NEETS Module 17 − Radio−Frequency Communications Principles
4−1− to 4−10,
1-1 shows the radio-frequency spectrum broken down into nine bands used by the
military. Propagation of radio waves varies widely at different frequencies. Frequencies
and equipment are chosen to meet the communications application desired. We will
discuss the radio-frequency spectrum in the following paragraphs.
Table 1-1. - Radio-Frequency Spectrum
Extremely Low-Frequency Communications
The purpose of the Extremely Low-Frequency (elf) communications system is to
send short "phonetic letter spelled out" (PLSO) messages from operating authorities
in the continental United States (CONUS) to submarines operating at normal mission
speeds and depths. Elf has the ability to penetrate ocean depths to several hundred
feet with little signal loss. This ability allows submarines to be operated well
below the immediate surface and enhances submarine survivability by making detection
This is a one-way communications system from the operating authority to submarines
at sea. The large size of elf transmitters and antennas makes elf transmission from
The communications commitments of the Navy now cover the face of the earth. New
sea frontiers to the north have opened a four-million-square-mile, ice-covered ocean
of strategic importance. Our Navy must maintain control of the operating forces
in an ever expanding coverage area. This additional area requires changes in communications
capacity, range, and reliability. Additional needs have been particularly great
in the North Atlantic and the newly opened Arctic Ocean. High-frequency circuits
are too unreliable in these areas because of local atmospheric disturbances.
Very-Low-Frequency (VLF) transmissions provide a highly reliable path for communications
in these northern latitudes as well as over and under all oceans and seas of the
world. At present, practically all Navy VLF transmitters are used for fleet communications
or navigation. The VLF transmission is normally considered a broadcast, that is,
one-way transmission, no reply required. The VLF transmitter normally transmits
VLF is currently used for communications to large numbers of satellites and as
a backup to shortwave communications blacked out by nuclear activity. Our Navy depends
on VLF for crucial communications during hostilities.
Secondary applications of the VLF range include worldwide transmission of standard
frequency and time signals. Standard frequency and time signals with high accuracy
over long distances have become increasingly important in many fields of science.
It is essential for tracking space vehicles, worldwide clock synchronization and
oscillator calibration, international comparisons of atomic frequency standards,
radio navigational aids, astronomy, national standardizing laboratories, and communications
A VLF broadcast of standard time and frequency signals has more than adequate
precision for the operation of synchronous cryptographic devices, decoding devices,
and single-sideband transmissions.
The Low-Frequency (lf) band occupies only a very small part of the radio-frequency
spectrum. This small band of frequencies has been used for communications since
the advent of radio.
Low-frequency transmitting installations are characterized by their large physical
size and by their high construction and maintenance costs. Another disadvantage
is low-frequency signal reception being seriously hampered by atmospheric noise,
particularly at low geographical latitudes. Over the years, propagation factors
peculiar to the low-frequency band have resulted in their continued use for radio
communications. Low-frequency waves are not so seriously affected during periods
of ionospheric disturbance when communications at the high frequencies are disrupted.
Because of this, the Navy has a particular interest in the application of low frequencies
at northern latitudes.
The Navy's requirement to provide the best possible communications to the fleet
requires operation on all frequency bands. Constant research is being done to improve
existing capabilities and to use new systems and developments as they become operationally
In the past, the fleet broadcast system provided ships at sea with low-frequency
communications via CW telegraph transmissions. As technology advanced, the system
was converted to single-channel radio teletypewriter transmission. Today If communications
is used to provide eight channels of frequency- division multiplex RTTY traffic
on each transmission of the fleet multichannel broadcast system.
The MEDIUM-Frequency (mf) band of the radio-frequency spectrum includes the international
distress frequencies (500 kilohertz and approximately 484 kilohertz). Some ships
have mf equipment. If desired the distress frequencies may be monitored. When this
is done the transmitter usually is kept in the standby position. Ashore, the mf
receiver and transmitter equipment configuration is usually affiliated with search
and rescue organizations, which are generally located near the coast.
Only the upper and lower ends of the mf band have naval use because of the commercial
broadcast band (AM) extending from 535 to 1,605 kilohertz. Frequencies in the lower
portion of the mf band (300 to 500 kilohertz) are used primarily for ground-wave
transmission for moderately long distances over water and for moderate to short
distances over land. Transmission in the upper mf band is generally limited to short-haul
communications (400 miles or less).
