Wednesday, May 19, 2010

Tactical Antennas


Critical Tasks: 01-5705.07-0003
01-5879.07.9001

OVERVIEW

LESSON DESCRIPTION:

In this lesson, you will learn about the types of tactical antennas and their radiation patterns. In addition, you will learn about fabricating field expedient antennas using various repair techniques.

TERMINAL LEARNING OBJECTIVE

ACTION:Explain basic antenna theory.
CONDITION:Given this lesson.
STANDARD:To demonstrate competence, you must achieve a minimum of 70 percent on the subcourse examination.
REFERENCES:The material in this lesson was derived from FM 11-32, FM 11-64, FM 24-18, FM 24-19, and TC 24-24.

INTRODUCTION

Tactical antennas are designed for efficiency and ease-of-use, and are ruggedized to take the abuse they receive in the field. Some antennas are easy to use, such as a whip antenna that is used in high mobility operations. Others, like directional antennas, require a working knowledge of antenna engineering. All antennas either release or capture electromagnetic radiation.

1. General. Most practical transmitting antennas are divided into two basic classifications-half-wave antennas and quarter-wave antennas. An antenna operates some distance above the ground and may be polarized either vertically or horizontally. A quarter-wave antenna operates with one end grounded. Quarter-wave antennas are used both below and above 2 MHz. Half-wave antennas are used at the higher frequencies (above 2 MHz).

2. Half-wave antenna. The half-wave antenna operates on the principle that the wavelength to which any wire will electrically tune depends upon its physical length. It is center-fed. Its total wire length equals a half of the wavelength of the signal to be transmitted. The maximum radiation emanates perpendicular from the axis to the half-wave antenna. The half-wave antenna is also known as a doublet, a dipole, or a Hertz antenna. It can be erected in a vertical, horizontal, or slanting position between trees or with upright supports from a kit. The half-wave antenna is used for voice or RATT messages when the tactical situation permit stationary operation. It is used for operating in the 2-to 30-MHz frequency range, and it extends the signal range to 300 miles and beyond by using sky wave propagation. The half-wave antenna operates at high frequencies when used on an aircraft or vehicle. In such cases the aircraft or vehicle chassis becomes the effective ground for the antenna. The AN/GRA-50 antenna group that is used in erecting a half-wave system is shown in Figure 2-1. Two configurations for the AN/GRA-50 half-wave antenna are shown in Figures 2-2 and 2-3.

Figure 2-1.  AN/GRA-50 antenna group

Figure 2-1. AN/GRA-50 antenna group

Figure 2-2.  Half-wave doublet antenna with two upright supports

Figure 2-2. Half-wave doublet antenna with two upright supports

Figure 2-3.  Half-wave doublet antenna supported by trees

Figure 2-3. Half-wave doublet antenna supported by trees

3. Whip antenna. The efficient half-wave antenna is not practical for use in mobile operations, particularly with vehicular-mounted radio set. The whip is the most common antenna used for both manpack and mobile vehicle operations. It is electrically short and vertically positioned. To achieve an efficiency comparable to that of a half-wave antenna, the height of the vertical radiator should be a quarter wavelength. To attain this, a loaded whip is used. This loading increases the electrical length of the vertical radiator to a quarter wavelength. The other quarter wavelength of the antenna is supplied by the ground, a counterpoise, or any conducting surface that is large enough. Some common whip antenna applications are shown in Figure 2-4.

Figure 2-4.  Whip antennas

Figure 2-4. Whip antennas

a. Whip antennas used with HF tactical radio sets can be as long as 15 feet, such as the whip used on a RATT rig. The whip antenna provides an omnidirectional pattern during mobile operations. When the RATT rig arrives at its destination, its crew switches to the half-wave antenna.

b. The whip antenna used with lightweight portable FM radios is 3 ft long for the semi-rigid steel tape antenna and 10 ft long for the multisection whip antenna. It is shorter than a quarter wavelength to keep it at a practical length. (A quarter wavelength antenna for 5 MHz would be over 46 feet long.) An antenna tuning unit, built into the radio set or supplied with it, compensates for the missing antenna length. The tuning unit (called a matching unit when mounted on the antenna) varies the electrical length of the antenna to accommodate a range of frequencies.

