INTRODUCTION TO TACTICAL RADIO COMMUNICATIONS THEORY
CRITICAL TASKS: 01-5878.04-0005,
LESSON DESCRIPTION :
In this lesson you will learn the basic theory of tactical radio communications including types of modulation (frequency modulation (FM) and amplitude modulation (AM)), transmission characteristics for various frequency ranges, antenna characteristics, and how to select an antenna for a specific communications task.
TERMINAL LEARNING OBJECTIVE :
You will be given information from this lesson.
To demonstrate competency of the terminal learning objective, you must achieve a minimum score of 70% on the subcourse examination.
The material contained in this lesson was derived from the following publication: FM 24-18.
The advent of modern transportation has changed the face of the battlefield immensely. Gone are the days when the messenger could run through the trenches to the headquarters tent with on urgent message for the Officer in Charge. Today that headquarters may be as much as 500 miles away, and that is a little too far even for the hardiest marathon runner. We must now rely on other more sophisticated means to get messages from the front lines to the "Headquarters Tent." In addition, the introduction of aircraft to the modern battlefield has added a third dimension to the way we must think about coordinating our forces.
The medium that brings all the elements of the modern battlefield together and ensures a coordinated effort is radio. The radio has become the central nervous system of the battlefield, keeping each element informed of the progress of the whole. As signal officers, it is your job to ensure that these nerves function properly and efficiently. You must be able to make the very best use of each and every piece of equipment assigned to you and to units under your cognizance. In order to do this you must first have a thorough understanding of those capabilities, and the best place to start is with the basics of radio communication theory.
1. Radio Waves.
Radio waves make up one portion of the electromagnetic spectrum. We identify a particular radio wave by its frequency. The frequency of a radio wave is the number of oscillations the wave makes in one second. The unit of measurement for radio wave frequency is the hertz (Hz) or cycles per second. One hertz equals one cycle per second. Another way to measure radio waves is by their wavelength. The wavelength of a wave is simply the distance that the wave travels as it goes through exactly one cycle. Figure 1-1 illustrates the concept of wavelength, and figure 1-2 shows a comparison of two waves of different frequencies. The relationship between a wave's frequency and its wavelength is:
In each of these expressions, the number 300,000,000 represents the speed of light in meters per second.
Figure 1-1. Wavelength of a radio wave
Figure 1-2. Comparison of two waves of different frequency
2. Propagation Methods of Radio Waves.
In order to use radio waves as a communications medium, the waves must travel from the sending station to the receiving station. Thus, it is important that you understand something of the propagation methods of radio waves. Electromagnetic waves travel in a straight line unless they are reflected or refracted by some outside force. In the case of radio waves, the basic paths of transmission are the ground wave and the sky-wave. Figure 1-3 illustrates these transmission paths.
- Three Wave Types. All ground waves fall into one of three wave types. The first type is the direct wave. Direct waves travel along a line of sight (LOS) path. There must be a clear path between two stations in order to communicate using this transmission path. Communications with aircraft, satellites, and stations within sight of each other generally take place along a direct path. The second type of ground wave is similar to the direct wave and is called a ground reflected wave. Ground reflected waves also travel in straight lines. The difference between this type and the direct wave is that the ground reflected wave is reflected off the ground at some point between the sending and receiving stations. Ground reflected waves can sometimes interfere with a direct wave signal if both waves arrive at the receiving station 180 degrees out of phase.
Some of the lower frequency ranges are affected by the electromagnetic properties of the earth's surface and will actually bend around the curvature of the earth. These are surface waves. You can communicate with stations not in your LOS using systems that take advantage of these lower frequencies.
