Radio Operator's Handbook

Transcription

1 MCRP B (Formery MCRP 6-22C) Radio Operator's Handbook U.S. Marine Corps DISTRIBUTION STATEMENT A: approved for pubic reease; distribution is unimited PCN

2 ERRATUM MCCDC (C 42) 10 Ju 2001 to MCRP RADIO OPERATOR'S HANDBOOK 1. For administrative purposes, the pubication short tite has been reidentified. Change MCRP 6-22C" to read: "MCRP B" of June 1999 wherever it appears in the Manua. PCN

3 DEPARTMENT OF THE NAVY Headquarters United States Marine Corps Washington, D.C June 1999 FOREWORD Marine Corps Warfighting Pubication (MCWP) 6-22, Communications and Information Systems, provides the doctrine and tactics, techniques, and procedures for the conduct of communications and information systems across the spectrum of Marine air-ground task force (MAGTF) operations. Marine Corps Reference Pubication (MCRP) B, Radio Operator s Handbook, compements and expands upon this information by detaiing doctrine, tactics, techniques, and procedures for operating singe-channe high frequency (HF), very high frequency (VHF), and utrahigh frequency (UHF) radios. The primary target audience for this pubication is Marine Corps radio operators and other users of singechanne radios. MCRP B describes Basic radio principes. Singe-channe radio. Equipment sighting and grounding techniques. Antennas. Interference. Radio operations under unusua conditions. Eectronic warfare. MCRP B provides the requisite information needed by Marine radio operators to understand, pan, and execute successfu singe-channe radio operations in support of the MAGTF.

4 MCWP B supersedes FMFM 3-35, Radio Operator s Handbook, dated 26 September Reviewed and approved this date. BY DIRECTION OF THE COMMANDANT OF THE MARINE CORPS J. E. RHODES Lieutenant Genera, U.S. Marine Corps Commanding Genera Marine Corps Combat Deveopment Command DISTRIBUTION:

5 Radio Operator s Handbook Tabe of Contents Page Chapter 1. Radio Principes Section I. Theory and Propagation Basic Components of Radio Equipment 1-2 Radio Waves 1-3 Radio Wave Propagation 1-6 Section II. Moduation and Singe Side Band Transmission Moduation 1-14 Singe Side Band Transmission 1-16 Chapter 2. Singe-Channe Radio Singe-Channe Radio Communications Equipment 2-1 High Frequency Radio 2-2 Very High Frequency Radio 2-6 Utrahigh Frequency Radio 2-11 Data Communications 2-15 Chapter 3. Equipment Siting and Grounding Techniques High Frequency 3-1 Very High Frequency and Utrahigh Frequency 3-3 Grounding Techniques 3-10 Data Communications 2-15 Chapter 4. Antennas High Frequency Antennas 4-1 Very High Frequency Antennas 4-6 Antenna Length 4-7

6 MCRP B Chapter 5. Interference Natura Interference 5-1 Manmade Interference 5-1 Poor Equipment Condition and Improper Usage 5-2 Frequency Interference and Intermoduation 5-2 Use of Unauthorized Frequencies 5-3 Frequency Reuse 5-3 Chapter 6. Radio Operations Under Unusua Conditions Operations in Desert Areas 6-1 Operations in Junge Areas 6-3 Operations in a Cod Weather Environment 6-5 Operations in Mountainous Areas 6-9 Operations in Specia Environments 6-9 Chapter 7. Eectronic Warfare Eectronic Attack Techniques 7-1 Eectronic Protection Techniques 7-6 Eectronic Warfare Support Techniques 7-10 Appendices A Map Coordinates A-1 B Time Zones B-1 C Prowords C-1 D Phonetic Aphabet D-1 E Phonetic Numeras E-1 F Prosigns F-1 G Instructions for Preparing Fied Messages G-1 HRadio Log H-1 I Metric System Conversion Tabe I-1 J Authentication J-1 K Internationa Morse Code K-1 L Frequency Prediction Means L-1 M Position and Navigation Systems M-1 N Size of Dipoe and Inverted L Antennas N-1 O Fied Repair and Expedients O-1 P Radio Operator s Checkist P-1 Q Gossary Q-1 R References and Reated Pubications R-1

7 Chapter 1 Radio Principes Communications and information systems (CIS) are any systems whose primary functions are to coect, process, or exchange information. The fundamenta requirement is to provide the Marine air-ground task force (MAGTF) commander with a reiabe, secure, fast, and fexibe communications network. Communications and information systems automate routine functions, thereby freeing commanders and staffs to focus on those aspects of command and contro that require experience, judgment, and intuition. These systems and the personne who insta, operate, and maintain them pay a key roe in the command and contro of the MAGTF. Communications and informations systems support the commander and every staff section in every phase of operations panning and execution. These systems faciitate information fow throughout the MAGTF and provide shared situationa awareness, informed decisionmaking, and rapid dissemination of decisions. The success of the MAGTF in the modern battespace depends on the effective empoyment of communications and information systems. Singe-channe radio (SCR) is one of the most important components of MAGTF CIS.

8 1-2 MCRP B Section I. Theory and Propagation BASIC COMPONENTS OF RADIO EQUIPMENT The radio equipment for communication between two stations and the path the signa foows through the air is caed a radio ink. A radio ink consists of seven components: the transmitter, power suppy, transmission ines, transmitting antenna, propagation path, receiving antenna, and receiver (see figure 1-1). The transmitter generates a radio signa. The power suppy (i.e., battery or generator) suppies power for the operating votage of the radio. The transmission ine deivers the signa from the transmitter to the antenna. PROPAGATION PATH TRANSMITTING ANTENNA TRANSMISSION LINES RECEIVING ANTENNA TRANSMITTER POWER SUPPLY Figure 1-1. Radio Link. RECEIVER

9 Radio Operator s Handbook 1-3 The transmitting antenna sends the radio signa into space toward the receiving antenna. The path in space that the radio signa foows as it goes to the receiving antenna is the propagation path. The receiving antenna intercepts or receives the signa and sends it through a transmission ine to the receiver. The receiver processes the radio signa so the human ear can hear it. When transmitting, the radio operator aims to provide the strongest possibe signa at the site of the receiving station. The best possibe signa is that signa which wi provide the greatest signa-to-noise ratio at the receiving antenna. To transmit the best possibe signa, seect or determine the Optimum frequency. Best antenna for that frequency based on the avaiabe space of the transmitting site. Proper propagation path. Propagation Veocity (Speed) RADIO WAVES Radio waves trave near the surface of the Earth and radiate skyward at various anges to the Earth s surface. These eectromagnetic waves trave through space at the speed of ight, approximatey 300,000 kiometers (km) or 186,000 mies (mi) per second. Waveength Waveength is the distance between the crest of one wave and the crest of the next wave (see figure 1-2 on page 1-4). It can aso be the ength of one compete cyce of the waveform. It is aso the distance traveed during one compete cyce. The ength of the wave is aways measured in meters.