c. The whip antennas used with tactical radio sets radiate an omnidirectional pattern in the horizontal plane. This radiation pattern is ideal for tactical operations because the stations in a radio net will lie in random directions and will frequently change their positions.

d. When a whip antenna is mounted on a vehicle, the metal of the vehicle affects antenna's operation. As a result, the direction in which the vehicle is facing may affect transmission and reception, particularly of distant or weak signals. A vehicle with a whip antenna mounted on the left rear side transmits its strongest signal in a line running from the antenna through the right front side of the vehicle. Likewise, an antenna mounted on the right rear side of the vehicle transmits its strongest signal in a direction toward the left front side. The best reception is obtained from signals traveling in the direction shown by the dashed arrows in Figure 2-5.

Figure 2-5.  Best directivity of whip antenna mounted on vehicle

Figure 2-5. Best directivity of whip antenna mounted on vehicle

e. The best direction for transmission can often be determined by driving the vehicle in a small circle until the best position is located. Normally, the best reception and transmission are achieved in the same direction.

4. Ground-plane antenna. This antenna is a vertical quarter-wave antenna that is used to increase the transmission and reception range of tactical FM radio sets. It uses radial elements (acting as a counterpoise) that serve as the ground. The coaxial cable is connected with the inner conductor feeding the vertical element, 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. The two tactical ground-plane antennas in wide use throughout the US Army are the RC-292 and the OE-254.

a. The RC-292 is a stationary, general-purpose, ground-plane antenna. The vertical radiating element and the ground-plane elements must be changed to the proper length for different operating frequencies. Its frequency range is between 20 and 76 MHz, and its planning range is about twice that of a radio set using a quarter-wave whip antenna. The RC-292 antenna can be erected at various heights up to 41 feet, depending on the number of mast sections used. Figure 2-6 shows an erected RC-292 antenna.

Figure 2-6.  RC-292 ground-plane antenna

Figure 2-6. RC-292 ground-plane antenna

b. The OE-254 broadband, omnidirectional VHF antenna system is replacing the RC-292. The OE-254 antenna operates in the 30-to 88-MHz frequency range without the need to manually drop and change out antenna elements. Its planning range is about 36 miles for average terrain. Like the RC-292, the OE-254 can be erected at different heights, depending on the number of mast section used. An erected OE-254 is depicted in Figure 2-7.

Figure 2-7.  OE-254 ground-plane antenna

Figure 2-7. OE-254 ground-plane antenna

5. Directional VHF log-periodic antenna.

a. This broadband, omnidirectional antenna provides an extended range and directivity for tactical radios. It is used to communicate in the 30-to 88-MHz frequency range and does not require any mechanical or electrical adjustments.

b. The log-periodic antenna can operate with either horizontal or vertical polarization. It has the capability of changing polarization in less than one minute.

c. The highly directional radiation pattern of the log-periodic antenna provides very effective electronic counter-countermeasures (ECCM) in a hostile electronic warfare (EW) environment. Since its radiated energy is focused in one direction, less transmitter power is needed, further enhancing ECCM.

d. This antenna can be erected in a geographical area no greater than 60 feet in diameter by two soldiers in 20 minutes. Its mechanical azimuth can be changed within one minute. This antenna can be mounted on a quick-erect mast either on a vehicle or a shelter, and transported by manpack or tactical vehicle when fitted into two packages (one for the antenna and one for the mast).

e. The log-periodic antenna is organic to battalion and higher level units for special applications. It is primarily used by forward units in command and intelligence nets to a higher headquarters. Because it is a directional antenna, its use is usually restricted to point-to-point communications.

f. Figure 2-8 illustrates the log-periodic antenna in three configurations.

