Harris AN-PRC150 User Guide

OPERATIONAL CONCEPT AND

PROCEDURES FOR HF RADIO IN

THE BRIGADE COMBAT TEAMS

PREPARED BY DAVID M

FIEDLER 732-532-3760

ELECTRONICS ENGINEER

OPM -TRCS

FORT MONMOUTH NJ 07703

TRANSFORMATION HIGH FREQUENCY (HF) RADIO SYSTEM (THFRS) -

AN/PRC-150 FAMILY STANDARD OPERATING PROCEDURE (SOP) AND

OPERATIONS CONCEPT (OC) FOR THE BRIGADE COMBAT TEAMS

PURPOSE: This SOP/OC is intended to describe the planning factors and operational procedures
required to successfully use THFRS radios in the Brigade Combat Team (BCT).

WHY HF RADIO FOR THE BCT: HF radio (radio signals in the 1.6-3OMhz frequency spectrum)
have the following characteristics that makes it an ideal communications system to support the fast
moving wide area operations that the Brigade Combat Team will participate in:

I - HF signals travel longer distances over the ground than the higher frequency VHF
(SINCGARS) or UHF (EPLRS/NTDR) signals do because they are less affected by factors such as
terrain or vegetation.

2 - HF signals can be reflected off the ionosphere (a layer of charged gases surrounding the earth
at high altitudes) in a way that will cover beyond line of sight (BLOS) areas at distances out to 400 miles
without gaps in communications coverage.

3 - HF signals can be reflected off the ionosphere to cover distances of many thousands of miles
for "reach back" communications.

4 - HF signals do NOT require the use of either SATCOM or retransmission (RETRANS) assets.

5 - HF equipment provided to the Brigade can be used either fixed station or on the move (OTM).

6 - HF systems can be engineered to operate independent of intervening terrain or mamnade
obstructions.

HF (2-3OMhz) 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 for this SOP/OC 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. Figure 1 shows
how radio energy (signal) 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 I/d
squared). 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
radios 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 of these effects will be used to provide gap free tactical HF
radio communications throughout the Brigades area of operations and back to its sustaining base. It is
important to recognize that how the radio antenna shapes the signal pattern and the system operating
radio frequency(s) are the two most critical factors in assuring HF communications for the Brigade.

POSSIBLE TRANSMISSION PATHS WITHIN THE BRIGADE OPERATIONAL AREA: Fig
2 shows possible radio paths between two stations located in the Brigade area of operations. It is
assumed that most combat units in the Brigade will be located within a maximum distance of 400 miles
from each other for purposes of this SOP/OC. Circuits of greater distances (reach back) will be covered
under other sections of this SOP/OC. Fig 2 shows three possible low angle radio paths located along or
near the surface of the earth. These paths are called ground-wave paths because they are close to the
earth's surface or in contact with it. They consist of the (1) direct 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 (LOS) mode of propagation. Maximum LOS distance
depends upon the height of the antenna 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 6-8 miles before the curve of the earth blocks the sig nals. Direct wave
communications can go much further if stations are located high on hilltops with no intervening
obstructions so control of high ground and antenna height is important when using direct wave
communications. (2) The ground reflected path like the direct path travels through the atmosphere but due
to the lower "take off' angles from the transmitting antenna, the signal energy is reflected off the earth
while traveling from the transmitting antenna to the receiving antenna. Depending upon 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 upon 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 will usually not
be the predominate mode of communications in the BCT. The (3) surface wave path is the transmitted
radio energy that travels along the boundary between the atmosphere and the earth's surface and is in
actual contact with the earth's surface. The surface wave is greatly affected by the electrical conductivity
of the earth in the path of propagation. With a good conductor such as seawater surface wave
communications out to 1 00+ 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 or mountainous 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 utilize surface wave signals. Space wave communicatio ns will
predominate only when communicating from high ground to other high ground locations along the line of
sight (LOS). Vertical monopole (whip) man-pack and vehicle antennas of various lengths are the
antennas provided to produce the low take off angle energy needed to generate ground wave signals.
Figure 3 shows the antenna energy pattern of the vertical monopole (whip) antenna. Note that the signal
is along the surface of the earth and on the lower angles. There is much less energy on the higher angles
and none directly overhead (vertical angles). The pattern resembles a doughnut so operationally, it can be
very difficult to communicate with aircraft that are directly overhead while you can talk to aircraft many
miles away that are receiving low angle energy from a vertical 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 for ces 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 the gas ions tend to "bunch" in distinct "layers" of ions at heights of between 30 and 300 miles shown
in fig 4. These charged areas will reflect radio signals back to earth if they strike the ionosphere at
particular angles using particular frequency bands. Radio engineers have labeled these layers the D, E, F I
and F2 layers (see fig 4). 3 factors determine whether a radio signal will be reflected back to earth and
can be used by Brigade THFRS communications systems. They are (1) the higher the radio frequency
the more likely the signal will penetrate the io nosphere rather than be reflected by it, (2) the current ion
density determined by the amount of sun light (time of day, season, solar activity) at the time
communications is desired, and (3) the angle at which the radio wave contacts the ionosphere. See figure
5 for details. Note - that at any time of the day, year, or solar activity (sunspot) cycle there is always
available a band of radio frequencies that can be reflected off the ionosphere and will support HF
communications. The Automatic Link Establishment (ALE) feature of THFRS will find these frequencies
for the operator from the 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 wave front contacts the
reflecting layer is determined by the radios antenna system. Low angles of radiation are produced by the
OE-5 05 and AT- IO 1 1 vertical whips and high angle radiation is produced by bending the whips into the
horizontal position with the whip tilt adaptor or by using the RF- 1 912 or RF- 1 941 wire dipole antennas
30 feet OR LESS above ground.