10 1-4 MCRP B ONE CYCLE WAVELENGTH STRENGTH 0 PEAK PEAK TIME OR DISTANCE Figure 1-2. Radio Waves. Radio Frequency The frequency of a radio wave is the number of compete cyces that occur in one second. The onger the cyce, the onger the waveength and the ower the frequency. The shorter the cyce, the shorter the waveength and the higher the frequency. Frequency is measured and stated in units caed hertz (Hz). One cyce per second is stated as 1 hertz. Because the frequency of a radio wave is very high, it is generay measured and stated in thousands of hertz (kiohertz [KHz]) or in miions of hertz (megahertz [MHz]). One KHz is equa to 1,000 cyces per second, and 1 MHz is equa to a miion cyces per second. Sometimes frequencies are expressed in biions of hertz (gigahertz [GHz]). One GHz is equa to a biion cyces per second. For practica purposes, the veocity of a radio wave is considered to be constant, regardess of the frequency or the ampitude of the transmitted wave. Therefore, to find the frequency when the waveength is known, divide the veocity by the waveength. Frequency (hertz) 300 miion (meters per second) = Waveength (meters) To find the waveength when the frequency is known, divide the veocity by the frequency. Waveength (meters) = 300 miion (meters per second) Frequency (hertz)

11 Radio Operator s Handbook 1-5 RADIO HF VHF UHF IR VISIBLE UV X-RAY GAMMA- RAY COSMIC- RAY 3 MHz 30 MHz 300 MHz 3 GHz Figure 1-3. Eectromagnetic Spectrum. Within the radio frequency spectrum (see figure 1-3), radio frequencies are divided into groups or bands of frequencies. The radio frequency spectrum is part of the eectromagnetic spectrum. Most tactica radio sets operate within a 2- to 400-MHz range within the frequency spectrum. Each frequency band has certain characteristics. The ranges and power requirements shown in tabe 1-1 are for norma operating conditions (proper siting and antenna orientation and correct operating procedures). The ranges wi change according to the condition of the propagation medium and the transmitter output power. Tabe 1-1. Frequency Range Characteristics. Band Ground Wave Range Sky Wave Range Power Required HF 0-50 mies mies.5-5 kw VHF 0-30 mies mies.5 or ess kw UHF 0-50 mies N/A.5 or ess kw

12 1-6 MCRP B IONOSPHERE SKY WAVES GROUND WAVES Figure 1-4. Principe Paths of Radio Waves. RADIO WAVE PROPAGATION There are two principa paths by which radio waves trave from a transmitter to the receiver (See figure 1-4): ground wave which traves directy from the transmitter to the receiver and sky wave which traves up to the ionosphere and is refracted (i.e., bent downward) back to the Earth. Short-distance, utrahigh frequency (UHF), and upper very high frequency (VHF) transmissions are by ground waves. Long-distance, high frequency (HF) transmission is principay by sky waves. Singe-channe radio sets can use ground wave or sky wave propagation for communications. Ground Wave Propagation Radio communications which use ground wave propagation do not use or depend on waves that are refracted from the ionosphere (i.e., sky waves). Ground wave propagation is affected by the eectrica characteristics of the Earth and by the amount of diffraction (i.e., bending) of the

13 Radio Operator s Handbook 1-7 DIRECT WAVE GROUND REFLECTED WAVE SURFACE WAVE Figure 1-5. Ground Wave Propagation. waves aong the curvature of the Earth. The strength of the ground wave at the receiver depends on the power output and frequency of the transmitter, the shape and conductivity of Earth aong the transmission path, and the oca weather. The foowing paragraphs describe the components of a ground wave. See figure 1-5. Direct Wave. The direct wave is that part of the radio wave which traves directy from the transmitting antenna to the receiving antenna. This part of the wave is imited to the ine of sight (LOS) distance between the transmitting and receiving antennas, pus the sma distance added by atmospheric refraction and diffraction of the wave around the curvature of the Earth. This distance can be extended by increasing the height of the transmitting antenna, the receiving antenna, or both. Ground Refected Wave. The ground refected wave is that portion of the radio wave which reaches the receiving antenna after being refected from the surface of the earth. Canceation of the radio signa can occur when the ground refected component and the direct wave component arrive at the receiving antenna at the same time and are 180 out of phase with each other. Surface Wave. The surface wave, which foows the curvature of the Earth, is that part of the ground wave which is affected by the conductivity and dieectric constant of the Earth.

14 1-8 MCRP B F1 & F2 COMBINE AT NIGHT F2 F1 E D SUN DAYLIGHT POSITIONS F2 F1 E D F km ( km at night) F km E km D km Sky Wave Propagation Figure 1-6. Layers of the Ionosphere. Radio communications that use sky wave propagation depend on the ionosphere to provide the signa path between the transmitting and receiving antennas. Ionospheric Structure. The ionosphere has four ayers (see fig. 1-6). In order of increasing heights and decreasing moecuar densities, these ayers are abeed D, E, F1, and F2. During the day, when the rays of the

15 Radio Operator s Handbook 1-9 Sun are directed toward that portion of the atmosphere, a four ayers may be present. At night, the F1 and F2 ayers seem to merge into a singe F ayer, and the D and E ayers fade out. The actua number of ayers, their height above the Earth, and their reative intensity of ionization varies constanty. The foowing are ayers of the ionosphere: D exists ony during dayight hours and has itte effect in bending the paths of high frequency radio waves. The main effect of the D ayer is to attenuate high frequency waves when the transmission path is in sunit regions. E used during the day for high frequency radio transmission over intermediate distances (ess than 2,400 km [1,500 mi]). At night, the intensity of the E ayer decreases, and it becomes useess for radio transmission. F exists at heights up to 380 kiometers (240 mi) above the Earth and is ionized a the time. It has two we-defined ayers (F1 and F2) during the day, and one ayer (i.e., F) during the night. At night, the F ayer remains at a height of about 260 kiometers (170 mi) and is usefu for ong-range radio communications (over 2,400 km [1,500 mi]). The F2 ayer is the most usefu of a ayers for ong-range radio communications, even though its degree of ionization varies appreciaby from day to day. Factors Affecting the Ionosphere. The movements of the Earth around the Sun and changes in the Sun s activity contribute to ionospheric variations. There are two main casses of variations: reguar, which is predictabe; and irreguar, which occurs from abnorma behavior of the Sun. Reguar Variations of the Ionosphere. The reguar variations are Daiy caused by the rotation of the Earth. Seasona caused by the north and south progression of the Sun. 27-day caused by the rotation of the Sun on its axis. 11-year caused by the sunspot activity cyce going from maximum to minimum, back to maximum eves of intensity.