Figure 1-3. Radio wave transmission paths
- Sky-Waves. The other type of transmission path is the sky-wave. Sky-waves are waves that have been transmitted upward and reflected back to the earth by the ionosphere. The ionosphere is a series of four layers (in daylight hours) of ion concentration in the earth's atmosphere called the D, E, Fl, and F2 regions. Figure 1-4 illustrates the ionosphere regions and their approximate heights above the earth's surface. The D region of the ionosphere serves only to attenuate the strength of radio waves and does not provide any useful reflection of the waves. This region fades out at night. The E region also fades at night but provides some reflection of radio waves during the day. Sky-waves which bounce off the E region can provide communications up to about 2,400 kilometers (1,500 miles). The F regions of the ionosphere do not fade out at night but they do combine to form a single region. You can communicate using F region sky-waves over distances of over 2,400 kilometers. This region is especially useful at night when the two intervening regions (D and E) have faded. The ionosphere is not constant and, therefore, sky-wave communications are not completely predictable. The ionosphere constantly undergoes variations which are classified as regular or irregular.
Figure 1-4. Distribution of the ionosphere
(1) Regular variations. Regular variations in the ionosphere occur as a result of the earth being a satellite of the sun and rotating about its own axis. These variations are so called because the period of variation is fairly well known from previous observation. You must account for these variations when you plan your communications system. There are four basic types of regular variations, which are:
(a) Daily variations. These are caused by the rotation of the earth.
(b) Seasonal variations. These are caused by the seasonal tilt of the earth on its axis.
(c) 27-day variations. These are caused by the rotation of the sun on its axis.
(2) Irregular variations. Irregular variations in the ionosphere occur as a result of random events. Because these variations occur randomly, you cannot anticipate or plan for them. There are three basic types of irregular variations, which are:
(a) Sporadic E. This is caused by the E region becoming highly ionized and blocking out the reflections from the F regions. This can completely blank out sky-wave signals or it can result in signals traveling much further than you would normally expect.
(b) Sudden ionospheric disturbance (SID). This is caused by bright solar eruptions. The eruption causes abnormal ionization of the D region, absorbing all frequencies above approximately 1 megahertz (MHz). This results in receivers seeming to go dead, and it can last for several hours. Since it is associated with the D region of the ionosphere, this phenomenon is limited to daylight hours and does not occur after dark.
(c) Ionospheric storms. These are caused by meteorological disturbances in the ionosphere. These storms can involve the entire ionosphere and can last from several hours to several days. This phenomenon can cause low intensity in sky-wave signals and can cause a type of random "flutter fading" in sky-wave signals.
The range of sky-wave radio transmissions depends largely on the density of the ionospheric regions and the frequency of the radio signal. Because the frequency of a radio wave and its energy level are proportional, higher levels of ionization must exist in the ionosphere in order to reflect the waves back to earth. As a result, there is at any given time a frequency above which radio waves will not be reflected back to earth. This frequency is the critical frequency. The critical frequency is not a fixed value because the level of ionization in the ionosphere is constantly changing. Another limiting factor associated with the ionosphere is the critical angle. The critical angle is that angle of incidence (angle at which the radio wave meets the ionosphere) above which the radio wave will not be reflected, but will pass through the ionosphere and be lost in space.
Two other important terms you should understand when you are dealing with sky-wave transmissions are skip distance and skip zone. The skip distance is the distance that a sky-wave travels from its transmission point to the point where it returns to the earth's surface. The skip zone is the area in which no usable radio signal can be received because it is shorter than the skip distance but longer than the ground wave range. Figure 1-5 illustrates these concepts.
Figure 1-5. Skip zone and skip distance
You can use a piece of equipment called the AN/TRQ-35(V) Ionospheric Sounder to determine which frequencies are best for sky-wave transmission at any time of day or night.
3. Useful Frequencies.
The number of useful frequencies in radio communications is very large, spanning a range of about thirty kilohertz (30 kHz) to about 300 Gigahertz (300 GHz), or 30,000 to 300,000,000,000 Hz. Since the transmission characteristics of radio waves change as the frequency changes, it is useful to break this wide range of frequencies into smaller groups called bands. We divide the radio frequency (RF) spectrum into bands of frequencies which have similar transmission characteristics. Table 1-1 shows these frequency bands and their respective frequency ranges.
Most tactical radio sets operate within the medium frequency (MF) to UHF bands.
