Use of radio in IBCT logo

Planning for the use of high-frequency radios in the brigade combat teams and other transformation Army organizations

by David Fiedler

Over the past three years, the Army has begun to �transform� itself into a 21st-century combat force. Central to this transformation are the new brigade combat teams being organized and equipped at Fort Lewis, Wash., and other Army posts.

An examination of the BCT�s operations and organizational concept will show that tactical long-distance/wide-area communications will be a major factor in the BCT�s success or failure. To equip the new formations for their mission, the Army is both developing new systems and recapitalizing on older systems and concepts to meet new requirements. Modernized high-frequency radios (Transformation High-Frequency Radio System), shown in Army Communicator�s Winter 2001 edition, will go a long way in meeting BCT tactical wide-area communications needs � particularly in rough terrain and urban environments � if a few basic concepts are understood.

Why HF radio for the BCT

HF radio (radio signals in the 1.6 to 30 megahertz frequency spectrum) has the following characteristics that make HF an ideal communications system to support the fast-moving, wide-area operations the BCT will participate in.

HF signals travel longer distances over the ground than the higher frequency very-high-frequency (Single-Channel Ground and Airborne Radio System) or ultra-high-frequency (Enhanced Position-Location Reporting System or near-term digital radio) signals do because they�re less affected by factors such as terrain or vegetation.
HF signals can be reflected off the ionosphere (a layer of charged gases surrounding the earth at high altitudes) at high angles that will allow beyond-line-of-sight communications at distances out to 400 miles without gaps in communications coverage.
HF signals can be reflected off the ionosphere at low angles to communicate over distances of many thousands of miles for reachback communications.
HF signals do not require the use of either satellite-communications or retransmission assets.
HF equipment provided to the brigade can be used either fixed station or on-the-move.
HF systems can be engineered to operate independent of intervening terrain or manmade obstructions.

HF (2-30 mhz) radio-wave propagation

Radio propagation is the process by which electromagnetic energy (signal) moves from one point to another. Since radio waves propagate (move) the same way light waves do, we can think of radio waves in terms of light. As with light rays, radio energy (signal) can travel from a point source outward in all directions, just as a light spreads from a light bulb. For radio waves, this is called an omni-directional signal.

The figure below shows how radio energy decreases as distance from the source increases. Note that as the distance (range) doubles, the signal strength is reduced to one quarter of what it was (proportional to 1/d squared).

Radio signal compared to light

How radio energy spreads can be metaphorically compared to how light spreads. The farther away someone is from the source of light (or radio signal), the weaker the light (or signal) is.

Also as with light, radio signals can be focused to travel in a single direction similar to a flashlight beam. This is called a directional signal. The shaping of the radio signal is a function of the radio�s antenna system. Just as with light, radio signals can also be blocked by obstructions and bent (diffracted) over solid obstructions. This is similar to seeing the small amount of light that can be detected from a source behind a wall.

All these effects can be used to provide gap-free tactical HF radio communications throughout the brigade�s operations area and back to its sustaining base. It�s important to recognize that the system operating radio frequency(s) and how the radio antenna shapes the signal pattern are the two most critical factors in assuring HF communications for the brigade.

Possible transmission paths within the brigade�s operational area

The figure below shows possible radio paths between two stations located in the brigade AO. We�ll assume for this article that most combat units in the brigade will be located no more than 400 miles from each other. Circuits of greater distances (reachback) will be covered under other sections.

Ground-wave components

Ground-wave components.

The above figure shows three possible low-angle radio paths located along or near the earth�s surface. These paths are called ground-wave paths because they�re close to the earth�s surface or are in contact with it. They consist of the direct-wave path, ground-reflected path and surface-wave path.

The direct wave consists of radio-frequency energy that travels through the atmosphere and near the earth directly from one antenna to another. This is called the line-of-sight mode of propagation. Maximum LOS distance depends on the antenna�s height above the ground and whether or not the path is obstructed by terrain that will block radio signals. On flat ground, direct-wave paths suitable for THFRS communications can be expected out to six to eight miles before the earth�s curve blocks the signals. Direct-wave communications can go much farther if stations are located high on hilltops or have masts with no intervening obstructions. Control of high ground and antenna height is important when using direct-wave communications.