16 1-10 MCRP B Irreguar Variations of the Ionosphere. The current status of the four reguar variations must be anticipated when panning a communications system. There are aso unpredictabe, irreguar variations that must be considered. They have a degrading effect (at times bocking communications) which cannot be controed or compensated for at present. Some irreguar variations are Sporadic E. When it is excessivey ionized, the E ayer often bocks out the refections back from the higher ayers. It can aso cause unexpected propagation of signas hundreds of mies beyond the norma range. This effect can occur at any time. Sudden ionospheric disturbance (SID). A sudden ionospheric disturbance coincides with a bright soar eruption and causes abnorma ionization of the D ayer. This effect causes tota absorption of a frequencies above approximatey 1 MHz. It can occur without warning during dayight hours and ast from a few minutes to severa hours. When SID occurs, receivers seem to go dead. Ionospheric storms. During these storms, sky wave reception above approximatey 1.5 MHz shows ow intensity and is subject to a type of rapid basting and fading caed futter fading. These storms may ast from severa hours to days and usuay extend over the entire Earth. Sunspots. Sunspots generate bursts of radiation that cause high eves of ionization. The more sunspots, the greater the ionization. During periods of ow sunspot activity, frequencies above 20 MHz tend to be unusabe because the E and F ayers are too weaky ionized to refect signas back to earth. At the peak of the sunspot cyce, however, it is not unusua to have wordwide propagation on frequencies above 30 MHz. Frequency Characteristics in the Ionosphere. The range of ong-distance radio transmission is determined primariy by the ionization density of each ayer. The higher the frequency, the greater the ionization density required to refect radio waves back to Earth. The upper (i.e., E and F) ayers refect the higher frequencies because they are the most highy ionized. The D ayer, which is the east ionized, does not refect frequencies above approximatey 500 KHz. Thus, at any given time and for each

17 Radio Operator s Handbook 1-11 ionized ayer, there is an upper frequency imit at which radio waves sent verticay upward are refected back to Earth. This imit is caed the critica frequency. Radio waves directed verticay at frequencies higher than the critica frequency pass through the ionized ayer out into space. A radio waves directed verticay into the ionosphere at frequencies ower than the critica frequency are refected back to Earth. Radio waves used in communications are generay directed towards the ionosphere at some obique ange, caed the ange of incidence. Radio waves at frequencies above the critica frequency wi be refected back to Earth if transmitted at anges of incidence smaer than a certain ange, caed the critica ange. At the critica ange and a anges arger than the critica ange the radio waves wi pass through the ionosphere if the frequency is higher than the critica frequency. When the ange of transmission becomes smaer, the radio waves wi be refected back to Earth. Transmission Paths. The distance from the transmitting antenna to the pace where the sky waves first return to Earth is caed the skip distance. The skip distance is dependent on the ange of incidence, the operating frequency, and the height and density of the ionosphere. The antenna height, in reation to the operating frequency, affects the ange that transmitted radio waves strike and penetrate the ionosphere and then return to Earth. This ange of incidence can be controed to obtain the desired area of coverage. Lowering the antenna wi increase the ange of transmission and provide broad and even signa patterns in a arge area. The use of near-vertica transmission paths is known as near-vertica incidence sky wave (NVIS). Raising the antenna wi ower the ange of incidence. Lowering the ange of incidence can produce a skip zone in which no usabe signa can be received. This area is bounded by the outer edge of usabe ground wave propagation and the point nearest the antenna at which the sky wave returns to Earth. In most communications situations, the skip zone is not a desirabe condition. However, ow anges of incidence make ong-distance communications possibe. When a transmitted wave is refected back to the surface of the Earth, part of its energy is absorbed by the Earth. The remainder of its energy is refected back into the ionosphere to be refected back again. This means

18 1-12 MCRP B of transmission by aternatey refecting the radio wave between the ionosphere and the Earth is caed hops, and it enabes radio waves to be received at great distances from the point of origin. Maximum Usabe and Lowest Usabe Frequencies. There is a maximum frequency at which a radio wave wi return to Earth at a given distance when a given ionized ayer and a transmitting antenna with a fixed ange of radiation is used. This frequency is caed the maximum usabe frequency (MUF). It is the monthy median of the daiy highest frequency that is predicted for sky wave transmission over a particuar path at a particuar hour of the day. The MUF is aways higher than the critica frequency because the ange of incidence is ess than 90. If the distance between the transmitter and the receiver is increased, the maximum usabe frequency wi aso increase. Radio waves ose some of their energy through absorption by the D ayer and the portion of the E ayer of the ionosphere at certain transmission frequencies. The tota absorption is ess and communications are more satisfactory as higher frequencies are used up to the eve of the MUF. The absorption rate is greatest for frequencies ranging from approximatey 500 KHz to 2 MHz during the day. At night, the absorption rate decreases for a frequencies. As the frequency of transmission over any sky wave path is decreased from high to ow frequencies, a frequency wi be reached at which the received signa just overrides the eve of atmospheric and other radio noise interference. This is caed the owest usefu frequency (LUF) because frequencies ower than the LUF are too weak for usefu communications. The LUF aso depends on the power output of the transmitter as we as the transmission distance. When the LUF is greater than the MUF, no sky wave transmission is possibe.