Table 1-1. Frequency bands
Each of these frequency bands has different transmission characteristics. The frequency range of the band determines how the waves propagate and how far they travel. Lower frequency bands (VLF and LF), for example, will travel as surface waves and as sky-waves. Frequencies in the UHF and higher ranges, on the other hand, propagate only as direct waves. The other side of the coin is that, because the wave energy is proportional to the wave frequency, lower frequency transmitters must use higher transmission power to get a usable signal strength. Table 1-2 shows the relative ranges and transmission powers required for some of the frequency bands. These ranges are approximate and do not take into account such variables as ionospheric variations, antenna siting problems, and usable antenna orientation or polarization.
Table 1-2. Frequency band characteristics
4. Forms of Radio Communications.
Radio communications can take one of several forms. Messages can be in the form of speech, data, radio teletypewriter (RATT), or telegraphic code. Let's first consider how a radio set transmits speech. The frequency range of normal speech is about 50 Hz to 500 Hz. Although these frequencies could be directly converted into electromagnetic energy and transmitted, the antenna required would be close to 5,000 miles long! Thus, you can see that it is not practical to conduct radio transmissions using this method. Instead, the signal used to transmit speech over radio waves is a combination of a higher frequency carrier wave and the lower frequency modulator wave. The sound of speech is converted to electromagnetic energy and this signal is used to modulate the carrier signal. Using this method you can transmit low frequency speech signals using the transmission characteristics of the higher frequency radio waves. There are two basic types of modulation in radio communications.
Frequency Modulation. The first type of modulation is frequency modulation. In FM transmissions, the modulating signal is used to vary the frequency of the carrier wave. The rate at which the carrier frequency varies is equal to the frequency of the audio signal, and the amount of deviation from the carrier frequency is equal to the amplitude of the audio signal. Thus, an FM transmission consists of a constant-amplitude wave with frequency varying about a central rest frequency. Between the transmitter and receiver the FM transmission may pick up amplitude variations due to outside electromagnetic interference. To compensate, most FM receivers use a limiter to exclude these amplitude variations from the received signal. Because of the variances of the carrier frequency, FM transmissions usually have fairly large bandwidths. For this reason, FM is generally used in VHF and higher frequency bands where bandwidth is not as significant as in the lower bands.
Amplitude Modulation. The second type of modulation is amplitude modulation. AM is the variation of RF power output of a transmitter at an audio rate. Simply put, AM is the process of varying the amplitude (and thus the output energy) of the carrier wave by superimposing the signal wave on it. In an AM transmission, the rate at which the carrier amplitude varies is equal to the frequency of the audio signal, and the amount of variation in the carrier amplitude is equal to the amplitude of the audio signal. In addition to the amplitude variations of the carrier, the superposition of the audio signal produces new RF signals with frequencies near that of the carrier frequency. For example, assume a 600 kHz carrier is modulated by a .1 kHz audio signal. The two new RF frequencies developed will be 600 kHz +/- .1 kHz, or 599.9 kHz and 600.1 kHz. These two new frequencies are called sidebands. The lower frequency is the lower sideband and the higher frequency is the upper sideband. Thus for a range of audio frequencies, the frequency range of the sidebands would be the carrier frequency plus or minus the highest and lowest audio frequencies. The total space occupied by both sidebands and the carrier frequency of an AM signal is called a channel, and the range of frequencies is the channel bandwidth. Figures 1-6 and 1-7 illustrate the concepts of AM, FM, and sidebands.
Figure 1-6. AM and FM wave shapes
Figure 1-7. AM carrier with sidebands
You may have guessed from the previous information that the sidebands of an AM transmission contain duplicate information. In fact, the entire audio signal is contained in each sideband. Because of this, we can eliminate the carrier frequency and one sideband frequency from the transmitted signal and still transmit all the information needed for communications. This type of transmission is amplitude modulation/single sideband (AM/SSB) and is further classified as upper sideband (AM/USB) and lower sideband (AM/LSB). Standard AM transmission is also called AM double sideband (AM/DSB) transmission. One of the main advantages of an AM/SSB system is that by eliminating one sideband and the carrier frequency you make room in the frequency spectrum for extra communications channels. Other advantages of a SSB system are:
(1) AM/SSB provides greater reliability than AM/DSB.
