Also known as: shortwave radio, SW broadcasting, HF broadcasting, international broadcasting
Shortwave broadcasting is international and long-distance radio broadcasting in the high-frequency (HF) range of roughly 3–30 MHz, using designated broadcast segments scattered across the band.1 Its defining feature is not its modulation — which is mostly ordinary double-sideband AM — but its reach: HF signals refract off the ionosphere as a sky wave, returning to earth hundreds or thousands of kilometres from the transmitter and enabling a single station to cover continents.
How it works
Below the maximum usable frequency, HF waves striking the ionosphere are bent back toward the ground rather than escaping to space, so a wave launched at a shallow angle returns as a “hop” far downrange; multiple hops can circle much of the globe. Because ionospheric density varies with the sun, the usable bands shift dramatically between day and night: lower shortwave bands (e.g. 49 m, 41 m) carry best after dark, while higher bands (19 m, 16 m, 13 m) open during daylight. Broadcasters therefore change frequency by time of day and season, and coordinate schedules through the HFCC. Signals also fade (QSB) as multiple paths interfere, and long multi-hop paths add characteristic distortion.
The dominant mode is double-sideband AM, chosen for receiver simplicity across a huge and cheap installed base, typically with a few kHz of audio bandwidth. A minority of transmissions use single-sideband or the digital DRM system, which packs a COFDM waveform into the same ~10 kHz channel to deliver near-FM audio quality where propagation allows.
Close to the transmitter, a ground wave provides reliable local coverage, but the long-haul reach that defines shortwave comes entirely from the sky wave. Because a given path only supports a limited range of frequencies at any moment — bounded above by the maximum usable frequency and below by absorption in the lower ionosphere — broadcasters simulcast the same programme on several bands so listeners can find whichever one is propagating. This constant band-hopping, and the audible flutter of multi-hop paths, are the sonic signature of HF broadcasting.
Relevance to SDR
Shortwave is a rewarding SDR band because a wideband HF SDR — a direct-sampling receiver or an upconverter feeding a VHF dongle — can display the whole 3–30 MHz range at once, letting an operator watch broadcast bands open and close with the ionosphere. AM demodulation is trivial (envelope or synchronous detection), and the same receiver reveals utility stations, amateur traffic, and DRM blocks that decoding software can turn back into audio. It is a live demonstration of ionospheric propagation that VHF/UHF listening never shows.
GopherTrunk is a VHF/UHF trunked land-mobile decoder and does nothing with HF broadcasting; the two occupy different worlds of the spectrum. Shortwave is included here as essential propagation context — the clearest everyday example of the sky-wave mechanism that governs why some signals travel far beyond the horizon and others do not.
Sources
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Shortwave radio — Wikipedia, for the HF broadcast bands, sky-wave ionospheric propagation, day/night band selection, and the use of AM with some DRM. ↩