19 Radio Operator s Handbook 1-13 Section II. Moduation and Singe Side Band Transmission Radio communications equipment is used primariy to transmit voice and data. Athough sound can be converted to audio frequency eectrica energy, it is not practica to transmit it in this energy form through the Earth s atmosphere by eectromagnetic radiation. For exampe, efficient transmission of a 20-hertz audio signa woud require an antenna amost 8,000 kiometers (5,000 mi) ong. This woud not appy when radio frequency eectrica energy is used to carry the inteigence. When radio frequency eectrica energy is used, great distances can be covered; efficient antennas for radio frequencies are of practica engths; and antenna power osses are at reasonabe eves. The frequency of the radio wave affects its propagation characteristics. In the ow frequency band (.03 to.3 MHz), the ground wave is very usefu for communications over great distances. The ground wave signas are quite stabe and show itte seasona variation. In the medium frequency band (.3 to 3.0 MHz), the range of the ground wave varies from about 24 kiometers (15 mi) at 3 MHz, to about 640 kiometers (400 mi) at the owest frequencies of this band. Sky wave reception is possibe during the day or night at any of the ower frequencies in this band. At night, the sky wave is receivabe at distances up to 12,870 kiometers (8,000 mi). In the high frequency band (3 to 30 MHz), the range of the ground wave decreases as frequency increases, and the sky waves are greaty infuenced by ionospheric considerations. In the very high frequency band (30 to 300 MHz), there is no usabe ground wave and ony sight refraction of sky waves by the ionosphere at the ower frequencies. The direct wave provides communications if the transmitting and receiving antennas are eevated high enough above the surface of the Earth. In the utrahigh frequency band (300 to 3,000 MHz), the direct wave must be used for a transmissions. Communications are imited to a short distance beyond the horizon. Lack of static and fading in these bands makes ine of sight reception very satisfactory. Antennas that are highy directiona can be used to concentrate the beam of radio frequency (RF) energy, thus, increasing the signa intensity.

20 1-14 MCRP B MODULATION Both ampitude moduation (AM) and frequency moduation (FM) transmitters produce RF carriers. The carrier is a wave of constant ampitude, frequency, and phase which can be moduated by changing its ampitude, frequency, or phase. Thus, the RF carrier carries inteigence by being moduated. Moduation is the process of superimposing inteigence (voice or coded signas) on the carrier. Ampitude Moduation Ampitude moduation is the variation of the RF power output of a transmitter at an audio rate. In other words, the RF energy increases and decreases in power according to the audio frequencies superimposed on the carrier signa. When audio frequency signas are superimposed on the radio frequency carrier signa, additiona RF signas are generated. These additiona frequencies are equa to the sum of, and the difference between the audio frequencies and the radio frequency used. For exampe, assume a 500- KHz carrier is moduated by a 1-KHz audio tone. Two new frequencies are deveoped, one at 501 KHz (the sum of 500 KHz and 1 KHz) and the other at 499 KHz (the difference between 500 KHz and 1 KHz). If a compex audio signa is used instead of a singe tone, two new frequencies wi be set up for each of the audio frequencies invoved. The new frequencies resuting from superimposing an audio frequency (AF) signa on an RF signa are caed side bands. When the RF carrier is moduated by compex tones such as speech, each separate frequency component of the moduating signa produces its own upper and ower side band frequencies. The side band that contains the sum of the RF and AF signas is caed the upper side band. The side band that contains the difference between the RF and AF signas is caed the ower side band. The space occupied by a carrier and its associated side bands in the radio frequency spectrum is caed a channe. In ampitude moduation, the width of the channe (bandwidth) is equa to twice the highest

21 Radio Operator s Handbook 1-15 moduating frequency. For exampe, if a 5,000 KHz (5 MHz) carrier is moduated by a band of frequencies ranging from 200 to 5,000 cyces (.2 to 5 KHz), the upper side band extends from 5,000.2 to 5,005 KHz. The ower side band extends from 4,999.8 KHz to 4,995 KHz. Thus, the bandwidth is the difference between 5,005 KHz and 4,995 KHz, a tota of 10 KHz. Frequency Moduation Frequency moduation is the process of varying the frequency (rather than the ampitude) of the carrier signa in accordance with the variations of the moduating signas. The ampitude or power of the FM carrier does not vary during moduation. The frequency of the carrier signa when it is not moduated is caed the center or rest frequency. When a moduating signa is appied to the carrier, the carrier signa wi move up and down in frequency, away from the center or rest frequency. The ampitude of the moduating signa determines how far away from the center frequency the carrier wi move. This movement of the carrier is caed deviation; how far the carrier moves is caed the amount of deviation. During reception of the FM signa, the amount of deviation determines the oudness or voume of the signa. The FM signa eaving the transmitting antenna is constant in ampitude but varies in frequency according to the audio signa. As the signa traves to the receiving antenna, it picks up natura and man-made eectrica noises that cause ampitude variations in the signa. A of these undesirabe ampitude variations are ampified as the signa passes through successive stages of the receiver unti the signa reaches a part of the receiver caed the imiter. The imiter is unique to FM receivers as is the discriminator. The imiter eiminates the ampitude variations in the signa, then passes it on to the discriminator which is sensitive to variations in the frequency of the RF wave. The resutant constant ampitude, frequency-moduated signa is then processed by the discriminator circuit which changes the

22 1-16 MCRP B frequency variations into corresponding votage ampitude variations. These votage variations reproduce the origina moduating signa in a headset, oudspeaker, or teetypewriter. Frequency moduation is generay used by radioteephone transmitters operating in the VHF and higher frequency bands. SINGLE SIDE BAND TRANSMISSION The inteigence of an AM signa is contained soey in the side bands. Each side band contains a the inteigence needed for communications. Therefore, one side band and the carrier signa can be eiminated. This is the principe on which singe side band (SSB) communications is based. Athough both side bands are generated within the moduation circuitry of the SSB radio set, the carrier and one side band are removed before any signa is transmitted. The side band that is higher in frequency than the carrier is caed the upper side band (USB). The side band that is ower in frequency than the carrier is caed the ower side band (LSB). Either side band can be used for communications as ong as both the transmitter and the receiver are adjusted to the same side band. Most SSB equipment operates in the USB mode. The transmission of ony one side band eaves open that portion of the RF spectrum normay occupied by the other side band of an AM signa. This aows more emitters to be used within a given frequency range. Singe side band transmission is used in appications when it is desired to Obtain greater reiabiity. Limit size and weight of equipment. Increase effective output without increasing antenna votage. Operate a arge number of radio sets without heterodyne interference (e.g., whistes and squeas) from radio frequency carriers. Operate over ong ranges without oss of inteigibiity because of seective fading.

23 Chapter 2 Singe-Channe Radio SINGLE-CHANNEL RADIO COMMUNICATIONS EQUIPMENT Singe-channe radio is the principa means of communications support for MAGTF maneuver units. SCR communications equipment is easy to operate. The networks are easiy estabished, rapidy reconfigured, and, most importanty, easiy maintained on the move. SCR provides secure voice communications and supports imited data information exchange. SCR in the VHF and UHF bands is normay imited to ine of sight. In the HF band, SCR can support ong-range communications. SCR sateite communications (SATCOM) provides mobiity, fexibiity, and ease of operation with unimited range. Limitations of SCR incude susceptibiity to enemy eectronic warfare (i.e., cosite, terrain, and atmospheric interference); the requirement for cose coordination and detaied panning (i.e., a need for common timing, frequency, and equipment); and imited spectrum avaiabiity. The atter is particuary critica in the case of SATCOM. MAGTF SCR equipment is fieded in many configurations and incudes hand-hed, manpack, vehice-mounted, bench-mounted, and shetered radios. These radios operate in simpex and haf-dupex modes. The most widey empoyed tactica radios provide integrated communications security (COMSEC) and jam resistance through frequency hopping. Tactica SCRs operate in the three miitary radio frequency bands shown in Tabe 2-1 on page 1-2.

