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LESSON 1
EMPLOYMENT OF TACTICAL RADIO SETS
CRITICAL TASKS: S1-5878.04-9001, 01-5704.04-9001, 01-5704.07-0001
LESSON DESCRIPTION:
In this lesson you will learn how to configure the various tactical radio sets available and how to select a particular set for a given communications task. You will also learn the field expedient measures used to repair or replace damaged antennas, or to configure an antenna for a specific task.
TERMINAL LEARNING OBJECTIVE:
| ACTIONS: |
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| CONDITION: |
You will be given information from this lesson. |
| STANDARD: |
To demonstrate competency of the terminal learning objective, you must achieve a minimum score of 70% on the subcourse examination. |
| REFERENCES: |
The material contained in this lesson was derived from the following publications: FM 11-32, FM 24-18, and TC 24-24. |
The modern battlefield is a huge place that can span thousands of miles over virtually impassable terrain. It can also extend from below the ground to outer space. Radio communications is the only hope that a modern commander has of maintaining perspective (and thus control) over such a vast area. The communications tasks involved are many and varied, and require every resource that the modern signal officer can bring to bear in support of this effort. As that signal officer, it is vital that you be familiar with the equipment and personnel at your disposal, and that you be able to use whatever equipment is best suited for any given communications situation. The following paragraphs outline the capabilities and uses of several pieces of modern tactical communications equipment. Some of the equipment discussed may not be widely in circulation any more, but it is important that you be familiar with it nonetheless. You never know when you may be forced to use an old piece of equipment as a backup to a new piece that fails unexpectedly. Appendix B is equipment annexes that provide drawings of most of the equipment presented here. Refer to these drawings as you read about the various pieces of radio equipment. This course is not designed to provide in-depth knowledge about the operation of individual equipment, and should never be used as a replacement for the equipment technical manuals.
1. Radio Sets AN/PRC-25, AN/VRC-53, and AN/GRC-125.
This family is the earliest group of FM transmitter-receiver radio sets still in use. All three versions utilize the receiver-transmitter (RT) -505 receiver-transmitter operating in the lower very high frequency (VHF) band (30 to 52.95 megahertz (MHz) and 53 to 75.95 MHz). The PRC-25 is a portable manpack version. The VRC-53 adds amplifier OA-3633 to make a vehicular-based unit, and the GRC-125 is a portable or vehicular based model. All three configurations are used for short-range communications. This family of radio sets has no secure transmission capability.
2. Radio Sets AN/PRC-77, AN/VRC-64, and AN/GRC-160.
This family of radio sets is replacing the PRC-25, VRC-53, and GRC-125 series respectively. The transmission characteristics are exactly the same as the earlier radio sets. The difference is that the newer sets use receiver-transmitter RT-841 which is completely transistorized. Another feature incorporated into the new sets is the capability of secure voice transmission using the VINSON device.
3. AN/VRC-12 Family of Radio Sets.
The radio sets in the AN/VRC-12 family are short-range vehicular and fixed radio sets designed for general tactical use. They provide frequency modulation (FM) voice communications and can be used with secure voice and digital data equipment using the X MODE facility or VINSON. Two of the sets (AN/VRC-45 and AN/VRC-49) have retransmission capability. The radio sets of the AN/VRC-12 family will net with each other and with other FM radio equipment operating in the 30 to 75.95 MHz frequency range. Each of the eight configurations in this family is made up of a combination of receiver-transmitters RT-246/VRC and RT-524/VRC and receiver R-442/VRC along with support equipment. Table 1-1 shows the equipment configurations for each of the various AN/VRC-12 series radios.
Table 1-1. AN/VRC-12 family configurations
a. Receiver-Transmitter RT-246/VRC and RT-524/VRC. These two receiver-transmitters are the heart of the AN/VRC-12 family of radios. They operate in two bands; Band A - 30 to 52.95 MHz, and Band B - 53 to 75.95 MHz. They are capable of transmission up to about 41 kilometers (km). Each member of the AN/VRC-12 family contains at least one of these units. The primary differences between the two units are that the RT-246 has ten automatic channel presets and the RT-524 has a built-in loudspeaker.
b. Receiver R-442/VRC. This receiver is used in several of the AN/VRC-12 configurations to allow monitoring of one net while transmitting in another. The R-442 operates in the same frequency range as the RT-246 and RT-524.
