Shortwave radio

Shortwave radio communication uses high-frequency radio waves, generally 3–30 MHz. Signals can travel beyond the horizon when the ionosphere refracts them back toward Earth.

High-frequency radio: The radio-frequency band from 3 to 30 MHz, designated HF by the International Telecommunication Union. Shortwave broadcasting and communication occupy this frequency band.
International broadcasting: Radio transmission intended to reach audiences in countries beyond the broadcaster's own. Shortwave transmitters can carry programming across national borders without local relay stations.
Radio spectrum: The range of electromagnetic frequencies used for radio transmission and related technologies. Shortwave is one portion of this broader spectrum.
Heinrich Hertz: German physicist whose experiments in the 1880s demonstrated the existence of electromagnetic waves. His experiments established the physical basis for radio transmission.
Ionospheric propagation: Radio propagation in which charged layers of the ionosphere refract waves back toward Earth. It enables shortwave signals to reach locations beyond the geometric horizon.
Amateur radio: Noncommercial radio communication and experimentation conducted by licensed operators. Amateurs use HF shortwave bands for long-distance contacts and technical experimentation.
Medium frequency: The radio-frequency band from 300 kHz to 3 MHz, which includes the traditional medium-wave broadcast band. Its lower frequencies distinguish medium-wave broadcasting from shortwave.
Guglielmo Marconi: Italian inventor and engineer who developed practical wireless telegraphy systems. His long-distance radio experiments helped establish shortwave as a communications medium.
Skywave: Radio energy that travels upward and returns to Earth after refraction in the ionosphere. Shortwave long-distance links commonly use one or more skywave hops.
Single-sideband modulation: An amplitude-modulation method that transmits one sideband while suppressing the carrier and other sideband. It conserves bandwidth and transmitter power for voice communication on shortwave.

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