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FM 24-18: Tactical Single-Channel Radio Communications Techniques

CHAPTER 3
ANTENNAS

SAFETY WARNING
Be extremely careful when putting up, taking down, or moving antennas located near high voltage or commercial power lines. Antenna contact with these can and may result in electrocution or sever injury to personnel holding the antenna or the connecting guy wires and cables.

Section I.
Requirement and Function

3-1. Necessity

All radios, whether transmitting or receiving, require some sort of antenna. Single-channel radios normally send and receive radio signals on one antenna. This is called one-way-reversible (OWR) or simplex operation. During duplex (DX) operation two antennas are used, one for transmitting and the other for receiving. In either case, the transmitter generates a radio signal. A transmission line delivers the signal from the transmitter to the antenna. The transmitting antenna sends the radio signal into space toward the receiving antenna. The receiving antenna intercepts the signal and sends it through a transmission line to the receiver. The receiver processes the radio signal so that it can either be heard or used to operate a recording device such as a teletypewriter (fig 3-1).

Figure 3-1. Simple radio communications network.

3-2. Function

The function of an antenna depends on whether it is transmitting or receiving. A transmitting antenna transforms the output RF energy produced by a radio transmitter (RF output power) into an electromagnetic field that is radiated through space. In other words, the transmitting antenna converts energy from one form to another form. The receiving antenna reverses this process. It transforms the electromagnetic field into RF energy which is delivered to a radio receiver.

3-3. Gain

The gain of an antenna depends mainly on its design. Transmitting antennas are designed for high efficiency in radiating energy, and receiving antennas are designed for high efficiency in picking up energy. On many radio circuits, transmission is required between a transmitter and only one receiving station. In this case, energy may be radiated in one direction because it is useful only in that direction. Directional receiving antennas increase the energy pickup or gain in the favored direction, and reduce the reception of unwanted noise and signals from other directions. The general requirements for transmitting and receiving antennas are that they have small energy losses and that they be efficient as radiators and receptors.

Section II. Characteristics

3-4. Electromagnetic Radiation

Radiation Fields.

When RF power is delivered to an antenna, two fields are set up: one is an induction field, which is associated with the stored energy; the other is a radiation field. At the antenna, the intensities of these fields are large and are proportional to the amount of RF power delivered to the antenna. At a short distance from the antenna and beyond, only the radiation field remains. This radiation field is composed of an electric component and a magnetic component (fig 3-2).

Figure 3-2. Components of electromagnetic waves.

The electric and magnetic fields (components) radiated from an antenna form the electromagnetic field. The electromagnetic field is responsible for the transmission and reception of electromagnetic energy through free space. A radio wave is a moving electromagnetic field that has velocity in the direction of travel, and with components of electric intensity and magnetic intensity arranged at right angles to each other.

Radiation Patterns.

The radio signals radiated by an antenna form an electromagnetic field having a definite pattern, depending on the type of antenna used. This radiation pattern is used to show the directional characteristics of an antenna. A vertical antenna theoretically radiates energy equally in all directions (omnidirectional); a horizontal antenna is mainly bidirectional. There are also unidirectional antennas. These antennas theoretically radiate energy in one direction. In practice, however, the patterns usually are distorted by nearby obstructions or terrain features.

The full- or solid-radiation pattern is represented as a three-dimensional figure that looks somewhat like a doughnut with a transmitting antenna in the center (fig 3-3). The upper pattern in the figure is that of a quarter-wave vertical antenna; the center pattern is that of a half-wave horizontal antenna, located one-half wavelength above the ground. The bottom pattern is that of a vertical half-rhombic antenna.

Figure 3-3. Solid radiation patterns from quarter-wave, half-wave, and vertical half-rhombic antennas.

3-5. Polarization

The polarization of a radiated wave is determined by the direction of the lines of force making up the electric field. If the lines of electric force are at right angles to the surface of the Earth, the wave is said to be vertically polarized (fig 3-4). If the lines of electric force are parallel to the surface of the Earth, the wave is said to be horizontally polarized (fig 3-5). When a single-wire antenna is used to extract (receive) energy from a passing radio wave, maximum pickup results if the antenna is oriented so that it lies in the same direction as the electric field component. Thus, a vertical antenna is used for efficient reception of vertically polarized waves and a horizontal antenna is used for the reception of horizontally polarized waves. In some cases, the field rotates as the waves travel through space. Under these conditions, both horizontal and vertical components of the field exist and the wave is said to have elliptical polarization.

Figure 3-4. Vertically polarized signal.

Polarization Requirements for Various Frequencies.

At medium and low frequencies, ground-wave transmission is used extensively and it is necessary to use vertical polarization. Vertical lines of force are perpendicular to the ground, and the radio wave can travel a considerable distance along the ground surface with a minimum amount of loss. Because the Earth acts as a relatively good conductor at low frequencies, horizontal lines of electric force are shorted out and the useful range with the horizontal polarization is limited.

At high frequencies, with sky wave transmission, it makes little difference whether horizontal or vertical polarization is used. The sky-wave, after being reflected by the ionosphere, arrives at the receiving antenna elliptically polarized. Therefore, the transmitting and receiving antennas can be mounted either horizontally or vertically. Horizontal antennas are preferred however, since they can be made to radiate effectively at high angles and have inherent directional properties.

Figure 3-5. Horizontally polarized signal.

For frequencies in the very-high or ultra-high range, either horizontal or vertical polarization is satisfactory. Since the radio wave travels directly from the transmitting antenna to the receiving antenna, the original polarization produced at the transmitting antenna is maintained as the wave travels to the receiving antenna. Therefore, if a horizontal antenna is used for transmitting, a horizontal antenna must be used for receiving.

Satellites and satellite terminals use circular polarization. Circular polarization describes a wave whose plane of polarization rotates through 360° as it progresses forward. The rotation can be clockwise or counterclockwise (see fig 3-6). Circular polarization occurs when equal magnitudes of vertically and horizontally polarized waves are combined with a phase difference of 90°. Depending on their phase relationship, this causes rotation either in one direction or the other (see app L).