Figure 2-8.  Log-periodic antenna

Figure 2-8. Log-periodic antenna

6. VHF half-rhombic antenna OE-303.

a. The half-rhombic antenna is used mostly for special purposes by forward units over extended distances on command and control and intelligence nets. The OE-303 antenna is a vertically polarized antenna which, when used with the current VHF-FM tactical radios, considerably extends the transmission range. It provides some degree of ECCM protection not offered by the current VHF-FM omnidirectional antenna. When properly employed, the half-rhombic antenna decreases VHF-FM radio susceptibility to hostile EW operations and enhances the communications ranges of the deployed radio set. This effect is realized by directing the maximum signal strength in the direction of the desired friendly unit.

b. This high-gain, lightweight, directional antenna is capable of operating in the 30-to 88-MHz frequency range without having to be physically tuned by the operator. Its highly directional pattern makes it especially suited in providing point-to-point communications. It is oriented in the direction of the desired transmission by using a compass and the appropriate map sheet.

c. The OE-303 antenna is rugged enough to withstand moving (erection and teardown) every four to six hours. It can operate in harsh climatic conditions. It can be erected by two soldiers in 20 minutes or less in a geographical area 175 ft in diameter or less, depending on the frequency used. This antenna can be mounted on any structure about 50 feet in height, and is capable of azimuth directional change within one minute. Mast assembly AB-1244 is the primary antenna support structure used with the half-rhombic VHF antenna. Using its mast assembly, this antenna is 30 feet high.

d. The antenna and all the ancillary equipment (guys, stakes, tools, mast sections) are contained in two carrying bags for manpack or vehicular transportation. Figure 2-9 depicts the half-rhombic antenna.

Figure 2-9.  Half-rhombic VHF antenna

Figure 2-9. Half-rhombic VHF antenna

7. Near vertical incidence sky wave (NVIS). NVIS antenna AS-2259/GR is a lightweight sloping dipole, omnidirectional antenna used with AM radios (AN/GRC-106) and improved high frequency radios (IHFR)(AN/PRC-104A and AN/GRC-213/193A) that operate in the HF range of 2 to 30 MHz. Like the doublet antenna, the NVIS is used when the tactical situation allows stationary operations. The NVIS extends radio range up to 300 miles by using sky wave propagation. Polarized horizontally and vertically at the same time, it can be erected by two soldiers in about five minuets. Figure 2-10 shows an operator using an NVIS antenna wit an AN/PRC-104A portable radio.

Figure 2-10.  NVIS antenna

Figure 2-10. NVIS antenna

8. Combat net radio (CNR). CNR is designed around three separate radio systems--IHFR, Single-Channel Ground and Airborne Radio System (SINCGARS), and single-channel tactical satellite (TACSAT) radios.

a. The IHFR is replacing the older HF manpack (AN/PRC-70/74) and vehicular (AN/GRC-106) radios. User-owned and operated, it employs both ground and sky wave propagation for short and medium-range communications to pass voice and data signals. The IHFR uses the manpacked whip antenna, the vehicle-mounted whip antenna, the AN/GRA-50 doublet antenna, and the AS-2259 NVIS antenna.

(1) The AT-271A/PRC, one type of collapsible whip antenna for manpack operation, is depicted in Figure 2-11. This antenna is easily assembled and broken down. The cord running through the sections provides the tension needed to keep them together.

Figure 2-11.  Collapsible whip antenna AT-271A/PRC with antenna base

Figure 2-11. Collapsible whip antenna AT-271A/PRC with antenna base

(2) The AS-3683/PRC, another whip antenna used with the AN/PRC-104A and the SINCGARS radios, is shown in Figure 2-12. This is a very flexible antenna especially suited for use in thick brush or jungle. Its flexible goose-neck antenna base allows the operator to orient it to obtain optimum reception. However, using the goose neck antenna base is optional.

Figure 2-12.  Flexible whip antenna AS-3683/PRC

Figure 2-12. Flexible whip antenna AS-3683/PRC

(3) A whip antenna used with the AN/GRC-213 in a vehicle-mounted configuration is shown in Fig 2-13. When installed with the vehicle installation kit, this antenna is the most convenient to use because it allows communications while the vehicle is moving and it has a omnidirectional (360°) radiation pattern. It also requires no support except for the vehicle mount. A disadvantage is that its propagation range is shorter than either the doublet or NVIS antennas.

Figure 2-13.  Vehicle-mounted whip antenna

Figure 2-13. Vehicle-mounted whip antenna

b. SINCGARS operates in the VHF range.