The Navy began using High FREQUENCIES for radio communications around World War
I when only a few communications systems were operated on frequencies near 3 megahertz.
When we look at the extensive present-day use of high frequencies for long-distance
communications, the fact that those Navy systems were intended for very short-range
communications of a few miles seems curious. The general belief at the time was
that frequencies above 1.5 megahertz were useless for communications purposes.
One of the prominent features of high-frequency, long-distance communications
is the variable nature of the propagation medium. (You studied this in NEETS, Module
10, Introduction to Wave Propagation, Transmission Lines, and Antennas). Successful
transmission of HF signals over a long distance is dependent upon refraction of
radio waves by layers of the ionosphere. The height and density of these layers
is formed mainly by ultraviolet radiation from the sun. They vary significantly
with the time of day, season of the year, and the eleven-year cycle of sunspot activity.
Because of these variations, you must generally use more than a single frequency,
sometimes up to four or five, to maintain communications on a circuit.
In spite of the difficulties we encounter with HF propagation, the economic and
technical advantages of using high frequencies have led to rapid expansion of the
use of the HF band. Because the number of users has increased, the HF spectrum is
The HF band is shared by many domestic and foreign users, and only portions scattered
throughout the band are allocated to the military services. Like other agencies,
Navy requirements have grown; the capacity of the Navy's assigned portion of the
HF spectrum has become severely taxed. The use of single- sideband equipment and
the application of independent sideband techniques have increased the capacity,
but not enough to catch up with the demand. Some predict that satellite communications
will eventually relieve congestion in the HF band and that, for some types of service,
it will replace HF for long-distance communications. We will present more information
to you concerning satellite communications in chapter 3. Even with new technology
the HF spectrum most likely will continue to be in high demand for some time.
Naval communications within the HF band can be grouped into four general types
of services: point- to-point, ship-to-shore, ground-to-air, and fleet broadcast.
All but the fleet broadcast are normally operated with two-way communications. Some
of these services involve ships and aircraft that present special problems because
of their physical characteristics and mobility. Generally, the less than optimum
HF performance of this shipboard equipment is at least partially offset by powerful
transmitters and sensitive receiving systems at the shore terminals.
Point-to-Point. - Point-to-point systems are established to
communicate over long-distance trunks or links between fixed terminals. a trunk
is normally a message circuit between two points that are both switching centers
or individual message distribution points. a link is a transmitter-receiver system
connecting two locations.
Generally, enough real estate is acquired at the terminals to permit the use
of large, high-gain antennas aimed at opposite terminals of each link. This increases
the effective radiated power and the sensitivity of the receiving system; it also
reduces susceptibility of a circuit to interference.
With the path length and direction fixed, other propagation factors are simplified
and highly reliable communications can be achieved.
Ship-to-Shore. - This application of the HF band is more difficult
than point-to-point since the ship is moving and constantly changing its position.
In ship-to-shore the path length and direction are variable. Aboard ship, limited
space and other restrictions prohibit installation of large, efficient HF antennas.
Because of the mobility of ships, shipboard antennas are designed to be as nearly
omnidirectional as possible.
Our problems are not as severe at the shore terminal where we have sufficient
space for efficient omnidirectional antennas or arrays designed for coverage of
large areas of the earth. At shore stations, rotatable, high-gain antennas or fixed,
point-to-point antennas are used. For example, a rhombic antenna ashore may work
well for long-haul, ship-to-shore communications when the ship is within range of
Several frequencies are usually assigned for each circuit. Therefore, a frequency
can be selected that best matches the propagation path conditions between the shore
terminal and the ship.
Ground-to-Air. - The use of HF radio for ground-to-air communications
is similar to ship-to- shore. The only exception is an aircraft moves more rapidly
than a ship. All major circuit improvements must be made at the ground station.
For example, higher powered transmitters, lower noise receivers, and more efficient
antennas must be used on the ground.
Fleet Broadcasts. - As the name implies, this service involves
broadcast area coverage from shore-based transmitters to ships at sea. Messages
to be sent to ships are delivered by various means to the proper broadcast station.
They are then broadcast for shipboard reception. To overcome propagation problems,
naval communicators send the messages on several frequencies at once. This is known
as frequency-diversity transmission. This type of transmission allows the ship to
choose the best frequency for reception. Space-diversity with physically separated
receive antennas also helps to overcome this problem.
