INTRODUCTION
GAIN ATTENTION
PURPOSE
INDIVIDUAL TRAINING STANDARDS
Ref: FMFM 1-3b, FMFM 0-11, MEF SOTG URBAN SNIPER COURSE
8541.3.5
MISSION PERFORMANCE STANDARDS
Ref: FMFM 1-3b, FMFM 0-11, MEF SOTG URBAN SNIPER COURSE
8541.1.18
TERMINAL LEARNING OBJECTIVE
Without the aid of references, the student will be able to design, construct and employ a field expedient antenna in an urban area.
ENABLING LEARNING OBJECTIVES
1. Describe propagation in reference to the type of radio being utilized.
2. State the environmental restrictions of field expedient antennas in an urban area.
3. State the employment characteristics of a field expedient antenna where the operator has control.
4. METHOD AND MEDIA
5. This period of instruction will be taught through lecture and practical application of the utilization of field expedient antennas in the MOUT facility.
6. TESTING
7. This period of instruction will be evaluated during practical application.
TRANSITION
BODY
1. Frequency Bands
2. There are four radio frequency bands used by Marine Corps communications. Of the four bands, we will concentrate on three (The POI will exclude SHF, micro wave or troposcatter). They are as follows.
a) High Frequency (HF) (2-39 mhz)
b) Very High Frequency (VHF) (30-76/88 Mhz)
c) Ultra High Frequency (UHF) (225-400 Mhz)
3. Propagation
Propogation is the process by which a radio signal travels through the atmosphere from one antenna to another. .Different frequency bands utilize different methods of propogation to transmit their signals.
a) HF propogation can be accomplished in two manners: ground-wave propogation and sky-wave propogation.
1) HF ground-wave propogation involves the transmission of radio waves on, or near the surface of the earth. The ground wave can be divided into four parts.:
(a) Direct-wave which travels through the atmosphere from one antenna to another in what is called the line of sight (LOS) mode. Maximum LOS distance is dependent on the height of the antenna. Obstructions, such as hills or buildings can disrupt or block the signals.
(b) Reflected-wave signals reflect off the earth while travelling from one antenna to another. Together, the direct-wave and the reflected wave are called the space wave.
(c) Refracted-wave signals are in essence the same as reflected waves. The difference is that refracted waves radio signals are bent back to earth prior to contact with the atmosphere. This is due to an obstruction to LOS.
(d) Surface-wave signals are the usual means of ground-wave communications. They are very dependent on the surface over which they travel. The conductivity of the surface will determine how well the signal will be received. Sea water is a highly conductive surface where frozen ground or sand are very poor conductors.
2) HF sky-wave propogation - Beyond the range covered by the ground-wave signal, HF communications are possible through sky-wave propogation. Sky-wave propogation is possible by the bending of the radio signal by a section of the atmosphere known as the ionosphere. Although the ionosphere extends from 37 miles up to 620 miles, we are only concerned with the four layers that affect radio-wave propogation.
(a) The "D" region is the closest to earth and exists only during daylight hours. It does not have the capability to bend radio waves back to earth but plays an important role in HF communications. The "D" region absorbs energy from the radio signal passing through it, thereby reducing the strength of the received signal. It is for this reason that HF communications are better during the hours of darkness.
(b) The "E" region is the next higher region and is present 24 hours a day. However, it is much weaker during the hours of darkness than during the day. The "E" region is the first region with enough charge to bend radio signals. At times, parts of the "E" region become highly charged and can either help, hinder, or completely block-out HF communications. These highly charged areas are called sporadic E and occur mostly in the summer.
(c) The "F1" and "F2" regions are always present and are the most dependent factor in HF communications. It is these two regions that are primarily responsible for bending the radio waves back towards earth. The bending of a radio signal by the ionosphere depends on the frequency of the radio signal and the degree of ionization in the atmosphere (called the "take off angle"). It is possible for a frequency of less than 2 MHz to be completely absorbed by the "D" region, however, a frequency of greater than 30 MHz can punch completely through the atmosphere into space. For these reasons, it is a good rule of thumb to use higher frequencies during daylight hours and lower frequencies at night.