(1) SINCGARS uses broadband antennas that do not have to be changed when frequencies are changed, such as the OE-254 ground-plane and the AS-3900 and AS-3684 vehicular whip antennas. The output frequency can change in a wide range between hops due to the frequency hopping nature of SINCGARS. The narrow band RC-292 ground-plane antenna cannot be used.

(2) Like the IHFR, SINCGARS uses the manpack whip antenna (AS-3683/PRC) for communicating in heavy vegetation or when the transmitting range is deliberately limited. It also operates using the AS-3684/VRC whip antenna, a 10-foot long antenna that consists of two antenna elements and a matching unit base (Figure 2-14). The base spring allows the antenna to bend when it strikes an obstruction

Figure 2-14.  AS-3684/VRC vehicular antenna

Figure 2-14. AS-3684/VRC vehicular antenna

c. TACSAT operates in the UHF range. The AN/PSC-3 and AN/VSC-7 communications systems are lightweight, highly compact, and deployable in quick-reaction situations where extended communication range is essential to mission effectiveness. They can operate on-the-move/line-of-sight (LOS) at 2 watts or in the at-halt/satellite mode at 35 watts. They can transmit or receive in voice or data formats in both modes. Figure 2-15shows the AN/PSC-3 radio set. This medium gain, collapsible parabolic antenna can be set up in minutes and is highly reliable. The AN/VSC-7 TACSAT radio set (Figure 2-16) used as a net control station and is mounted in a tactical truck or an S-280 communications shelter. It can serve up to 15 AN/PSC-3 terminals in a communications net with the selection of conference or individual call-codes. Its low-gain omnidirectional whip antenna provides LOS. The AN/VSC-7's high-gain antenna enhances transmission and reception.

Figure 2-15.  AN/PSC-3 TACSAT radio set

Figure 2-15. AN/PSC-3 TACSAT radio set

Figure 2-16.  AN/VSC-7 TACSAT radio set

Figure 2-16. AN/VSC-7 TACSAT radio set

9. UHF antennas. These highly directional antennas are used with mobile subscriber equipment (MSE) and the older multichannel systems. They concentrate radiation in a given direction and minimize radiation in other directions. Their high directivity also aids in obtaining a slight degree of transmission security, making enemy direction finding more difficult, and reducing noise and interference.

a. The corner-reflector (flyswatter) antenna consists of an adjustable reflector and an antenna element. It is used in multichannel systems with radios that operate in the 200-to 1000-MHz range. Highly directional, its reflector angle can be adjusted to operate on any frequency within its range. This antenna is usually used with radio sets that have RF duplexing capabilities. The RF duplexer permits the radio set to transmit and receive, using one coaxial cable between the antenna and the radio set. The corner-reflector is shown is both vertical and horizontal polarization positions in Figure 2-17.

Figure 2-17.  Typical corner reflector antenna

Figure 2-17. Typical corner reflector antenna

b. The horn-type antenna is a directional antenna with a modified dipole element mounted in a ridge-loaded horn. The horn-type antenna used in multichannel systems are pyramidal (rectangular and flared in both planes). The horn is designed to get the flare, length, and aperture that will give the best performance throughout the horn's entire frequency range. This combination of flare, length, and aperture emits a narrow beam of high-intensity RF energy in the forward direction. Horn antennas can be vertically or horizontally polarized, and can be mounted in pairs on the same mast or singularly on separate masts. When used with a radio set having an RF duplex, a single horn can be used to transmit and receive radio signals. Horn-type antennas are used with some radio sets that operate on frequencies from 601.5 to 1849.5 MHz. The horn antenna is broadband and no adjustments are made when changing frequencies. Elevation or depression angles may be made at the antenna's rear, as needed. A rear view of a pair of horn-type antennas mounted in a vertical polarization is shown in Figure 2-18.

Figure 2-18.  Typical horn-type antenna

Figure 2-18. Typical horn-type antenna

c. The parabolic reflector antenna consists of a saucer-like reflecting surface (parabola) and a dipole (feed device) placed at its focal point. There are numerous types of reflector antennas used, depending on the frequency. The MSE LOS radios use parabolic reflector antennas mounted on 15-meter masts. Figure 2-19 shows two examples of parabolic antennas.

