As illustrated below, the RF spectrum generally refers to frequencies between 3 kHz and 300 GHz. Microwave commonly refers to the subset of the RF spectrum with wavelengths shorter than 30 cm and frequencies above 1 GHz.
Microwave ovens, wireless access points, and TV broadcasters all generate RF energy. For example, Wi-Fi devices use frequencies within the ISM (Industrial Scientific and Medical) and/or UNII (Unlicensed National Information Infrastructure) band. The ISM band used by 802.11b/g/n starts at 2.4 GHz and is divided into 11 unique channels. Many other consumer electronic devices transmit within the ISM band, including cordless telephones, Bluetooth peripherals, baby monitors – and microwave ovens.
Microwave ovens generate RF energy at a frequency of 2.45 GHz, using a magnetron to convert electrical energy into radio waves that bounce around inside the oven to heat food. Microwave ovens are designed to contain those radio waves, but it is possible for some leakage to occur, especially in older ovens. This is why microwave ovens can degrade nearby Wi-Fi performance -- any RF leakage that occurs competes for frequencies used by many 802.11b/g/n access points.
Note that "microwave" and "microwave oven" are not synonymous. Microwave ovens generate microwaves, but not all microwaves are generated by ovens.
This was first published in June 2009