MAXIMUM USEABLE FREOUENCY (MUF)- LOWEST USEABLE FREOUENCY (LUF). '
Each layer of the ionosphere has a frequency that is the highest that the layer will reflect. The
exact frequency is determined by the amount of ions in the layer. As seen in fig 5. the lower
frequencies are reflected by the lower layers while the higher frequencies penetrate the lower
layers and are reflected back by the higher layers. To cover the largest tactical area of

operations possible the highest frequency that will reflect should be used since the higher the reflecting
layer the wider the area covered by the reflection (see fig 5.). Since the ionosphere is always changing a
general rule when in manual operation is to select a frequency 15% lower than the actual MUF to avoid
problems. This frequency is called the frequency of optimum traffic (FOT). 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 utilize higher frequencies for
communications because they will be reflected. The ALE mode of THFRS will automatically prevent
signals with a frequency above the @ from being selected for operations. A-LE will select the best radio
frequency for communications on a continuous basis if 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 man-made sources. Noise on a particular system depends mainly on locatio n and
season. For each situation there is a frequency (LUF) below which there is to

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. LLTF is defined for this SOP as

the lowest frequency at which a 90% probability of communications exists. The ALE, MODEMS, and VOCODER
features of the THFRS are designed to make the LUF as low as possible by being able to operate 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 fig.7. Note the range of frequencies between the @ and the LUF over the entire day.
Under almost every circumstance there is a range of HF radio frequencies that will be suitable for Brigade
communications. It is the responsibility of the operator and the system manager to obtain frequency assignments in
this range for operations. To aid in frequency selection sky wave and ground wave predictions and prediction
software are available through frequency management channels. It is the responsibility of the Brigade S-6 frequency
manager to predict HF radio frequency requirements, obtain authorized frequencies between the predicted @ and
LUF, and provide them to the THFRS 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. In order to select the best antenna for a particular
Brigade operation, the following concepts must be understood by the THFRS operator and system manager.

Wavelength and frequency - For best radio perfon-nance, there is 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 I cycle of its motion. In order to calculate this distance (in meters) the
speed of light (in meters) has to be divided by the operational frequency in cycles per second (cps). After
simplifying the math wavelength (in meters) is equal to 300 divided by the frequency in Mega-hertz (millions of cycles
per second) As an example, the wavelength of a 3 Mhz HF signal is 300 divided by 3 (300/3) or 100 meters. This
means that in the time it takes to complete I cycle at 3 Mhz the signal has traveled I 00 meters. Knowing how to
calculate wavelength is important because signal strength depends upon the length of the antenna and the amount
of current flowing through it. For maximum current (signal) at a given frequency, the antenna needs to be !/2
wavelength or, multiples of V2 wavelength long.