This configuration includes one RT-246, one R-442, and two antennas. You can use this configuration to monitor one net while you conduct communications in another. You can easily switch transmitting frequencies of the R-246 to transmit in the R-442's net.
This configuration consists of one RT-246 and one antenna and it is used for communications in a single radio net.
This configuration consists of one RT-246, two R-442s, and three antennas. With this set you can communicate in one net and monitor two additional nets. As with the VRC-12, you can change transmitting frequency on the RT-246 and transmit into either of the other two nets.
This configuration consists of two RT-246s, two antennas, and a C-2299/VRC Retransmission Cable Kit. You should use this configuration to provide retransmission facilities for two stations that are too far apart to talk to each other directly. You can also use this set to retransmit information from one net into another net by tuning the two RT-246s to different frequencies. The MK-456/GRC cable kit can also perform the retransmission functions for the AN/VRC-45 if the C-2299/VRC cable is not available.
This configuration uses one RT-524 and one antenna and is essentially identical in function to the AN/VRC-43.
This configuration uses one RT-524, one R-442, and two antennas; It functions the same way as the AN/VRC-12.
This set consists of one RT-524, two R-442s, and three antennas. It functions the same way as the AN/VRC-44.
This configuration consists of two RT-524s, two antennas, and the C-2299/VRC. It serves the same retransmission functions as the AN/VRC-45.
All the AN/VRC-12 family configurations are secure voice capable. You can secure the sets using either VINSON (KY-57) or NESTOR (KY-38) secure devices.
4. Radio Set Control AN/GSA-7.
The AN/GSA-7 is an electronic switching device. You use it to interface FM radio equipment with local push-to-talk wire telephone circuits. The GSA-7 is the basis of the net radio interface (NRI) system. The unit acts as an automatic keying device so that when a remote telephone operator keys his telephone set he also keys the transmitter of the FM radio set. This allows his wire telephone message to transmit over the FM frequency to a receiver that can also be attached to another GSA-7 to convey the message into another wire telephone net at a remote site. You can control a radio set with the GSA-7 from as far away as 16 km (10 miles (mi)).
5. Control Group AN/GRA-6.
Occasionally you may want to locate your radio transmitter in a site where communications are good, but the site does not offer a good tactical position. When this happens the best thing to do is set the radio up so you can operate it remotely. The GRA-6 is the piece of equipment that allows you to do this. The GRA-6 consists of a local and remote unit used at the radio site and the remote control site respectively. The two units are connected by field wire and can be separated by up to 3.2 km or 1.5 miles.
6. Radio Set Control Group AN/GRA-39B.
The GRA-39B is another remote control set similar to the GRA-6. It also is used with FM radio sets and has a range of 3.2 km. The principal difference between the GRA-39B and the GRA-6 is that the 39B is fully transistorized and the 6 is not.
7. Retransmission Cable Kit MK-456A/GRC.
This kit is used with the AN/PRC-25 and AN/PRC-77 series of radios for retransmission. You can also use it with the AN/VRC-12 series if the standard C-2299/VRC kit is unavailable.
8. Radio Set AN/GRC-19.
The AN/GRC-19 is a medium-power voice and continuous wave radio set designed for vehicular installations. It forms the central unit for several of the older radio teletypewriter (RATT) sets. The GRC-19 is being phased-out of use and replaced with the newer GRC-106 radio set. You can also use the-GRC-19 to perform retransmission functions. Both transmitter and receiver operate in the high frequency (HF) band. The transmitter operates between 1.5 and 20 MHz, and the receiver operates between 0.5 and 32 MHz. The AN/GRC-19 operates in amplitude modulation/double sideband (AM/DSB) mode.
As stated previously, the AN/GRC-106 is replacing the AN/GRC-19 in its mobile retransmission role. It is also the central unit for the newer family of radio teletypewriter sets currently in use. The GRC-106 operates in the HF band between 2.0 and 29.999 MHz and uses the amplitude modulation/single sideband (AM/SSB) mode. There are two basic configurations of this equipment, the GRC-106 and GRC-106A. The former uses receiver-transmitter RT-662/GRC which has a channel spacing of 1 kilohertz (kHz). The 'A' variation uses receiver-transmitter RT-834/GRC which has channel spacing of 100 hertz (Hz). You can see from this that the 'A' variation has ten times as many channels available as the standard GRC-106. The AN/GRC-106 has both voice and continuous wave (CW) capability.
