospheric ionization because of the varying distance of the sun to certain areas of the earth and the varying angle of radiation on those areas. TESTThe height of the F2 layer increases greatly in summer and decreases in winter. Also F2 ionization density is stronger and rises earlier in the day during winter. Ionization density in the D, E, and F1 layers is weaker in the winter. What this means is that in winter we have a wider range of critical frequencies and less absorption of all frequencies. There is more variation between nighttime and daytime operating frequencies during winter than during summer.

Sunspot effects. Both the 27-day sunspot variations and the 11-year sunspot cycle have major effects on atmospheric ionization. They are called regular variations because they are predictable and occur at regular intervals. Sunspots are dark or cooler spots on the sun that change the sun's radiation intensity. TESTWhen the number of sunspots on the surface of the sun facing the earth is high, the amount of solar radiation from the sun is much higher, resulting in higher critical frequencies for the E, F1, and F2 layers, and higher absorption in the D layer. The number of sunspots on the surface of the sun facing the earth varies every 27 days due to the sun's rotation.

Along with the 27-day variations, sunspot activity varies in 11-year cycles. The overall effect on HF communications is that there will be higher critical frequencies occur during years of maximum sunspot activity. Operating frequencies will will normally be higher during these years, too (fig.(figure 3-6).

Irregular ionospheric variations. There are a variety of irregular variations of the ionosphere that affect our ability to communicate by sky waves (see fig.ure 3-7). The main cause of most irregular ionospheric disturbances is solar flares.

Solar flares. Solar flares are large, sudden releases of energy on the sun that are indicated by relatively short-lived brightening of localized regions. They erupt suddenly and have lifetimes from a few minutes to several hours (average of a half-hour), and they can have energy outputs equivalent to that of an explosion of a billion H-bombs (fig.(figure 3-8). When this energy enters the ionosphere, variations occur. Some of these variations help while others hinder sky-wave communications. Increased X-ray radiation, which often accompanies solar flares, can increase ionization in the D layer and decrease HF signal strength. The signal strength decrease varies with the intensity of the flare, the location of the HF path relative to the Sun, and the design characteristics of the HF system. Duration of signal strength decrease may be 45 -to 90 minutes for a large flare.

The two major types of phenomena associated with solar flares are the immediate effects and the delayed effects. The immediate effects occur simultaneously with the visible flare and consist of phenomena known as solar radio bursts and sudden ionospheric disturbances. The delayed effects occur 15 minutes to 72 hours after the flare and consist of ground-level events, polar cap absorption, geomagnetic

disturbances, and aurora. Immediate effects are most pronounced with large flares, but important effects may also occur with subflares.

The two major types of phenomena associated with solar flares are the immediate effects and the delayed effects. The immediate effects occur simultaneously with the visible flare and consist of phenomena known as solar radio bursts and sudden ionospheric disturbances. The delayed effects occur 15 minutes to 72 hours after the flare and consist of ground-level events, polar cap absorption, geomagnetic disturbances, and aurora. Immediate effects are most pronounced with large flares, but important effects may also occur with subflares.

Solar radio bursts. Solar radio emission originates as background radiation and as enhancements from bright regions and transient disturbances such as flares. Many flares are accompanied by increased emissions at the radio frequencies. The cause of this increased emission is believed to be the passage of particle streams through the earth's atmosphere.

Sudden ionospheric disturbances. The high degree of radiation from solar flares produces abnormally high ionization in all layers of the ionosphere. This increase in ionization occurs very suddenly throughout the daylight area of the earth and is called a sudden ionospheric disturbance (SID). SIDs occur almost simultaneously with a flare and may last from minutes to several hours. Normally, frequencies from about 1 or 2MHz to about 20MHz are made useless because of high absorption in the D layer. During a SID, higher frequencies (above 20MHz) may be used for long-distance communications. These frequencies refract off the increasingly denser F layers instead of passing through the ionosphere as they normally would. Lower frequencies, however, are will be absorbed so much by the denser ionosphere that they are will only be useful for short-distance ground-wave communications. SIDs only occur during daylight and are the most unusual of all atmospheric disturbances.TEST

