A wave.

A wave is a disturbance that travels from one place to another without actually transporting any matter. The source of all waves is something that is vibrating, moving back and forth at a regular, and usually fast rate.

Waves have high points called crests and low points called troughs. The vibration between two extreme points-the crest and the trough-is generally an "oscillation." Amplitude refers to the maximum distance on either side of the midpoint of the wave. The distance from the top of one crest or tough to the next is called the wavelength. How frequently a point on a wave passes a particular point per unit time is described by its frequency. The unit of frequency is the hertz (hz) after Heinrich Hertz, who demonstrated the existence of radio waves in 1886. Once per second is 1 hertz, twice per second is 2 hertz, etc. One thousand hertz is expressed as a kilohertz (kHz). One million hertz is a megahertz (MHz). One billion hertz is a gigahertz (GHz). The time it takes for an object to make a complete vibration-for instance, the time it takes a pendulum to swing once each way--is called its period. Frequency and period are reciprocals of each other.

All electromagnetic waves move at the same speed and differ from one another in their frequency. Radio waves are described as electromagnetic waves with a particular range of frequencies. AM radio waves are electromagnetic wave with frequencies in the order of several thousand hertz and are expressed in kilohertz (for instance, 630 kHz). FM radio has frequencies that are expressed in megahertz (for instance, 99.1 MHz).

Radios that aircraft pilots and air traffic controllers use to communicate with each other at particular assigned frequencies. Radio beams for the various landing systems also broadcast at particular frequencies that pilots can receive.

Earliest radar operated in the high-frequency range. Some more recent radar operates in the microwave frequency range from 1,000 to 300,000 MHz, which is above the frequency that is normally used for radio. In general, each radar application has a particular frequency band to which it is best suited.

Frequency designations refer to a range of frequencies and wavelengths as follows: