Electromagnetic waves travel in a straight line or by reflection from the ionosphere or from a communications satellite. A typical radio communication system has two main components, a transmitter and a receiver. The transmitter generates electrical oscillations at a radio frequency called the carrier frequency. Information is imposed on a carrier wave by varying its amplitude, frequency, or duration in a process called modulation. An amplitude-modulated signal consists of the carrier frequency plus two sidebands resulting from the modulation.
Frequency modulation produces more than one pair of sidebands for each modulation frequency.
These changes in the otherwise constant frequency of the wave carry the desired information from transmitter to receiver. Duration modulation is a simpler system, in which a constant tone is transmitted for a certain period of time, shut off, and then transmitted again. Morse code utilizes this system. Amateur Radio Interest in amateur radio arose around the turn of the century, shortly after the Italian inventor Guglielmo Marconi successfully sent the first transatlantic wireless signal in 1901. The interference of amateur broadcasts with commercial and military transmissions led to the institution of government control in 1911.