Resonance - The strength of a signal radiated from an electrical conductor that has a radio frequency (RF) current
flowing depends on the length of the conductor and the amount of the current. For a given frequency, maximum
current flows and maximum signal is produced when the conductor (antenna) is !/2 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 (AT1912, RF-1941) provided are constructed using
lengths that are close to resonance and are therefore very efficient. Mobile antenna lengths can range from less than
10 feet to as much as 32 feet. These antennas are physically to short to be resonant. In order to make the short

antennas radiate as strong signal as possible, antenna couplers such as the RF-382 or RF5830 are
provided. Couplers allow RF current to flow to the short antenna and dissipate energy that is 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 (VSWR). It is important to keep this ratio low
(less than 2: 1) for highest efficiency. High VSWR will not physically damage the THFRS equipment.
Antennas whose length is close to resonance do not 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 the power at the antenna. This is acceptable for mobile operations or when at the
brief halt. It is Brigade policy that whenever possible more efficient ground mounted (resonant) wire
antennas will be used. 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 to the antenna via the cables provided even though
efficiency is reduced slightly.

Polarization - polarization is the directional relationship of radio energy coming from an antenna to the
surface of the earth. 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 electric field is short­circuited by the earth. A vertically polarized signal does not lose strength nearly as quickly because it does
not contact the earth as much. In the Brigade, ground-wave communications will be the primary mode of
short distance (0-20miles) communications. Man-pack, 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) sky-wave communications. These
antennas provide the high take off angles necessary for beyond line of sight 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-6's 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
the electrical properties of seawater will greatly reduce the signal strength of a horizontally polarized
surface wave signal. Figure 7 shows the concept of vertical and horizontal polarization.

Vertical (whip) antennas. - Ground wave HF communications are most effective when using vertical
polarization over a good conductive ground. BCT man pack radios are provided the I 0-foot long OE-5 05
antenna and vehicular radios are provided the 32-foot long AT- IO 1 1 antenna. Whip antennas are most
efficient when they are between ¼ and 5/8 wavelength long at the lowest operating frequency. At HF
frequencies normally used in the Brigade the whips are far to short for efficient operation. Tuning devices
(such as the RF382 antenna coupler) are provided to electrically match a physically short or long antenna
to the radio and the transmission line. The Brigade will use the physically longest antenna possible under
the operational conditions in order to achieve best performance. For

Example, the 10-foot OE-505 man-pack 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- 1 941 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.
In order to further improve antenna efficiency and increase signal strength on the lower (surface wave)
radiation angles, radios in man-pack 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 are
not provided but can be locally fabricated from computer ribbon cable, communications wire, or ground
strap braid. "Tails" should be as long as possible but not interfere with the carrying of the radio. The
man -pack "tail" concept is shown in fig 8. Along with height orientation is also very important when
operating in the man-pack 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 (see Fig 8). It is also
important to operate from areas that do not have energy robbing obstructions such as trees and
buildings when possible (see fig 9). When ever possible man-packed radios should be removed from
the operators back and operated from the ground. This will reduce the capacitive coupling to ground
effects of the operators body that reduces signal strength. In addition, when the man-pack 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 1/4wavelength long and secured to the earth on the ends by
means of nails, stakes etc. Distribution of the radials should be symmetrical. In operational terms for
the brigade, 4 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 (see fig 10).
Using ground radials improves vertical antenna performance (gain) by allowing more current to flow in
the antenna circuit and by lowering the takeoff angle of the antenna pattern. This produces an increase
in ground-wave signal strength on the low angles where it is the most useful for tactical communications
(see fig I 1). For vehicular operation both fixed and on the move the 32-foot AT-101 1 antenna is
provided. Under operational conditions it will not 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 onmi-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 mass of the vehicle 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 (see fig 12).

Half-wave Doublet or Wire Dipole antenna - The THFRS provides two types of wire horizontal dipole
antennas for fixed location operations at beyond ground wave distances.