N this unit we briefly look at the historical development of radio communications and introduce the principles of electromagnetic (EM) radiation. We'll also discuss the characteristics and properties of radio waves, and take a detailed look at the electromagnetic spectrum. From the first radio transmissions between St. John's, Newfoundland, and Cornwall, England, radio communications have undergone continued improvements, evolving into the sophisticated systems now used to communicate with space vehicles millions of miles from earth and nuclear submarines cruising the ocean's depths. Research continues today as powerful radio transmissions are probing deep into the universe in an attempt to establish contact with alien life. You will discover radio communications are a unique and reliable form of communications and will continue to play a vital role in military, commercial, and scientific advances.

Let's start by defining communications as a means or system by which we exchange our thoughts, opinions, information, and intelligence with others. We're all familiar with many methods of communication. These methods may be simple and direct or highly technical. For example, people engaged in conversation, either directly or by telephone, are using a common and simple means of exchanging ideas.

Most of our civilization's great discoveries and inventions were made accidentally--not so with radio. The discovery of radio waves and the invention of the numerous tubes, transistors, resistors, and other components that make transmission and reception possible were part of an almost evolutionary process. This evolutionary process started more than a hundred years ago and has not ended. The invention of radio can neither be attributed to any one person nor traced back to a specific date. There are however, certain individuals who made important contributions to its initial development.

The 1800's. Before the discovery and development of electricity and radio, people used simple and crude methods for transmitting intelligence. The early Indians used smoke signals and drum beats to convey messages from one tribe to another. Although this type of communication was adequate for early man, it proved to be increasingly archaic as man progressed. The invention of the telegraph and telephone became milestones in the history of communications progress since they were radically different from any previous communications system. These systems used electrical devices for both the sender and the receiver, and a wire or cable as the medium for the transmission. Thus, it became possible to communicate between any two points on the face of the earth that could be bridged by a cable or wire.

The next significant stage in the process of message transmission was the development of a system of communication called the wireless. The wireless was superior to the telegraph and telephone since it used the air as a transmission medium rather than a wire or cable. Today wireless is known as radio communications. In 1865, a Scottish physicist, James Clerk Maxwell, made a startling prediction. He stated that any electrical or magnetic disturbance created in free space could be propagated (transmitted) through space as an electromagnetic wave. He went on to predict that this would be a transverse wave; that is, one in which the disturbance is at right angles to the direction of travel. In addition, he concluded that the speed of such waves would be approximately the speed of light: 186,000 miles per second. Maxwell also suggested that such waves could be created by setting up electrical vibrations in a wire capable of conducting electricity. These revolutionary predictions were proven to be correct.

During the last part of the 19^th century, a German scientist, named Heinrich Hertz, performed a series of experiments based on Maxwell's theories. His work revealed that electromagnetic waves could in fact be produced. He also proved that these waves were invisible and moved at the speed of light. Hertz's experiments further showed that while light waves are only a few thousandths of an inch long, electromagnetic waves vary in length from millimeters to thousands of miles long. Simultaneously, many other scientists throughout the world were working and experimenting with the propagation of electromagnetic waves. Most of their experiments were of little or no practical value. Nevertheless, they all contributed to the eventual development of radio. Their research proved their proposed systems did not work and thus saved other scientists, like Hertz, time and experimentation.

By 1895, enough information was available to enable an Italian scientist and inventor, Guglielmo Marconi, to develop a crude, but working, radio-telegraph system. In 1901, Marconi succeeded in transmitting the letter S (three dots in Morse code-- . . .) across the Atlantic Ocean. This was the start of transoceanic radio communications. For his invention, Marconi was awarded a Nobel prize in physics.

Due to equipment limitations, the first radio sets operated at the low frequency (LF) and medium frequency (MF) end of the radio-frequency (RF) spectrum. These two frequency bands offered good voice and low-speed teletype communications, but their transmission distance was limited due to the rather low power outputs available at the time. In the 1890's, experiments began with the use of the higher frequencies. Unfortunately, the ideas for the use of the higher frequencies were born quite a while before science could produce the components required for reliable operation.

The 1900's. High-frequency (HF) communication was first made practical in the 1920's when the first actual radio system was installed in Europe. In County Galway, on Ireland's western fringe, Italian radio pioneer Guglielmo Marconi set up one of the first transatlantic wireless stations. On 15 June 1919, with generators fueled by peat, the station notified London of the successful flight of two British aviators. This was the first nonstop transatlantic flight.

The desire to go to higher frequencies was caused by the need for longer range, higher capacity circuits. Until HF came about, transatlantic communication was by cable or mail. Cable systems were very limited in capacity and sending messages was extremely expensive; mail was rather slow. With HF radio, transatlantic communication became faster, had greater capacity, and was less expensive. From this point in history to the present, radio technology increased dramatically. World War II had a profound impact on the use of the radio-frequency spectrum. Military leaders realized higher capacity communications were needed. Naturally, the solution was to go to even higher frequency bands. During the early part of the war, a system called radar was developed. The development of components and equipment to operate at the higher radar frequencies led to the development of higher frequency radio systems.