Ionospheric storms. Ionospheric storms are actually strong magnetic disturbances in the upper atmosphere from 18 to 72 hours after SIDs. The occurrence of SIDs doesn't mean that ionospheric storms will follow. Unlike SIDs, ionospheric storms can be present during the day or night. Critical frequencies will drop below normal, which limits use of higher frequencies. The worst effects occur in the auroral zones near the north and south poles, decreasing toward the equator. These storms can last from a few minutes to several hours and all effects will disappear in a few days. During ionospheric storms, communicators should use lower operating frequencies, especially at the higher latitudes.TEST

After you complete these questions, you may check your answers at the end of the unit.

We have learned that radio waves can take several paths from a transmitter to a receiver. Long-distance HF radio transmission uses sky waves; and short-distance transmission uses ground waves as figure 3-9 shows. Ground-wave propagation is affected by the earth's electrical characteristics and by the amount of diffraction around the curvature of the earth. These characteristics vary in different localities, but they're relatively constant with respect to time and season. Sky-wave propagation is variable, since the constantly changing state of the ionosphere has a definite effect on the refraction of the waves.

Polarization. Radio waves consist of an electric field and a magnetic field. The polarization of an electromagnetic wave is defined as the plane of vibration of its electric field. For instance, a wave with a vertical electric field is said to be vertically polarized. A wave with a horizontal electric field is said to be horizontally polarized. The electric lines of force and the corresponding magnetic lines are always at right angles to each other and to the direction of propagation. A horizontally polarized wave has an electric field parallel to the earth's surface. A vertically polarized wave has an electric field perpendicular to the earth's surface. Figure 3-10 shows horizontally and vertically polarized waves.

The polarization of the propagated wave is determined initially by the type and arrangement of the transmitting antenna. As a rule, a vertical conductor radiates a vertically polarized wave. A horizontal conductor radiates a horizontally polarized wave. More complex forms, such as circular and elliptical polarization, in which the direction of maximum voltage rotates in space at the frequency of transmission, are also possible. These complex waves are generated by special antennas (e.g., a helix antenna), or may be developed accidentally when linearly polarized waves pass through nonuniform media such as the ionosphere.

Horizontally polarized waves are weakened more rapidly in traveling over the ground than are vertically polarized waves. At high frequencies, the polarization of sky waves usually varies, sometimes quite rapidly, and often is elliptical because the wave splits into several components that follow different paths. Ground waves usually retain the polarization characteristics they had when the wave left the antenna.

The performance of a receiving antenna is improved if it can be oriented to take advantage of the polarization of the incident wave. However, as a consequence of random changing of the polarization of HF waves as they travel through the ionosphere, matching the polarization of transmitting and remote receiving antennas is not essential. If possible, both vertically and horizontally polarized antennas should be tried. Where circuit requirements dictate surface-wave propagation, it's important that the antennas at both ends of the path have the same polarization. However, vertically polarized antennas provide the most effective surface-wave coverage.

Polarization. Radio waves consist of an electric field and a magnetic field. The polarization of an electromagnetic wave is defined as the plane of vibration of its electric field. For instance, a wave with a vertical electric field is said to be vertically polarized. A wave with a horizontal electric field is said to be horizontally polarized. The electric lines of force and the corresponding magnetic lines are always at right angles to each other and to the direction of propagation. A horizontally polarized wave has an electric field parallel to the earth's surface. A vertically polarized wave has an electric field perpendicular to the earth's surface. Figure 3-10 shows horizontally and vertically polarized waves.

Horizontally polarized waves are weakened more rapidly in traveling over the ground than are vertically polarized waves. At high frequencies, the polarization of sky waves usually varies, sometimes quite rapidly, and often is elliptical because the wave splits into several components that follow different paths. Ground waves usually retain the polarization characteristics they had when the wave left the antenna.