(d) The "Skip Zone" is the area between where the furthest reaching ground-wave and the nearest reaching sky-wave signals touch down. The primary means of defeating the skip zone during HF communications is the use of Near Vertical Incidence Skywave (NVIS) propogation. NVIS communications are specifically useful for short range HF communication circuits. NVIS antennas are designed for take off angles that are usually greater than 60 degrees. This is a result of the short range and increase in take off angle. This is comparable to throwing a ball at the ceiling, the sharper the angle thrown up, the sharper the angle it bounces down. Keep in mind that a 250 to 350 mile HF circuit is considered short. NVIS communications generally perform quite well for frequencies below 8 MHz.
b) VHF propogation is completely dependent on a ground-wave circuit. VHF communications to distant stations (beyond LOS) are very difficult unless R/T power output is increased or a retrans (repeater) chain is established. VHS LOS ground wave communications are influenced by these factors.
1) The direct wave: In the 39 NHz range, VHF will often act like HF ground-wave. Circuit distance is nearly 100% dependent on antenna height.
2) The reflected-wave: Same as HF.
3) The refracted-wave: Same as HF.
4) The diffracted-wave: The diffracted wave scatters around obstacles and permits communications in the shadow region behind obstacles. Lower frequencies scatter (diffract) more than higher frequencies, so it is not uncommon for a lower frequency signal to diffract across an obstacle and result in reliable communications and result in to a receiver antenna not far below the LOS, while at the same time, a signal of a higher frequency will not be heard.
c) UHF propogation is usually limited to ground-to-air and air-to-air communications. Although communications are limited to LOS, they may extend for more than 250 miles, as long as the aircraft is high enough to be within LOS.
4. Antenna
5. Of all the communications equipment the Marine Corps uses, the antenna is quite close to being the only variable that is user controlled. Many factors need to addressed when considering antenna construction. The list below, or "matrix" will help enable you to select the proper antenna to gain and maintain communications. The matrix also identifies when a positive communications circuit is not probable.
a) Tactical situation - As always, the tactical situation will dictate most, if not all, of the communications planning for your operation. This includes:
(1) Radios to be used based on availability.
(2) Frequencies to be utilized (SPEED, PROPHET).
(3) Radio retrans/relay teams.
(4) Communications plan/no comm plan.
(5) NEVER FORGET that the need to gain and maintain communications is not an excuse for compromise!
b) Site size - when constructing an antenna, wavelength and geometry (design) of your antenna will be dictated by the size and layout of the site. For example, a dipole antenna requires at least two types of supports to support the running ends. If no supports are readily available, or the site is not wide enough to accommodate the wavelength for your frequency, a different antenna selection will need to be made.
(1) Wavelength - In radio frequency communications, there is a definite relationship between antenna length and transmitter frequency wavelength. This is very important when constructing antennas for a specific frequency, or frequency range. The wavelength of a radio signal is equal to the distance traveled in the time it takes to complete one cycle.
(2) Defining wavelength - Wavelength is equal to the speed of light (the speed at which radio signals travel) divided by the frequency. An example of 3 MHz is 300,000,000 M/S 1 Wavelength = 100m or 328 ft. (3,000,000 Hz). This means that in the time it takes to complete one cycle at 3 MHz, the signal travels 100 meters, or 328 ft. As shown, the lower the frequency, the larger/longer the antenna, which can be very difficult to employ tactically with reduced site size.
(3) Computing wavelength - An antenna's wavelength can be computed by the formula below. A general rule of thumb is - the longer the antenna - the more resonant (frequency effective) it will be.
(a) Quarter-wavelength: Divide 234 by the operating frequency in MHz.
(b) Half-length: Divide 468 by the operating frequency in MHz.
(c) Full-wavelength: Divide 936 by the operating frequency in MHz.
*It is important to note the "most" antennas should be at least one half-wavelength long and within =/- 2% of the intended frequency length to be considered resonant.
d) Materials - Obviously, the materials available will dictate what antenna designs are feasible for the specific situation. The materials below are a generic list. A communications kit can be extravagant or simple - it is the user's choice. Most of the following items can be acquired through the logistics system, open purchased, or improvised.
1) Conductor - The type of wire (conductor) used to construct your antenna needs to only meet these minimal standards:
(a) 12-20 gauge copper wire.
(b) Use of longer spools limits the number of splices and is easier to work with.
(c) Stranded wire is more pliable (solid core wire holds a specific shape more readily).