Figure 3-6. Circular polarized wave.

Advantages of Vertical Polarization.

Simple vertical half-wave and quarter-wave antennas can be used to provide omnidirectional (in all directions) communications. This is desirable in communicating with a moving vehicle. Its disadvantage is that it radiates equally to the enemy and friendly forces.

When antenna heights are limited to 3.05 meters (10 ft) or less over land, as in a vehicular installation, vertical polarization provides a stronger received signal at frequencies up to about 50 MHz. From about 50 MHz to 100 MHz, there is only a slight improvement over horizontal polarization with antennas at the same height. Above 100 MHz, the difference in signal strength between vertical and horizontal polarization is small. However, when antennas are located near dense forests, horizontally polarized waves suffer lower losses than vertically polarized waves.

Vertically polarized radiation is somewhat less affected by reflections from aircraft flying over the transmission path. With horizontal polarization, such reflections cause variations in received signal strength. An example is the picture flutter in a television set when an aircraft interferes with the transmission path. This factor is important in areas where aircraft traffic is heavy.

When vertical polarization is used, less interference is produced or picked up from strong VHF and UHF transmissions (television and FM broadcasts) because they use horizontal polarization. This factor is important when an antenna must be located in an urban area that has television or FM broadcast stations.

Advantages of Horizontal Polarization.

A simple horizontal half-wave antenna is bidirectional. This characteristic is useful in minimizing interference from certain directions.

Horizontal antennas are less likely to pick up man-made interference, which ordinarily is vertically polarized.

When antennas are located near dense forests, horizontally polarized waves suffer lower losses than vertically polarized waves, especially above 100 MHz.

Small changes in antenna location do not cause large variations in the field intensity of horizontally polarized waves when an antenna is located among trees or buildings. When vertical polarization is used, a change of only a few feet in the antenna location may have a significant effect on the received signal strength.

3-6. Directionality

Vertical receiving antennas accept radio signals equally from all horizontal directions, just as vertical transmitting antennas radiate equally in all horizontal directions. Because of this characteristic, other stations operating on the same or nearby frequencies may interfere with the desired signal and make reception difficult or impossible. However, reception of a desired signal can be improved by using directional antennas.

Horizontal half-wave antennas accept radio signals from all directions, with the strongest reception being received in a line perpendicular to the antenna (that is, broadside); and, the weakest reception being received from the direction of the ends of the antenna. Interfering signals can be eliminated or reduced by changing the antenna installation so that either end of the antenna points directly at the interfering station.

Communications over a radio circuit is satisfactory when the received signal is strong enough to override undesired signals and noise. The receiver must be within range of the transmitter. Increasing the transmitting power between two radio stations increases communications effectiveness. Also, changing the types of transmission (for example, changing from radiotelephone to CW), changing to a frequency that is not readily absorbed, or using a directional antenna aids in communications effectiveness.

Directional transmitting antennas concentrate radiation in a given direction and minimize radiation in other directions. A directional antenna may also be used to lessen interception by the enemy and interference with friendly stations.

3-7. Ground Effects

Since all practical antennas are erected over the Earth and not out in free space, except for those on satellites, the presence of the ground will alter the free space radiation patterns of antennas. The ground will also have an effect on some of the electrical characteristics of an antenna. It has the greatest effect on those antennas that must be mounted relatively close to the ground in terms of wavelength. For example, medium- and high-frequency antennas, elevated above the ground by only a fraction of a wavelength, will have radiation patterns that are quite different from the free-space patterns.

Grounded Antenna Theory.

The ground is a good conductor for medium and low frequencies and acts as a large mirror for the radiated energy. This results in the ground reflecting a large amount of energy that is radiated downward from an antenna mounted over it. Using this characteristic of the ground, an antenna only a quarter-wavelength long can be made into the equivalent of a half-wave antenna. A quarter-wave antenna erected vertically, with its lower end connected electrically to the ground (fig 3-7), behaves like a half-wave antenna. Under these conditions, the ground takes the place of the missing quarter-wavelength, and the reflections supply that part of the radiated energy that normally would be supplied by the lower half of an ungrounded half-wave antenna.

Types of Grounds.

When grounded antennas are used, it is especially important that the ground has as high a conductivity as possible. This reduces ground losses and provides the best possible reflecting surface for the down-going radiated energy from the antenna. At low and medium frequencies, the ground acts as a sufficiently good conductor. Therefore, the ground connection must be made in such a way as to introduce the least possible amount of resistance to ground. At higher frequencies, artificial grounds constructed of large metal surfaces are common.

The ground connections take many forms, depending on the type of installation and the loss that can be tolerated. In many simple field installations, the ground connection is made by means of one or more metal rods driven into the soil. Where more satisfactory arrangements cannot be made, ground leads can be connected to existing devices which are grounded. Metal structures or underground pipe systems are commonly used as ground connections. In an emergency, a ground connection can be made by forcing one or more bayonets into the soil.

When an antenna must be erected over soil with low conductivity, treat the soil to reduce its resistance. The soil should be treated with substances that are highly conductive when in solution. Some of these substances, listed in order of preference, are sodium chloride (common salt), calcium chloride, copper sulphate (blue vitriol), magnesium sulphate (Epsom salt, and potassium nitrate (saltpeter). The amount required depends on the type of soil and its moisture content.

WARNING: When these substances are used, it is important that they do not get into nearby drinking water supplies.

Figure 3-7. Quarter-wave antenna connected to ground.

For simple installations, a single ground rod can be fabricated in the field from pipe or conduit. It is important that a low resistance connection be made between the ground wire and the ground rod. The rod should be cleaned thoroughly by scraping and sandpapering at the point where the connection is to be made, and a clean ground clamp should be installed. A ground wire can then be soldered or joined to the clamp. This joint should be covered with tape to prevent an increase in resistance because of oxidation.

