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Amateur radio, also known as Ham radio, is the use of specific bands and frequencies within the radio spectrum for non-commercial communication, technical experimentation, self-training, recreation, radiosport, contesting, and emergency communications. A surge of popular interest in radio experimentation in the early 20th century necessitated that parts of the radio spectrum be allocated to non-professionals. In 1927 a radio amateur was defined as "a duly authorized person interested in radioelectric practice with a purely personal aim and without pecuniary interest," meaning without monetary or similar reward. The definition distinguished amateur activity from commercial broadcasting, public safety services, and professional two-way radio uses such as maritime, aviation, and taxi communication.

The amateur radio service (amateur service and amateur-satellite service) is established by the International Telecommunication Union (ITU) through its Radio Regulations. National governments set technical and operational rules for transmissions and issue individual station licences with unique call signs. Call signs must be used in transmissions, at least every ten minutes and at the end of the conversation. Amateur radio operators must hold an amateur radio licence, granted after an examination that tests knowledge of radio theory, electronics, and national regulations.
Operators are authorised to transmit on frequency ranges allocated internationally as the amateur radio bands. Within these bands they may use any frequency, though some are restricted to particular modes of voice, text, image, or data communications. This allows communication locally, nationally, worldwide, and even with stations in space. In many countries, operators may also exchange radio communications between computers or transceivers connected through virtual private networks on the Internet.
Amateur radio is represented by the International Amateur Radio Union (IARU), which has three regions and includes national amateur radio societies in most countries. In 2011 the American Radio Relay League (the U.S. national society) estimated that about 2 million people worldwide were active in amateur radio. About 830,000 amateur radio stations are in Region 2 (the Americas), about 750,000 in Region 3 (South and East Asia and the Pacific), and about 400,000 in Region 1 (Europe, the Middle East, the CIS and Africa). Radio amateurs have been credited with a number of advancements, notably the discovery of skywave propagation.

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