(2) Insulators - Nearly any item that does not conduct electricity may be used as an insulator. Store bought insulators are inexpensive and are reusable. A few improvised insulators include dry wood, plastic and glass.
(3) Resistors - Resistors make an antenna directional. To be effective a resistor needs 500-700 ohms of resistance and be rated at half the power output of the R/T being used. When no conventional resistors are available the following expedients:
(a) BA-30woth a nail driven through the center.
(b) Nail wound tightly with copper wire.
(c) Plastic earplug case filled with salt water.
(4) Grounding - Grounding wire must be a part of any antenna kit. A properly ground antenna will always out-perform an ungrounded antenna. A proper ground can take many forms, a few examples include:
(a) Driving a knife or metal stake into moist ground.
(b) Utilizing a building's existing pipes or supports (as long as you are sure that they eventually lead underground).
(c) Plugging directly into the ground of a standard 3-hole socket.
(5) Dual banana clip connectors (DBCs), or called cobra heads, are fabricated antenna connectors which transfer R/T power to the radiating elements via RG-59/59 coaxial cable. By using a cobra head and coaxial cable, a very limited amount of power loss occurs in the expedient transmission line. To further reduce power loss, the minimum amount of coaxial cable should be used, 50 ft. or less is acceptable. If additional cable is on the deck, it should be zigzagged across the floor rather than looped. (DBC's NSN 5935-410-1399).
(a) When cobra heads are unavailable, tightly twisted slash wire (wd-1) will suffice. The tighter the slash wire is twisted the more it acts as a transmission line and the less it will act as a radiating element. The length of the twisted pair should not exceed 10 ft (if minimal power loss is expected).
(6) Counterpoise - When an actual ground connection cannot be used due to high resistance of the soil, a counterpoise may be used to replace the usual ground connection. There are two different types of counterpoise:
(a) Horizontal - A horizontal counterpoise should be at least equal to, or longer, than the length of the antenna. The counterpoise should be placed directly beneath the antenna, a short distance above the ground. Insulate the wire at each end.
(b) Vertical - When an antenna is mounted vertically, the counterpoise should be made into a simple geometric pattern. Perfect symmetry is not required. The counterpoise appears to the antenna as an artificial ground that helps produce the required radiation pattern.
d) Distance - The distance of your proposed circuit will not only dictate your radio/antenna combination, but also the operation's overall communications plan. Distance will coincide extremely closely with takeoff angle, as well as terrain.
e) Terrain - Nearly all the communications assets an operator uses were not designed for use in an urban environment. In an "average" American city, the local radio stations push 20,000 to 75,000 watts of power output. Conversely, the AN?PRC-104 pushes 10 watts (with a fresh battery). LOS/groundwave distances can be extremely limited due to metal building structures and building height. A few objects to avoid include:
(1) Solid steel bridges and underpasses.
(2) High tension wires and telephone poles.
(3) Main supply routes (MSR's) as ignition and electrical systems emissions by vehicles will introduce noise, hum and interference to a circuit.
(4) Log periodic, microwave, repeater, and other high-power antennas.
f) Take off angle - Take off angle of an antenna is the angle above the horizon that an antenna radiates the largest amount of energy. For VHF communication, antennas are designed so that energy is radiated parallel to the earth. In HF communication, the take off angle of an antenna can determine whether a circuit is successful or not. HF sky-wave antennas are designed for specific take off angles depending on the circuit distance. High take off angles are used for short range communications. Low take off angles are used for long range communications. Consulting the TOA vs. Distance chart will aid in this computation.
g) Directionality - Antennas are classified according to how radio energy is radiated. The three classifications are:
1) Omni-directional - These radiate energy equally in all directions. It is used to communicate or receive from stations in several directions. Circuit range is limited due to power blanketing a large area. It also allows interference to weaken the signal from every direction.
2) Bi-directional - These produce a stronger signal in two favored directions while reducing the signal in other directions. Tactical bi-directional antennas are usually field expedient antennas. Bi-directional antennas are usually employed in point to point circuits and in situations where the null can be positioned to reduce or block out interfering signals.
3) Uni-directional - A directional antenna will concentrate nearly all the radio signal in one specific direction. A uni-directional antenna must be carefully oriented. Reflectors can be used behind the main lobe to boost the incoming or out-going signals.
4) QUESTIONS
5) SUMMARY
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