Developments during the war led to the development of very high frequency (VHF) and ultrahigh frequency (UHF) radio systems. Along with these systems came the idea of line-of-sight (LOS) microwave and tropospheric (TROPO) scatter systems. Unfortunately, it was found that using these higher frequency bands caused the distance range to be shorter than with HF. So until the late 1950's, long-range radio communication had to remain in the HF band, even with its limitations.

With the advent of the space program, radio engineers realized they could now get long-range communications at the higher frequencies by using satellites as radio relay stations. Thus came the development of satellite communications systems. Today, practically all of our long-range communication goes through satellite links. Since the first communications satellite was placed in orbit, satellites have been thought of as "the" communications system. However, as seen from a military viewpoint, satellite systems--and most other radio systems--have some weaknesses.

Because higher frequency systems have weaknesses associated with their method of radio-wave propagation, lower frequency systems are taking on more importance. Studies and experiments have indicated that if there is a nuclear blast, most--if not all--of our higher frequency systems would be adversely affected. Since the military always requires communications, low frequency (LF), very low frequency (VLF), and extremely low frequency (ELF) communications systems have been undergoing development since the early 1960's.

As you can see, radio communication has changed markedly over the last century. There's still much to be learned, and research continues in all areas of the electromagnetic spectrum.

407. Nature's force fields and electromagnetic radiation

There are three major force fields in nature: gravitational, electric, and magnetic. In radio, we are concerned with the electric and magnetic fields. No one knows the exact composition of these fields, but scientists have gathered sufficient information to be able to predict how they behave. Although an accurate representation can't be pictured, you can visualize a field as consisting of lines of force. In school, you may have seen the experiment with magnetic lines of force, where iron filings were scattered on a piece of cardboard. When a magnet was placed under the cardboard the filings arranged themselves in a pattern that outlined the magnetic lines of force between the two poles of the magnet (fig. 2-1). Even before Maxwell's time, it was known that the following relationships existed between the electric and magnetic fields:

• When an electric field is changing, a magnetic field is being created.
• When a magnetic field is changing, an electric field is being created.

Changing electric fields. To discuss the phenomenon of changing electric fields, we must briefly explain the principle of electric current. There are two types of electric current: direct current (DC) and alternating current (AC). Direct current is simply a flow of electrons through a wire from a negative to a positive charge. A flashlight battery, for example, produces direct current. Alternating current, on the other hand, is not a steady flow. Rather, it is continually changing in magnitude and periodically changing in direction. The rate at which the current changes direction is twice the frequency of the alternating current. This means that if the current alternates (changes direction) 120 times per second, its frequency is 120 divided by 2, or 60 cycles per second (cps). (The term "cycle" means one complete set of events or phenomena that occurs periodically. Thus, a cycle consists of two complete alternations.) Incidentally, the standard frequency for generation of electrical power in the United States is 60 cps.

Electromagnetic fields. Radio transmission is made possible because alternating current changes in magnitude and reverses its direction during each cycle. As you recall, radio waves are electromagnetic fields of force; that is, magnetic fields generated by continually changing electric fields. As alternating current moves back and forth in a wire, magnetic fields are created around the wire. With each current increase, the magnetic fields expand; and with each current decrease, they collapse. The magnetic field collapses completely at the instant current is at the zero point, just before starting the next alternation. Figure 2-2 shows a magnetic field around a conducting wire. This field is continuous along the wire, not just in sharply defined circles as shown in the illustration. It also expands and collapses with changes in the direction and magnitude of the AC current.

Electromagnetic propagation. The magnetic fields created around a wire conducting a 60-cps current are very small. They extend only a short distance from the wire, and collapse with each current reversal. As the current increases in frequency, however, the magnetic fields have more and more difficulty in collapsing completely with each alternation. When the frequency reaches a certain point, somewhere around 10,000 cps, the magnetic fields no longer have time to collapse completely between alternations. Instead, they are pushed away from the wire by the fields produced by the succeeding alternations. This is the principle of electromagnetic propagation.

Alternating current in a conductor creates magnetic fields that expand and collapse with each alternation. At frequencies below approximately 10,000 cps, these fields collapse completely between alternations. However, at frequencies above 10,000 cps, these fields no longer collapse completely. Instead, they are pushed away (radiated) from the conductor. They travel out from the conductor as electromagnetic energy, commonly known as radio waves. This radiation is composed of two perpendicular waves: one electrostatic in nature, the other magnetic. Both of these waves are at right angles to the direction of propagation as shown in figure 2-3. These two waves are in time phase with each other and travel at a constant speed through space. This speed (186,000 statute miles per second, 162,000 nautical miles per second, or 3 × 10⁸ meters per second) is the speed of light.

Before we go any farther, remember that frequency is normally expressed in "Hertz" (Hz), not cycles per second (cps), even though both terms have identical meaning. This was done to honor Heinrich Hertz who, in 1887, demonstrated that electromagnetic energy could be sent out into space as radio waves. One Hz equals one cps. Other terms used to express frequency are "kilo" (thousand) hertz (kHz);

"mega" (million) hertz (MHz); "giga" (billion) hertz (GHz); and a new designation, "tera" (trillion) hertz (THz). This last portion of the electromagnetic spectrum (electro-optical) is being used for communications using light waves transmitted by laser beams.