The ground-reflected path, like the direct path, travels through the atmosphere, but due to the lower takeoff angles from the transmitting antenna, signal energy is reflected off the earth while traveling from the transmitting antenna to the receiving antenna. Depending on the composition of the ground at the reflecting point, the reflected energy can be considerably reduced when it arrives at the receiving antenna. Signals reflected off seawater lose almost no energy, while signals reflected off a sandy desert become quite weak.

When summed together, the direct wave and the reflected wave are referred to as the space wave. As the two combine, they can result in either a stronger or weaker total signal, depending on the timing difference of the two signals as they arrive. The difference in signal phasing is caused by the longer distance traveled by reflected wave. Space-wave signals won�t usually be the dominant communications mode in the BCT.

The surface-wave path is the transmitted radio energy that travels along the boundary between the atmosphere and the earth�s surface, and it�s in actual contact with the earth�s surface. The surface wave is greatly affected by the electrical conductivity of the earth in the propagation�s path. With a good conductor such as seawater, surface-wave communications out to 100-plus miles are possible. With a poor surface such as sand or frozen ground, surface-wave communications are greatly reduced. Surface-wave signals are also greatly reduced by heavy vegetation and mountainous or urban terrain. Surface-wave signals can be made stronger over poor ground by using techniques that improve the conductivity of the earth near the antenna.

Most HF ground-wave communications within the BCT will use surface-wave signals. Space-wave communications will predominate only when communicating from high ground to other high-ground locations along the LOS. Vertical monopole (whip) manpack and vehicle antennas of various lengths are the antennas provided to produce the low takeoff-angle energy needed to generate ground-wave signals.

The figure below shows the antenna-energy pattern of the vertical monopole (whip) antenna. Note that the signal is mostly along the earth�s surface and on the lower angles. There�s much less energy on the higher angles and none directly overhead (vertical angles). The pattern resembles a doughnut, so operationally you see it can be very difficult to communicate with aircraft that are directly overhead (reduced signal), while you can talk to aircraft many miles away that are receiving low-angle energy from a vertical antenna.

Vertical whip antenna's radiation pattern

Radiation pattern of a quarter-wave vertical whip antenna.

The ionosphere

The ionosphere is an electrically charged region of atmospheric gases that surround the earth. Ionization (electric charge) happens when solar radiation bombards atmospheric gas molecules and forces them to detach electrons, leaving the gas molecule with a positive electrical charge called an ion and leaving free electrons in the atmosphere. Since positive electrical charges repel each other, gas ions tend to �bunch� in distinct �layers� of ions at heights of between 30 and 300 miles � shown in the figure below. These charged areas will reflect radio signals back to earth if they strike the ionosphere at particular angles using particular frequency bands.

Earth's atmosphere (simplified)

The earth's atmosphere (simplified).

Radio engineers have labeled these layers the D, E, F1 and F2 layers (figure below). Three factors determine whether a radio signal will be reflected back to earth and can be used by brigade HF communications systems. They are:

The higher the radio frequency, the more likely the signal will penetrate the ionosphere rather than be reflected by it;
The current ion density determined by the amount of sunlight (time of day, season, solar activity) at the time communications is desired; and
The angle at which the radio wave contacts the ionosphere.

See the following figure for details.

Skywave transmission paths

Skywave transmission paths. As illustrated on the diagram's left side, radio waves that pass through all layers are lost.

Note that at any time of the day, year or solar-activity (sunspot) cycle, there�s a band of radio frequencies always available that can be reflected off the ionosphere and will support HF communications. The automatic-link-establishment feature of the new Army HF radios (AN/PRC-150 family) will find these frequencies for the operator from a list of authorized frequencies in the radio database. Signals on these frequencies can be used for brigade tactical HF communications over distances of hundreds of miles unless very unusual and rare solar activity is occurring.

