Lower side band is an spectrum-efficient way to communicate by voice over the amateur radio, and is used on the 160, 80, and 40 meter bands. While use of lower side band is not expressly required by FCC rules, amateurs are expected to transmit voice in this mode when operating in certain ham radio bands.
Lower side band is part of a mode of voice operation used by amateur radio on certain high-frequency bands called single side band (SSB). This mode is more efficient than AM or FM, and can travel further and is more audible when band conditions are poor than either of those modes. AM transmissions are made up of two different signals, which are placed on top of a radio signal and varied -- or modulated -- between the two side bands. SSB only broadcasts on one of these bands, while the other is suppressed.
World governments struck agreements to govern when and where each SSB type should be used. Lower side band transmissions can be found on 160, 80, and 40 meters, while upper side band is used on 60, 20, 17, 15, 12, and 10 meters on the HF bands. Modern radios automatically set the transmission type to the sideband used for a particular band, but operators with older radios will need to set the emission type (located on a dial on the radio itself) to "LSB" when on 160, 80 or 40 meters.
A SSB transmission -- either lower or upper side band -- uses spectrum more efficiently. It uses half the bandwidth of an AM transmission and allows more amateur radio operators to communicate on a given band. Lower sideband use also requires less power: a 250 watt SSB transmission has the same range as a 1000 watt AM transmission. This keeps amateur radio stations in compliance with FCC rules which state "an amateur station must use the minimum transmitter power necessary to carry out the desired communications." (editor: see § 97.313 (a))
The use of SSB is not expressly required by FCC rules nor the type of SSB used. The only exception is the 60 meter band, where upper sideband must be used for all voice transmissions. This means it is up to the individual to operate their station in line with accepted amateur practice. Failure to use lower side band mode where necessary results in problems in communicating with other amateurs and complaints on the operation of an amateur radio station.