Radio-wave creation. As previously mentioned, the creation of radio waves requires a current whose frequency is at least 10,000Hz. That is, it must be in the radio-frequency (RF) range. Mechanical generators, which are capable of developing frequencies up to 100kHz, are not adequate to cover the useful RF range (10kHz to 300GHz). The solution to this problem has been found in the electronic oscillator. The oscillator is an electronic device for creating voltages that can be made to surge back and forth at whatever frequency is desired. (Since the output of an electronic oscillator is RF energy, we normally refer to it as an RF oscillator.) When RF energy is applied to a conductor (antenna), the antenna resonates (vibrates). The antenna provides a means of radiating the electromagnetic (EM) waves into the air. Thus, we are well on the way to having a complete radio circuit. Later in this volume, specific transmitter components and various sections of the electromagnetic spectrum are covered in detail.

Understanding how to create and radiate EM waves is only the first step toward comprehending radio communications. Once a radio wave leaves the antenna, there are many factors that need to be understood and considered before actual communication can take place. As a start, let's look at the properties of a radio wave as it propagates through space.

Wave motion. The EM wave form is most commonly illustrated as in figure 2-4. This wave form represents the wavelength and amplitude characteristics of an EM wave. By tracing along the wave form through points A, B, C, D, and E, one complete cycle is outlined. If it takes one-thousandth (1/1,000) of a second for this cycle to occur, the wave form would then represent a frequency of 1,000 hertz.

The movement of radio waves through space can best be explained by comparing them to the movement of waves in a pond. (Figure 2-5 illustrates some of the similarities.) When you toss a rock into the center of a pond, a disturbance is created in the water. This disturbance (ripples or waves) spreads rapidly in equally spaced circles, and soon the leaves that are floating on the water begin to bob up and down. In other words, the energy that was transferred to the water by tossing the rock into the pond has now been transmitted (propagated) to the leaves, making them move up and down.

An electromagnetic signal radiated from an antenna creates an electromagnetic disturbance (wave) that spreads outward from an antenna through space. As water molecules were used to propagate energy in figure 2-5, controlled EM disturbances in space are used to propagate the waves created by the antenna. When these waves reach another antenna, some of their energy is transferred to that antenna, just as wave energy was transferred to the leaves floating on the water in the pond. These EM waves set up a small current in the antenna, which is then amplified and reproduced as a radio signal by a radio receiver.

Wavelength. Wavelength is a measure of the distance between two successive crests or any corresponding points on two consecutive cycles of a sine wave; it is also the distance traveled by a wave during the time interval of one cycle. This distance normally is expressed in meters or feet. All waves of electric and magnetic fields travel at the speed of light, which is 186,000 miles per second, or 300,000,000 meters per second, or 984,300,000 feet per second. Since speed is constant, the more cycles that pass a given point in a given amount of time, the higher their frequency, the shorter their wavelength. The relationship between frequency and wavelength is illustrated in figure 2-6. The equation used to determine wavelength is expressed as:

With this formula, you can calculate the distance a wave travels in one cycle (the wavelength) or the length an antenna needs to be to resonate at a specific frequency.

In free space (no atmosphere) the speed of light expressed in feet is about 984,300,000 feet per second, or 300,000,000 meters per second. Thus, to find the length of a full wavelength antenna use the above formula substituting 984,300,000 to find the distance in feet or 300,000,000 to find the distance in meters--that is, if your antenna was in free space.

Since our antennas aren't erected in free space, they don't operate in the most efficient manner and must be adjusted accordingly. In the HF band, a practical antenna is about 5 percent shorter than the same antenna in free space. Therefore, the previous formula is adjusted downward 5 percent. 300,000,000 mps now becomes 285,000,000 mps, and 984,300,000 fps now becomes 936,000,000 fps. (For practical purposes these feet per second figures are normally rounded off to their nearest million. To be specific, 984,300,000 less 5 percent is 935,085,000 fps.)

As a result of this adjustment, we use these formulas in computing the wavelengths of terrestrial antennas in feet:

To find the length in meters, substitute meters per second for feet per second. Understand that dipole/doublet antennas are center-fed, half-wave antennas. Therefore, to find the overall length of the antenna, use the half-wave formula (for example, 468/MHz); but to find the length of each leg of the antenna use either the half-wave formula and divide the product by 2, or use the quarter-wave formula. On the other hand, the full wavelength formula is used in figuring the length of long-wire antennas. We'll look at wavelength again in the text on antennas. Remember, no matter what part of the frequency spectrum used, wavelength is inversely proportional to frequency. That is, the higher the frequency, the shorter the wavelength, and, the lower the frequency, the longer the wavelength.

Frequency is defined as the number of complete cycles per unit of time for a periodic phenomenon. The EM waveform and thus the EM frequency spectrum are categorized by their periodic characteristics. The entire spectrum (fig. 2-7) extends from direct current (DC) with zero cps to cosmic rays above 10²³Hz. (The term "hertz" (Hz), the international unit of frequency, is now more commonly used than cps.)

Frequency band designators. Each frequency range has a band designator and each range of frequencies behaves differently and performs different functions. The following is a descriptive designation of international designators:

While you may be familiar with many of the systems that operate within these frequency bands, a few examples in figure 2-8 give a better understanding of the types of intelligence they carry.