Also note that the angle at which the wavefront contacts the reflecting layer is determined by the radio�s antenna system. The OE-505 and AT-1011 vertical whips produce low angles of radiation. Bending the whips into the horizontal position with the whip-tilt adaptor, or by using the RF-1912 or RF-1941 wire-dipole antennas 30 feet or less above ground, produces high-angle radiation.

Maximum useable frequency, lowest useable frequency

Each layer of the ionosphere has a frequency that�s the highest the layer will reflect. The exact frequency is determined by the amount of ions in the layer. As you may see in the above figure, the lower layers reflect the lower frequencies, while the higher frequencies penetrate the lower layers and are reflected back by the higher layers. To cover the largest tactical AO possible, use the highest frequency that will reflect, since the higher the reflecting layer, the wider the area covered by the reflection.

Since the ionosphere is always changing, a general rule when in manual operation is to select a frequency 15 percent lower than the actual maximum useable frequency to avoid problems. This frequency is called the frequency of optimum traffic. Signals on frequencies that exceed the MUF go through the ionosphere and are lost in outer space.

The MUF is also different for different angles of reflection. Signals on lower takeoff angles can use higher frequencies for communications because they�ll be reflected. The ALE mode of modern HF radios will automatically prevent signals with a frequency above the MUF from being selected for operations. ALE will select the best radio frequency for communications on a continuous basis if it�s used.

A limitation of HF radio is the high-radio-noise (static) level on HF frequencies. Radio noise comes from sources in outer space, lightning in the earth�s atmosphere and manmade sources. Noise on a particular system depends mainly on location and season. For each situation, there�s a frequency (lowest useable frequency) below which there is too high a noise level for communications. LUF is affected by transmitter power, antenna gain and directivity and absorption of signal by the lower layers of the ionosphere. LUF is defined for as the lowest frequency at which a 90-percent probability of communications exists.

The new radios� ALE, modems and vocoder features are designed to make the LUF as low as possible by enabling operation in a high-noise environment. This widens the range of operational frequencies available for communications. A typical plot of MUF/FOT/LUF is shown in the figure below. Note the range of frequencies between the MUF and the LUF over the entire day. Under almost every circumstance, there are a range of HF radio frequencies that will be suitable for brigade communications.

Frequency propagation chart

Important features of a typical propagation chart.

It�s the responsibility of the operator and the system manager to obtain frequency assignments in this range for operations. To aid in frequency selection, skywave and ground-wave predictions and prediction software are available through frequency-management channels. It�s the responsibility of the brigade S-6 frequency manager to predict HF RF requirements, obtain authorized frequencies between the predicted MUF and LUF and provide them to operators and system managers. When using ALE, the radio itself will test the propagation conditions and select the best operational frequency. ALE in the BCT will be set to accomplish this every half hour under normal operating conditions.

Antennas

The single most important factor in reliable tactical HF communications is the antenna. At HF frequencies, this is especially true. To select the best antenna for a particular brigade operation, the following concepts must be understood by the operator and system manager.

Wavelength and frequency. For best radio performance, there�s a specific relationship between antenna length and operational frequency. All radio signals travel at the speed of light. The wavelength at a particular frequency is the distance traveled by light as it completes one cycle of its motion. To calculate this distance (in meters), the speed of light (in meters) must be divided by the operational frequency in cycles per second. After simplifying the math, wavelength (in meters) is equal to 300 divided by the frequency in mhz (millions of CPSs).

As an example, the wavelength of a three-mhz HF signal is 300 divided by 3 (300/3), or 100 meters. This means that in the time it takes to complete one cycle at three mhz, the signal has traveled 100 meters.

Knowing how to calculate wavelength is important because signal strength depends on the antenna�s length and the amount of current flowing through it. For maximum current (signal) at a given frequency, the antenna needs to be one-half a wavelength or multiples of a half-wavelength long.

Resonance. The strength of a signal radiated from an electrical conductor that has an RF current flowing depends on the conductor�s length and the current�s amount. For a given frequency, maximum current flows and maximum signal are produced when the conductor (antenna) is half a wavelength long, or multiples of that length. An antenna that radiates most of the energy flowing in it is said to be resonant.