(3) AM/SSB systems provide increased output without increasing antenna voltage.
(4) AM/SSB systems make it possible to operate a larger number of radio sets without heterodyne interference (whistles and squeals) from interfering RF carriers.
(5) AM/SSB systems can operate over longer ranges without loss of intelligence of the signal due to selective fading.
Because of their narrower bandwidth, AM/SSB systems are used in all frequency ranges, but are especially useful in the HF and lower frequency bands.
5. Basic Transmitter.
Now that you have learned the basic theory of radio signals, the next thing you need to learn about is how a radio set actually converts an input signal such as your voice into a radio signal that can travel to a distant station, and how the receiver at that station can convert that signal back into a recognizable voice pattern. We will start with a basic transmitter. Figure 1-8 illustrates the basic components of a simple continuous wave (CW) transmitter. The transmitter consists of a power supply, a keying device, an oscillator, and an antenna. Let's briefly look at each of these components.
Figure 1-8. Block diagram of a simple radio transmitter
Power Supply. The power supply is common to all radio sets. It simply provides electrical power to all the other components in the radio set. The power supply may consist of transformers and convertors and may have multiple output voltages.
Oscillator. The oscillator is the device that actually produces the RF signal to be sent. In the early days of radio, these oscillators consisted of crystals that vibrated at a certain frequency when stimulated by an electric current. Some older radio sets still use crystal chips in the oscillator, but most modern radios use some type of electronic appurtenance to perform this function. Most tactical radio sets have oscillators that can be tuned to a certain frequency or channel for transmission on that channel. An oscillator may also contain filters to limit the bandwidth of the transmission to avoid interference with other radio sets.
Antenna. The antenna is nothing more than the device that converts RF electrical energy into radio waves that travel to the receiving station. You will learn more about antennas later in this lesson.
Keying Device. The keying device is the device you use to generate a message to be transmitted. In the CW transmitter, the key serves to interrupt the power to the oscillator and antenna. Thus, when the key is depressed, the power is sent to the oscillator and antenna and a signal is transmitted. When the key is released, power is interrupted and the signal stops. Using this method you can send Morse code messages by radio.
In order to transmit messages containing something besides Morse code, you need a transmitter with a few more components. Figure 1-9 illustrates a basic radiotelephone transmitter. You can see that a microphone and modulator have replaced the keying device, and that a buffer and an RF amplifier have been added. Let's look at each of these new components briefly.
Figure 1-9. Block diagram of a radiotelephone transmitter
Buffer. The buffer is nothing more than a series of electronic filters and stabilizers that take the output from the oscillator and ensure that it is as stable as possible. Since the RF signal from the oscillator is what produces the carrier for the radio signal, you can see that it is extremely important for this signal to be constant and stable.
Microphone and Modulator. The microphone and modulator in this transmitter serve the same purpose as the keying device in the CW transmitter previously described. The microphone converts speech into electrical signals and the modulator converts these signals into an audio modulating signal. This signal can then be applied to the carrier to produce the modulated radio wave that can be received and understood by a remote receiver.
RF Amplifier. The RF amplifier is the stage in the transmitter where the carrier and modulating signals are combined to produce the radio signal to be transmitted. Depending on the type of transmitter (AM, FM, etc.), this combination of signals will take one of the forms we discussed previously in this lesson. The amplifier then amplifies the combined signal and sends it to the antenna for transmission.
Transmitting a radio signal is useless unless there is a receiver somewhere to receive and understand the message. The receiver, then, is equally as important to radio communications as the transmitter. Figure 1-10 shows a typical radio receiver. As with the transmitter, we will look at each of its components individually.
Figure 1-10. Block diagram of a radio receiver
Receiver Antenna. The antenna on the receiver serves much the same function that it does on the transmitter. The principal difference is that the receiver antenna absorbs the radio waves and converts them to an electronic signal to be used by the receiver.
- RF Amplifier. In most transmitters, the energy that is transmitted through the antenna is transmitted in several directions. If the receiving station is more than a small distance from the transmitter, only a small fraction of the radiated energy of the transmitter will reach the receiver. You can guess from this that the electronic signal produced by the receiver antenna will be very small. For this reason, receivers have an RF amplifier attached to the antenna. The amplifier takes the incoming signal and amplifies it to a level that can be processed by the receiver's other components.
