Radio amateurs use Various modes of transmission to communicate. Voice transmissions are most common, with some such as frequency modulation (FM) offering high quality audio, and others such as single sideband (SSB) offering more reliable communications when signals are marginal and bandwidth is restricted. Radiotelegraphy using Morse code remains popular, particularly on the shortwave bands and for experimental work, with its inherent signal-to-noise ratio advantages. Morse, using internationally agreed code groups, also facilitates communications between amateurs who speak different languages.
Modern personal computers have led to a boom in digital modes such as radioteletype, which previously required cumbersome mechanical equipment. Hams led the development of packet radio, which has since been augmented by more specialized modes such as PSK31 to facilitate real-time, low-power communications on the shortwave bands. Other modes, such as WSJT, are used for weak signal modes including meteor scatter and moonbounce communications. Similarly, fast scan amateur television, once considered rather esoteric, has exploded in popularity thanks to cheap camcorders and video cards in home computers. Because of the wide bandwidth and stable signals required, fast scan amateur television is normally limited to 100 km (about 60 miles) range. However, with HAM TV repeaters linked together the video may travel across several states. Amateur radio station with vintage vacuum tube gear featuring separate transmitter, receiver and power supplyOn VHF and higher frequencies, automated relay stations, or repeaters, are used to increase range. Repeaters are usually located on the top of a mountain or tall building. A repeater allows the radio amateur to communicate over hundreds of square miles using a low power hand-held transceiver. Repeaters can also be linked together, by use of other amateur radio bands, wireline, or the Internet. Repeater stations are either owned, maintained and operated by clubs or individuals. Other operators pursue specialized interests such as providing emergency communications for community emergency response teams; designing new antennas; communicating via amateur satellites; severe weather spotting; DX communication to far away countries; using the Internet Radio Linking Project (IRLP) to connect radio repeaters via the Internet; tracking vehicles using the Automatic Position Reporting System (APRS), which integrates with the GPS; engaging in the sports of contesting, Amateur Radio Direction Finding and High Speed Telegraphy; or trying low-power operation.
In times of crisis and natural disasters, Amateur radio provides emergency communications when wireline, cell phones and other means of communications fail. Unlike commercial systems, Amateur radio is not as dependent on terrestrial facilities that can be destroyed. It also dispersed throughout a community without "choke points" such as celluar telephone sites that can become overloaded. Amateur radio operators are also experienced in improvising antennas and power sources and most equipment can be powered by an automobile battery. Annual "Field Days" are held in many countries to practice these emergency improvisational skills. Amateur radio operators can use hundreds of frequencies and can quickly establish networks tying disparate agencies together to enhance interoperability. Recent examples include the 2001 attacks on the World Trade Center the 2003 North America blackout, the 2004 Indian Ocean earthquake[9] and Hurricane Katrina, where amateur radio was used to coordinate disaster relief activities when other systems failed.