Very-High-Frequency and Above Communications
Frequencies above 30 megahertz are not normally refracted by the atmosphere and
ground-wave range is minimal. This normally limits our use of this frequency spectrum
to line of sight. The exception to this is increased range through the use of tropospheric
scatter techniques. Some communications using VHF and above frequencies use a technique
called forward propagation by tropospheric scatter (fpts). This method will be discussed
in more detail in chapter 5.
Certain atmospheric and ionospheric conditions can also cause the normal line-of-sight
range to be extended. Frequencies at the lower end of this band are capable of overcoming
the shielding effects of hills and structures to some degree; but as the frequency
is increased, the problem becomes more pronounced. Reception is notably free from
atmospheric and man-made static. (The Very-High- Frequency (VHF) and ULTRAHigh-Frequency
(UHF) bands are known as line-of-sight transmission bands.) Because this is line-of-sight
communications, the transmitting antenna is in a direct line with the receiving
antenna and not over the horizon. The line-of-sight characteristic makes the VHF
band ideal for amphibious operations (beach landing from sea craft) and the UHF
well suited for tactical voice transmissions (maneuvering of ships traveling together).
The SUPERHigh-Frequency (SHF) band is used for radar and satellite communications,
whereas the Extremely High-Frequency (EHF) band is used only in the experimental
Q9. he majority of VLF transmitters are used for what purpose?
Q10. Today the Navy uses lf communications as a segment of what operational
Q11. Why does the Navy only use the upper and lower ends of the mf
Q12. What are the four general types of communications services in
the HF band?
Q13. a message transmitted on several frequencies at the same time
is an example of what type of transmission?
Q14. Physically separating receive antennas is an example of what
Q15. When using frequencies above 30 megahertz, you are normally
limited to using what range?
Now that we have learned the Navy's fundamental use of the various frequency
bands, we should look at the types of communications links and their modes of operation.
The Navy uses many modes of operation; the type used is based upon the function
of the circuit or network. These modes (or functions) are combined to form a communications
link. We will also discuss some of the actual networks the Navy uses on a daily
A complex of links forms a major communications system. The naval communications
system is broken down into strategic and tactical groups.
Strategic communications are generally world-wide in nature. They are operated
on a common-user (Navy, Army, Department of Defense, and so on) or special-purpose
basis. a strategic system may be confined within a specified area or limited to
a specific type of traffic, but the configuration is designed so that combined operations
with other strategic systems are possible. As an example, we will look at the automatic
voice network, automatic digital network, and the defense special security communications
system later in this chapter.
Tactical communications are usually limited to a specific area of operations
and are used to direct or report the movement of specific forces. Some tactical
networks are used only for operational traffic; others may be used for operational
and administrative traffic. For instance, the task force, task-group, and air-control
networks are ordinarily used for operational traffic. Ship-to-shore networks and
broadcast networks serve both types of traffic.
Modes of Operation
Communications links have numerous modes of operation. In our discussion, a mode
of operation is identified as a link or path between two or more points that is
capable of providing one or more channels for the transmission of intelligence.
Let's take a look at the five most common modes of operation.
SIMPLEX. - The simplex (SPLX) mode uses a single channel or
frequency to exchange information between two or more terminals. Communications
is in one direction only.
Half DUPLEX. - The half-duplex (HDX) mode has one-way flow of
information between terminals. Technical arrangements often permit transmission
in either direction, but not simultaneously. This term must be qualified to show
s/o (send only), r/o (receive only), or s/r (send or receive).
SEMIDUPLEX. - The semiduplex (SDX) uses an arrangement of equipment
where one terminal is simplex configured and the other uses two channels or frequencies
in full duplex. a clarifying example is
a ship in a simplex mode terminated full duplex with a shore station. The ship
may send or receive but not do both at the same time.
FULL DUPLEX. - The full-duplex (FDX) mode is a method of operation
in which telecommunications between stations takes place simultaneously in both
directions using two separate frequencies. In other words, a ship may send and receive
different messages at the same time. The term "full duplex" is synonymous with "duplex."
Broadcast. - Broadcast (BC) is the type of operation in which
one station transmits information on one or more channels directed to more than
one station and/or unit. The broadcast system has no provision for receipt or reply;
however, special arrangements may require the receiving station to reply or receipt
for the message at a later time by other means. Broadcasts are the primary means
of delivering messages to the fleet. Since Navy units copying broadcasts are not
required to receipt for messages received, they can maintain radio silence while
still receiving essential messages.