Detector. Once the received signal is amplified, the next step is to convert the modulated signal back into an audio signal that will be intelligible to the operator. The detector is the component that does this. The detector, like the modulator in the transmitter, serves to uncouple or "demodulate" the radio signal. In an FM receiver, this component is called a discriminator. As in the transmitter, the exact function of the detector depends on the type of receiver it serves.
Audio Amplifier. When the detector separates the audio and carrier signals, the resulting audio signal is quite small. In order to raise the audio signal to a usable level (one that can be heard), we use an audio amplifier between the detector and the speaker or headphone. This amplifier simply amplifies the audio signal so that it can drive the speaker or headphone and reproduce the sound of the original transmission.
Most modern radio sets perform the functions of transmitting and receiving in the same unit. These radios have a transmitting section and a receiving section which generally use the same antenna for both functions, though not simultaneously. Additionally, most radio sets are designed to operate in a particular frequency band and to use a particular type of modulation. For example, the AN/VRC-12 series radio set operates in the VHF band and uses frequency modulation. There are newer radio sets in use today however, that offer much more flexibility to their users. The AN/PRC-70 series radio, for example, operates in AM, SSB, and FM modes and can transmit and receive in both HF and VHF bands.
One very important thing you must consider when planning radio communications is the compatibility of the radios that will be communicating with each other. There are four basic conditions which you must meet if you desire to conduct radio communications. They are communications range, operating frequency, method of communication, and type of modulation. If two radio stations are 200 miles apart and one can only transmit up to 100 miles, it cannot be heard by the other station and communications are not possible. Similarly, a VHF radio cannot communicate with an HF radio, nor an AM radio with an FM radio. A CW radio could transmit to a RATT radio but the receiving radio would not be able to interpret the signal and would produce no output.
The object of radio communications is to be able to convey intelligence over long distances without having to use wires to carry the signal. The component of a radio set that makes this possible is the antenna. You learned previously that the antenna converts electronic signals into radio waves in a transmitter and converts radio waves to electronic signals in a receiver. The antenna, then, is the element that takes the place of thousands of miles of wires in transferring messages from radio station to radio station. It is helpful in learning about the various types and uses of antennas if you first learn some basic concepts and terminology associated with them.
Antenna Gain. Antennas come in many different configurations and some work much better for certain applications than others. The term we use to talk about the efficiency of an antenna is antenna GAIN. Antenna gain is simply a measure of an antenna's efficiency at transmitting or receiving certain signals. An antenna that is more efficient is said to have a higher gain than one that is less efficient.
Antenna Polarization. Another term you commonly hear associated with antennas is polarization. Polarization refers to the orientation of the electromagnetic fields that make up a radio wave. If the fields are perpendicular to the earth's surface, the wave is vertically polarized. If the fields are parallel to the earth's surface, the wave is horizontally polarized. Some transmitters, most notably satellites, produce fields that constantly change orientation with respect to the earth's surface. These are circularly polarized waves. If an antenna has a better gain in receiving or transmitting a certain type of wave, we say the antenna is polarized in a certain direction. Thus antennas, as well as radio waves, can be horizontally, vertically, or circularly polarized. You will learn the particular uses of each type of polarization in an antenna later in this lesson.
Directional Antennas. You will learn that some types of antennas have a higher gain in a certain direction than they do in another. This is called the directionality of the antenna and is dependent on the type of antenna and its orientation. We call antennas of this type directional antennas. Directional antennas can be very useful because they transmit and receive in the same direction. You could use a directional antenna to prevent a signal from being intercepted by an enemy on your flank, or to prevent a particularly noisy (RF speaking) industrial area from interfering with your reception. Another term associated with directional antennas is azimuth. The azimuth is the orientation of the directional axis of the antenna with respect to true North. You will measure azimuth in degrees. Azimuth can be very critical if the antenna you use is highly directional. It is important to know where you are in relation to the station you want to communicate with.