For communications purposes, the usable frequency spectrum now extends from about 3Hz, through 300GHz, and up to about 100THz, where research on laser communications is taking place. This frequency spectrum is shared by civil, government, and military users of all nations according to International Telecommunications Union (ITU) radio regulations. In radio operations, we're mainly concerned with the audio-frequency and radio-frequency ranges.

Audio-frequency range. Frequencies that are ordinarily heard by the average person are said to be in the audio range. Although the audio range of any two persons may be very different, it is considered to be those frequencies between 15Hz and 20,000Hz. For example, the lowest note on a piano is approximately 32Hz, while the frequencies of human speech fall approximately between 200 and 2,500Hz. The range of a pipe organ is from about 16Hz to 5,000Hz, and the highest fundamental note of the flute is about 4,000Hz. The high-pitched whine from a jet engine may be above 10,000Hz.

RF range. The RF range extends from about 10,000Hz to over 300,000,000,000 (300GHz). For convenience, the Federal Communications Commission (FCC) has divided the RF spectrum into different bands. These frequency bands, their uses, their characteristics, and their advantages and disadvantages are addressed in detail later in this unit. As weather personnel, we're mainly concerned with radio operations in the HF, VHF, UHF, and sometimes SHF frequency bands for our communications. However, with the added emphasis of satellite communications to our career field, expect more involvement with communications equipment operating through the EHF frequency band.

In selecting a communications system for use, we must consider many factors. Besides frequency, power, and equipment configuration, we must also consider such effects as atmospheric absorption, rain, vegetation, and terrain. Then, there's the susceptibility of the various RF bands to electronic warfare. As you can see, it's not an easy task to determine the best mode of transmission for your traffic. (Sometimes it'll be easy--your radio will only have one mode of operation, you'll only have so many authorized frequencies, and your equipment configuration will be fixed.) However, all these considerations are secondary to the two principal elements of radio-wave propagation: the type of radio wave and the transmission path(s) of the radio wave.

Radio waves travel through free space at the speed of light and can be reflected, refracted, or diffracted. The effect of the atmosphere on radio waves is a problem which has complicated the use of communications systems since before World War II. In our discussion of radio systems and antennas we normally discuss antennas designed and constructed to direct radio waves in a specific direction. These types of antennas work by focusing and directing the EM energy into specific patterns of radiation and thus forming radio "beams."

Types of radio waves. Primarily, there are three types of radio waves, the ground wave, sky wave, and direct wave. Since ground waves travel near the surface of the earth, they're greatly affected by the earth's conductivity and by any obstruction (such as mountains or buildings) on its surface. Ground-wave transmission is used mainly for local communications.

The sky wave is an electromagnetic wave propagated at such an angle that it travels up through the atmosphere, strikes its upper layer (the ionosphere), and refracts back toward the earth. Sky-wave transmission is used in long-distance communications.

Figure 2-9 shows the various radio-wave paths. The radio waves (beams) represented are simplified, of course. All radio waves emitted by an antenna have the components shown in figure 2-9; that is, surface, ground-reflected, direct, refracted, and sky. However, radio waves of different frequencies are affected by the environment in different ways. As an example, lower frequency waves are easier to propagate by surface wave than any other means because they follow the contour of the earth and penetrate obstacles more easily. Higher frequency waves propagate more easily via sky and direct wave because they are easily absorbed by obstacles. For these reasons, a particular type of antenna is usually used for a given radio system.

The propagation of electromagnetic radiation depends on the conditions existing within the atmosphere, including variations in temperature and pressure, as well as the various components making up the atmosphere. The way radio waves react to atmospheric conditions depends on the radio frequency being used.

RF transmission paths. Radio waves are classified according to the paths they take from transmitter to receiver: ground waves (surface waves) along the surface of the earth, sky waves reflected back to earth from the troposphere and ionosphere, and direct waves from antenna to antenna (line-of-sight).

The act of a radio wave traveling from one point to another is called propagation. When a radio wave is radiated from an antenna it may start its journey in a variety of directions. However, we normally use only one path to reach the station with which we want to communicate (distant end). These transmission paths can be short or long, may travel along the ground, or be reflected from the upper parts of the atmosphere. The primary transmission path of a radio wave is determined by the propagating characteristics of its frequency and the direction and manner in which it is radiated by the antenna. Remember, all radio waves are propagated by one of three primary transmission paths: direct waves, ground waves, or sky waves. Again, figure 2-9 illustrates these transmission paths.

Direct waves. Direct waves are those which travel through the air in a straight line (line-of-sight or LOS) from the transmitting antenna to the receiving antenna. Direct waves continue to travel in a straight line until they are interrupted by an object or weaken over a great distance. The average distance of direct-wave communications is therefore limited by the height of the transmit or receive antenna. At frequencies greater than 30MHz (VHF and above) with antennas at ground level a direct wave is normally limited to under 20 miles. This is due to the curvature of the earth. Of course, if you increase the height of either antenna you will be able to increase the distance between the antennas. By eliminating obstructions, long-range UHF or SHF satellite communications or VHF/UHF communications with aircraft are possible using direct waves.

