Types of Modulation

Independent from frequency, the receiver must also be capable of receiving the modulation type being transmitted. AM, Singlesideband and CW are common modulations. Types of modulation frequently used in the shortwave frequency range are:

• AM: amplitude modulation. Most commonly used for shortwave broadcasting.

• SSB: Single sideband: This is used for long-range communications by ships and aircraft, for voice transmissions by amateur radio operators, and for broadcasting. LSB (lower sideband) is generally used below 9 MHz and USB (upper sideband) above 9 MHz.

• CW: Continuous wave, which is used for Morse code communications.

• NBFM: Narrow-band frequency modulation. Primarily military NFM transmissions occur in the higher HF frequencies (typically above 20 MHz). Because of the larger bandwidth required, NBFM is much more commonly used for VHF communication. NBFM is poorly suited for long range SW broadcasting due to the multiphasic distortions created by the ionosphere.

• DRM: Digital Radio Mondiale: digital modulation for use on bands below 30 MHz.

• Various radioteletype, fax, digital, slow-scan television--or other systems, which require software or special equipment to decode.

Users

Some major users of the shortwave radio band include:

• Domestic broadcasting in countries with a widely dispersed population with few longwave, mediumwave, or FM stations serving them;

• International broadcasting to foreign audiences (also known as "world band radio");

• Speciality political, religious, and conspiracy theory radio networks, individual commercial and non-commercial paid broadcasts for the North American and other markets;

• Utility stations transmitting messages not intended for a general public, such as aircraft flying between continents, encoded or ciphered diplomatic messages, weather reporting, or ships at sea;

• Clandestine stations. These are stations that initiate broadcasts on behalf of various political movements, including rebel or insurrectionist forces, and are normally unauthorized by the government-in-charge of the country in question. Clandestine broadcasts may emanate from transmitters located in rebel-controlled territory or from outside the country entirely, using another country's transmission facilities;

• Numbers Stations Officially these stations do not exist as they are unlicensed and untraceable yet regularly appear and disappear all over the shortwave radio band range. It is believed that Numbers Stations are operated by government agencies and are used by these agencies to communicate with clandestine operatives working within foreign countries however, no definitive proof of such use has emerged. As the vast majority of these broadcasts contain nothing but the recitation of blocks of numbers, in various languages, with the occasional 'burst' of music, they have become known as 'Number Stations', although they actually lack any official designation. Perhaps the most famous example of a Number Station is the Lincolnshire Poacher (numbers station) named after the 18th century English folk song, which is broadcast just prior to the recitation of the number sequences.;

• Amateur radio operators;

• Time signal stations: In North America, WWVand WWVH operate on these frequencies: 2500 kHz, 5000 kHz, 10000 kHz, 15000 kHz, and 20000 kHz. CHU Canada operates on these frequencies: 3335 kHz, 7335 kHz, and 14670 kHz. Other similar stations operate on various shortwave and longwave frequencies throughout the world;

• Over-the-horizon radar From 1976 to 1989, the Russian Woodpecker over the horizon radar system blotted out countless shortwave broadcasts daily.

The term DXing, in the context of listening to radio signals of any user of the shortwave band, is the activity of monitoring distant stations. In the context of amateur radio operators, the term 'DXing' refers to the two-way communications with a distant station, using shortwave radio frequencies.

The Asia-Pacific Telecommunity estimates that there are approximately 600,000,000 shortwave broadcast radio receivers in use in 2002. WWCR claims that there are 1.5 billion shortwave receivers worldwide.

Shortwave listening

Many hobbyists listen to shortwave broadcasters without operating transmitters. In some cases, the goal is to hear as many stations from as many countries as possible (DXing); others listen to specialized shortwave utility, or "ute", transmissions such as maritime, naval, aviation, or military signals. Others focus on intelligence signals from numbers stations, or the two way communications by amateur radio operators.

Many listeners tune the shortwave bands for the programmes of stations broadcasting to a general audience (such as Radio Canada International, Voice of America, BBC World Service, Radio Australia, Radio Netherlands, etc.). Today, through the evolution of the Internet, the hobbyist can listen to shortwave signals via remotely controlled shortwave receivers around the world, even without owning a shortwave radio. Many international broadcasters (such as Radio Canada International, the BBC and Radio Australia) offer live streaming audio on their websites.

Shortwave listeners, or SWLs, can obtain QSL cards from broadcasters, utility stations or amateur radio operators as trophies of the hobby. Some stations even give out special certificates, pennants, stickers and other tokens and promotional materials to shortwave listeners.

Utility stations

Utility stations are stations that do not broadcast to the general public. There are shortwave bands allocated to the use of merchant shipping, marine weather, and ship-to-shore stations; for aviation weather and air-to-ground communications; for military communications; for long-distance governmental purposes, and for other non-broadcast communications. Many radio hobbyists specialize in listening to "ute" broadcasts, which often come from places without shortwave broadcasters.