Editing History of radio (section)

==== Wavelength and frequency  ====
In early radio, and to a limited extent much later, the transmission signal of the radio station was specified in meters, referring to the [[wavelength]], the length of the radio wave. This is the origin of the terms [[long wave]], [[medium wave]], and [[short wave]] radio.<ref>{{Cite web|title=Radio Waves and the Electromagnetic Spectrum|url=https://radiojove.gsfc.nasa.gov/education/lesson_plans/lesson4.pdf|website=radiojove}}</ref> Portions of the radio spectrum reserved for specific purposes were often referred to by wavelength: the [[40-meter band]], used for [[History of amateur radio|amateur radio]], for example. The relation between wavelength and frequency is reciprocal: the higher the frequency, the shorter the wave, and vice versa.

As equipment progressed, precise frequency control became possible; early stations often did not have a precise frequency, as it was affected by the temperature of the equipment, among other factors. Identifying a radio signal by its frequency rather than its length proved much more practical and useful, and starting in the 1920s this became the usual method of identifying a signal, especially in the United States. Frequencies specified in number of cycles per second (kilocycles, megacycles) were replaced by the more specific designation of [[hertz]] (cycles per second) about 1965.