Magazine article UNESCO Courier

From Short Wave to Satellite

Magazine article UNESCO Courier

From Short Wave to Satellite

Article excerpt

Do satellite and digital radio spell the end of short-wave radio?

For almost seventy years, short-wave transmission has been the most efficient method of delivering programmes across long distances. Today, however, the development of new technologies such as satellite transmission, digital audio broadcasting and interactive two-way cable is forcing many international broadcasters to reconsider the choice of short wave as their principal medium for the future

The phenomenon known as "short-wave radio" is actually the result of a combination of transmission technology and the electromagnetic properties of the earth's atmosphere Just as the waves of the ocean are caused by the vibration of water molecules, radio waves result from the vibration of electromagnetic particles. The rate at which they vibrate is much faster than that of ocean waves - several thousand times per second, or even several hundred thousand times per second.

The number of vibrations (or cycles) per second is the frequency of the wave. The cycle per second is named the hertz (abbreviated Hz) after the German physicist Heinrich Hertz. The higher the frequency of a radio wave, the shorter its wavelength - the distance between two consecutive cycles. Radio waves of different lengths are used for different purposes, because they have distinct physical properties. Radio stations are allocated frequencies according to a system ratified by a series of international conventions. They are often identified by their number on the dial, which corresponds to a frequency in kilohertz (kHz) or megahertz (MHz).

Hopping round the earth

Short-wave transmissions are in the frequency range of 3 to 30 MHz. They are used for international broadcasting because their physical and electromagnetic characteristics (wavelength, energy level, etc.) allow them to be reflected by the ionosphere, a layer of electrically charged particles in the upper atmosphere. Unlike domestic broadcast transmissions, which are beamed at the horizon and cover a broad but local area, international short-wave broadcasts are pointed at the sky. When a radio wave emitted from a transmitting antenna hits the ionosphere, it "bounces" back towards the earth. This is called a "hop", and carries the signal a distance between 2,000 and 4,000 kilometres.

The wave is then reflected back towards the ionosphere by the earth's surface, making another "hop" as it again returns to earth. After several "hops" the signal can reach the other side of the globe. The more powerful the transmitter, the farther the signal can travel.

But the ionosphere also causes problems for long-distance broadcasts. The generation of radio waves in the ionosphere is strongly affected by the time of day, the season and sunspot activity. Small but rapid changes in the ionosphere's altitude and makeup alter its reflecting abilities, affecting the quality of reception.

Static also interferes with short-wave reception. It can be caused by electrical charges such as those produced by ignition systems in automobile and aircraft engines. But lightning is the most common culprit. Radio frequency interference produced by these atmospheric discharges can travel thousands of kilometres. Since there is almost always a thunderstorm somewhere within a few thousand kilometres of any radio, static is a constant disturbance. This is why short-wave broadcasters often send the same programme simultaneously on several frequencies, and periodically change these frequencies. But as a consequence, the spectrum of frequencies is often overloaded, increasing the likelihood of interference and restricting the introduction of new services.

Technological innovations and improved co-operation among countries are now making it possible for international broadcasters to provide alternative solutions to ensure reception and improve sound quality. For example, they now often transmit programmes by satellite to local FM stations, which retransmit them as part of their domestic schedule. …

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