At the frequencies most used by the brigade for fixed communications, the wire antennas (AT-1912, RF-1941) the Army provides are constructed using lengths that are close to resonance and are therefore very efficient.

On the other hand, mobile antenna lengths can range from less than 10 feet to as much as 32 feet. These antennas are physically too short to be resonant. To make the short antennas radiate as strong a signal as possible, antenna couplers such as the RF-382 or RF-5830 are provided. Couplers allow RF current to flow to the short antenna and dissipate energy that�s not radiated as signal but is instead reflected back from the antenna towards the radio.

The ratio of radiated power to reflected power is called the voltage standing-wave ratio. It�s important to keep this ratio low (less than 2:1) for highest efficiency. High VSWR won�t physically damage the radio equipment, but it will reduce the radio signal�s strength.

Antennas whose length is close to resonance don�t require couplers to function since the antenna radiates all energy. When a coupler is needed to match an antenna, it should be located as close to the antenna as possible for best efficiency. When configured for mobile operation, the coupler may be located near the transmitter, reducing power at the antenna. This is acceptable for mobile operations or when at a brief halt. However, it�s wise that whenever possible, use more efficient ground-mounted (resonant) wire antennas.

Antenna couplers may also be dismounted and located at the antenna feed point to reduce signal loss when practical. When not practical, due to operational constraints, antenna couplers will remain on the vehicle and the coupler output connected directly to the antenna via cables (provided), even though efficiency is reduced slightly.

Polarization. Polarization is the directional relationship of radio energy coming from an antenna to the earth�s surface. As a rule, antenna fields are vertical if the antenna is physically vertical and horizontal if the antenna is physically horizontal. The intensity of a horizontal signal traveling in contact with the ground (ground-wave/surface-wave) drops rapidly because in effect the earth short-circuits the electric field. A vertically polarized signal doesn�t lose strength nearly as quickly because it doesn�t contact the earth as much.

In the brigade, ground-wave communications will be the primary mode of short distance (0-20 miles) communications. Manpack, ground-mounted and vehicular vertical antennas are provided for this purpose. Horizontal antennas and adaptors that �tilt� vertical antennas into a horizontal position are provided for long distance (0-400 miles) skywave communications. These antennas provide the high takeoff angles necessary for BLOS HF communications.

All antennas in a brigade radio net must have the same polarization. Mixing polarization of antennas in a net as a rule will result in significant loss of signal strength due to cross-polarization. S-6s will therefore assure that all stations in a net will have the same (horizontal or vertical) antenna polarization when possible. Surface-wave communications over seawater should always use vertical polarization because seawater�s electrical properties will greatly reduce the signal strength of a horizontally polarized surface-wave signal.

The following figure shows the concept of vertical and horizontal polarization.

Vertical and horizontal polarization

Vertical polarization, top. Horizontal polarization, bottom.

Vertical (whip) antennas. Ground-wave HF communications are most effective when using vertical polarization over good conductive ground. BCT manpack radios are provided the 10-foot long OE-505 antenna, and vehicular radios are provided the 32-foot long AT-1011 antenna.

Whip antennas are most efficient when they�re between one-quarter and five-eighths a wavelength long at the lowest operating frequency. At HF frequencies normally used in the brigade, the whips are far too short for efficient operation. Tuning devices (such as the RF-382 antenna coupler) are provided to electrically match a physically short or long antenna to the radio and the transmission line. Operators should use the longest antenna physically possible under the operational conditions to achieve best communications performance.

For example, the 10-foot OE-505 manpack antenna can be replaced by a vertical wire tied to a support, such as a high tree branch, under many conditions to improve antenna efficiency. Any good heavy-wire conductor can be used, including field-telephone wire or the wire from the RF-1941 wire-dipole antenna kit provided with the radios. The end of the vertical wire must be insulated from the support. The feed end of the wire antenna is connected to the radio via the wire adaptor provided with the radio.