24 2-2 MCRP B Tabe 2-1. SCR Equipment. Frequency Band MAGTF SCR Equipment Used Operating Frequency Range Typica Appication HF VHF UHF AN/PRC-104 AN/GRC-193 AN/MRC-138 AN/TSC-120 AN/VRC-12 famiy: AN/PRC-68 AN/PRC-77 SINCGARS famiy: AN/PRC-119 AN/VRC-88 (A, D) AN/VRC-89 (A, D) AN/VRC-90 (A, D) AN/VRC-91 (A, D) AN/VRC-92 (A, D) AN/GRC-213 AN/MRC-145 AN/PRC-113 AN/VRC-83 AN/PRC-113 AN/VRC-83 AN/GRC-171 AN/PSC-3 AN/PSC MHz Radio ine of sight and beyond/ong range MHz Radio ine of sight and reay/retransmission MHz Critica ine of sight (ground to air) MHz Critica ine of sight (ground to air) SATCOM footprint HIGH FREQUENCY RADIO HF radio equipment is capabe of both ong- and short-range secure voice and data communications. Data communications capabiity is typicay imited to rates of 2.4 kiobits per second (kbps). Data transmission requires modems specificay designed for operation in this band of the radio spectrum. The AN/PRC-104 is capabe of remote operation by using the anaog AN/GRA-39B radio remote contro. See fig. 2-1.

25 Radio Operator s Handbook 2-3 Figure 2-1. AN/PRC-104 HF Radio. High frequency communications are capabe of traveing around the word under the right conditions. This accounts for the arge number of signas and noise in the receiver (e.g., thunderstorms). Conversey, the HF transmission may be intercepted and traced by the enemy who is many hundreds of mies away. VHF and UHF communications are normay imited to ine of sight; therefore, their range is restricted. UHF transmissions may aso be used in sateite communications, increasing ranges to thousands of mies. High Frequency Radio Empoyment Considerations. The primary advantage of using HF radio is its capabiity to provide ong-range, over the horizon (OTH) communication. Successfu data communications over the HF range depends on severa factors: equipment siting, proper equipment grounding, types of antennas used, and other considerations such as tactica empoyment of radio equipment, path assessment and anaysis, and frequency panning and assignment. When commercia data termina equipment (DTE) is used, users empoying HF radio equipment need to be aware of radio interference and potentia shock hazards that can easiy affect unprotected DTE. Whenever possibe, HF radio equipment shoud be remoted from DTE.

26 2-4 MCRP B High Frequency Radio Environmenta Limitations The primary imiting factors when using HF radios are frequency aocation and management and bandwidth avaiabiity. Frequency aocation and management is concerned with frequency, time of day, time of year, and ocation. The abiity to refect HF radio waves off the ionosphere to a distant ocation is in a constant state of fux because of activity in the ionosphere. The Sun s radiation causes disturbances in the ionosphere, with most changes taking pace in what is known as the F ayer (see chapter 1 for more detais). Sunrise and sunset can be the most difficut times for HF communications. The F ayer spits into two separate ayers around sunrise and recombines into one ayer around sunset. These spits affect transmission distances as the area skipped over increases and decreases. At times, soar storms can eiminate a HF communications. HF transmission paths must be constanty monitored to achieve a dependabe HF ink. HF radio data communications capabiities are imited by the bandwidth that is imposed by ega constraints and the physics of the spectrum. The bandwidth avaiabe in the HF spectrum imits the channe bandwidth, which imits data throughput. High Frequency Propagation There are two modes of propagation in HF: ground wave and sky wave. See figure 2-2. Ground Wave. Ground wave propagation invoves the transmission of a signa aong the surface of the ground. The maximum ground wave range for most tactica HF communications is about 20 to 30 kiometers (12 to 22 mies) for manpack equipment and 80 to 100 kiometers for high-power vehicuar and van equipment. The range may be decreased by heavy vegetation (e.g., Camp Lejeune), mountainous terrain (e.g., Camp Pendeton), or dry desert soi (e.g., Twenty-nine Pams). A ground wave circuit wi generay be free of fading and may ast for the entire 24-hour period without the need to change frequencies.

27 Radio Operator s Handbook 2-5 IONOSPHERE SKY WAVE SKIP ZONE SKY WAVE GROUND WAVE SKIP ZONE Sky Wave. Beyond this range, it is necessary to communicate by sky wave. Sky wave propagation invoves the bending of the signa by the ionosphere. Frequencies are very important, as those above a certain vaue wi not bend back to earth but wi punch through the ionosphere into outer space. On the other hand, ower frequencies are noisier and become absorbed by the ionosphere. The refective nature of the ionosphere wi change when sunight hits it each day. As a resut, at east two frequencies are usuay required during a 24-hour period: a ow, night frequency and a higher, day frequency. Skip Zone Figure 2-2. HF Propagation. A skip zone is where no signas wi be received from a particuar transmitter for a particuar frequency. Skip zones are formed when the nearest point at which a sky wave is received is beyond the furthest point at which a ground wave is received. By using an antenna with a high

28 2-6 MCRP B radiation take-off ange (i.e., the ange measured from the Earth s surface to horizon up to the direction of propagation towards the ionosphere), HF radio waves can be bounced off the ionosphere and come back to earth coser than they can with more commony used antennas. This can cause the skip zone to disappear if the waves do not punch through. The use of high radiation take-off anges is caed near-vertica incident sky wave (NVIS) communications. The imit of the effective range of NVIS communications is usuay about 300 mies. NVIS communications require a horizontay poarized antenna and are done over frequencies between 2 and 12 MHz. Launch anges can be changed by atering the antenna s height above ground, but for most tactica appications one-quarter waveength above ground is sufficient. NVIS communications are particuary usefu because they can be transmitted from moving vehices. Used correcty, NVIS provides reiabe, continuous communications beyond the range of HF ground wave and VHF and UHF ine of sight. Mutipath interference occurs when both the sky wave and the ground wave signas from the transmitter arrive at different times at the receiver. See figure 2-3. More detaied information on HF propagation and antennas may be found in MCRP 6-22D, Antenna Handbook. VERY HIGH FREQUENCY RADIO The primary MAGTF VHF radio is the singe-channe ground and airborne radio system (SINCGARS). SINCGARS is a famiy of ightweight combat radios that serves as the primary means of communications for command and contro and fire support on the battefied. SINCGARS is the standard VHF-FM tactica radio for the Marine Corps, repacing the AN/PRC-77 and the AN/VRC-12 famiy. The system provides high security against threat eectronic warfare (EW) by using frequency hopping with integrated COMSEC. It is capabe of voice and data transmission (up to 16 kbps under optimum conditions and over imited distances) over the VHF-FM frequency range of 30 to MHz. See MCRP 6-22A, Tak II SINCGARS Mutiservice Communications Procedures for the Singe-Channe Ground and Airborne Radio System, for more detais.

29 Radio Operator s Handbook 2-7 IONOSPHERE SKY WAVE GROUND WAVE Figure 2-3. Mutipath Interference. There are seven different SINCGARS configurations avaiabe, depending on the requirements of the user. These configurations incude the manpack AN/PRC-119 (see figure 2-4 on page 2-8), typicay used in infantry operations, and vehice-mounted variants. The radio provides voice communications ranges of up to 8 km for the manpack and 35 km for vehicuar configurations. SINCGARS is capabe of remote operation by using the anaog AN/GRA-39B radio remote contro, the digita HYX-57 wire-ine ADAPTER, or the digita C (C)/U remote contro unit (RCU). The SINCGARS radio has undergone a systems improvement program (SIP). This radio is referred to as the SINCGARS SIP. The primary improvements reate to the data transmission capabiities of the system. A forward error correction appiqué was impemented in the receiver and/or transmitter, and a new packet data mode was created to better support packet networks. In addition, an improved channe access protoco was added, which optimizes data throughput performance whie minimizing impact on voice communications on the same SINCGARS channe.

30 2-8 MCRP B Figure 2-4. AN/PRC-119 SINCGARS Radio. THE SINCGARS SIP radio is aso avaiabe in a downsized version the resut of an advanced systems improvement program (ASIP). This radio is referred to as the SINCGARS ASIP. This radio wi retain a the functionaity of the fu-size SIP radio but is haf the size. It weighs 7.6 pounds (incuding the battery). The radio is interchangeabe with previous SINCGARS versions, incuding the capabiity to be mounted in oder vehicuar adapter assembies. A new feature of the SINCGARS ASIP provides a retransmission capabiity whie operating in the packet data mode and wi aso empoy a new, fast-channe access protoco for improved operations in shared voice or data nets. The AN/ARC-210 mutipurpose radio supports singe-channe air-to-air, air-to-ground, and ground-to-air communications in tactica Navy and Marine Corps fixed- and rotary wing aircraft. It can transmit and receive VHF-FM, VHF-AM, and UHF signas. It is compatibe with SINC- GARS, HAVE QUICK, and HAVE QUICK II frequency hopping UHF radios, and it can accept 25 preset, singe-channe frequencies. The AN/ ARC-210 requires a TSEC/KY-58 encryption device to encrypt transmissions and decrypt received signas.

31 Radio Operator s Handbook 2-9 Hand-Hed Very High Frequency Radios Radio operators may have the opportunity to use various commercia off-the-shef (COTS) VHF radios in the Feet Marine Force (FMF). A of these radios have been open-purchased by the user units and are not part of the officia Marine Corps tabe of equipment. Therefore, they have not been assigned a tabe of authorization materia contro number (TAMCN). These hand-hed radios are typicay sma, ightweight, battery-powered equipment which provides cear (and in some cases secure) voice communications on up to 100 different channes. Some modes come with headsets and microphones. Hand-hed radios are mosty used at the infantry-squad eve or in maritime prepositioning force (MPF) offoads. Very High Frequency Radio Empoyment Considerations Operator maintenance of the radio equipment, antennas, cabe assembies, and equipment grounding as we as proper panning and seection are essentia to reiabe communications. Frequency separation, radio antenna separation, remote rekeying when using COMSEC, and power output are significant empoyment factors. SINCGARS may be imited to the singe-channe mode when operating with some Navy ships. When SINCGARS is empoyed in the frequency hopping mode, the foowing operating factors need to be taken into account: hopset (i.e., frequency segment aocation), net sych time and mission date, antenna pacement (cosite interference is more of a concern than in the singe-channe operating mode), and power setting. SINCGARS radios configured for different hopsets that dia into the same numbered net wi not be abe to communicate. MCRP 6-22A provides detaied information on the empoyment of SINCGARS. VHF SCR is the primary communications system for combat and combat support units whie on the move. The predominant mode of operation is secure voice. However, use of VHF radio for data communications wi increase with the fieding of tactica information systems at the battaion eve and beow. Sma, hand-hed VHF radios are used at the sma-unit eve in the MAGTF. These radios are often commercia items

32 2-10 MCRP B that ack compatibiity with SINCGARS and do not have integrated COMSEC. Their use shoud be governed accordingy. Very High Frequency Radio Environmenta Limitations The primary imiting factors when using VHF radios are range and frequency avaiabiity. VHF radios can provide reiabe communications for ranges of up to 10 mies, depending on the equipment operating constraints and the operating environment. Unit ocation must be considered when empoying radios that operate in the VHF spectrum. Most circuits are imited to radio ine of sight, known as four-thirds earth curvature. VHF radio signas essentiay foow the curvature of the earth to a distance that is approximatey one-third greater than the distance to the horizon. Foiage interferes with VHF signas and may reduce norma operating ranges to significanty ess than 10 mies. Very High Frequency Propagation Radios in the SINCGARS famiy are the principa VHF transceivers used by the Marine Corps. The mode of communications used in this range is frequenty referred to as frequency moduation. VHF wi extend sighty beyond ine of sight due to diffraction or bending of the signa by the atmosphere (see fig. 2-5). At frequencies in the 30-MHz range, VHF wi often act ike HF ground wave. The range of reiabe communications is generay no more than 15 to 20 kiometers (9.3 to 12.4 mi) under norma fied conditions for manpack equipment. Vehicemounted equipment may communicate farther because of higher transmitter power and better antennas. DIFFRACTION ANGLE TRANSMITTING ANTENNA EARTH DIFFRACTED WAVE RECEIVING ANTENNA Figure 2-5. VHF Diffraction.