The polarization of the propagated wave is determined initially by the type and arrangement of the transmitting antenna. As a rule, a vertical conductor radiates a vertically polarized wave. A horizontal conductor radiates a horizontally polarized wave. More complex forms, such as circular and elliptical polarization, in which the direction of maximum voltage rotates in space at the frequency of transmission, are also possible. These complex waves are generated by special antennas (e.g., a helix antenna), or may be developed accidentally when linearly polarized waves pass through nonuniform media such as the ionosphere.

The performance of a receiving antenna is improved if it can be oriented to take advantage of the polarization of the incident wave. However, as a consequence of random changing of the polarization of HF waves as they travel through the ionosphere, matching the polarization of transmitting and remote receiving antennas is not essential. If possible, both vertically and horizontally polarized antennas should be tried. Where circuit requirements dictate surface-wave propagation, it's important that the antennas at both ends of the path have the same polarization. However, vertically polarized antennas provide the most effective surface-wave coverage.

Ground-wave propagation. A propagated ground wave takes three separate paths to the receiver. They are the direct wave, the ground-reflected wave, and the surface wave, as shown in figure 3-11. The effectiveness of ground waves depends on the radio frequency, transmitter power, transmitting antenna characteristics, electrical characteristics (conductivity and dielectric constant) of the terrain, and electrical noise at the receiver site. Low and very low frequencies are propagated much better by surface path than are higher frequencies. TESTWhen high-powered transmitters and efficient antennas are used, the surface path has a maximum range of about 500km (300 miles) at 2MHz. Surface path range decreases as frequency increases. About 80km (50 miles) represents the usual minimum range.

Direct path. The direct wave is the ground-wave component that travels directly from the transmitting antenna to the receiving antenna. In terrestrial communications, the direct path is limited by the distance to the horizon from the transmitter. This is essentially line-of-sight distance. It can be extended by increasing the height of the transmitting antenna, the receiving antenna, or both. The direct path is also useful for extraterrestrial communications. It's useful in air/ground/air communications because most short-distance air/ground services are now on VHF or UHF.

Ground-reflected path. The ground-reflected path reaches the receiving antenna after being reflected from the ground or sea. Upon reflection from the earth's surface, the ground wave undergoes an 180°-degree phase shift. Since the reflected path travels longer reaching its destination, a phase displacement somewhat greater than the 180°-degree shift caused by reflection results. The net result near the ground is a weakening of the direct wave. This weakening is roughly equal to the strength of the reflected wave.

Surface-wave path. The surface path is the ground-wave component that's affected mainly by the conductivity and the dielectric constant of the earth. When both transmitting and receiving antennas are close to the ground, the direct and ground-reflected paths tend to cancel each other. The surface path is not confined to the earth's surface. It extends up to considerable heights, diminishing in strength with increased height. Its intensity becomes negligible at about 1one wavelength over ground and 5five to 10ten wavelengths over sea water.

The ground absorbs part of the surface path's energy. The ground attenuates the electric intensity of the surface wave. This attenuation depends on the conductivity of the terrain over which the wave travels. Figure 3-12 shows the relative conductivity for various types of terrain. The best type of surface for surface-wave transmission is sea water. The electrical properties of the terrain that determine the attenuation of the surface-wave field intensity vary little. This type of transmission has relatively stable characteristics.

The earth's short-circuiting effect severely attenuates the electric field of a horizontally polarized wave. It has much less effect on vertically polarized waves. That's why we normally transmit a vertically polarized wave when we use the surface path for communications. Radio waves travel slower over the earth's surface than in the air, resulting in a forward tilt of the wave-front. This forward tilt means that the wave is being directed toward the surface. This explains the satisfactory performance of antennas only slightly above the ground. Poor conduction surfaces cause high loss and greater tilt.