Ground waves. Radio waves that travel close to the earth are called ground waves. When these are transmitted over the earth, they take three separate paths to the receiver: a direct path, a surface path, and a ground-reflected path. Depending on the conductivity of the earth, the surface path may be more useful for communications from one ground station to another when lower frequencies are used. Conductivity is a measure of the ability of a medium to conduct electric current, or the efficiency with which a current is passed. The earth's conductivity is determined by the type of soil and water in the propagation path. Soil with poor conductivity quickly attenuates (weaken) radio signals. Note the conductivity characteristics in figure 2-10 and you can see quite a difference between types of terrain. If a ground wave was transmitted over sea water, the direct path would only travel the short line-of-sight distance, but the surface path might travel up to 700 miles.

All radio waves can reflect, to some degree, off certain surfaces. The higher the frequency, the shorter the wavelength, and so the greater the chance that the wave can be deflected or reflected. In ground-wave communications, the most probable reflector is the ground. If ground-reflected waves reach the receive antenna out of phase with the direct wave, there'll be some fading of the received signal. On the other hand, if the direct waves and reflected waves arrive in phase, the received signal is increased. These effects are called "multipath effects."

Sky waves. Sky waves are those waves that travel upward and are redirected by atmospheric properties back to the earth. High above the earth these radio waves meet the ionosphere, which consists of layers of gases ionized by the ultraviolet rays of the sun. Passing through these ionized layers, radio waves are bent from their original course. Sky-wave communications become possible when the bending of the waves is great enough to return them to earth. Sky-wave transmissions are very effective for long-distance communications in the high-frequency range (3 to 30MHz).

In this lesson we look at the following frequency bands:

• Extremely low frequency.
• Very low frequency.
• Low frequency.
• Medium frequency.
• High frequency.
• Very high frequency.
• Ultrahigh frequency.
• Superhigh frequency.
• Extremely high frequency.

Extremely low frequency (ELF). The ELF frequency range is from 3 to 30Hz, and it can transmit signals 5,000 miles or more. As currently used, ELF propagates through the earth's substrate. ELF waves produce high-power sounds that can penetrate ocean depths to several hundred feet. ELF communications systems require enormous transmit antennas covering thousands of acres and operating at very high transmitting powers--in the 100-megawatt range. Called transducers, these "antennas" transfer the transmitted RF energy to the earth and vice versa. The distance range of ELF is greater than that of any other terrestrial communications system, and it is not greatly affected by atmospheric disturbances. This area of the frequency spectrum is used primarily for underwater communications.

Operating in the range of audible sound, ELF is capable of only very low transmission rates. This slow data rate makes ELF transmissions impractical for normal character message transmission and impossible to use with current communications security (COMSEC) devices. ELF traffic is used mainly to communicate with submerged submarines. Messages are only one or two characters in length and are transmitted by interrupted continuous wave (ICW).

Very low frequency (VLF). The VLF frequency range is from 3 to 30kHz. Like ELF, VLF transmissions can span 5,000 miles or more and penetrate vegetation and water. VLF is used mainly for navigation and to communicate by low-speed secure teletypewriter with submarines at sea when they're submerged at shallow depths (about 10 feet). While VLF transmitters are normally shore-based, certain command and control (C2) aircraft such as airborne command posts may have a VLF capability, using long trailing wire antennas and transmitters powered in the 100 to 200 kilowatt (kW) range.

While VLF transmissions are capable of higher data rates than ELF transmissions, they're still limited. VLF broadcast systems use minimum shift keying (MSK) and operate low-speed, 50 baud, secure teletypes. A very common mode of operation on VLF circuits is ICW. An anti-jam (AJ) capability does exist, but it reduces the data transmission rate dramatically--to about three characters every 12 seconds.

Low frequency (LF). The LF range is from 30 to 300kHz and can span distances of 1,000 to 5,000 miles. LF is used for medium-distance communications, particularly with submarines and surface ships at sea, and for navigation. Airborne operations can be conducted efficiently using LF. While LF can penetrate vegetation and water, it is less effective than ELF or VLF. Current shore-based LF communications systems use 50 to 100kW transmitters and use frequency shift keying (FSK) for secure teletypewriter or International Morse Code (IMC) for communications operations.

Using FSK and appropriate COMSEC equipment, LF can transmit in a secure teletype mode at 75 baud (which equates to approximately 100 words per minute (wpm)).

Medium frequency (MF). The MF range is from 300 to 3,000kHz. MF propagates by ground wave, direct wave, and sky wave. MF can span from 100 to 1,000 miles by ground wave and from 1,000 to 3,000 miles by sky wave, depending on the transmitter output power and the atmospheric conditions. The main uses of the MF band include medium-distance communications, radio navigation, and amplitude modulation (AM) broadcasting.

The 550 to 1,600-kHz part of this frequency band is mainly used for AM broadcasting. A 10kHz separation standard between stations, results in 105 available audio channels. The MF band can support low-capacity multichannel circuits for both voice and teletype operations, with the latter limited to 75 baud (100 wpm). Security is available through voice and data COMSEC devices.

High frequency (HF). The HF part of the spectrum can transmit signals by ground-wave or sky-wave propagation. Ground-wave propagation is effective from 30 to 300 miles. Sky-wave propagation can span the world--depending on atmospheric conditions and the frequency used. HF is widely used for long-distance communications, short-wave broadcasting, over-the-horizon (OTH) radar, and amateur radio. HF transmitter power can range from as low as 2 watts to above 100kW, depending on the intended use.