10. Radio Set AN/FRC-93.
The FRC-93 is a commercial type of AM/SSB (upper or lower) radio set adapted for military use. You can also perform CW communications using the FRC-93. The radio set operates in two regions of the HF band, from 3.4 to 5.0 MHz and from 6.5 to 30 MHz. This type of radio set uses crystal sets for tuning.
11. Radio Set AN/PRC-74.
The AN/PRC-74 is a low-power, transistorized AM/SSB/upper sideband (USB) radio set designed for voice and CW communications. The PRC-74 is configured as a manpack and is primarily designed for communications in areas where line of sight (LOS) communications is not possible due to terrain or other obstructions. The AN/PRC-74 comes in four variations: basic, A, B, and C. The basic and A versions operate between 2.0 and 11.999 MHz. The B and C versions extend the upper frequency limit to 17.999 MHz. The AN/PRC-74 adds the versatility of being able to communicate with remote sites that you could not communicate with using the AN/PRC-25 or 77 series of FM radios.
12. Radio Set AN/PRC-70.
The AN/PRC-70 was originally designed to give special forces, ranger, long-range reconnaissance patrol, and selected engineering units a lightweight multimode means of communications. The unit operates over a range of 2.0 to 75.999 MHz in the HF and very high frequency (VHF) bands. The PRC-70 can operate in AM/DSB, AM/SSB, FM, CW, and frequency-shift keying (FSK) modes. This flexibility allows the unit using this device to travel almost anywhere they need to go and still be able to provide real-time intelligence information to command and control units in the corps and theater areas. The AN/PRC-70 usage has been expanded to battalion communications stations as well as forward deployed units. The AN/PRC-70 can be operated in secure mode using both VINSON and NESTOR secure devices. Another great advantage of the PRC-70 is that it can communicate with any of the AM, AM/SSB, or FM radios discussed previously in this lesson. You can also set up retransmission stations using the PRC-70 and the MK-456/GRC control cable. The combination of its light weight, ruggedness, and wide range of capabilities makes the AN/PRC-70 an ideal radio set for units that require great freedom of movement and activity. In a fixed site, one AN/PRC-70 can serve as a backup for almost all other tactical communications systems.
13. Teletypewriter Set AN/PGC-1.
The PGC-I is the most basic RATT system currently in use. It is a small transportable unit consisting of a keyboard and a printing device. The PGC-1 is used in fixed and transportable configurations.
14. Teletypewriter Set AN/FGC-20.
The FGC-20 is similar to the PGC-1 with the exception of being larger and more durable. This unit is found primarily in fixed sites where portability is not a requirement.
15. Radio Teletypewriter Sets AN/GRC-46, AN/VSC-1 and AN/VRC-29..
The GRC-46, VSC-1, and VRC-29 are older RATT sets that are used in mobile communications applications. They all use the AN/GRC-19 radio set as the basic transmitter-receiver, and they have the same operating frequencies and modes as that unit. The Army is replacing these units with the AN/GRC-106 based RATT sets.
a. GRC-46. The GRC-46 consists of one GRC-19 and two teletypewriter units. One of these units is a tape reperforator type unit. The GRC-46 is shelter-housed and found in 3/4-ton mobile communications stations.
b. VRC-29. The VRC-29 is identical to the GRC-46 except that it is not shelter-housed. This version is carried in armored personnel carriers (APCs).
c. VSC-l. The VSC-1 contains the AN/GRC-19 and a single teletypewriter (non-reperforating). This set is designed to be lightweight and is carried in jeeps.
16. Radio Set AN/GRC-26D.
The GRC-26D is a high-power RATT station designed for use in fixed, semifixed, or mobile applications. The set consists of a transmitter, two receivers, and three teletypewriters. The GRC-26D operates in the same frequency bands and operational modes as the GRC-46 family. In its mobile configuration, the GRC-26D is shelter-mounted in a 21 ton vehicle. The GRC-26D is generally considered obsolete and has been replaced in regular units by the GRC-106 based AN/GRC-122.
