AM radio is radio broadcasting using amplitude modulation. This was the dominant system of radio in the first two thirds of the 20th century, and remains important today.
AM radio technology is simpler than other types of radio, such as FM radio and DAB. An AM receiver detects the power of the radio wave, and amplifies changes in the power measurement to drive a speaker or earphones. The earliest crystal radio receivers used this principle.
AM radio was used for small scale voice and music broadcasts before World War I. The great increase in the use of AM radio came the following decade. The first commercial radio services began on AM in the 1920s. Radio programming boomed during the "Golden Age of Radio." Dramas, comedy and all other forms of entertainment were produced, as well as broadcasts of news and music.
AM Radio is broadcast in frequency bands ranging from 144 to 30,000 kHz:
- Long wave is 144 - 351 kHz
- Medium wave is 530 - 1,700 kHz
- Short wave is 1,700 - 30,000 kHz, divided into 14/15 "broadcast bands".
Medium wave is by far the most used for commercial radio broadcasting; this is the "AM radio" that most people are familiar with.
For the long and medium wave bands, the wavelength is long enough that the wave diffracts around the curve of the Earth by ground wave propagation, giving AM radio, in particular long wave and medium wave at night, a long range.
Short wave is used by radio services intended to be heard great distances away from the transmitting station; the far range of short wave broadcasts comes at the expence of lower audio fidelity. The mode of propagation for short wave is different, see High frequency.
Frequencies between the broadcast bands are used for other forms of radio communication, such as baby minders, walkie talkies, cordless telephones, radio control, amateur radio etc.