The term, Radio, refers to a class of time-varying electromagnetic fields created by varying voltages and/or currents in certain physical sources. These sources may be man made, such as electrical power and electronic circuits, or natural, such as the atmosphere (lightning) and stars (sunspots). The electromagnetic field variations radiate outward from the source forming a pattern called a Radio Wave. Thus, a radio wave is a series of electromagnetic field variations traveling through space. Although, technically, any varying source of voltage or current produces a varying field near the source, here the term “radio wave” describes field variations that propagate a significant distance from the source.
Like sound, a radio wave can be described by its frequency and its amplitude. The frequency of a radio wave is the time rate of the field variations measured in Hertz (Hz), where 1 Hz equals 1 cycle-per-second. The radio spectrum, or range of frequencies, extends from a few Hertz through the Kilohertz (KHz) and Megahertz (MHz) ranges, to beyond the Gigahertz (GHz) range. The suffixes kHz, MHz, and GHz refer to thousands, millions, and billions of cycles-per-second respectively.