Sky-wave propagation. HF wave propagation takes place by ionospheric refraction. As an oblique radio wave enters a region of increasing electron density, its phase velocity increases in proportion to the density. This increase in phase velocity results in a refraction of the wave away from the direction of increasing electron density. Refraction gives us the ability to communicate by radio waves beyond the optical line-of-sight. The normal HF propagation methods are by refraction in the F layer for the single hops and by reflection between the ground and the F layer for multiple hops (fig.(figure 3-13). Notice the difference between refraction and reflection. Radio waves are refracted by the ionized layers and reflected by the earth. Refraction decreases with an increase in frequency. This course follows the usual practice of referring to reflected waves when no ambiguity in meaning will results.

Propagation distance also depends on the angle at which the wave enters the propagation medium (in HF, the ionosphere). TESTThis angle is called the angle of incidence. As the angle of incidence increases, the amount of wave refraction decreases. From this you can see that we can change the frequency, the angle of incidence, or both to get different degrees of refraction.

Propagation distance also depends on the angle at which the wave enters the propagation medium (in HF, the ionosphere). TESTThis angle is called the angle of incidence. As the angle of incidence increases, the amount of wave refraction decreases. From this you can see that we can change the frequency, the angle of incidence, or both to get different degrees of refraction.

Virtual height. The point of the ionosphere from which a radio wave appears to have been refracted is called the virtual height of the ionosphere. Thus, virtual height is the altitude that refraction occurs. (See point H in fig.ure 3-14.)

Critical angle. Figure 3-14 shows the sky-wave signal that is not bent back to earth because of its high takeoff angle (line CD). The critical angle of a given frequency is the highest angle at which you can send a radio wave into the ionosphere and have it return to earth. Unlike the critical frequency, the critical angle is not applicable to any single ionospheric layer. The critical angle applies to the refraction of a single frequency from any part of the ionosphere. When radio electromagnetic energy is radiated from an antenna, it travels as a wave-front that's like the contour of a balloon.

Waves radiate from the antenna at many different angles. The wave front that's at the right angle of radiation is useful to sky-wave communications. Waves above the critical angle will pass through the ionosphere. Those angled too low are will be absorbed before refraction can occur. In figure 3-15, frequency "A" is shown entering the ionosphere at two angles. At a radiation angle of 55° degrees, frequency "A" passes through the ionosphere. At a radiation angle of 45° degrees, the same frequency is refracted back to earth. Figure 3-15 also shows a frequency (frequency "B") that's not refracted even at 45° degrees, because it's a higher frequency. That means that the critical angle of radiation for frequency "A" is somewhere between 45° degrees and 55° degrees. The critical angle of radiation for frequency "B" is below 45° degrees.

In HF communications, the takeoff angle of an antenna can determine whether a circuit is successful or not. HF sky-wave antennas are designed for specific takeoff angles and are used for short-range communications; low takeoff angles are used for long-range communications.

Our antennas give us some control over the takeoff angle of our signals. Sometimes we can change their angle of radiation to match our operational frequencies. However, most of the time our antenna radiation patterns are predetermined and can't be easily changed. Those antennas are also directed toward certain receiving stations that may not be reached if takeoff angles are changed.

In HF communications, the takeoff angle of an antenna can determine whether a circuit is successful or not. HF sky-wave antennas are designed for specific takeoff angles are used for short-range communications and low takeoff angles are used for long-range communications.

Our antennas give us some control over the takeoff angle of our signals. Sometimes we can change their angle of radiation to match our operational frequencies. However, most of the time our antenna radiation patterns are predetermined and can't be easily changed. Those antennas are also directed toward certain receiving stations that may not be reached if takeoff angles are changed.

Frequencies also have a bearing on our critical angle of radiation. If we know that one of our regular operating frequencies will will not refract off the ionosphere (due to atmospheric variations) at the angle of radiation required for our communications path, we can simply change to a frequency that will. To do this we have to know the maximum usable frequency (MUF) for that particular path. The MUF is the highest frequency that allows a wave to reach a particular destination on a given path. The MUF for a certain path will variesy according to regular and irregular ionospheric variations. Later we'll discuss two ways to predict ionospheric variations and to determine the critical frequencies, critical angles of radiation, and maximum usable frequencies.