To further improve antenna efficiency and increase signal strength on the lower (surface wave) radiation angles, radios in manpack operation should be given a �tail� wire connected to the radio ground post. The �tail� will provide a low-resistance return path for antenna currents. Tail wires aren�t provided but can be locally fabricated from computer-ribbon cable, communications wire or ground-strap braid. Tails should be as long as possible but shouldn�t interfere with carrying the radio. The manpack-tail concept is shown in the figure below.

Illustration of whip antenna positions and best results

If operating ground wave, best results are obtained with the whip vertical. Also use a dangling ground-plane enhancement tail.

Along with height, physical orientation is also very important when operating in the manpack configuration. The antenna must be kept as vertical as possible to produce the best surface-wave signal and also to avoid losses due to cross-polarization (figure above). It�s also important when possible to operate from areas that don�t have energy-robbing obstructions such as trees and buildings (figure below).

Illustration of clearing vs clutter

Where possible, find a clearing, no matter if you're using ground wave or NVIS.

Whenever possible, manpacked radios should be removed from the operator�s back and operated from the ground. This will reduce the capacitive coupling-to-ground effects of the operator�s body that reduce signal strength. Also, when the manpack radio (AN/PRC-150) is operated from the ground, the ground-stake kit should be connected to the radio ground terminal and driven into the earth. This kit is provided with every radio and is designed to provide a low-resistance return path for ground currents. This dramatically improves signal strength and communications efficiency.

Signal strength can be improved even more by connecting �radial� wires to the ground. Radials need to be constructed from insulated wire and connected on one end to the radio ground terminal. Ideally, radials should be one-quarter wavelength long and secured to the earth on their ends by means of nails, stakes, etc. Distribution of the radials should be symmetrical. In operational terms for the brigade, four wires (more if possible) of a practical length should be crossed in the center (X), and the center connected to radio ground. The wires should be spread by 90 degrees and secured (figure below).

Illustration of using ground screen with manpack radio

Manpack operation at halt. Note use of ground screen.

Using ground radials improves vertical antenna performance (gain) by allowing more current to flow in the antenna circuit and by lowering the antenna pattern�s takeoff angle. This produces an increase in ground-wave signal strength on low angles, where it�s the most useful for tactical communications (figure below).

Whip performance illustration

Whip performance.

For vehicular operation, both fixed and OTM, the Army provides the 32-foot AT-1011 antenna. Under operational conditions, it won�t always be possible to use all 32 feet of this antenna and keep it in the vertical position for best ground-wave performance. The antenna should always be kept as vertical as possible and as long as possible under the operational circumstances.

The radiation pattern for a vehicular-mounted vertical whip is essentially omni-directional; however, the mass of the prime mover under the antenna will distort the antenna pattern in the direction of the vehicle mass and provide signal gain in that direction. This can be exploited by pointing the vehicle�s mass in the direction of the weakest station in a net or in the direction of the highest-priority station in a net to improve system operations (figure below).

Illustration of vehicle chassis used as counterpoise

Vehicle chassis as a counterpoise for surface-wave operation.

Half-wave doublet or wire-dipole antenna. THFRS provides two types of wire horizontal dipole antennas for fixed-location operations at beyond-ground-wave distances. These antennas will overcome problems encountered when using vertical antennas in unsuitable situations (figure below). The antennas are the RF-1941 lightweight wire dipole and the AT-1912 dipole with 30-foot mast kit. The AT-1912 is provided only with the 400-watt base-station configuration.

vertical antenna problems

Problems associated with vertical antennas.

A horizontal dipole consists of two one-quarter wavelengths of wire supported at the ends and connected to the radio in the center (top figure below). If the antenna is kept physically one-quarter wavelength or less off the ground at the operating frequency, or is laid on the ground, or is even buried under the ground, the antenna pattern produced is that of an �inverted teardrop� (bottom figure below). The bulk of the energy radiated is on angles between 30 and 90 degrees.

Horizontal dipole antenna

Horizontal dipole antenna.

