Damped wave

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A damped wave is an electromagnetic wave whose amplitude of oscillation decreases with time, eventually going to zero. This term also refers to an early method of radio transmission from spark gap transmitters, or other types of transmitters having characteristic decrement similar to spark transmitters, in which a carrier wave is switched on and off. This is generally referred to as "Class B" emission. Information is carried in the rhythm and spacing with which the signal is sent. Damped waves were used for a form of on-off keying (OOK). However, such transmitters were noisy and generated a wide emission bandwidth.

There is a currently an international prohibition against the use of Class B, damped wave emission. This prohibition forms part of the International Telecommunication Union regulations and is a throwback to the days when spark gap transmitters were employed. The prohibition against Class B emissions was intended to prevent the generation of wasteful radio frequency energy and the use of inefficient transmission techniques. However, ironically there is no longer a clear definition of a Class B, damped wave emission^{[citation needed]}.