Shortwave

Some frequencies of shortwave are capable of reaching the other side of the planet by bouncing a signal off the ionosphere. The selection of a frequency to use to reach a target area depend on several factors:

• The distance from the transmitter to the target receiver
• Time of day. During the day, higher shortwave frequencies can travel longer distances than shorter; at night, this property is reversed.
• Season of the year.
• Solar conditions, including the number of sunspots, solar flares, and overall solar activity. Solar flares can prevent the ionosphere from reflecting or refracting radio waves.

There are several classes of primary users of the shortwave radio band:

• Domestic broadcasting in countries with a widely-dispersed population with few longwave, mediumwave or FM stations serving them
• International broadcasting to foreign audiences
• Utility stations transmitting messages not intended for a general public, such as aircraft flying between continents, encoded or ciphered diplomatic messages, weather reporting, or ships at sea
• Amateur radio operators

The Asia-Pacific Telecommunity estimates that there are approximately 600,000,000 shortwave radio receivers in use in 2002.

The World Administrative Radio Conference[?] (WARC), organized under the auspices of the International Telecommunications Union, allocates bands for various services in conferences every few years. The next WARC is scheduled to take place in 2003. At the WARC conference in 1997, the following bands were allocated to international broadcasters:

5,900-5,950 kHz

7,300-7,350 kHz (7,100-7,350 kHz in the Americas)

9,400-9,500 kHz

11,600-11,650 kHz

12,050-12,100 kHz

13,570-13,600 kHz

13,800-13,870 kHz

15,600-15,800 kHz

17,480-17,550 kHz

18,900-19,020 kHz

Shortwave broadcasting channels are allocated with a 5 kHz separation. International broadcasters, however, may operate outside the normal WARC-allocated bands or use off-channel frequencies to attract attention in crowded bands.

The power used by shortwave transmitters ranges from less than one watt for some experimental transmissions to 500 kilowatts and highter for intercontinental broadcasters. Shortwave transmitting centers often use specialized antenna designs to concentrate radio energy on a bearing aimed at the target area.

International broadcasting

See International broadcasting for details on the history and practice of broadcasting to foreign audiences.

Amateur radio

In the USA, the privilege of operating shortwave radio transmitters is open to amateurs licensed with the Federal Communications Commission. U.S. citizens do not need licenses to own or operate shortwave receivers. Recently the FCC has added an amateur radio license which requires no knowledge of Morse code, making it easier for beginners to get involved; however, a working knowledge of Morse code is required to operate on shortwave bands.

Amateur radio operators have made several technical advancements in the field of radio and make themselves available to transmit emergency communications when normal communications channels fail. Some amateurs practice operating off the power grid so as to be prepared for power loss.

Shortwave radio: the future

The development of direct broadcasts from satellites has reduced the demand for shortwave receivers, but there are still a great number of shortwave broadcasters. A new digital radio[?] technology, Digital Radio Mondiale[?], is expected to improve the quality of shortwave audio from very poor to standards comparable to the FM broadcast band.