17. Radio Teletypewriter Sets AN/GRC-122, AN/GRC-142, AN/VSC-2 and AN/VSC-3.
These RATT sets are replacing the older AN/GRC-19 based systems described previously. They are based on the more modern and versatile AN/GRC-106 transmitter-receiver, and they have the same frequency and mode characteristics as that equipment. The following paragraphs briefly discuss the differences in each of these pieces of equipment.
a. AN/GRC-122. The GRC-122 has replaced the AN/GRC-26D as the long-distance shelter mounted RATT station for fixed, semifixed, and mobile applications. It consists of two receiver-transmitters and three teletypewriters. The GRC-122 is much lighter than the GRC-26D and can be carried on a 3/4 ton vehicle.
b. AN/GRC-142. The GRC-142 is replacing the AN/GRC-46 in the 3/4 ton vehicular installations. In addition to the ability to use AM/SSB, the GRC-142 also has the added ability to perform secure voice communications which the older equipment does not. The GRC-142 consists of one receiver-transmitter and two teletypewriter units.
c. AN/VSC-3. The VSC-3 is replacing the AN/VRC-29 for installation in armored personnel carriers. The set consists of one receiver-transmitter and two teletypewriters. Except for being shelter-mounted, the VSC-3 is essentially identical to the GRC-142.
d. AN/VSC-2. The VSC-2 is replacing the VSC-1 as the jeep-mounted version of the RATT set. Like its predecessor, the VSC-2 consists of a receiver-transmitter and a non-reperforating teletypewriter. As with the more robust members of its family, the VSC-2 is capable of AM/SSB secure voice communications.
18. SINCGARS.
SINCGARS equipment is replacing the AN/PRC-77 and AN/VRC-12 series equipment currently in use. SINCGARS is a modular system, allowing components of one piece of equipment to serve as replacements for another. This feature provides increased reliability of communications overall. SINCGARS has two basic modes of communication: single-channel or frequency hopping (FH). In the single-channel mode, it can communicate with any of the older VHF-FM equipment currently in use. In this mode, SINCGARS equipment can hold up to eight single-channel frequencies. When you shift the SINCGARS equipment to FH mode, it shifts operating frequencies about 100 times per second in the 30-88 MHz range. The audio signal is then modulated on the shifting carrier to produce an output signal that moves about the VHF spectrum. This "hopping" of the frequency makes the signal very difficult for enemy forces to detect, locate, or jam. If the changing of frequencies was completely random, there would be no way for two sets to communicates The receiving set has to change frequencies in the same manner as the transmitter. To accomplish this synchronous frequency hopping, the SINCGARS equipment stores up to six separate hopsets. Each hopset is a code that tells the unit how and when to change frequencies in a repeating pattern. In order to establish communications, two stations would make contact in single-channel mode and then switch to their designated hopset to conduct FH operations. As with the single-channel presets, the SOP defines the hopsets to be stored. One additional feature of SINCGARS equipment that makes it better than its predecessors is the ability to control output power of the unit. You can change power settings to transmit from 300 meters to 35 km (22 mi). This feature is advantageous both in field applications where detection is undesirable and in command post applications where many radios are operated simultaneously. You should always take care to operate at the minimum power required to ensure effective communications. The following paragraphs will describe the components and some of the various configurations of SINCGARS equipment.
a. Receiver-Transmitters RT-1523 and RT-1439. All ground-based SINCGARS configurations use either the RT-1523 or the RT-1439 as the basic component. Both units transmit and receive between 30 and 79.999 MHz. Both units are capable of FM, FSK, FH, and digital data input modes of operation. The RT-1523 has an internal integrated communications security (ICOM) module built into it. The RT-1439 is a non-ICOM radio but it can be secured using the KY-57 VINSON secure device. As stated previously, both units can store up to eight single-channel presets and six hopsets. While in FH mode, the units will also receive a manual frequency. This frequency is used to allow other radios to contact the net and join after the net has been established.