Critical frequency. As we increase the frequency of the transmitted signal at vertical incidence, the wave is returned to earth from successively higher ionospheric layers. As the increase in frequency continues, we reach a frequency that will penetrates the F2 layer and won't return to earth. The highest frequency at which a vertical signal will will be returned to earth is known as the critical frequency. Frequencies higher than this critical frequency pass into space. TESTFrequencies that are too low are absorbed in the D layer, and frequencies between the two boundaries are refracted back to earth (fig.(figure 3-16). Since the critical frequency increases with altitude, a signal that has passed through the E layer might be returned from the F1 or F2 layer. The critical frequency also varies, for a given layer, at different locations on the earth's surface. Generally, it's higher near the equator, where more of the sun's radiation is intercepted by the earth's atmosphere.

Maximum usable frequency (MUF). The MUF is the highest frequency that allows reliable long-range HF radio communication between two points by ionospheric refraction. The highest frequency that can be refracted depends on the angle of incidence, and hence, for a given layer height, on the horizontal length of the hop. The maximum frequency that can be refracted back for a given transmission path is the MUF for that path. The MUF is closely related to the critical frequency. Like the critical frequency, it changes with the time of day, season, solar activity, and geographic location. There is a range of usable frequencies, between the MUF and the lowest usable frequency (LUF), that needs to be predicted for operator use. The MUF and LUF vary with solar activity, season, and time of day. At times, the available range of usable frequencies may be reduced to zero.

Frequency of optimum transmission. Because of the strong increase of absorption with decreasing frequency, it is desirable to use as high a frequency as possible. From a purely physical point of view, a frequency very close to the MUF would be most suitable. This is impractical, since the MUF changes considerably from day to day and (for operational reasons) the working frequency can't be adapted to these changes. Even the monthly median of the MUF is unsuitable, since it's reached only during 50 percent of the days. In practice we chose a frequency that corresponds to a 90 percent probability of refraction. It's considered to be roughly 0.85 times the monthly median value of the MUF. The frequency thus chosen is called the frequency of optimum transmission (FOT). FOT is also called the optimum working frequency or the optimum traffic frequency.

Ordinary and extraordinary waves. In general, a wave propagating perpendicular to the earth's magnetic field has will have components of the electric field both parallel and perpendicular to the magnetic field. Such a wave in the ionosphere will splits into two refracted waves that travel different paths with different time delays. The process is termed magnetonic splitting. The waves are called the ordinary and the extraordinary. wave. TESTThe ordinary and the extraordinary waves are shown in figure 3-17. The extraordinary wave suffers greater absorption at higher frequencies and has a slightly higher critical frequency than the ordinary wave.

Measurement of layer heights and critical frequency. The simplest method of measuring heights of ionospheric layers is by vertical-incidence sounding. TESTA vertical-incidence sounding station is a combination of a transmitter and a receiver placed side-by-side, often using the same antenna. The transmitter sends out pulses of electromagnetic energy, which, during a period of about a minute, sweep in frequency through the range from 0.75 to 25MHz. The receiver is synchronized with the frequency of the transmitter. The output of the receiver is coupled to a display unit (usually, a cathode-ray tube). The display unit indicates the time required for the transmitted pulses to be returned as a function of the sweep frequency of the transmitter. We can get a permanent record by photographing the display. The travel time is used to determine the virtual height of the ionized layers. A plot of the measured height versus frequency is called an ionogram.

[Figure 3-175. Ordinary and extraordinary waves.