In the HF range, two-way voice and data (record) communications can be supported in various ways. This includes point-to-point broadcast and air/ground/air operating modes using upper or lower sidebands. Besides long-range communications, HF is also widely used in tactical environments to supplement communications when LOS radio isn't possible or feasible.

Another HF mode is short-range near-vertical-incidence sky wave (NVIS) used with the NVIS antenna. The NVIS is useful when stations are separated by obstacles (such as mountains). When direct communication isn't possible, a NVIS antenna can radiate an HF signal almost straight up for reflection down (over a mountain peak) to another station only a few miles away. NVIS operations are most effective when using the lower HF frequencies (2 to 6MHz).

HF can accommodate IMC, voice, and teletypewriter operating modes and can operate in secure modes using a variety of available COMSEC devices. HF radios can be mounted in vehicles, ships, or aircraft and can be fixed, portable, or manpack configured. Transmissions are normally in either the single sideband (SSB) or independent sideband (ISB) mode.

HF sky-wave propagation is extremely vulnerable to intercept, particularly the high-powered, long-haul systems. The HF part of the spectrum is currently the frequency band most susceptible to jamming. Electronic countermeasure (ECM) jammers far from the receiver can jam or disrupt HF sky-wave communications. Proper use of COMSEC devices and burst transmission techniques can reduce this vulnerability however. Without some form of anti-jam protection, HF communications aren't considered suitable for critical C2 systems.

Very high frequency (VHF). The VHF frequency range is from 30 to 300MHz, and its signals propagate principally by line-of-sight (LOS). Although LOS restrictions limit the ground range of VHF systems, LOS is an effective means of ground communication for distances up to 25 to 50 miles (depending on terrain and antenna height) without using a repeater. By placing repeaters properly along an intended communications path, we can get long-range VHF transmissions through a series of short LOS hops. Remember, the higher the antenna, the greater the LOS distance possible in each link. Depending on the use, range, and number of channels intended, VHF transmitter power can range from 0.25 watts for a portable hand-held FM radio to 120 watts for a 12/24 multichannel LOS system. A rule to remember here is--the higher the frequency, the less power required to transmit VHF signals over a given distance.

Single-channel VHF radios are portable, vehicular, or airframe mounted, and can usually be operated in motion. The larger multichannel systems are commonly mounted aboard ships and on 2 1/2 or 5-ton trucks in shelters, and require careful siting of directional antennas. Typical uses include short-range FM combat radio nets, radar, radio navigation, wideband LOS multichannel systems (repeatered or nonrepeatered) and television broadcasting. VHF links can provide excellent circuit quality, comparable to cable systems with up to 99 percent reliability. VHF links can handle either analog or digital voice and data transmissions in single/multichannel modes. Data rates may vary from 45.5 to 75 bits per second (bps) for mobile VHF radio nets to 1.2 to 9.6 thousand bits per second (Kbps) per channel for LOS multichannel radio relay systems.

Ultrahigh frequency (UHF). The UHF range is from 300 to 3,000MHz. The main propagation methods include tropospheric scatter, satellite, air/ground/air, and LOS. Due to the flexibility of UHF communications, the distance range varies significantly as follows:

• Line-of-sight (LOS)--15 to 100 miles, terrain dependent, 300+ nautical miles (nm) LOS from aircraft.
• Satellite--thousands of miles, depending on altitude, power, and antenna configuration.

Transmitter power can range from a low of 10 to 100 watts for LOS and satellite systems while troposcatter systems operate in the 2,500 to 10,000-watt range.

Many UHF systems are transportable by vehicle, aircraft, or ship. Some UHF satellite terminals are small enough and lightweight enough to be manpack portable. Common UHF applications are seen daily in local ambulance, fire, and police radio nets, with repeater operations being typical. On military installations, the non-tactical intrabase radio (IBR) nets are usually VHF or UHF. UHF systems are capable of high-quality, reliable, and high-capacity transmissions with data rates of 2.4Kbps and higher. UHF is used widely to provide secure/nonsecure voice, record, data, and facsimile service in both mobile and fixed configurations. Along with VHF, UHF is the band preferred for television.

Superhigh frequency (SHF). The SHF range, from 3 to 30GHz. It is used mainly for high-data-rate LOS microwave, multichannel radio relay, troposcatter, and satellite systems. Distances for SHF systems range from line-of-sight for terrestrial microwave links to thousands of miles for satellite connectivity. Here are some nominal distances for SHF systems:

Properly engineered, LOS microwave systems provide reliable, high-capacity, long-distance communications through radio relay sites.

SHF carrier signals permit large bandwidths, so they can handle significant amounts of data over multiplexed voice channels and television. High-speed data with rates of 2.4Kbps and more (250Kbps data rates are possible) can be transmitted by SHF systems. Some military satellite terminals and troposcatter terminals have been designed for tactical use. These systems are transportable by 2 1/2 to 5-ton truck and have antenna dishes of varying size.

Extremely high frequency (EHF). The EHF frequency range is from 30 to 300GHz. Military application of this band is the subject of continuing research and development. Two types of experimental applications appear to offer attractive advantages. EHF satellite systems and millimeter-wave (MMW) transmissions.