b. Manpack Radio AN/PRC-119. This SINCGARS manpack radio is replacing the AN/PRC-25 and AN/PRC-77 manpack radios currently in use. The unit consists of one receiver-transmitter and a battery pack. If you use the RT-1439 then you must also use the VINSON security device.
c. Vehicular Short-Range Radio AN/VRC-87. The VRC-87 is replacing the AN/GRC-53 and AN/GRC-64 as the short-range vehicular tactical radio. This unit is essentially the same as the PRC-119 with extra cabling and no battery.
d. Dismountable Short-Range Radio AN/VRC-88. The VRC-88 adds the components to the VRC-87 necessary to make it a portable unit. This unit is replacing the AN/GRC-125 and AN/GRC-160.
e. Vehicular Long-Range/Short-Range Radio AN/VRC-89. The VRC-89 adds a second receiver-transmitter and a power amplifier to the VRC-87 configuration. This configuration allows the unit to monitor one net while communicating in another. This configuration is replacing the AN/VRC-12 and AN/VRC-47 configurations.
f. Vehicular Long-Range Radio AN/VRC-90. The VRC-90 is essentially a VRC-87 with a power amplifier added for long-range capability. The VRC-90 is replacing the AN/VRC-43 and AN/VRC-46.
g. Vehicular Short-Range/Long-Range Dismountable Radio AN/VRC-91. This unit is essentially identical to the VRC-89 except that it adds the necessary components to be operated as a manpack radio, providing manpack long-range capability. The VRC-91 does not replace any current equipment. The closest current configuration would be an AN/PRC-77 and either a VRC-43 or a VRC-46 in the same vehicle.
h. Vehicular Dual Long-Range/Retransmission Radio AN/VRC-92. The VRC-92 replaces the AN/VRC-45 and AN/VRC-49 as the vehicular station capable of acting as a retransmission station. It is the same as the AN/VRC-89 with an additional power amplifier. This configuration provides high-power capability for both receiver-transmitters.
i. Intravehicular Control Unit (IVRCU) C-11291. This device allows the operator to control up to three separate radios in armored vehicles. It can control all radio functions and can be used with ICOM and non-ICOM radios. The unit can also be set up so that three different operators can control the radio from their respective positions in the vehicle.
j. SINCGARS Remote Control Unit (SRCU). This device allows you to remotely operate your radio from up to 4 km (2.4 mi) away. The SRCU allows for secure remote operation including the control lines from the radio to the SRCU. It also provides intercom facilities between the radio and the remote site. All radio functions can be controlled remotely with the SRCU. The AN/GRA-39 can also be used to remotely control SINCGARS radios, but it only allows remote keying of the radio set.
k. Data Fill Devices MX-10579 and MX-18290. Data fill devices contain hopset and transmission security key (TSK) information for use with SINCGARS in FH mode. The TSK information actually tells the unit how to control the frequency hopping sequence. The MX-10579 holds up to 13 hopsets and 2 TSKs and you must use it with non-ICOM radios only. The MX-18290 holds 13 hopsets and 6 TSKs. Currently, you must have two fill devices to operate a secure SINCGARS radio in FH mode. One fill device is needed to load the radio and the second loads the security device (VINSON or ICOM).
l. VINSON and ICOM Secure Devices. Secure devices provide the means for secure voice communications between remote stations. The ICOM device is built into the ICON transmitter. It contains one traffic encryption key (TEK) for each hopset. This means that in addition to frequency changing constantly, the information being passed is also encrypted. The VINSON device is an external device and is used with the non-ICON radios. It has six preset positions, five for TERs and one for a key encryption key (KEK). The five TEKs used with the radio allow secure operations in up to five different networks at one time. The KES allows the VINSON device to be loaded by over-the-air (radio transmission), fill. In other words, several field units could carry VINSON devices and not carry any encryption codes. A transmitter located at a safe location could transmit the TEK information to all field units simultaneously. Thus, the units could set up secure networks without carrying any cryptographic codes with them. Both the VINSON device and the ICOM radio set are considered cryptographic material and should be destroyed if a unit possessing them is overrun.
