This page is about the radiation; for the appliance, see microwave oven.
Microwaves, also known as super-high frequency (SHF) signals, have wavelengths approximately in the range of 30 cm (frequency = 1 GHz) to 1 mm (300 GHz). However, the boundaries between far infrared light, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. The existence of electromagnetic waves, of which microwaves are part of the higher frequency spectrum, was predicted by James Clerk Maxwell in 1864 from his famous Maxwell's equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building apparatus to produce radio waves.
Note: above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque to higher frequencies of electromagnetic radiation, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.
Microwaves can be generated by a variety of means, generally divided into two categories: solid state devices and vacuum-tube based devices. Solid state microwave devices are based on semiconductors such as silicon or gallium arsenide, and include field-effect transistors (FET's), bipolar junction transistors (BJT's), Gunn diodes, and Impatt diodes. Specialized versions of standard transistors have been developed for higher speed which are commonly used in microwave applications. Microwave variants of BJT's include the heterojunction bipolar transistor (HBT), and microwave variants of FET's include the MESFET, the HEMT (also known as HFET), and LDMOS transistor. Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron, klystron, travelling wave tube (TWT), and gyrotron.
- A microwave oven uses a magnetron microwave generator to produce microwaves at a frequency of approximately 2.45 GHz for the purpose of cooking food. Microwaves cook food by causing molecules of water and other compounds to vibrate. The vibration creates heat which warms the food. Since organic matter is made up primarily of water, food is easily cooked by this method.
- Microwaves are used in communication satellite transmissions because microwaves passes easily through the earth's atmosphere with less interference than longer wavelengths. There is also much more bandwidth in the microwave spectrum than in the rest of the radio spectrum.
- Radar also uses microwave radiation to detect the range, speed, and other characteristics of remote objects.
- Wireless LAN protocols, such as Bluetooth and the IEEE 802.11g and b specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses an ISM band in the 5 GHz range. Licensed long-range (up to about 25 km.) Wireless Internet Access services can be found in many countries (but not the USA) in the 3.5 - 4.0 GHz range.
- Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies. Some cellphone networks also use the lower microwave frequencies.
- Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. NASA worked in the 1970s and early 1980s to research the possibilities of using Solar Power Satellite (SPS) systems with large solar arrays that would beam power down to the Earth's surface via microwaves.
Microwave frequency bands
The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave Frequency Bands are defined in the table below:
|L band||1 to 2 GHz|
|S band||2 to 4 GHz|
|C band||4 to 8 GHz|
|X band||8 to 12 GHz|
|Ku band||12 to 18 GHz|
|K band||18 to 26 GHz|
|Ka band||26 to 40 GHz|
|Q band||30 to 50 GHz|
|U band||40 to 60 GHz|
|V band||50 to 75 GHz|
|E band||60 to 90 GHz|
|W band||75 to 110 GHz|
|F band||90 to 140 GHz|
|D band||110 to 170 GHz|
The above table reflects Radio Society of Great Britain (RSGB) usage. The term P band is sometimes used for UHF frequencies below L-band. For other definitions see Letter Designations of Microwave Bands
History and research
For some of the history in the development of electromagnetic theory applicable to modern microwave applications see the following figures:
- Michael Faraday.
- James Clerk Maxwell.
- Heinrich Hertz.
- Nikola Tesla.
- Guglielmo Marconi.
- Samuel Morse.
- Sir William Thomson, later Lord Kelvin.
- Oliver Heaviside.
- Lord Rayleigh.
- Oliver Lodge.
Specific significant areas of research and work developing microwaves and their applications:
|Work carried out by||Area of work|
|Barkhausen and Kurz||Positive grid oscillators|
|Hull||Smooth bore magnetron|
|Varian Brothers||Velocity modulated electron beam → klystron tube|
|Randall and Boot||Cavity magnetron|