The EHF Military Strategic Tactical and Relay (Milstar) satellite system provides worldwide coverage using geosynchronous space segments in both equatorial and polar orbits. The range of EHF satellite systems with cross-satellite linking is global. EHF transmissions passing through the atmosphere are susceptible to being attenuated by rain and other atmospheric conditions.

EHF systems can transmit secure voice and high-speed data at rates of up to 100Mbps (million bits per second). These systems operate in either the single-channel or multichannel mode. The extensive bandwidths available in the EHF range permit up to as many as 600 channels per link, depending on the type of multiplexing equipment used. EHF also offers increased capacity, jam resistance, electromagnetic pulse (EMP) protection, low power, narrow beam width, and excellent mobility advantages.

The basic idea of spectrum management in foreign areas is that the RF spectrum is a natural resource within the boundaries of any sovereign nation and may be used only with the consent of that nation. Each nation must consent to the use of the RF spectrum. Allocation and use of the RF spectrum requires international understanding and cooperation. With that in mind we look at international spectrum management, U. S. spectrum regulation and management, Department of Defense (DoD), and spectrum management within unified/specified commands, treaty organizations, and other foreign areas

International spectrum management. The premise of international spectrum management is that the radio-frequency spectrum is a natural resource within the boundaries of any sovereign nation and may be used only with the consent of that nation. National plans must be tailored to the international allocation pattern. We can show the necessity for these international agreements by a few examples:

1. International flights must be able to communicate always with at least one checkpoint along each route.
1. Communications-electronics (C-E) equipment developed by the U. S. military should be usable by troops deployed worldwide.
1. We can't allow unrestricted spectrum use during wartime, since various countries have allies with whom they must cooperate.
1. The need for compatibility and interoperability of C-E equipment is particularly important in allied joint commands such as the North Atlantic Treaty Organization (NATO).
1. During wartime, certain civil safety services require continued protection.

International allocations of the RF spectrum and registration of frequency assignments are handled by the International Telecommunications Union (ITU). ITU Radio Regulations--ratified by member nations--have treaty status.

U. S. spectrum regulation and management. Regulation of radio spectrum use within the United States is predicated on the idea that responsibility for orderly use of spectrum space by a nation's citizens lies with the government of that nation. National regulation is necessary so each country will be able to live up to international agreements that have treaty status in the world political arena.

The Communications Act of 1934 governs the use of the RF spectrum in the United States. This act established two branches of spectrum management. The President is the final authority for controlling spectrum usage by government-owned radio stations (in this case, government-owned refers to the federal government). The Federal Communications Commission (FCC) is the responsible agent for regulation of the non-government part of the spectrum, which includes civil users, state and local governments.

The need for efficient use of spectrum resources on a national basis is urgent and must be recognized by all spectrum users. Efficient use requires intelligent planning, management, and technical advances in communications equipment. To put together these facets of planning, management, and equipment requires sound and effective spectrum management at the national level.

The Communications Act of 1934 established the FCC, with responsibility for regulation and management of non-government interstate and foreign telecommunications originating in the United States including:

1. Assignment of space in the RF spectrum among private users.
1. Regulation of the use of that space.
1. Authorization of alien amateur operators licensed by their governments for operation in the U. S. under reciprocal agreements.

Recognizing the constitutional powers of the Office of the President, the Act puts control of government radio stations in that office. The President:

1. Assigns frequencies to radio stations "belonging to and operated by the U. S."
1. Authorizes foreign governments to construct and operate radio stations in fixed service at the United States seat of government and assigns frequencies to these stations, provided these actions are determined to be in the national interest, and the foreign governments grant reciprocal privileges to the United States.
1. Has the power to control all frequency resources in the United States when the nation is in a wartime posture.

By Executive Order, the President established the National Telecommunications and Information Administration (NTIA) under the Department of Commerce to act on his behalf in discharging his telecommunications responsibilities. The President (or NTIA) and the FCC are the sole authorities for frequency assignments in the United States and possessions. Additionally, the FCC and NTIA assist and advise the Department of State in negotiations regarding international telecommunications policy.

Spectrum management within the Department of Defense (DoD). The military services and the defense agencies, acting as agents of the Secretary of Defense and the Assistant Secretary of Defense for Telecommunications, are responsible for management and operational direction of telecommunications resources within the DoD. Multilateral government channels coordinate DoD spectrum management matters with non-DoD agencies in the United States. Military channels are used for all other military frequency management matters worldwide.

The Director, Command, Control, Communications and Intelligence (C3I), is the principal staff assistant to the Secretary of Defense on telecommunications matters. This is the focal point for coordinating telecommunications policy within the DoD and with organizations that work together with the DoD on telecommunications matters. The Director is also responsible for reviewing and monitoring policies, plans, programs, and budgets for telecommunications within the DoD.

The Military Communications-Electronics Board (MCEB) is the primary agency for coordination of military C-E matters among DoD components. They also coordinate between the DoD and other government departments and agencies, and between the DoD and representatives of foreign nations. It functions under the policies and directives of the Secretary of Defense and the Joint Chief of Staff (JCS), acting under the Secretary of Defense. The MCEB is responsible for providing DoD guidance and direction in preparing and coordinating detailed and technical joint and combined directives and agreements in various C-E activities, including authorizing spectrum allotments of resources allotted by the NTIA to DoD.