19. IHFR.
The IHFR system is another new-technology modular system being introduced into operation. The system is designed to replace the AN/PRC-70, AN/PRC-74, and AN/GRC-106 systems currently in operation. The following paragraphs describe the major components and configurations for IHFR equipment.
a. Receiver-Transmitter RT-1209. The heart of the IHFR system, RT-1209 is an AM/SSB (USB or lower sideband (LSB)) radio set. It is compatible with the AX/SSB sets currently in use but is not directly compatible with the older AM/DSB equipment. An experienced DSB operator may still be able to pick up the SSB transmissions however. The RT-1209 operates between 2 and 30 MHz in the HF-band.
b. Manpack Radio AN/PRC-104A. This portable unit will replace the AN/PRC-70 and AN/PRC-74 radios currently in use. It consists of a basic receiver-transmitter and support equipment. The PRC-104A also uses a tunable antenna and has an automatic antenna tuning feature built into the unit.
c. Low-Power Vehicular/Manpack Radio AN/GRC-213. The GRC-213 combines the features of the older GRC-106 into a unit capable of vehicular or manpack use. Depending on the application, different amplifiers, power supplies, and antennas are attached. An important note on these radio sets is that your operators should always avoid excessive keying of the radio. The low-power IHFR radio has a maximum key down time of one minute and a ratio of one minute transmit to nine minutes receive. Exceeding this key down time can damage the radio set.
d. High-Power Vehicle Radio AN/GRC-193A. The GRC-193A is a basic RT-1209 with a high-power supply and support components. The AN/GRC-193A is replacing the AN/GRC-106.
20. Retransmission Stations.
When two stations that are far apart need to communicate, one way they can do so is to use more powerful transmitters. This is not desirable because it increases the electromagnetic signature of both stations and makes them more susceptible to enemy action. Another way for two remote stations to communicate is to use a retransmission station. A retransmission station consists of two radio sets and some type of interconnecting device. You have already learned about the AN/VRC-45 and -49, the AN/PRC-70, and the AN/VRC-92 SINCGARS retransmission sets. You have also learned about the C-2299/VRC and the MK-456/GRC retransmission cable kits used with these radio sets. In order to better understand the actual process of using a retransmission station, it is also important for you to understand a few details about the retransmission process.
a. Interference Elimination. A retransmission station consists of two radio stations physically located in close proximity. The radio stations are of the same type (single-channel, SINCGARS, etc.). This has the obvious drawback of being the ideal situation for cosite interference. To avoid this, your frequency planners should ensure that the two radios used for retransmission functions are assigned frequencies at least 10 MHz apart. Also the two antennas for the retransmission station should be physically located as far apart as possible. In a vehicular or airborne retransmission station, this is obviously not very far. Similarly, timesharing is not a viable option since both radios must operate simultaneously in order for the station to function properly. This leaves frequency separation as the only means of eliminating cosite interference.
b. Retransmission Mode. All the retransmission sets you have learned about can perform their functions in the VHF-FM single-channel mode. The AN/VRC-92 can also perform retransmission functions in the FH mode or in a combination of single-channel to FH modes or vice versa.
c. Employment. Although the primary use for a retransmission station is to extend the range of a medium powered radio set, it also has other uses that you should be aware of. Sometimes you may be in a headquarters that is geographically screened from forward units by a high hill in order to protect it from enemy fire. The geographical situation makes it impossible for you to communicate with forward groups using VHF-FM. If you establish a retransmission station on the top of the hill, however, both your radio station and the radios of forward units will be able to transmit to the retransmission station, and thus to talk to each other. Another infrequent use of retransmission stations is to provide NRI functions. You will learn more about the NRI system in lesson 3.
21. Types of Tactical Radio Operation.
The operation of tactical radio systems falls into three basic categories: point-to-point, networks, and net radio interface. You already know about tactical radio networks and NRI will be covered in Lesson 3. This section will teach you about the third category, point-to-point operation.
As a communications officer, you may occasionally encounter situations where you would like to set up a dedicated communications channel for just one station. This type of operation is called point-to-point communications. Point-to-point has the advantage of placing you in direct contact with a particular station at a moment's notice. The operator at the remote station has only to key his radio to talk to you. He does not have to worry about entering a network or obtaining permission to pass traffic; he just does it. This type of setup is excellent for communications with a vital intelligence station that needs to pass emergent intelligence information directly to headquarters as soon as possible. In the modern battlefield, rapid reporting of intelligence can determine courses of events on a very large scale. Point-to-point communications also has its drawbacks. Since only one station has access to the communications frequency, the channel may sit idle for significant periods of time. This is not efficient use of the electromagnetic spectrum. Given the potential importance of a point-to-point system, however, the disadvantages are outweighed by the benefits.