Counterpoise.

When an actual ground connection cannot be used because of the high resistance of the soil or because a large buried ground system is not practical, a counterpoise may be used to replace the usual direct ground connection. The counterpoise (fig 3-8) consists of a device made of wire which is erected a short distance above the ground and insulated from it. The size of the counterpoise should be at least equal to or larger than the size of the antenna.

When the antenna is mounted vertically, the counterpoise should be made into a simple geometric pattern. Perfect symmetry is not required. The counterpoise appears to the antenna as an artificial ground that helps to produce the required radiation pattern.

Figure 3-8. Wire counterpoise.

In some VHF antenna installations on vehicles, the metal roof of the vehicle (or shelter) is used as a counterpoise for the antenna.

Small counterpoises of metal mesh are sometimes used with special VHF antennas that must be located a considerable distance above the ground.

Ground Screen.

A ground screen consists of a fairly large area of metal mesh or screen that is laid on the surface of the ground under the antenna. There are two specific advantages in using ground screens. First, the ground screen reduces ground absorption losses that occur when an antenna is erected over ground with poor conductivity. Second, the height of the antenna can be set accurately. As a result of this, the radiation resistance of the antenna can be determined more accurately (See TM 11-666, para 61).

3-8. Antenna Length

The length of an antenna must be considered in two ways. It has both a physical and an electrical length, and the two are never the same. The reduced velocity of the wave on the antenna and a capacitive effect (known as end effect) make the antenna seem longer electrically than it is physically. The contributing factors are the ratio of the diameter of the antenna to its length and the capacitive effect of terminal equipment (insulators, clamps, etc.) used to support the antenna.

To calculate the physical length of an antenna, use a correction of 0.95 for frequencies between 3.0 and 50.0 MHz. The figures given below are for a half-wave antenna.

Length (meters)  =  150 X 0.95 / Frequency in MHz 
                  =  142.5 / Frequency in MHz

Length (feet) = 492 X 0.95 / Frequency in Mhz = 468 / Frequency in MHz

The length of a long-wire antenna (one wavelength or longer) for harmonic operation is calculated by using the following formula.

Length (meters) = 150(N-0.05) / Freq MHz

Length (feet) = 492 (N-0.05) / Freq MHz

Where N = number of half-wave lengths in the total length of the antenna.

For example, if the number of half-wavelengths is 3 and the frequency in MHz is 7, then:

Length (meters) =150(N-0.05) / Freq MHz = 150(3-.05) / 7 = 150 x 2.95 / 7 = 442.50 / 7 = 63.2 meters

3-9. Antenna Orientation

Azimuth.

If the azimuth of the radio path is not provided, the azimuth should be determined by the best available means. The accuracy required in determining the azimuth of the path is dependent upon the radiation pattern of the directional antenna. If the antenna beam width is very wide (for example, 90° angle between half-power points, fig 3-9), an error of 10° in azimuth is of little consequence. In transportable operation, the rhombic and V antennas may have such a narrow beam as to require great accuracy in azimuth determination. The antenna should be erected for the correct azimuth. Great accuracy is not required in erecting broad-beam antennas. Unless a line of known azimuth is available at the site, the direction of the path is best determined by a magnetic compass. Figure 3-10 is a map of magnetic declination, showing the variation of the compass needle from the true north. When the compass is held so that the needle points to the direction indicated for the location on the map, all directions indicated by the compass will be true.

Figure 3-9. Beam width measured on relative field strength and relative power patterns.

Figure 3-10. Magnetic declination over the world.

Improvement of Marginal Communications.

Under certain situations, it may not be feasible to orient directional antennas to the correct azimuth of the desired radio path. As a result, marginal communications may suffer. To improve marginal communications, follow the procedure presented below.

  • Check, tighten, and tape cable couplings and connections.
  • Retune all transmitters and receivers in the circuit.
  • Check to see that antennas are adjusted for the proper operating frequency.
  • Try changing the heights of antennas.
  • Try moving the antenna a short distance away and in different locations from its original location.
  • Separate transmitters from receiving equipment, if feasible.

Transmission and Reception of Strong Signals.

After an adequate site has been selected and the proper antenna orientation obtained, the signal level at the receiver will be proportional to the strength of the transmitted signal.

If a high-gain antenna is used, a stronger signal can be obtained. Losses between the antenna and the equipment can be reduced by using a high quality transmission line, as short as possible, and properly matched at both ends.

WARNING: Excessive signal strength may result in enemy intercept and interference or in your interfering with adjacent frequencies.

Section III.
Types of Antennas

3-10. Tactical Considerations

Tactical antennas are specially designed to be rugged and permit mobility with the least possible sacrifice of efficiency. They are also designed to take abuse. Some are mounted on the sides of vehicles that have to move over rough terrain; others are mounted on tops of single masts or suspended between sets of masts. The smallest antennas are mounted on the helmets of personnel who use the radio sets. All tactical antennas must be easy to install. Large ones must be easy to take apart and pack and they must be easy to transport.

Several types of transmitting and receiving antennas are shown in figure 3-11.

  • A of the figure is a rhombic antenna.
  • B is a half-wave Hertz antenna.
  • C is an end-fed, vertical antenna, also called a whip antenna.
  • D is a loop antenna that receives a strong signal in directions as shown and almost no signal in other directions.
  • E is an antenna group OE-254/GRC which is an omnidirectional, biconical antenna designed for broadband operation.
  • F is a long-wire antenna.
  • G is a vertical half-rhombic antenna.
  • H is a directional half-rhombic antenna.

Figure 3-11. Types of antennas.

Figure 3-11. Types of antennas (cont).

Figure 3-11. Types of antennas (cont).

