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 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
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.