22. Field Expedient Antennas.
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 1-1 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. Even if you do not break your antenna, you may find that you have need of a different type of antenna from the one you are carrying. For example, enemy jamming or high radio frequency (RF) noise levels may dictate the use of a directional antenna when all you are carrying is a whip antenna. Remember, the best tools to have in an emergency situation are your knowledge and your imagination. The following paragraphs will teach you the basic steps to fabricate some of the more common types of field antennas and in what situation each might be useful.
Figure 1-1. Field expedient insulators
a. 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 the correction factor is 0.95. Knowing this you can calculate the required length of your antenna using the following formula (for half-wave antennas):
For quarter-wave antennas, the formula is the same, except that the constant is 234 instead of 468. Thus for a quarter-wave antenna, the length formula is:
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.
b. Omnidirectional Antennas. An omnidirectional antenna transmits the same power level in all directions. This is usually the simpler type of antenna and is much more common among tactical radio systems than the directional antenna. Because of this, it stands to reason that you can expect to experience more problems with the failure of an omnidirectional antenna than with a directional one. You will not normally fabricate a field-expedient omnidirectional antenna unless your current antenna breaks. The advantages of a field expedient omnidirectional antenna over the standard antenna provided with your radio set will generally be minimal. Tactical omnidirectional antennas are of two basic types: whip antennas and ground plane antennas.
The whip antenna is the simplest of the Marconi family of antennas. 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. The whip antenna is the most portable type of antenna and, therefore, the most widely used in tactical radio communications. Because of the simplicity of its design, the whip antenna is also the easiest to fabricate a replacement for, should it break. The first step in fabricating a whip antenna is to cut a length of wire a quarter-wavelength long. Use the formulas you learned above to calculate the proper antenna length for your communications frequency. Next, using an insulator, attach one end of the antenna wire to a rope. Attach the free end of the antenna to your radio set (do not forget to ground your radio). Now all you have to do is hoist the end of the antenna attached to the rope so that the antenna is vertical and you have your replacement. If trees are nearby, you can throw one end of your rope over a branch and hoist your antenna that way. You can also attach the rope to a long pole such as a broom handle and use that for an antenna mast.
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 its own reflection. 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 fact that the ground plane antenna has more parts also means that there are more parts to break. Radiating elements, reflecting elements, masts, guys, and insulators are all subject to damage in a real-world installation. The steps to fabricate a field expedient ground plane antenna vary depending on which portion of the antenna has been broken. If you are replacing a broken mast, for example, you can use a long wooden pole as you would for a whip. You can also attach the radiating element of the antenna to a rope using an insulator, and suspend the antenna from a tree or other tall object. This type of field expedient allows you to easily adjust the height of the antenna and also allows for quick setup and teardown.
You may occasionally find yourself in a situation where your whip antenna is not sufficient for your communication needs, and a more powerful antenna is necessary. In these situations you can fabricate a 292-type ground plane antenna out of antenna wire and other available materials. Using the formulas you have learned, calculate the length of wire required for a quarter-wavelength radiating element and three reflecting elements of the same length. Cut your antenna wire into these four sections. Cut three spacing sticks the same length. Connect the three spacing sticks to form a triangle and tie or tape the ends together. Attach an insulator to each corner of the triangle and attach one end of each reflecting element to each insulator.
Strip about three inches of insulation from the free end of each reflecting element wire and twist all three wires together. Attach the twisted ends of the reflecting elements to another insulator. Attach one end of the radiating element to the other side of this insulator, and connect the other end of the radiating element to a rope with an insulator (that's five insulators in all). Use the rope over a tree branch or other suitable object to suspend your antenna. Make the connection to your radio as short as possible; excess length reduces system efficiency. Connect the end of the radiating element that is attached to the insulator with the reflecting elements to the antenna terminal on your radio set using WD-1 field wire or an appropriate substitute. Connect the twisted ends of the reflecting elements to the case of your radio set, also using WD-1. Tape all your electrical connections. Your field-expedient ground plane antenna is now ready to operate. Remember that your antenna is tuned for a particular operating frequency. If you need to operate on a different frequency you should adjust the length of your antenna elements accordingly. Figure 1-2 illustrates an assembled field-expedient 292-type ground plane antenna.