Most practical transmitting antennas come under one of two classifications, Hertz antennas or Marconi antennas. A Hertz antenna is operated some distance above the ground and may be either vertical or horizontal. A Marconi antenna operates with one end grounded (usually through the output of the transmitter or the coupling coil at the end of the feed line). Hertz antennas are generally used at higher frequencies (above about 2 MHz) while Marconi antennas are generally used at the lower frequencies. Marconi antennas, when used on vehicles or aircraft, operate at high frequencies. In these cases, the aircraft or vehicle chassis becomes the effective ground for the antenna.

3-11. The Hertz Antenna

The operation of the Hertz antenna is based on the fact that the wavelength to which any wire will electrically tune depends directly upon its physical length. The basic Hertz antenna is center fed and its total wire length is equal to approximately one half of the wavelength of the signal to be transmitted. This type of antenna, which is also known either as a doublet, a dipole, an ungrounded, or a half-wave antenna, can be mounted in either a vertical, horizontal, or slanting position. Two typical military half-wave, center-fed Hertz antennas are shown in figures 3-12 and 3-13. These antennas are used for transmitting and receiving frequencies from 2 to 30 MHz.

Figure 3-12. Center-fed Hertz antenna with two upright supports.

Figure 3-13. Center-fed Hertz antenna with three upright supports.

The Mast Base AB-155 is employed for upright supports with the centerfed doublet antenna. Refer to TM 11-5820-256-10. Antenna Group AN/GRA-50 can also be used to install a center-fed doublet antenna by using trees, buildings, or other existing means of support. Refer to TM 11-5820-467-15. For all practical purposes, the length (in feet) of a half-wave doublet antenna is determined by using the formula, 468 divided by the frequency in megahertz.

Employment.

The half-rhombic antenna is employed with the current VHF/FM family of radios. Because the half-rhombic antenna is very directional, its employment is restrictive in nature, usually providing only point-to-point communications.

Training.

The introduction of the half-rhombic antenna into the Army tactical inventory minimally impacts on the Army training program. The operators of VHF-FM radio nets are responsible for the installation, operation, and teardown of the half-rhombic antenna in all applications. The fielding of the half-rhombic antenna does not require any additional support personnel. Half-rhombic antenna installation/training may consist of a short period of on-the-job training (OJT) conducted by the using unit.

3-12. The Marconi Antenna

If the lower half of a vertical Hertz antenna is replaced by an extensive conducting plane, no disturbance is caused in the propagated waves from the upper half. In other words, the remaining quarter-wave continues to radiate much in the same way as a half-wave antenna, providing a large conducting plane is used. The Marconi antenna is a practical form of this kind of radiating system in which the antenna proper provides one-quarter wavelength and the soil supplies the additional quarter-wavelength. The total effective (or electrical) length is then one-half wavelength.

The main advantage of the Marconi antenna is that, for any given frequency, it is physically much shorter than the Hertz antenna. This is particularly important in all field and vehicular radio installations. Typical Marconi antennas are the inverted L, the whip, the ground plane, and the modified ground-plane antennas.

3-13. The Whip Antenna

At the lower frequencies where wavelengths are longer, it is impractical to use resonant-length tactical antennas with portable radio equipment, especially with vehicular-mounted radio sets. Tactical whip antennas are electrically short, vertical, baseloaded types, fed with a nonresonant coaxial cable of about 52 ohms impedance (fig 3-14).

If the tactical whip antenna is to attain an efficiency comparable to that of a half-wave antenna, the height of the vertical radiator should be a quarter wavelength. However, this is not always possible, so the loaded whip is used instead. The loading increases the electrical length of the vertical radiator to a quarter wavelength. The missing quarter-wavelength of the antenna is supplied by the ground, a counterpoise, or any conducting surface that is big enough.

Figure 3-14. Common types of whip antennas.

The whip antenna supplied with military radio sets is usually 4.5 meters (15 ft) long for the high-frequency tactical radio sets. The whip antenna used with the lightweight portable FM radios is 0.9 meters (3 feet) long for the semirigid steel tape antenna and 3 meters (10 feet) long for the multisection whip antenna. It is made shorter than a quarter-wavelength to keep it a practical length. (A quarter-wavelength antenna for 5.0 MHz would be over 14 meters (46 ft) long.) An antenna tuning unit, either built into the radio set or supplied with it, compensates for the missing length of antenna. The tuning unit varies the electrical length of the antenna to accommodate a range of frequencies.

Whip antennas are used with tactical radio sets because they radiate equally in all directions in the horizontal plane (fig 3-15). Since stations in a radio net lie in random directions and change their positions frequently, the radiation pattern is ideal for tactical communications.

When a whip antenna is mounted on a vehicle, the metal of the vehicle affects the operation of the antenna. As a result, the direction in which the vehicle is facing may also affect transmission and reception, particularly of distant or weak signals.

Figure 3-15. Radiation pattern of a whip antenna.

Figure 3-16. Best directivity of whip antenna mounted on vehicle.

A vehicle with a whip antenna mounted on the left rear side of the vehicle transmits its strongest signal in a line running from the antenna through the right front side of the vehicle. Similarly, an antenna mounted on the right rear side of the vehicle radiates its strongest signal in a direction toward the left front side (fig 3-16). The best reception is obtained from signals traveling in the direction shown by the dashed arrows on the figure.

In some cases, the best direction for transmission can be determined by driving the vehicle in a small circle until the best position is located. Normally, the best direction for receiving from a distant station is also the best direction for transmitting to that station.

There are times when a whip antenna mounted on a vehicle must be left fully extended so that it can be used instantly while the vehicle is in motion. The base mounted insulator of the whip is fitted with a coil spring attached to a mounting bracket on the vehicle. The spring base allows the vertical whip antenna to be tied down horizontally when the vehicle is in motion and when driving under low bridges or obstructions. Even in the vertical position, if the antenna hits an obstruction, the whip usually will not break because most of the shock is absorbed by the spring base.

WARNING: When an antenna must be left fullyextended while in motion, contact with overhead powerlines must be avoided. Death or serious injury can result if a vehicular antenna strikes a high-voltage transmission line. If the antenna is tied down, be sure the tip protector is in place.