Message traffic is normally sent to the fleet by three methods: broadcast, intercept,
and receipt. The first two are "do not answer" methods; the third, as its name implies,
requires a receipt from the addressee (addee) for each message. Broadcast and intercept
methods allow the fleet to preserve radio silence, which is a great advantage from
the standpoint of security. By the intercept method, a shore radio station transmits
messages to another shore station that repeats them back. Ships intercept and copy
all of this message traffic.
Broadcast is preferable to intercept chiefly because it is faster. It is the
method by which nearly all fleet traffic is handled. It uses radiotelegraph, radiotelephone,
radio teletypewriter, and facsimile.
There is some similarity between civilian and naval broadcasts. Just as commercial
stations in the broadcast band transmit programs to radio receivers in the homes
in their communities, Navy communications stations broadcast messages to fleet units
in their particular geographic areas. The resemblance between Navy and commercial
stations ceases there. Information broadcast by naval communications stations is
contained in chronologically numbered messages addressed to the ships. The messages
are copied by the fleet units, which check the serial numbers to ensure they have
a complete file. This checks and balances system ensures the ship has not missed
any of the broadcast message traffic.
Fleet broadcasts follow regular schedules. Messages are placed on the schedules
in order of precedence. If a message of higher precedence is given to a transmitter
station while a lower precedence message is being transmitted, the latter message
may be interrupted to transmit the message of higher precedence. All ships copy
all messages appearing on the broadcast schedule they are guarding.
Messages are normally transmitted on several frequencies to make sure they are
received. This gives the receiving station the choice of frequency selection when
considering time of day and atmospheric conditions for best reception.
Q16. The naval communications system is made up of what two groups
Q17. What are the five most prominent communications modes of operation?
The defense communications system (DCS) is composed of all worldwide, long-haul,
government- owned and leased point-to-point circuits, trunks, terminals, switching
centers, control facilities, and tributaries of military departments and other defense
activities. In essence the DCS combines into a single system all the elements that
make up the naval communications system and the Army and Air Force equivalent.
The switched networks discussed in this section, automatic voice network, automatic
secure voice communications, automatic digital network, and the defense special
security communications system, are part of the DCS and are managed by the Defense
Communications Agency (DCA). You should not confuse these DCS networks with the
HICOM (high-command communications network) and NORATS (Navy operational radio and
telephone switchboard) networks. We will discuss both of these Navy-only networks
later in this chapter.
Automatic Voice Network (AUTOVON)
The DCS AUTOVON offers rapid, direct interconnection of DOD and certain other
government installations through worldwide telephone exchanges. AUTOVON is a worldwide,
general-purpose direct dialing telephone system. The goal of the AUTOVON system
is to complete connections between two points anywhere in the world in about two
seconds and to complete regular connections with push-button speed.
The AUTOVON system is made up of several installations comparable in function
to commercial telephone exchanges. An installation is referred to as an AUTOVON
switch, or simply switch. Within individual areas we have local command, control,
and administrative voice communications systems. These systems connect into the
worldwide AUTOVON through manually operated telephone switchboards or automatic
dial exchanges by using direct in and out dialing.
Normal AUTOVON service allows your station to call other stations on a worldwide
basis for day- to-day communications by using the telephone.
Automatic Secure Voice Communications (AUTOSEVOCOM)
Another close relative to the AUTOVON system is the AUTOSEVOCOM a worldwide,
switched telephone network. It provides authorized users with a means for exchanging
classified information over communications security (COMSEC) circuitry or over other
approved circuitry. The system consists of both manual and automated networks within
a single system.
For subscribers to the AUTOSEVOCOM network, telephone directories containing
subscriber listings, general instructions for placing calls, and trouble-reporting
procedures are provided.
Automatic Digital Network (AUTODIN)
The DCS AUTODIN is a fully automatic, digital system. The system converts word
messages to digital form for transmission.
AUTODIN is used to furnish instantaneous, error-free, and secure communications
around the world to several thousand directly connected subscriber terminals. Daily
capacity of the system is about five- million average-length messages.
AUTODIN switching centers are interconnected through a network of high-frequency
radio channels, submarine cables, microwave and tropospheric channels, and a variety
of wire lines.
The whole concept of AUTODIN is to reduce manual handling of messages to a minimum
by the use of automated equipment. This system has reduced message delivery times
and delay anywhere in the world to a matter of seconds rather than minutes or hours.