Measurement of layer heights and critical frequency. The simplest method of measuring heights of ionospheric layers is by vertical-incidence sounding. TESTA vertical-incidence sounding station is a combination of a transmitter and a receiver placed side-by-side, often using the same antenna. The transmitter sends out pulses of electromagnetic energy, which, during a period of about a minute, sweep in frequency through the range from 0.75 to 25MHz. The receiver is synchronized with the frequency of the transmitter. The output of the receiver is coupled to a display unit (usually, a cathode-ray tube). The display unit indicates the time required for the transmitted pulses to be returned as a function of the sweep frequency of the transmitter. We can get a permanent record by photographing the display. The travel time is used to determine the virtual height of the ionized layers. A plot of the measured height versus frequency is called an ionogram.

Skip zone. The skip zone is the area between the most distant point reached by the ground waves of a particular signal and the point at which the ionospheric wave first returns to the earth. Close to the antenna, strong ground-wave signals would be received. The signal strength of the ground wave drops off as you move away from the transmitting antenna. At a certain distance out, it is so weak that it is of no use. You now have reached the limit of the ground-wave range. Now, if you were to continue to travel outward, you would experience a zone of silence, called the skip zone. In the skip zone, no radio signals are received. Eventually, you would again begin to receive strong signals. This is the point at which the sky wave first returns to earth.TEST

Skip distance. The skip distance of a frequency is the distance from the transmitter to the point at which the refracted sky wave first returns to earth. Figures 3-9 and 3-15 illustrated both the skip distance and the skip zone of a signal. The skip distance is determined by many factors. The two most important factors are the frequency and the angle of radiation. These two factors determine the specific ionospheric layer in which refraction will occurs and the subsequent skip distance that will results. The relationships between these two factors and the skip distance are easily understood. The higher the frequency, the higher the ionospheric layer required to refract the wave, and consequently, the longer the skip distance (direct relationship). The lower the radiation angle, the further out the wave must travel before reaching the ionosphere and being refracted, and consequently the greater the skip distance (inverse relationship). Figure 3-18 shows skip distances at various angles of radiation.

Multihop paths. The wave paths in figures 3-13 and 3-17 have shown a single refraction from the ionosphere. In multihop transmissions, radio waves are refracted from the ionosphere, and they are then reflected from the earth's surface. Multihop transmission occurs when radio energy returns to earth, is reflected back into the ionosphere, and is refracted back to earth again. As a result, the radio wave reaches a distant receiving point after two or more hops (fig.(figure 3-14). Several such refraction/reflections can take place, and paths involving multiple refraction/reflections are called multihop paths. TESTEach time an additional hop is made, considerable signal strength loss occurs. This loss results primarily from absorption.

Multihop transmissions cause the radio wave to enter and re-enter the D layer as well as reflect off the earth's surface. Signal loss can be significant, and noise levels will normally increase. For effective communication with a station at a given point, it's desirable to use as few hops as possible. To a certain degree, we can control the number of hops by varying the angle of radiation. This is shown in the radiation angles of frequencies "A" and "B" in figure 3-14. This angle, in turn, depends on the frequency and the type of antenna used.

Long circuits may require multihop paths, but each reflection increases the signal loss and increases the possibility of multipath waves. Where possible, use a higher frequency for communications, and thus avoid a larger number of hops.

Multipath effects. A multipath signal occurs when a transmitted signal travels over two or more separate paths during transmission. When a signal is refracted more than once in different layers of the ionosphere, one refraction may return to the earth slightly ahead of the other. If your receiving antenna picks up this signal out of phase, distortion, fading, and complete cancellation of the signals can occur. On the other hand, if the two signals arrive in phase, the signal is strengthened. Looking back at figure 3-13, we can see that the receive antenna is receiving a multipath signal caused by the multihop and single-hop transmissions. In a previous section, you learned that multipath effects also occur in ground-wave transmissions and are caused by multiple reflections from the ground or other objects. No matter what the cause, multipath transmissions are extremely critical when you're transmitting digital or data traffic. You should try to avoid multipath transmissions by changing frequencies if possible.TEST

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