Figure 3-17. Ground-plane antenna.

Some of the energy leaving a whip antenna travels downward and is reflected by the ground with practically no loss. To obtain greater distance in transmitting and receiving, it may be necessary to raise the whip antenna. However, when a whip antenna is raised, its efficiency decreases because it is further from the ground. Therefore, when we use a whip antenna at the top of a mast, we must supply an elevated substitute for the ground (ground plane).

3-14. The Ground-Plane Antenna

The ground-plane antenna is a whip antenna that includes radial elements which serve as the ground. The coaxial feeder is connected with the inner conductor feeding the vertical element (whip), and the braid of the coaxial cable is connected to the radials (the ground plane) to keep them at ground potential. The ground-plane antenna is a broad-tuned type that radiates efficiently over a wide range of frequencies.

Figure 3-17 shows a ground-plane antenna presently used with tactical FM sets. It can be tuned by using the proper length of antenna sections in the vertical element and in the ground-plane elements. Refer to TM 11-5820-348-15.

Figure 3-18. Radiation pattern of V antenna.

3-15. The V Antenna

The V antenna consists of two wires arranged to form a V, with their ends at the apex (where the legs come together) attached to a transmission line (fig 3-18A). Radiation lobes off each wire combine to increase gain in the direction of an imaginary line bisecting the apex angle (See fig 3-18A). The pattern is bidirectional. Adding terminating resistors to the far end of each leg will make the pattern unidirectional (See fig 3-18B). For details on the angle and resistor values see TM 11-666, paragraph 89.

3-16. The Broadband Omnidirectional Antenna

The broadband omnidirectional VHF antenna system OE-254 (fig 3-19) is an improved tactical antenna that will replace the present antenna, RC-292. The antenna permits operation in the 30 to 88 MHz frequency range without the need to manually drop and retrieve the antenna each time the operating frequency is changed. The antenna system is being fielded for use with current tactical VHF-FM net radios. The antenna system is being issued on a one-for-one replacement for the RC-292.

This omnidirectional vertically polarized VHF antenna system--

  • Operates in the 30 to 88 MHz range without any physical adjustments.
  • Has input impedance of 50 ohms unbalanced with an average voltage standing wave ratio (VSWR) of 3:1 or less at RF power levels up to 350 watts.
  • Is capable of being assembled and erected by one individual.
  • Operates at a distance equal to that of the present antenna equipment RC-292.
  • Meets the broadband and power handling requirements of the 2-port and 5-port transceiver multiplexer (See appendix D).
  • Is capable of being used with the Steerable Null Processing Group OL-275()/VRC (SNAP-1) (See appendix E).

The antenna system (Figure 3-19) includes:

  • A standard lightweight mast having a maximum height of 41 feet and 9 inches.
  • A canvas carrying bag similar to that issued with antenna equipment RC-292. Assembly and erection procedures are provided with the bag.
  • All necessary guying assemblies, stakes, base plate, tools, and other ancillary equipment required to make a complete antenna system.
  • All necessary coaxial cables and cable adapters for Radio Sets AN/PRC-25 and AN/PRC-77 (series) and the AN/VRC-12 (family).

Figure 3-19. Broadband omnidirectional antenna system OE-254

3-17. The Directional VHF Log-Periodic Antenna

As the mobility of the Army increases, the need for extended range and directivity for the tactical radios in the field also increases. The Army has long awaited the advent of an effective electronic counter-countermeasures (ECCM) device/procedure that allows continued radio communications in a hostile electronic warfare (EW) environment. This need will be partially met by use of the log-periodic antenna (see fig 3-20). Use of the log-periodic antenna with nominal transmitter power output satisfies the need for extended range. Additionally, the use of the log-periodic antenna allows the use of reduced transmitter power with the resultant reduction of signals in all directions other than the antenna main beam. This satisfies the need of providing a degree of ECCM protection. Finally, the log-periodic antenna operates with the present VHF-FM tactical net radios (AN/VRC-12/PRC-25 series).

Figure 3-20. Log periodic antenna.

Operation.

The log-periodic antenna operates in the 30 to 88 MHz frequency range without any mechanical or electrical adjustments. This makes it a broadband antenna. It operates with power levels up to 250 watts and has a maximum of 2:1 VSWR at the RF input connectors over the frequency range. It presents a nominal input impedance of 50 ohms over the frequency range of 30 to 88 MHz.

Capabilities.

The log-periodic antenna is--

  • Capable of being erected in a geographical area no greater than 18.3 meters (60 ft) in diameter.
  • Capable of being mounted on a quick-erect mast on either a vehicle or a shelter.
  • Capable of a mechanical azimuthal directional change within a 1-minute time frame.
  • Capable of being assembled by two individuals in 20 minutes.
  • Capable of being transported by manpack or tactical vehicle when fitted into two packages (one for the antenna and one for the mast).
  • Capable of operation with either the 2-port or 5-port transceiver multiplexer.
  • The antenna system operates with either horizontal or vertical polarization with the capability of changing polarization in less than 1 minute of time.

Antenna Assembly.

The antenna assembly consists of a collapsible frame which supports a log-periodic antenna assembly. No tools are required to reassemble the antenna for operation. The antenna is capable of being mounted on top of the mast, polarized either vertically or horizontally. The antenna system includes all necessary guying assemblies, stakes, base plate, tools, and other ancillary equipment required to make a complete antenna system. It also includes all necessary coaxial cables and cable adapters for Radio Sets AN/PRC-25 and AN/PRC-77 (series), and the AN/VRC-12 (family).

Mast Assembly.

The mast assembly is capable of being mounted on either a vehicle or on a shelter.

Use.

The log-periodic antenna is organic to battalion and higher level units for special applications. It is task assigned, where needed, to subordinate units within the battalion. Its primary usage is by forward units in command and intelligence nets to a higher headquarters. Because the log-periodic antenna is directional, its employment is usually restricted to point-to-point communications.

