High-Frequency Communications

1-13 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.