Inverted teardrop pattern of antenna If a horizontal dipole antenna is one-quarter wavelength or less off the ground at the operating frequency, or is laid on the ground, or is even buried under the ground, the antenna pattern produced is that of an "inverted teardrop." The bulk of the energy radiated is on angles between 30 and 90 degrees.

Since much of the radio signal is directed upward, where it can be reflected back to earth by the ionosphere, this mode of propagation is called the near-vertical-incidence skywave mode. The relationship between antenna height above real electrical-conducting ground and signal gain is shown in the following figure.

Cut half-wavelength dipole

Cut half-wavelength dipole at various heights over perfect and average ground. The bottom row of numbers is in mhz.

Stations will try to elevate dipole antennas to 30 feet and leave them there, since the best average high-angle gain is attained in the NVIS frequency band at this height. The NVIS frequency band is, as a rule, two to four mhz at night and four-eight mhz in the day. Exception: in desert and arctic areas, the ground isn�t very conductive. This means the antenna may perform better if it�s physically lower or even on the ground, since real conducting ground could be many feet below the surface in these areas.

Dipole heights must be adjusted to match actual operating conditions. The basic NVIS inverted-teardrop antenna pattern remains the same for all dipole heights one-quarter wavelength or less. Only the signal strength (gain) will change. Once a radio signal on a frequency that will be reflected is selected and the dipole is at a correct height, the signal will return to earth in an omni-directional pattern with a radius of hundreds of miles.

Note that dipoles can be made directional off their broad sides by putting them close to one-half a wave above ground. However, operators won�t normally erect dipoles this high, so omni-directional communications will be used for most operations.

The NVIS signal after reflection has no holes and no �dead spots� or �skip zones,� since all the energy is coming down from above. This makes NVIS an ideal mode for brigade-and-larger size operations over wide areas and at extended distances. The bottom figure below shows the distance that can be expected by radiating signals on all angles.

Contrast the top figure below, which shows strong high-angle NVIS signal patterns generated by dipoles on all angles above 45 degrees, and the bottom figure below, which shows that energy on all angles above 45 degrees will, when reflected, give a strong radio signal at distances from zero to 300 miles. This is a good match for brigade communications needs such as reachback and tactical-operations-center-to-TOC communications. Using NVIS will also make communications in urban areas easy, since all energy comes from above and won�t be as readily absorbed by urban structures. NVIS using ground-mounted wire-dipole antennas will be the most efficient means of HF communications when stations are located at BLOS (beyond ground-wave) distances from each other.

Pattern of horizontal dipole antenna

Inverted teardrop pattern of a horizontal dipole antenna one-quarter wavelength or less off the ground, laid on the ground or buried in the ground.

Radiation angle compared to radio range Radiation angle vs. range.

OTM NVIS operations. As I previously described, each THFRS vehicular radio is equipped with an AT-1011 32-foot (whip) antenna. When in the vertical position, this antenna does a good job radiating vertically polarized surface-wave HF signals when OTM.

The AT-1011�s length is often too long to be practical under operational conditions. In this case, shorten the AT-1011 by removing antenna sections until you find a practical length for the operational conditions. Shortening the antenna will make it less efficient for both transmitting and receiving, so operators shouldn�t make the antenna less than 10 feet long under most conditions.

The RF-382 antenna coupler will tune a short antenna without a problem, and the omni-directional antenna pattern will remain for short antennas; however, signal strength will be greatly reduced when using very short vertical antennas. This same antenna when �tipped� horizontally, either forward or backward, will also produce an NVIS (dipole) antenna pattern. To facilitate whip antenna �tipping,� antennas are located in a rear corner of either the vehicle or the shelter they�re mounted on.

The antenna base is also provided with a seven-position �whip tilt adaptor� that will allow any length of AT-1011 antenna to be �tipped� into either the forward-facing or rear-facing horizontal position. When the brigade is at a brief halt, the antenna can be tipped backward to form a classic dipole � the AT-1011 whip being one half and the vehicle/shelter forming the dipole antenna�s other half (following figure). When tipped backward, a classic �inverted teardrop� low-height dipole antenna pattern is produced.