Organizational Maintenance.

Organizational maintenance is performed by operators of radio equipment at the echelons where radio operator MOSs are authorized and by the user at all other echelons where authorized. It includes--

  • Performance of scheduled preventive maintenance services as directed by the technical manuals for organizational level.
  • Forwarding all unserviceable components, cables, elements, etc., to the designated direct support activity for repair.
  • Repair by replacing those faulty components that have been specified in the maintenance allocation charts (MAC) as replaceable at this level (for example, insulators, and cables).

3-18. The Half-Rhombic Antenna

The VHF half-rhombic antenna is a vertically polarized antenna which, when used with the current VHF-FM tactical radios, extends the range of transmission considerably and provides some degree of ECCM protection not offered by the current VHF-FM omnidirectional antenna. The current omnidirectional antennas, when employed in forward combat areas, transmit and receive signals equally in all directions and provide as strong a signal to the enemy EW units as they do to friendly units. The half-rhombic antenna, when properly employed, decreases VHF-FM radio susceptibility to hostile EW operations and enhances the communications ranges of the deployed radio sets. This effect is realized by directing the maximum signal strength in the direction of the desired friendly unit. It is made rugged enough to withstand the current doctrinal requirements of moving (erection and teardown) every 4 to 6 hours and be able to operate in the climatic conditions as stated in AR 70-38.

Operations.

The half-rhombic antenna operates in the 30 to 88 MHz frequency range without mechanical or electrical adjustments. It operates with power levels up to 200 watts and has a maximum of 2:1 VSWR over the entire frequency range. The antenna presents a nominal load impedance of 50 ohms over the frequency range 30 to 88 MHz.

Capabilities.

The half-rhombic antenna can--

  • Be erected in a geographical area of 53 meters (175 feet) in diameter or less depending upon the frequency.
  • Be mounted on any structure about 15 meters (50 feet) in height.
  • Be capable of azimuthal directional change within 1 minute of time.
  • Be installed by 2 individuals in 20 minutes or less.
  • Be transported by manpack or tactical vehicle when fitted into a package.
  • Operate with either the 2- or 5-port transceiver multiplexer.

Functional Description.

The half-rhombic VHF antenna is a high gain, lightweight, directional antenna capable of operation over the frequency range of 30 to 88 MHz without the need for physical tuning by the operator. The antenna and all the ancillary equipment (guys, stakes, tools, mast sections) can be packaged in a carrying bag for manpack or vehicular transportation. The AB-1244 mast assembly is used with the OE-303 half-rhombic antenna. The mast assembly (fig 3-21) consists of 12 tubular mast sections (five lower-mast sections, one mast transition adapter, five upper-mast sections, and antenna adapter), a mast base assembly and assorted ancillary equipment. When erected, the mast assembly is stabilized by a two-level, four-way guying system (fig 3-22). The weight of the half-rhombic VHF antenna including mast is approximately 18 kilograms (45 lb). This antenna handles radio frequency power levels up to 200 watts, matches a nominal 50 ohm impedance with a VSWR of no more than 2: 1 over the entire frequency range of the antenna, and meets the operation, storage, and transit requirements as specified in AR 70-38. Nuclear survivability has not been considered during the building of this antenna; however, due to the density of antennas in the field, replacement of damaged antennas may be made in a timely manner if the situation warrants. The half-rhombic antenna may be used with the radio set alone or with the 2- or 5-port multiplexer. Connections to the antenna and radio sets are made with connectors and cables provided with the antenna. This antenna, due to its simplicity and light weight, may be installed by two individuals in a time of 20 minutes or less. The simplicity of this antenna also makes it much easier to maintain in a tactical environment.

Figure 3-21. OE-303 antenna mast assembly (AB-1244( )/GRC).

Figure 3-22. Erected mast assembly, less antenna.

Unit Application.

In most instances, the half-rhombic antenna is used for special applications. It is task assigned as required. Its primary usage is by forward units on command control and intelligence nets to a higher headquarters. It must be available for use by units that habitually operate over extended distances from parent units and it must be available to units for special tasks or across the forward line of own troops (FLOT).

3-19. Near-Vertical Incidence Sky-Wave Antenna

The near-vertical incidence sky-wave (NVIS) antenna, AS-2259/GR is a lightweight sloping dipole omnidirectional antenna designed to operate in the 2- to 30-MHz frequency band (fig 3-23). It is employed with radio teletypewriter communications in a 0- to 483-kilometer (0- to 300-mile) range. The NVIS antenna is capable of operating with current AM/HF radio sets and will be an item of issue with the improved high frequency radio (IHFR). The antenna/coupler may be manually tuned at the antenna/coupler or remotely up to 61 meters (200 feet). Tuning time does not exceed 1 minute. The NVIS antenna is interchangeable with the AN/GRA-50 and the whip antenna when used with the above radio sets. Weight of the antenna is 5.44 kilograms (12 pounds) and it is constructed of chemical, biological resistant materials. It is horizontally and vertically polarized simultaneously and provides an omnidirectional pattern. The power handling capability is 1000 watts (PEP) or 500 watts (average). (See Table 3-1 and Figure 3-24.)

Maintenance is performed as outlined in AR 750-1. Organizational maintenance is performed by the Single-Channel Radio Operator, MOS 31C, at the echelons where this MOS is authorized and by the user at all other echelons where authorized. It includes performing scheduled preventive maintenance checks and services (PMCS) prescribed by organizational technical manuals, evacuating unserviceable repairable components to the designated direct support (DS) activity, and repairing by replacement of faulty components according to the MAC. DS will be performed by the Field Radio Repairer, MOS 29E. DS maintenance includes evaluation of faulty items received from organizational levels and performing repairs authorized at the DS level. Repaired items are returned to the user or replaced by a DX item. DS contact teams will repair on-site when possible to preclude the evacuation of items from the operating areas and to reduce the downtime of the HF system due to the antenna. There is no general support (GS) or depot maintenance.

