SWL meaning in Radio Science ?

Shortwave listening, or SWLing, is the hobby of listening to shortwave radio broadcasts located on frequencies between 1700 kHz and 30 MHz. Listeners range from casual users seeking international news and entertainment programming, to hobbyists immersed in the technical aspects of radio reception and collecting official confirmations (QSL cards) that document their reception of distant broadcasts (DXing). In some developing countries, shortwave listening enables remote communities to obtain regional programming traditionally provided by local medium wave AM broadcasters. In 2002, the number of households that were capable of shortwave listening was estimated to be in the hundreds of millions.

The practice of long-distance radio listening began in the 1920s when shortwave broadcasters were first established in the US and Europe. Audiences discovered that international programming was available on the shortwave bands of many consumer radio receivers, and a number of magazines and listener clubs catering to the practice arose as a result. Shortwave listening was especially popular during times of international conflict such as World War II, the Korean War and the Persian Gulf War.

Listeners use inexpensive portable world band receivers to access the shortwave bands, and some advanced hobbyists employ specialized shortwave communications receivers featuring digital technology as well as Digital signal processing designed for optimum reception of shortwave signals, along with outdoor antennas to enhance performance. Many hobbyists also choose to use Software-defined radio receivers for their benefits over traditional radios.

With the advent of the Internet, many international broadcasters have scaled back or terminated their shortwave transmissions in favor of web-based program distribution, while others are moving from traditional analog to digital broadcasting modes in order to allow more efficient delivery of shortwave programming. The number of organized shortwave listening clubs has diminished along with printed magazines devoted to the hobby; however, many enthusiasts continue to exchange information and news on the web.