33 Radio Operator s Handbook 2-11 VHF LOS can aso be pagued by mutipath interference when the direct ray and a refected ray traveing over a sighty onger path combine at the receiver antenna so that they periodicay cance or reinforce each other (see fig. 2-6). The signa fades in and out over a period of time as a resut. Fading is not as great a probem with immobie equipment because corrective action can be taken, but fading can cause significant probems when one or more of the units are mobie. TRANSMITTING ANTENNA DIRECT WAVE RECEIVING ANTENNA REFLECTED WAVE EARTH Figure 2-6. VHF LOS. ULTRAHIGH FREQUENCY RADIO Miitary UHF radio equipment operates in the 116 to 150 MHz upper- VHF frequency range and the 225 to 400 MHz miitary UHF radio spectrum. MAGTF UHF radio sets such as the AN/PRC-113 (see figure 2-7 on page 2-12) are capabe of data communications at 16 kbps under optima conditions. MAGTF ground and airborne UHF radios incorporate the HAVE QUICK Eectronic Counter-Counter Measures capabiity and operate in singe-channe and frequency hopping modes. The HAVE QUICK UHF radio is capabe of remote operation by using the AN/ GRA-39B or HYX-57. Utrahigh Frequency Radio Empoyment Considerations UHF radios are used for forward air contro (FAC) ground-to-air communication. Line of sight between radios is critica for reiabe communications. Significant range differences are encountered between UHF radios empoyed for ground-to-air and ground-to-ground communications. Greater range is achieved when empoyed from ground-to-air because of the increased ine of sight. When UHF radios are empoyed in

34 2-12 MCRP B Figure 2-7. AN/PRC-113. UHF Radio. the frequency hopping mode, the foowing operating factors must be understood for proper operation: hopset, time of day, antenna pacement, and power setting. Utrahigh Frequency Radio Environmenta Limitations The primary imiting factor when using UHF radios is range (i.e., critica ine of sight). Critica ine of sight can be described as what you see is what you get. As ong as the radio s antenna has optica ine of sight to another radio s antenna, the two wi be abe to transmit and receive. For this reason, UHF radios are used primariy in air-to-ground communications.

35 Radio Operator s Handbook 2-13 Utrahigh Frequency-Tactica Sateite The AN/PSC-5 is a portabe, battery-operated, haf-dupex UHF transceiver. It is empoyed for ong-range communications. It weighs approximatey 14 pounds incuding antenna and batteries. The AN/PSC-5 provides two-way voice and data communications by sateite. It operates on the UHF frequency band of 225- to 400-MHz range. It provides 2,400 to 16,000-bits per second (bps) data rate, depending on mode setting. Ony one operator is required to operate it. The United States Marine Corps UHF tactica SATCOM system supports and augments the high precedence command and contro and common-user, singechanne requirements of a Marine air-ground task force and its major subordinate headquarters. The space segments used by the AN/PSC-5 are the Feet Sateite Communications, eased sateite communications, and UHF foow-on sateites. A the sateites are ocated in geosynchronous orbits and permit interconnections among mobie, ground terminas. The one-way distance to servicing sateites is approximatey 25,000 mies, resuting in a round-trip propagation deay of approximatey one-quarter of a second. The shape of the sateite footprints is roughy circuar but eongated from north to south. This is caused by the ange at which the signa hits the Earth s surface and by the curvature of the Earth s surface. Mutipe-access schemes can operate either with fixed-channe assignments to the various users or with channes being assigned in varying fashion according to demand. The atter is caed demand assigned mutipe access (DAMA). With demand assignment, the user makes a channe request, and a channe is aocated after a brief time ag. The DAMA scheme of operation is empoyed on UHF-tactica sateite (TACSAT) to share avaiabe channes more efficienty. The radio systems are compatibe with the KY-57 (wideband mode ony), the KY-99 and ANDVT (narrowband mode ony), and the KG-84C (wideband or narrowband) COMSEC equipment. This radio equipment is aso capabe of remote operations by using the AN/GRA-39B (narrowband mode) or HYX-57 (wide-band mode).

36 2-14 MCRP B Figure 2-8. AN/PSC-5 UHF TACSAT Radio. TACSAT Radio Empoyment Considerations. Because of TACSAT s imited avaiabiity, the MAGTF empoys TACSAT primariy to support critica, ong-range communications requirements (e.g., communications support for deep reconnaissance operations or connectivity to the tactica echeon of a MEU[SOC] when depoyed ashore). The AN/PSC-5 is the primary DAMA-capabe, TACSAT radio avaiabe to the MAGTF (see fig. 2-8). TACSAT imitations incude the competition for avaiabe frequency resources and channe time on the sateite. If ony narrow band channes are avaiabe, channe-data rates are imited to 2,400 bps. Channe congestion, noise, and network saturation wi affect the information fow on sateite channes and wi require a significant reduction in the data transmission rates to sustain data communications. Transmit power seection can be critica. Increasing the transmit power can decrease net effectiveness. Larger directiona antennas provide increased signa gain, which increases the transmitted signa power. Antennas for these systems are ightweight and fragie and, therefore, require constant maintenance and inspection for proper operation. Sateites are shared resources. Exact frequency, bandwidth, and power of every carrier transmitted through the sateite is stricty controed by a higher authority. TACSAT Radio Environmenta Limitations. The primary environmenta imitations on TACSAT radios are signa propagation deay, ocation on the Earth, terrain masking, and weather effects. Timing between DTE can be a critica factor in SATCOM because the sateite, acting as a reay between radios, is about 25,000 mies away. There is

37 Radio Operator s Handbook 2-15 approximatey a one-fourth second propagation deay between sending and receiving stations. This deay can interfere with systems that automaticay retransmit if an acknowedgment is not received after a very short time-out period. As unit ocation changes, the ook ange (i.e., ange above the horizon) to the sateite can affect net reiabiity. The orbit of a sateite aows it to cover a certain footprint on the earth. Sateites in equatoria orbit can cover arge portions of the Earth, both north and south of the equator, but as the user moves coser to the Earth s poes, the TACSAT termina may exceed the sateite footprint. This wi cause intermittent or ost communications. Terrain can aso have this effect by interfering with the sateite and TACSAT termina ine of sight. Thunderstorms, heavy snowstorms, and hai aso affect sateite transmissions by damaging antennas and changing the eectromagnetic environment. Utrahigh Frequency Propagation UHF frequency propagation is used for ground-to-air, air-to-air, sateite, and tactica muti-channe communications. Communications are imited to LOS but may extend for more than 500 kiometers as ong as the aircraft is high enough to be within LOS (see fig. 2-9). It is even possibe for UHF communications ranges to a sateite to be more than 35,000 kiometers. DIRECT WAVE (LOS) TRANSMITTING ANTENNA EARTH Figure 2-9. UHF LOS. DATA COMMUNICATIONS SCR can aso transmit and receive data by using termina devices such as the digita message system (DMS) previousy caed and more commony known as the digita communications termina (DCT) and the tactica communications interface modue (TCIM). The DCT, data