Figure 2-19.  Parabolic reflector antennas

Figure 2-19. Parabolic reflector antennas

10. Field repair and expedients. Antennas are sometimes broken or damaged, resulting in failed or poor communications. In the days before cable television, viewers at home received a TV signal through an antenna, either strapped to the chimney or sitting on top of the set. If the antenna was missing or broken, an expedient loop antenna could be fashioned from a simple coat hanger. Reception was not the best, but it was tolerable. Remember, as a signal officer, your job is to get the message through. Knowing how to do a field expedient repair job may come in handy some day. The following antenna repair techniques have been proven to work.

a. Metallic whip antennas.

(1) Should a whip antenna break into two sections, you can lash the two sections together to provide a good mechanical connection. To ensure essential electrical conductivity, scrape the paint off of each section's end and tie them together with stripped field wire (WD-1). Solder the connection if you have the time and equipment. Place a stick, pole, or branch on each side of the break and wrap the splint tightly with field wire, rope, or tape, as shown in Figure 2-20.

Figure 2-20.  Spliced metallic whip antenna

Figure 2-20. Spliced metallic whip antenna

(2) If you lose your antenna and are left with just a stub, you can use another repair method to get back on the air. If the missing section is about 6 feet long, just find a pole the same length and lash it to the stub. Scrape off the paint from the top 2 inches of the whip's stub and wrap about 12 inches of bare wire around the stub's scraped portion. Wrap it tight and tape it securely. Attach a length of WD-1 along the pole's length with tape. The total length of the upright WD-1 and antenna stub should be the same length as the length of the original antenna. This antenna will not take much abuse, but it will send and receive signals. Figure 2-21 illustrates this technique.

Figure 2-21.  Spliced metallic whip antenna using WD-1 as part of the antenna

Figure 2-21. Spliced metallic whip antenna using WD-1 as part of the antenna

b. Vertical antennas transmit and receive omnidirectionally. Most tactical antennas used for vehicular and manpack radios are vertical. A vertical antenna can be improvised by using a metal pipe or rod of the correct length, held erect by guy lines. The antenna's lower end is insulated from ground by placing it on insulating material. Figure 2-22 shows vertical antennas made of wire and supported by a tree and a wooden pole. If the vertical mast is not long enough to support the wire upright, the connection can be modified at the antenna's top, as shown in Figure 2-23.

Figure 2-22.  Field substitutes for support of vertical wire antennas

Figure 2-22. Field substitutes for support of vertical wire antennas

Figure 2-23.  Additional means of supporting vertical wire antennas

Figure 2-23. Additional means of supporting vertical wire antennas

c. The electrical length of an end-fed, half-wave antenna is measured from the antenna terminal on the radio set to the antenna's far end. For optimum performance, the antenna should be constructed longer than necessary, and shortened as required until the best results are achieved. The ground terminal of the radio set should be connected to a good Earth ground for this antenna to operate efficiently. This antenna is depicted in Figure 2-24.

Figure 2-24.  End-fed half-wave antenna

Figure 2-24. End-fed half-wave antenna

d. The center-fed doublet antenna is a half-wave antenna consisting of two quarter-wavelength sections on each side of the center (Figure 2-25). Doublet antennas are directional broadside to their length; this makes the vertical doublet antenna essentially omnidirectional. The horizontal doublet antenna is bidirectional. A center-fed half-wave FM antenna can be supported by a wooden frame, shown in Figure 2-26. View A shows a horizontal antenna, and view B shows a vertical antenna. 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 2-25.  Half-wave doublet antenna

Figure 2-25. Half-wave doublet antenna

Figure 2-26.  Center-fed half-wave antenna

Figure 2-26. Center-fed half-wave antenna

e. Two field-expedient directional antennas are the vertical half-rhombic antenna (Figure 2-27) and the long-wire antenna (Figure 2-28). 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. If needed, the antennas will operate satisfactorily as low as 1 meter (about 3 feet) above the ground. The wire's far end is connected to ground through a noninductive 500-to 600-ohm resistor. To ensure that the transmitter's output power will not burn out the resistor, a resistor rated at least one-half of the transmitter's wattage output should be used. 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. These antennas are used primarily for transmitting or receiving HF signals.

