Radio communication - including communications by radio amateurs - is regulated on an international level by the International Telecommunication Union, ITU, a United Nations body. This organisation has assigned to each country in the world a unique code to identify the radio stations which it administers. The code consistes of a short number of characters.
Each amateur radio station is assigned a station identifier, the
callsign - which is unique for the entire world.
The call sign consists of two or three parts:
Sometimes it is possible for an amateur to operate his station on the territory of a country other than the one which initially issued his permanent licence. In these cases his permanent call may be preceeded by the prefix of the prefix of the host country.
Many radio amateurs started their hobby listing to shortwave radio. Starting with radio broadcasts from distant countries, they become interested in receiving weak or distorted radio signals on congested frequency bands. A very popular aspect of his hobby is to monitor radio communications on the many frequency bands, allocated by internation agreement to amateur radio.
Receiving other kinds of radio transmission, like standard time signals, may sometimes be very interesting. The reception of signals from radio beacons can provide usefull information about the propagation of radio waves.
The SWL very often builds his own equipment, from antennas to receivers and signal processors. And with the availability of personal computers, other communication modes, such as weathermaps, radio telex bulletins, slowscan television have become increasingly popular.