38 2-16 MCRP B automated communications termina (DACT), and the TCIM are critica in enabing data communications at the tactica eve over SCRs. Digita Message System-AN/PSC-2 The DMS-AN/PSC-2 is a hand-hed communications device that can be operated with either a standard miitary radio or teephone fied wire equipment. The DMS is used to compose, edit, dispay, transmit, and receive information. By menu seection, formatted text, free text, and digitized map messages are transmitted over tactica communications equipment. DMS is being used directy for air support, fire support coordination, reconnaissance, medica evacuation, and other functions. DMS uses a burst transmission capabiity which reduces the vunerabiity to enemy radio direction finding and jamming. Data Automated Communications Termina The DACT is a sma, tactica computer and communications termina which gives users the capabiity to receive, process, and transmit various messages, to incude text and symboogy, used by tactica data systems. The DACT wi effectivey repace the DCT when it achieves fu operationa capabiity in FY 03, and it wi provide much greater functionaity beow battaion eves. This wi incude an embedded goba positioning system (GPS) receiver, the abiity to share a common picture of the battespace, automated data exchange, and MAGTF command, contro, communications, computers, and inteigence (C4I) network connectivity. The DACT wi be transportabe by foot-mobie Marines and mounted in tactica or armored vehices. Tactica Communications Interface Modue The TCIM provides the communications ink between the tactica computers of the communications and information systems within the MAGTF and the oca and wide area networks, switched backbone (SBB), and radio nets. There are two versions of the TCIM card: an interna persona computer asynchronous transfer card to mount directy in the computer, and an externa version with the card mounted in a portabe chassis. TCIM software was deveoped for open-systems architectures. Sma computer systems interfaces (SCSI) provide interoperabiity between the TCIM and other Marine Corps computers.

39 Chapter 3 Equipment Siting and Grounding Techniques Two factors pay an important roe in equipment siting: optimum communications and camoufage. It is often difficut to find communications sites which are hidden from enemy view, fire, and direction finding and afford good communications connectivity. The idea ocation for a radio antenna is as far away from cover as possibe, such as a bare mountain top or in the midde of a arge fied. However, this goes against the commander s tactica requirement for troops and equipment to be camoufaged and conceaed as much as possibe. Therefore, panning the ocation of equipment must be detaied to achieve the best resuts. See Appendix A for a review on topographica maps and grid coordinates. HIGH FREQUENCY In the presence of his (without arge trees), the foowing guideines for ground wave inks shoud be used: Locate HF antennas just beow the top of the hi in the direction of desired communications. Often the signa wi be greater beow than on the top. This wi aso minimize interference and/or jamming from the opposite direction. Move the antenna back from the hi if a hi is between the operator and the distant station with which the operator wishes to communicate. The signa strength can vary widey in the region immediatey behind a hi. If it is necessary to set up behind a hi, then it may aso be necessary to set up a variety of antennas ocated at different distances from the hitop to see which one offers the best performance. Long-distance, HF sky wave signas of more than severa hundred kiometers are often best transmitted and received at anges just above the horizon eve. Obstaces on the horizon wi cause the signa to trave a

40 3-2 MCRP B ACTUAL PATH REQUIRED PATH Figure 3-1. Low HF Horizon Anges. higher path ange and may reduce the circuit reiabiity as a resut (see fig. 3-1). Wire fences between the operator and the horizon wi aso essen the chances of getting through (see fig. 3-2). An HF ground wave signa wi foow the terrain much better than higher frequency signas. It wi be weakened by trees (more so when Figure 3-2. Effect of Wire Fences and Power Lines.

41 Radio Operator s Handbook 3-3 SIGNAL PATH Figure 3-3. HF Ground Wave Path. they have eaves) and rugged terrain, but the signa may sti get through (see fig. 3-3). VERY HIGH FREQUENCY AND ULTRAHIGH FREQUENCY Obstaces such as trees, buidings, and his between a transmitter and receiver wi weaken the signa or stop it. Aircraft fying aong the path wi aso interfere with reception (see fig. 3-4). A cear signa path INTENDED PATH Figure 3-4. Aircraft Aong Signa Path.

42 3-4 MCRP B INTENDED DIRECTION BETTER GOOD BAD Figure 3-5. Antenna Obstaces. between the transmitter and the intended receiver, especiay for LOS communications, is preferred. The antenna must be positioned as high as possibe to overcome obstaces, especiay if communication is in the direction of trees or buidings (see fig. 3-5). Keep equipment as far back as possibe from obstaces in the direction of the signa path to prevent interference or damage to equipment. In some situations, soid obstaces may actuay improve a ink by providing a sharper surface to diffract over or refect from (see fig. 3-6). Figure 3-6. Diffraction Over Buiding.

43 Radio Operator s Handbook 3-5 Figure 3-7. Refection off Spherica Water Tower. Under certain conditions, spherica water towers and was of buidings (facing the proper direction) may enabe communication around interfering terrain or vegetative obstaces (see fig. 3-7). Transmitting over water aows VHF to go farther, but fading may occur. If communicating over water is unavoidabe, and fading occurs, the signa may be improved by raising or owering the antenna. The antenna may aso be positioned so a hi or rise is between it and the water but not high enough to bock the LOS to the other antenna (see fig. 3-8). DIRECT PATH REFLECTED PATH DIRECT PATH REFLECTED PATH TERRAIN SHIELDING Figure 3-8. Mutipath Fading and Terrain Shieding.

44 3-6 MCRP B Figure 3-9. Ridge Line Antenna Farm. Nothing is more compromising to a unit s ocation than an antenna farm stretched aong a ridge ine (see fig. 3-9). The enemy wi reaize that a major command post is nearby. The advantage of pacing an antenna on a ridge ine is the abiity to tak in many directions without and being in the way. If communication is needed in ony one direction away from the enemy put some terrain shieding between the antenna farm and the enemy (see figures 3-10 and 3-11). This way, the enemy won t be abe to intercept communications or jam circuits as easiy. However, it s not aways necessary to tak from hitop to hitop. Taking from hiside to hiside or aong the vaey foor may be a better option in some instances. The enemy wi certainy have a harder time ocating a unit this way. Figure Low Antenna Profie.