Shortwave radio operates between the frequencies of 3000 kHz and 30 MHz (30,000 kHz) and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than those commonly in use at that time. An alternate name is HF, or high frequency.
Short wavelengths are associated with high frequencies because there is an inverse relationship between frequency and wavelength.
Shortwave frequencies 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 depends on several factors:
The distance from the transmitter to the target receiver
Time of day. During the day, higher shortwave frequencies (> 10 MHz) can travel longer distances than lower; 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.
Some major users of the shortwave radio band include:
The Asia-Pacific Telecommunity estimates that there are approximately 600,000,000 shortwave radio receivers in use in 2002.
The World Radiocommunication Conference (WRC), organized under the auspices of the International Telecommunications Union, allocates bands for various services in conferences every few years. The next WRC is scheduled to take place in 2007. At the World Administrative Radio Conference (WARC) in 1997, the following bands were allocated to international broadcasters:
5,900-5,950 kHz (49 meters)
7,300-7,350 kHz (41m)(7,100-7,350 kHz outside of the Americas)
9,400-9,500 kHz (31m)
11,600-11,650 kHz (25m)
12,050-12,100 kHz (25m)
13,570-13,600 kHz (22m)
13,800-13,870 kHz (22m)
15,600-15,800 kHz (19m)
17,480-17,550 kHz (17m)
18,900-19,020 kHz (16m)
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.
The privilege of operating shortwave radio transmitters for non-commercial purposes is open to licensed amateurs. In the USA, they are licensed by the Federal Communications Commission (FCC). 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 numerous 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.
Numbers stations are shortwave radio stations of uncertain origin that broadcast streams of numbers, words, or phonetic sounds. Although officially there is no indication of their origin, radio hobbyists have determined that many of them are used by intelligence services as one-way communication to agents in other countries.