See appendix M for a discussion of the near-vertical incidence sky-wave concept.

Table 3-1. Specifications of NVIS Antenna AS-2269/GR.

Figure 3-23. NVIS antennas, AS-2259/GR.

Figure 3-24. Physical configuration and coordinate location of elements of AS-2259/GR antenna.

3-20. The Dummy Antenna

Using a radiating antenna during tuning may reveal the location of a transmitter to enemy radio direction finders and may cause interference with other stations operating on the same frequency. To eliminate the possibility of unauthorized signals going on the air during tune-up, a dummy antenna should be used. This device acts as a load for the transmitter and absorbs the signal without radiating it into space. Dummy antennas normally consist of a noninductive resistor large enough to absorb the transmitter output and dissipate it as heat. Some dummy antennas also have an RF wattmeter to check RF power output of the transmitter.

Section IV.
Field Repair and Expedients

3-21. Assessment of Damage

Antennas are sometimes broken or damaged, causing either a communications failure or poor communications. If a spare is available, there is no problem--replace the damaged antenna. When there is no spare, you may have to construct an emergency antenna. The following paragraphs are suggestions on repairing antennas and antenna supports and on constructing and adjusting emergency antennas. See also paragraph 7-7.

3-22. Repair Techniques

Whip Antennas.

When a whip antenna is broken into two sections, the portion of the antenna that is broken off can be connected to the portion attached to the base by joining the sections as shown in figure 3-25. Use the method illustrated in A, figure 3-25, when both parts of the broken whip are available and usable. Use the method shown in B when the portion of the whip that was broken off is lost or when the whip is 80 badly damaged that it is not fit for use. To restore the antenna to its original length, add a piece of wire that is nearly the same length as the missing part of the whip. Then, lash the pole support securely to both sections of the antenna. Clean the two antenna sections thoroughly to ensure good contact before connecting them to the pole support. If possible, solder the connections.

Figure 3-25. Emergency repair of broken whip antenna.

Wire Antennas.

Emergency repair of a wire antenna may involve the repair or replacement of the wire used as the antenna or transmission line; or, the repair or replacement of the assembly used to support the antenna.

When one or more wires of an antenna are broken, the antenna can be repaired by reconnecting the broken wires. To do this, lower the antenna to the ground, clean the ends of the wires, and twist the wires together. Whenever possible, solder the connection.

If the antenna is damaged beyond repair, construct a new one. Make sure that the length of the wires of the substitute antenna are the same length as those of the original.

Antenna supports may also require repair or replacement. A substitute item may be used in place of a damaged support and, if properly insulated, can be of any material of adequate strength. If the radiating element is not properly insulated, field antennas may be shorted to ground and be ineffective. Many commonly found items can be used as field expedient insulators. The best of these items are plastic or glass, to include plastic spoons, buttons, bottle necks, and plastic bags. Though less effective than plastic or glass but still better than no insulator at all are wood and rope, or both, in that order. The radiating element--the actual antenna wire-- should touch only the antenna terminal and should be physically separated from all other objects, other than the supporting insulator. Figure 3-26 shows various methods of making emergency insulators.

Figure 3-26. Improvised insulators.

Guys.

Lines used to stabilize the supports for an antenna are called guys. These lines are usually made of wire, manila rope, or nylon rope. If a rope breaks, it may be repaired by tying the two broken ends together. If the rope is too short after the tie is made, it can be lengthened by adding another piece of or a piece of dry wood or cloth. If a guy wire breaks, it can be replaced with another piece of wire. Figure 3-27 shows a method of repairing a guy line with wood.

Figure 3-27. Repaired guy lines and masts.

Masts.

Some antennas are supported by masts. If a mast breaks, it can be replaced with another of the same length. If long poles are not available as replacements, short poles may be overlapped and lashed together with rope or wire to provide a pole of the required length. Figure 3-27 shows a method of making an emergency repair to masts.

3-23. Tips on Construction and Adjustment

Constructing the Antenna.

The best kinds of wire for antennas are copper and aluminum. In an emergency, however, use any type that is available.

The exact length of most antennas is critical. The emergency antenna should be the same length as the antenna it replaces.

Antennas supported by trees can usually survive heavy wind storms if the trunk of a tree or a strong branch is used as a support. To keep the antenna taut and to prevent it from breaking or stretching as the trees sway, attach a spring or old inner tube to one end of the antenna. Another technique is to pass a rope through a pulley or eyehook, attach the rope to the end of the antenna and load the rope with a heavy weight to keep the antenna tightly drawn.

Guys used to hold antenna supports are made of rope or wire. To ensure that the guys made of wire will not affect the operation of the antenna, cut the wire into several short lengths and connect the pieces with insulators.

Adjusting the Antenna.

An improvised antenna may change the performance of a radio set. Use the following methods to find out if the antenna is operating properly.

A distant station may be used to test the antenna. If the signal received from this station is strong, the antenna is operating satisfactorily. If the signal is weak, adjust the height and length of the antenna and the transmission line to receive the strongest signal at a given setting on the volume control of the receiver. This is the best method of tuning an antenna when transmission is dangerous or forbidden.

In some radio sets, the transmitter is used to adjust the antenna. First, set the controls of the transmitter in the proper position for normal operation; then, tune the system by adjusting the antenna height, the antenna length, and the transmission line length to obtain the best transmission output.

Impedance-matching a load to its source is an important consideration in transmissions systems. If the load and source are mismatched, part of the power is reflected back along the transmission line towards the source. This reflection not only prevents maximum power transfer, but also can be responsible for erroneous measurements of other parameters, or even cause circuit damage in high-power applications.

The power reflected from the load interferes with the incident (forward) power causing standing waves of voltages and current to exist along the line. The ratio of standing-wave maxima to minima is directly related to the impedance mismatch of the load; therefore the standing-wave ratio (SWR) provides the means of determining impedance and mismatch.