Figure 2-27.  Vertical half-rhombic antenna

Figure 2-27. Vertical half-rhombic antenna

Figure 2-28.  Long-wire antenna

Figure 2-28. Long-wire antenna

f.The V antenna

(1) This antenna consists of two wires forming a V, with the open area of the V pointing toward the desired direction of transmission or reception (Figure 2-29). An easier way of constructing this antenna is to slope the legs downward from the apex of the V; this is called a sloping-V antenna (Figure 2-30).

Figure 2-29.  V antenna

Figure 2-29. V antenna

Figure 2-30.  Sloping-V antenna

Figure 2-30. Sloping-V antenna

(2) The angle between the legs varies with the length of the legs in order to achieve minimum performance. Table 2-1 can be used to determine the angle and the length of the legs.

Table 2-1.  Leg angle for V antennas

Table 2-1. Leg angle for V antennas

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

(4) To make the V antenna radiate in only one direction, noninductive terminating resistors are added 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 half that of the output power of the transmitter being used. The antenna will radiate bidirectionally, both front and back without the resistors.

11. Safety. Soldiers are still occasionally killed or seriously injured a result of antenna accident, in spite of repeated safety warnings tough briefings, publications, and messages. As a signal officer, you should develop a keen sense of field safety, especially as it relates to signal operations. If you know an antenna is too close to a power line, insist that it be dropped and moved. Safety tips and warnings that are found in many signal-related publications are shown in Figures 2-31 and 2-32, respectively. Study these safety tips and warnings and think back to experiences you have had during previous operations. Have you violated any of these warnings? If the answer is no, you are to be commended for your good approach to safety. If the answer is yes, then resolve to heed the warnings henceforth.

Figure 2-31.  Safety tips

Figure 2-31. Safety tips

Figure 2-32.  Safety warning

Figure 2-32. Safety warning

12. Summary. In this lesson, you learned about the types of tactical antennas, their radiation patterns, and how to fabricate field-expedient antennas using various repair techniques.

a. The half-wave antenna is also called a doublet, a dipole, or a Hertz antenna. It is center-fed. Its total wire length is one half of the wavelength of the signal to be transmitted.

b. The whip antenna provides an omnidirectional radiation pattern. It is a quarter-wave antenna used for manpack and vehicular operations.

c. The ground-plane antenna is a vertical quarter-wave antenna that increases the range of tactical FM radio sets. Its radial elements provide a counterpoise that simulates a ground. The older RC-292 has radial elements that must be changed accordingly with frequency changes. The OE-254 does not require any changes in elements when frequencies change.

d. The log-periodic antenna is very directional and is usually used in point-to-point communications.

e. The half-rhombic antenna OE-303 provides an extended range and affords some ECCM protection (not omnidirectional).

f. The NVIS is a sloping dipole that gives an omnidirectional pattern for AM radios. It can extend the range up to 300 miles.

g. The two CNR antennas are the AT-271A/PRC, a collapsible whip for manpack operations, and the AS-3683/PRC, a flexible antenna used in areas of heavy vegetation.

h. The three SINCGARS antennas are the OE-254, the AS-3684/VRC (vehicular mounted), and the AS-3683/PRC.

i. The two TACSAT antennas are the collapsible parabolic reflector for the AN/PSC-3 and the omnidirectional whip antenna for the AN/VSC-7.

j. There are several UHF antennas.

(1) The corner-reflector (flyswatter) antenna is highly directional and is used with multichannel systems.

(2) The horn-type antenna is another antenna used in multichannel systems. The flared design serves to direct the RF energy in a highly directional pattern.

(3) The parabolic reflector antenna has a reflecting surface and a dipole at its center. The reflector is used to capture or release signals.

k. There are several field expedient antennas.

(1) Metallic whip. Broken whips can be lashed together using WD-1, rope, and tape. If part of the antennas is missing, field wire can be used as the radiating element.

(2) Directional antennas. The vertical half-rhombic and the long-wire antennas are used for transmitting and receiving HF signals.

(3) V antenna. It is made of two wires forming a V with the open end pointing toward the direction of transmission. Using a resistor changes the V antenna from bidirectional to unidirectional.


Source: http://www.globalsecurity.org/military/library/policy/army/accp/ss0131/lsn2.htm