Figure 1-2. Field expedient ground plane antenna
c. 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. A directional can also provide the "extra punch" you might need in transmitting power to reach another station at the edge of your communications range. There are three basic types of directional antennas that you can easily employ in a field environment. They are the half-wave doublet, the long-wire antenna, and the vertical half-rhombic 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. From this you can see that the Hertz antenna oriented vertically would be essentially omnidirectional. 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. Fabricating a field-expedient Hertz antenna is fairly simple. First cut your antenna wire to the proper half-wavelength length for your operating frequency and attach insulators to both ends. Cut the wire in half and connect the two pieces with a third insulator. Strip a small section of insulation from the wires where they connect to the center insulator. Using WD-1, attach one of the halves to your radio's antenna connection, and attach the other half to your radio's casing or other grounded point. For a horizontal half-wave doublet, the next step is to determine the correct azimuthal orientation for your antenna. You should set the antenna up perpendicular to the azimuth of the radio station you are trying to contact. Attach the insulators on the ends of the antenna to ropes and string the antenna between-two supports so that it is oriented properly. For best results, the antenna lead going to the radio should be at right angles to the antenna for a length of at least six feet. For a vertical half-wave doublet, attach one end of the antenna to a rope and weight the other end with a rock or other heavy object. Alternately, you might want to tie the other end down to a stake or other fixed object. Since the vertical antenna transmits equally in all azimuthal directions, you do not need to worry about setting the antenna's orientation. Figures 1-3 and 1-4 show examples of field-expedient horizontal and vertical half-wave doublet antennas respectively.
Figure 1-3. Field expedient horizontal half-wave doublet antenna
Figure 1-4. Field expedient vertical half-wave doublet antenna
The long-wire antenna is simply what the name implies. It is an antenna you have constructed to a certain number of wavelengths (using previous formula). You should calculate your antenna length so that it is at least two wavelengths long. Attach insulators to both ends of the antenna. To obtain directionality, you connect the radio set to one end and terminate the other end with a grounded 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 set 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. Figure 1-5 shows a long-wire antenna with a terminating resistor. Without this resistor, the antenna will be bidirectional along its axis.
Figure 1-5. Long-wire antenna with terminating resistor
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 a wavelength long, and a horizontal segment that acts as a counterpoise. 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. To construct a field-expedient half-rhombic antenna, calculate the length of wire (at least two wavelengths) for your operating frequency and cut your antenna wire to that length. Attach insulators-to both ends of the wire. Attach your radio's antenna lead to one end of the wire, and fasten the other end with a grounded terminating resistor as you did for the long-wire antenna above. Attach the middle of the antenna to a tall pole or tree, and stake the two ends to the ground. The pole or tree should be high enough so that the angle of the elements is approximately 54 degrees from vertical. You should orient the terminated end of your antenna along the azimuth of the radio station you are trying to talk to. For added efficiency string a length of WD-1 about a foot off the ground and attach it to the ground side of both ends of the antenna wire. Attach this wire to your radio casing. This added wire acts as a counterpoise and helps to reflect the portion of the signal that is transmitted toward the earth. A counterpoise is especially helpful in areas where the soil has very poor conductivity such as deserts, frozen ground, and extremely rocky terrain. Figure 1-6 illustrates a vertical half-rhombic antenna with a terminating resistor and a counterpoise.
Figure 1-6. Vertical half-rhombic antenna with terminating resistor and counterpoise
d. Tips on Selecting Field Expedient Antennas. 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.
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.
23. Summary.
In this lesson you have learned how the various tactical radio sets are used in communications systems. You have learned about point-to-point communications and retransmission stations. You have also learned how to fabricate several different field-expedient antennas to use in the field. In the next lesson you will learn about the command and control structure of the division from a communications standpoint, and how networks are set up to provide for the various aspects of operating the division in the battlefield.
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