Spectrum management within unified/specified commands, treaty organizations, and other foreign areas. The RF spectrum is considered to be a natural resource within the boundaries of any nation. In foreign areas, the RF spectrum may be used only with the consent of the host nation. Any deviation from established frequency authorizations could affect relationships and negotiations with the host government.

Unified commands are normally established for missions that require significant assigned components of two or more services. Specified commands are normally established where a mission requires a force consisting primarily of units from a single service. Spectrum management in specified and unified commands is under the control of the highest command present, with policy guidance provided by the MCEB. MCEB policies allow unified and specified commanders to make frequency assignments for certain intracommand communications provided:

1. Appropriate coordination has been accomplished with host government agencies, such as FAA, FCC, or area frequency coordinators.
1. National or international protection is not desired or required.
1. NTIA and FCC jurisdictional areas are not involved.
1. Harmful interference with authorized users registered with NTIA will not result.

Unified and specified commanders, subject to host nation agreements, have overall management and control responsibility for all United States military use of the RF spectrum within their zones of operation. Direct military liaison channels have been established between the United States and the countries of the United Kingdom, New Zealand, Australia, and Canada through formation of the Combined Communications-Electronics Board (CCEB) which is staffed at the same level as the MCEB. Spectrum management for operations or planning within the Allied Treaty Organizations basically follows the pattern of the international command organization.

In NATO, the Allied Radio Frequency Agency Permanent Staff (ARFA P/S) is responsible for NATO plans, policies, and C-E requirement engineering. The United States maintains a permanent ARFA representative at United States Commander-in-Chief, Europe (USCINCEUR) Headquarters. An assistant representative at NATO Headquarters in Brussels is the major contact point for all United States C-E requirements at ARFA Headquarters.

Allied treaty organizations other than NATO have no equivalent to ARFA. Advanced spectrum planning is handled among the headquarters of the military departments concerned. United States spectrum planning for NATO and other Allied Treaty Organizations always includes coordination with United States authorities.

Communications play an indispensable role in the command and control network by providing decision-makers timely information to coordinate offensive and defensive activities. Information can come in several forms: voice communications, teletype, data link, or video transmissions. Also, information may be transferred by several methods: radio, microwave relay, tropospheric scatter systems, or satellite, to name a few.

We've covered a lot of information in a short time. From the early history of radio, we progressed into the principles, characteristics, and properties of the electromagnetic spectrum and radio waves (fig. 2-11). Due to the complexity of the material covered, let's emphasize some areas that are of major concern.

HF radio is a relatively low-frequency device used for long-range voice communications. HF radio waves can propagate along the surface of the ground and bend over the horizon, following the curve of the earth. They may also be reflected by the ionosphere. Transmitted skyward, HF waves can bounce between the ionosphere and the ground several times as they propagate from the transmitter to the receiver. HF's usefulness is limited by several factors. First, HF has a low capacity, with only four sidebands on each frequency. Second, it can't be depended on for full-time communication, because it's susceptible to a high-noise environment. Sunspot activity or high-altitude nuclear detonations also hinder down HF communications.

VHF and UHF radio provide LOS communication. By mixing signals (multiplexing), hundreds of voice channels can be transmitted simultaneously. They can also carry teletype, data link, or video transmissions. There are several ways to extend the LOS-limited range of UHF transmissions. Microwave relay stations can increase the range

and survivability of the communications system. By using directional, high-gain antennas, microwaves can be transmitted 20 to 40 miles by only 1 kilowatt of power. Rough terrain and inaccessible areas can be traversed more easily by relay stations than by telephone lines. In addition, since most of the equipment is inside buildings, the system is less susceptible to severe weather or bomb blast effects.

A tropospheric scatter system can also be used to extend UHF radio range. The atmosphere is made up of several layers that are constantly shifting but have sharply defined characteristics of temperature, moisture content, and refractive index. The index of refraction is the ratio of the velocity of a radio wave in free space to that of a wave in a different medium. The change in the index of refraction between atmospheric layers bends RF waves in the UHF band. Most of the transmitted energy continues forward, but enough is bent, or "scattered," back toward the earth to be usable (fig. 2-12). Because of the large losses, the transmitter requires a lot of power.

A troposcatter system can span up to 400 miles per link, where a microwave system would require many repeater stations to span the same distance. Like a microwave relay system, the troposcatter system can handle over 250 voice channels at ranges of 100 miles or less, but this number drops drastically as the range between links increases. For example, at ground ranges over 300 miles, the system can handle only 12 to 24 voice channels.

One final method for increasing the range of UHF radio transmissions is to use satellites, either as a repeater system or passive reflector. Because of reduced signal losses, satellite links can provide ground ranges more than 750 miles. Figure 2-13 shows how a satellite can greatly extend the range of a communications system over a troposcatter system. The resulting advantage is that one satellite could replace several ground-based troposcatter relay sites (which could be replacing several LOS repeater sites), effectively reducing the amount of equipment needed.

Note to Student: Consider all choices carefully, select the best answer to each question, and circle the corresponding letter.