Ground Effect. Except for satellites and aircraft, all antennas are set up on or near the earth's surface. This proximity almost always has some effect on the performance of the antenna. We call this phenomena ground effect. If the ground that an antenna is connected to is a good conductor, it will act like a mirror and reflect RF energy radiated downward by the antenna. If the antenna is grounded (electrically attached to the ground) this can have the effect of making the antenna behave like it is longer than it actually is. For example, you can make a quarter-wave antenna behave like a half-wave antenna by electrically grounding it. Figure 1-11 illustrates this concept.
Figure 1-11. Quarter-wave antenna connected to ground
Different types of soil may have very different conductivity levels. Damp soil near a river or in a pasture may have very good conductivity, while dry sandy soil such as in a desert, or ice and frozen ground, may have very poor conductivity. Sometimes you can improve the conductivity of the soil by adding salt or other agents to it. There are several ways to ground an antenna. You can attach it to other grounded structures or to metal rods driven into the ground. You can also attach the antenna to underground systems such as pipes. If you cannot sufficiently ground an antenna because of poor conductivity of the ground, you can use a device called a counterpoise. A counterpoise is a device generally made with wire that you erect between the antenna and the ground. The counterpoise should be insulated from the ground. The counterpoise then acts as a reflector the same way that soil of good conductivity would. The counterpoise is constructed in a simple geometric pattern and electrically connected to the antenna. Figure 1-12 illustrates different types of counterpoises. In order for a counterpoise to be effective, it should be at least as large as the antenna it is supporting, and preferably larger. If a counterpoise is not practical, you can sometimes use a large mesh screen laid over the surface of the ground under the antenna. This is called a ground screen.
Figure 1-12. Wire counterpoises
Antenna Length. The length of an antenna is a very important factor to consider when you are planning communications. The length of an antenna determines what frequencies it will transmit and receive efficiently. Antennas have two lengths: physical length and electrical length. Because of the reduced velocity of the radio wave on the antenna and the capacitive effect of the end of the antenna, the electrical length of the antenna is generally longer than its physical length. Thus, if you are designing an antenna for a specific communications task, you must consider the wavelength of the communications frequency you are using and the correction factor due to the difference in the physical and electrical length of the antenna. For frequencies between 3 and 50 MHz that correction factor is 0.95. Knowing this you can calculate the required length of your antenna using the following formula (for half-wave antennas):
If you desire to construct a long-wire antenna (one wavelength or longer) then the following formula applies:
Where N is the number of half-wavelengths in the total length of the antenna.
7. Types of Antennas Used for Tactical Radio Communications.
Now that you know the basic theory behind antenna operation you should learn about the different types of antennas you may need to use in tactical radio communications. Figure 1-13 shows several different types of antennas and indicates the directionality of each. The next several paragraphs will discuss the abilities and limitations of these antennas.
Figure 1-13. Types of antennas
Figure 1-13. Types of antennas (cont.)
Figure 1-13. Types of antennas (cont.)
Rhombic Antenna. The rhombic antenna is useful in conducting HF communications between fixed points. With the radio antenna lead attached at one apex and a terminating resistor at the opposite apex, the antenna is unidirectional and has very good directionality both in the transmit and receive modes. This feature can be useful if you are trying to limit the signal sent to unfriendly forces or prevent noise interference of a nearby RF noise source (e.g., an industrial area). The drawbacks associated with this antenna are that it is fairly large and not easy to set up. The wires must be the correct lengths, and the azimuth must be properly determined. Since the antenna is fixed, it is difficult to reorient it to a new azimuth to communicate with a different radio station.
Hertz Antenna. The Hertz or half-wave antenna is a common configuration for field communications. This antenna is also called a center-fed doublet. You can orient the antenna either horizontally or vertically and the polarization of the antenna corresponds to the orientation. The antenna is directional, with the primary direction being perpendicular to the antenna axis. One drawback of the antenna is that it transmits perpendicular to its axis in all directions. For example, if you are transmitting a strong signal directly behind you to a rear element, you are also transmitting a strong signal directly in front of you to possible hostile forces. When you construct a Hertz antenna you must take care to construct it to the proper length for the frequency you are using. Do this using the formulas previously discussed for a half-wave antenna. The obvious disadvantage is that the antenna is tuned only for a small band of frequencies. Hertz antennas are very portable and are easy to set up and take down. You can also change the orientation of a Hertz antenna with minimum difficulty.