Defense Special Security Communications System (DSSCS)
The defense special security communications system (DSSCS) was established for
the purpose of integrating the critical intelligence communications (CRITICOMM)
and the special intelligence communications (SPINTCOMM) networks into a single automated
communications network. In effect, the integration of DSSCS subscribers into AUTODIN
provides two separate systems within AUTODIN- one system for special intelligence
(SI) message traffic and the other for the AUTODIN regular message traffic.
Some networks are used by the Navy only. As mentioned previously, these are the
high command communications network (HICOM) and the Navy operational radio and telephone
switchboard (NORATS) networks. Let's look at some of their functions and purposes.
High Command Communications Network (HICOM)
The HICOM network provides a voice link between the Chief of Naval Operations
(CNO) and all subordinate commands ashore, afloat, and airborne. CNO is the master
control station and each fleet commander in chief has an area network control station.
All naval communications stations are members.
In cases where a fleet unit is suffering communications difficulties with normal
channels, HICOM is used on a not-to-interfere basis to restore communications. All
naval communications stations are required to guard HICOM for their respective area
networks and use this system.
Navy Operational Radio and Telephone Switchboard (NORATS)
The NORATS meets our need for a connection between Navy tactical voice systems
of the operating forces and the various fixed telephone services ashore. This system
extends tactical voice to shore-based operational commands. NORATS provides a connecting
point in the fleet center of each communication station. This point allows us to
connect or patch all ship-to-shore voice circuits and designated local shore telephone
systems and extensions. a combined HICOM/NORATS console exists at many naval communications
Q18. What four switched networks are part of the defense communications
Q19. What two elements support only designated Navy requirements?
Now that you have completed this chapter, a short review of what you have learned
is in order. The following summary will refresh your memory of radio-frequency communications
Telecommunications refers to transmission, emission, or reception
of signs, signals, writings, images, or sounds. This is done by visual, oral, wire,
radio, or other electromagnetic means.
Radio Communications is the term describing teletypewriter,
voice, telegraphic, and facsimile communications.
System is a combination of sets, units, assemblies, subassemblies,
and parts joined together to form a specific operational function or several functions.
Set is a unit or units and the assemblies, subassemblies, and
parts connected or associated together to perform a specific function.
Group is a collection of units, assemblies, subassemblies, and
parts. It is a subdivision of a set or system but is not capable of performing a
complete operational function.
Unit is an assembly or any combination of parts, subassemblies,
and assemblies mounted together. Normally capable of independent operation.
Assembly is a number of parts or subassemblies, or any combination
thereof, joined together to perform a specific function.
Subassembly consists of two or more parts that form a portion
of an assembly or a unit.
Part is one component or two or more components joined together.
It is not normally subject to disassembly without destruction.
Extremely Low Frequency is the band of frequencies up to 300
Very Low Frequency is the band of frequencies from 3 kilohertz
to 30 kilohertz.
Low Frequency is the band of frequencies from 30 kilohertz to
MEDIUM Frequency is the band of frequencies from 300 kilohertz
to 3 megahertz.
High Frequency is the band of frequencies from 3 megahertz to
Very High Frequency is the band of frequencies from 30 megahertz
to 300 megahertz.
ULTRA High Frequency is the band of frequencies from 300 megahertz
to 3 gigahertz.
SUPER High Frequency is the band of frequencies from 3 gigahertz
to 30 gigahertz.
Extremely High Frequency is the band of frequencies from 30
gigahertz to 300 gigahertz.
Answers to Questions Q1. Through Q19.
A1. Radio and wire.
A3. It is direct, convenient and easy to use.
A4. Static, enemy interference or a high local noise level.
A5. High speed automatic communications across ocean areas.
A6. The process used to transmit photographs, charts and other graphic
A7. Set, group, unit, assembly, subassembly, and part.
A9. Fleet communications or navigation.
A10. Fleet Multichannel Broadcast System.
A11. Due to the commercial broadcast (AM) band.
A12. Point-to-point, ship-to-shore, ground-to-air, and fleet broadcast.
A15. Line of sight.
A16. Strategic and tactical.
A17. Simplex, half-duplex, semiduplex, duplex, and broadcast.
A18. AUTOVON, AUTOSEVOCOM, AUTODIN, and DSSCS.
A19. HICOM and NORATS.
Posted August 3, 2021
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