Whip-tilt adaptor

Use of a whip-tilt adaptor.

If possible at longer halts, the antenna should be extended past 32 feet by replacing it with the wire from the RF-1941 antenna kit to make an even more efficient antenna. Ideal wire length will be one-quarter wavelength at the operational frequency.

When communicating OTM, the AT-1011 must be �tipped forward� over the vehicle for operational reasons. Again, the antenna should be as long as possible for best efficiency but practically can�t be much longer than the length of the vehicle (usually less than 20 feet). Again, shortening the antenna makes it less efficient, but in this configuration the antenna and vehicle form what engineers call a transmission-line antenna. While this antenna doesn�t have the ideal inverted teardrop NVIS shape that the wire dipole or rear-tipped whip has, it does produce enough energy on the near-vertical angles for NVIS communications. For missions such as motorized reconnaissance, movement-to-contact or convoy control, the bent-forward whip will be the antenna of choice for operations.

Antenna-location considerations. The brigade is a tactical fighting organization and, when engaged in combat operations, won�t always be able to locate its fixed and mobile radio assets at technically ideal positions for communications operations. Brigade HF communications planners should, however, attempt to comply with as many of the following criteria as possible to gain the best technical advantage for the tactical situation:

 Use ground radials and ground stakes under vertical antennas to improve antenna efficiency and lower takeoff angles for better ground-wave communications;
 Place vertical antennas on higher spots if possible to enhance ground-wave communications;
Place all antennas above reasonably smooth earth if possible to reduce antenna pattern discontinuities and distortion due to ground reflections;
Avoid placing vertical antennas behind metal fencing that will shield ground-wave signals;
Avoid placing vertical antennas near vertical conducting structures such as masts, lightpoles, trees or metal buildings. Antennas need to be at distances of at least one wavelength or more to eliminate major pattern distortions and antenna-impedance changes caused by induced currents and reflections; and
Separate antennas as far as practical to reduce interference effects between radio and antenna systems.

Remember that wire dipoles and tipped whips on vehicles can be placed in defilade since they radiate signals on high angles, while vertical whips will have their signals greatly reduced if they are in covered positions.

By following the concepts I discuss here, tactical communicators can provide reliable, gap-free, direct, wide-area communications BCTs need for operations in all types of environments and conditions. While HF radio will certainly not be the only type of tactical communications the BCT employs, it has been shown over and over again that HF will succeed under many conditions that will cause other means to fail. That�s why it will be so valuable to the success or failure of the BCTs in combat.

Mr. Fiedler � a retired Signal Corps lieutenant colonel � is an engineer and project director at the project manager for tactical-radio communications systems, Fort Monmouth, N.J. Past assignments include service with Army avionics, electronic warfare, combat-surveillance and target-acquisition laboratories, Army Communications Systems Agency, PM for mobile-subscriber equipment, PM-SINCGARS and PM for All-Source Analysis System. He�s also served as assistant PM, field-office chief and director of integration for the Joint Tactical Fusion Program, a field-operating agency of the deputy chief of staff for operations. Fiedler has served in Army, Army Reserve and Army National Guard Signal, infantry and armor units and as a DA civilian engineer since 1971. He holds degrees in both physics and engineering and a master�s degree in industrial management. He is the author of many articles in the fields of combat communications and electronic warfare.

Acronym QuickScan
ALE � automatic link establishment
AO � area of operations
BCT � brigade combat team
BLOS � beyond line of sight
CPS � cycles per second
FOT � frequency of optimum traffic
HF � high frequency
LOS � line of sight
LUF � lowest useable frequency
Mhz � megahertz
MUF � maximum useable frequency
NVIS � near-vertical-incidence skywave
OTM � on the move
PM �project manager
RF � radio frequency
SINCGARS � Single-Channel Ground and Airborne Radio System
THFRS � Transformation High-Frequency Radio System
TOC � tactical-operations center
VSWR � voltage standing-wave ratio

dividing rule

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04/04/12

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