Marconi Antenna. If you take a vertical half-wave antenna and replace the lower half with a conducting plane, the upper half will continue to radiate as a half-wave antenna even though it is only a quarter wavelength long. This type of antenna is called a Marconi or quarter-wave antenna. The conducting plane mentioned can be the ground, a vehicle body, or even a shelter roof. Since it is vertical the polarization of a quarter-wave antenna is almost always vertical. The whip and the ground plane antennas are typical examples of Marconi antennas. The obvious advantage of the quarter-wave antenna is that it is shorter than the half-wave antenna of the same frequency. This makes the quarter-wave antenna ideal for portable applications such as backpack and vehicular mounted radios.
Whip Antenna. The whip antenna is the simplest of the Marconi family of antennas. Some whip antennas are extremely short; much shorter than a quarter wavelength. These antennas are called baseloaded whip antennas and will have a coil attached to the base of the whip. The coil contains a conductor of sufficient length to make the antenna a quarter-wavelength long. Theoretically, the whip antenna is an omnidirectional antenna. When attached to a vehicle, however, the radiation pattern shows a certain directionality depending on the placement of the antenna on the vehicle body. Figure 1-14 illustrates this concept. The whip antenna is the most portable type of antenna and, therefore, the most widely used in tactical radio communications.
Ground Plane Antenna. If you take a whip antenna and add horizontal elements to the vertical radiator, the horizontal elements act as a ground reflector or counterpoise. This configuration is called a ground plane antenna. You can use a ground plane antenna on any type of soil because it creates it own reflection. The ground plane antenna is omnidirectional. This makes it ideal for communicating with mobile units that do not stay in one place for a very long time. In some configurations, you can tune the ground plane antenna to a certain frequency by changing the length of the radiating and reflecting elements. The broadband omnidirectional antenna system OE-254 is in common use today for VHF-FM tactical radio sets. It is an improvement over the earlier ground plane antennas because it does not have to be reconfigured physically when you change radio frequencies. The OE-254 kit comes with antenna, mast, all supporting equipment needed to set it up, and all electrical connectors needed to operate it with most tactical radio sets in use today.
Figure 1-14. Directivity of a vehicle-mounted whip antenna
Directional Antennas. While omnidirectional communications are good for communicating with mobile units, they do not provide very good communications security. Any station within range can pick up the signal transmitted from this type of antenna. Occasionally you may want to communicate with another station covertly. You will of course use coded transmissions, but even the fact that you are sending a signal out can give away your position if you are not careful. In this scenario the safest method of communications is to use a directional antenna. There are three basic types of directional antennas that you can easily employ in a field environment. They are the long-wire antenna, the vertical half-rhombic antenna, and the V antenna.
The long-wire antenna is simply what the name implies. It is an antenna that you have constructed to a certain number of wave-lengths (using the previous formula). To obtain directionality, you connect the radio set to one end and terminate the other end to ground with a resistor. You should always remember when choosing terminating resistors for any antenna that the rating of the resistor should be at least half of the rated output power of the radio transmitter you are using. This will keep you from burning out the resistor. Once your long-wire antenna is configured, you orient it by pointing the terminated end toward the azimuth of the radio station with which you are communicating. You orient the antenna horizontally and as high off the ground as possible. The antenna will work as low as three feet from the ground, but efficiency will increase as it is raised.
The vertical half-rhombic antenna is a vertically-polarized directional antenna that operates between 30 and 88 MHz. It consists of two sloping segments, each about two wavelengths long, and a horizontal segment that acts as a counterpoise (refer to figure 1-13). The antenna is easy to transport, set up, and tear down. You can also change the antenna azimuth quickly and easily. The use of the vertical half-rhombic antenna can not only provide directionality, but also provide extended range over the OE-254 antenna because it sends its energy out in only one direction.