WARNING: Serious injury or death can result from contact with the radiating antenna of a medium- or high-power transmitter.Turn the transmitter off while making adjustments to the antenna.

The matching unit of the standing-wave Ratio Power Meter ME-165/G (see fig 3-28), provides a noninductive dummy load of 52 ohms and, when connected between the transmitter and its load, permits direct readings of the transmitter power output and the SWR between the transmitter and its load.

3-24. Field Expedient Omnidirectional Antennas

Vertical antennas are omnidirectional; that is, they transmit and receive equally well in all directions. Most tactical antennas are vertical; for example, the manpack portable radio uses a vertical whip as do the Figure 3-28. Standing-wave Ratio Power Meter ME-165/G. vehicular radios in tactical vehicles. A vertical antenna can be improvised by using a metal pipe or rod of the correct length, held erect by means of guys. The lower end of the antenna should be insulated from the ground by placing it on a large block of wood or other insulating material. A vertical antenna may also be a wire supported by a tree or a wooden pole (fig 3-29). For short vertical antennas, the pole may be used without guys (if properly supported at the base). If the length of the vertical mast is not long enough to support the wire upright, it may be necessary to modify the connection at the top of the antenna (fig 3-30).

Figure 3-29. Field substitutes for support of vertical wire antennas.

Figure 3-30. Additional means of supporting vertical wire antennas.

End-Fed Half-Wave Antenna

An emergency, end-fed half-wave antenna can be constructed from available materials such as field wire, rope, and wooden insulators. The electrical length of this antenna is measured from the antenna terminal on the radio set to the far end of the antenna (fig 3-31). The best performance can be obtained by constructing the antenna longer than necessary, and then shortening it, as required, until best results are obtained. The ground terminal of the radio set should be connected to a good Earth ground for this antenna to function efficiently.

Figure 3-31. End-fed half-wave antenna.

Center-Fed Doublet Antenna

The center-fed doublet is a half-wave antenna consisting of two quarter wavelength sections on each side of the center. Construction of an improvised doublet antenna for use with FM radios is shown in figure 3-32.

Doublet antennas are directional broadside to their length, which makes the vertical doublet antenna essentially omnidirectional. This is because the radiation pattern is doughnut shaped (See fig 3-3 for quarter-wave antenna which has a similar radiation pattern.). The horizontal doublet antenna is bidirectional (See fig 3-3 for half-wave antenna).

Figure 3-32. Half-wave doublet antenna.

The length of a half-wave antenna may be computed by using the formula in paragraph 3-8. Cut the wires as closely as possible to the correct length because the length of the antenna wires is important.

A transmission line is used for conducting electrical energy from one point to another and it is used to transfer the output of a transmitter to an antenna. Although it is possible to connect an antenna directly to a transmitter, the antenna generally is located some distance away. In a vehicular installation, for example, the antenna is mounted outside and the transmitter inside the vehicle. A transmission line, therefore, is necessary as a connecting link.

Center-fed half-wave FM antennas can be supported entirely by pieces of wood. A horizontal antenna of this type is shown in figure 3-33A; a vertical antenna in figure 3-33B. These antennas can be rotated to any position to obtain the best performance. If the antenna is erected vertically, the transmission line should be brought out horizontally from the antenna for a distance equal to at least one-half of the antenna's length before it is dropped down to the radio set.

Figure 3-33. Center-fed half-wave antenna.

A similar arrangement for a short, center-fed half-wave antenna is shown in figure 3-34. The ends of this antenna are connected to a piece of dry wood, such as a bamboo pole, and the bend in the pole holds the antenna wire straight. Another pole, or bundle of poles, serves as the mast.

Figure 3-34. Bent bamboo antenna.

Figure 3-35 shows an improvised half-wave antenna. This technique is used primarily with FM radios. It is effective in heavily wooded areas to increase the range of portable radios. The top guy wire can be connected to a limb or passed over the limb and connected to the tree trunk or a stake.

Figure 3-35. Improvised vertical half-wave antenna.

3-25. Field Expedient Directional Antennas

The vertical half-rhombic antenna (fig 3-36) and the long-wire antenna (fig 3-37) are two field expedient directional antennas. These antennas consist of a single wire, preferably two or more wavelengths long, supported on poles at a height of 3 to 7 meters (10 to 20 feet) above the ground. The antennas will, however, operate satisfactorily as low as 1 meter (approximately 3 feet) above the ground. The far end of the wire is connected to ground through a noninductive resistor of 500 or 600 ohms. To ensure the resistor is not burned out by the output power of the transmitter, use a resistor rated at least one-half the wattage output of your transmitter. A reasonably good ground, such as a number of ground rods or a counterpoise, should be used at both ends of the antenna. The radiation pattern is directional. The antennas are used primarily for either transmitting or receiving high-frequency signals.

Figure 3-36. Vertical half-rhombic antenna.

Figure 3-37. Long-wire antenna.

The V antenna is another field expedient directional antenna. It consists of two wires forming a V with the open area of the V pointing toward the desired direction of transmission/reception (see fig 3-38). To make construction easier, the legs may slope downward from the apex of the V (this is called a sloping-V antenna) (fig 3-39).

The angle between the legs varies with the length of the legs in order to achieve maximum performance. Use table 3-2 to determine the angle and the length of the legs.

When the antenna is used with more than one frequency or wavelength, use an apex angle that is midway between the extreme angles determined by the chart.

To make the antenna radiate in only one direction, add noninductive terminating resistors from the end of each leg (not at the apex) to ground. The resistors should be approximately 500 ohms and have a power rating at least one half that of the output power of the transmitter being used. Without the resistors, the antenna radiates bidirectionally, both front and back.

The antenna must be fed by a balanced transmission line.

For further information, see TM 11-666 paragraph 89.

Table 3-2. Leg Angle for V Antennas.

Figure 3-38. V antennas.

Figure 3-39. Sloping-V antenna.




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