NAVY FREQUENCY BAND USE

Rapid growth in the quantity and complexity of communications equipment and increased worldwide international requirements for radio frequencies have placed large demands upon the radio-frequency spectrum. These demands include military and civilian applications such as communications, location and ranging, identification, standard time and frequency transmission, and industrial, medical, and other scientific uses.

The allocation, assignment, and protection of all frequencies used by any component of the Navy are the responsibility of Commander Naval Telecommunications Command (COMNAVTELCOM). Table 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 more difficult.

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 submarines impractical.

Very-Low-Frequency Communications

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 single-channel rtty.

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 systems.

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.

Low-Frequency Communications

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 reliable.

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.

Medium-Frequency Communications

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).

High-Frequency Communications

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 approaching saturation.

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 the antenna.

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 stage.

Q.9 The majority of vlf transmitters are used for what purpose?
Q.10 Today the Navy uses lf communications as a segment of what operational system?
Q.11 Why does the Navy only use the upper and lower ends of the mf band?
Q.12 What are the four general types of communications services in the hf band?
Q.13 A message transmitted on several frequencies at the same time is an example of what type of transmission?
Q.14 Physically separating receive antennas is an example of what technique?
Q.15 When using frequencies above 30 megahertz, you are normally limited to using what range?