The V antenna is very similar to the rhombic antenna discussed earlier in this lesson. It consists of two horizontal legs connected in a V with the radio set connected at the apex. The free ends are terminated in the same manner as the long-wire and vertical half-rhombic antennas. The combination of radiation patterns from the two elements forms a central directional beam that bisects the two legs. Figure 1-15 illustrates the radiation pattern of a V antenna. The signal from this antenna is horizontally polarized. This makes the antenna useful in an area where there is much vertically polarized RF interference.
Figure 1-15. Radiation pattern of a V antenna
You can see from this discussion that you can use the V and the vertical half-rhombic antennas to complement each other in areas of high RF noise. TM 11-666 talks in detail about the configuration of the V antenna for various frequency ranges, but table 1-3 gives optimum angles of separation for the antenna legs based on their length. Review of TM 11-666 is not required for completion of this subcourse.
Table 1-3. Leg angle for V antennas
You can see from this table that a longer antenna length provides a narrower beamwidth. This has the advantage of greater directionality and the disadvantage of requiring greater accuracy in setting the antenna azimuth.
8. Field Expedient Measures.
While you are field-deployed it will happen that some portion of your antenna will break. It may be the radiating element, the antenna mast, or simply an insulator. It is important that you be ready for this, and that you be able to improvise replacements for the failed component so that you do not lose communications capability. The most important thing to remember is that when a particular item breaks, look around for items with similar physical and electrical properties to replace it. Figure 1-16 shows that a tree makes a very good replacement for a broken antenna mast as does a wooden pole (e.g., broom handle). You can take ordinary antenna wire and fabricate almost all the antennas you have learned about in this section. Remember, the formulas for determining the length of your antenna and your field expedient can be just as good as the original.
Figure 1-16. Field substitutes for antenna supports
Figure 1-17 will give you an idea of the wide variety of items you can use to replace things like broken insulators. A knife or bayonet can also serve as a good substitute for a ground stake that has been lost or broken. You can repair a broken whip antenna by attaching a length of wire to it to make its total length a quarter wavelength again. Remember, the best tool to have in an emergency situation is your imagination.
9. Improvement of Marginal Communications.
Just as you are destined to have equipment failure, so are you destined to be in positions where communications is only marginal at best. This may be because of terrain obstructions, interference due to jamming or other RF noise, or even range limitations of your equipment. The following paragraphs provide some guidance on what to do when you find yourself in this situation. Interference can be caused by many different things. Electronic jamming, industrial machinery, air traffic, other radio sets operating in your area, high tension lines, and even solar and cosmic disturbances can interfere with your radio communications. One way to compensate for this is by checking your radio system for any loose connections and by making sure your radio and antenna are properly tuned. Sometimes switching to another channel may help, but you may also lose communications with the station you are trying to communicate with.
Figure 1-17. Field improvised insulators
If you can determine the source of the interference you may be able to use a directional antenna to eliminate some of it. This will only work, however, if the interference source and the station you are communicating with are on different azimuths.
Determining the source of interference may also tell you how the interfering signal is polarized. Most man-made RF noise is vertically polarized. Using a horizontally polarized antenna can do a lot to lessen the effect of interference. Likewise, in a forest vertically polarized waves tend to be absorbed by the trees more than horizontally polarized waves. RF interference caused by aircraft or commercial radio and television transmissions, on the other hand, is usually horizontally polarized. You should use a vertically polarized antenna if you are experiencing interference from one of these sources.
If you are having trouble communicating due to range fading, a directional antenna may help. It can boost your signal in the direction of the station you are talking to and will also increase your receiver sensitivity. If none of the previous measures improve your communications, you should also try moving your antenna or raising or lowering your antenna. If you are still having trouble communicating there may be nothing to do but move to a different location and try again.
In this lesson you have learned the basics of radio theory. You have learned how a basic radio transmitter and receiver work and how amplitude modulation differs from frequency modulation. You have also learned about the different types of antennas used in tactical radio communications and how to select and use the best antenna for a given job. In the next lesson you will learn about the different types of tactical radios in use and how to select a certain radio for a certain communications task.