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.]] Microwaves are Electromagnetic Waves with Wavelength s longer than those of Terahertz (THz) wavelengths, but relatively short for Radio Wave s. Microwaves have wavelengths approximately in the range of 30 Cm (frequency = 1 GHz) to 1 Mm (300 GHz). However, the boundaries between far Infrared light, Terahertz radiation, microwaves, and ultra-high-frequency Radio Wave s are fairly arbitrary and are used variously between different fields of study. A credible definition comes from Pozar's text "''Microwave Engineering''", which states that the term microwave "refers to alternating current signals with frequencies between 300 MHz (3 x 108 Hz) and 300 GHz (3 x 1011 Hz)." 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. The microwave range includes Ultra-high Frequency (UHF) (0.3-3 GHz), Super High Frequency (SHF) (3-30 GHz), and Extremely High Frequency (EHF) (30-300 GHz) signals. Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque , until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges. Generation 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 Transistor s (FET's), Bipolar Junction Transistor s (BJT's), Gunn Diode s, and IMPATT Diode s. 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 , Traveling Wave Tube (TWT), and Gyrotron . Uses throughout the entire gigahertz range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)]]
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: 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 Perhaps the first use of the term ''microwave'' occurred in 1931: :"When trials with wavelengths as low as 18 cm. were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon." ''Telegraph & Telephone Journal'' XVII. 179/1 Perhaps the first use of the word ''microwave'' in an astronomical context occurred in 1946 in an article "Microwave Radiation from the Sun and Moon" by Robert Dicke and Robert Beringer . For some of the history in the development of electromagnetic theory applicable to modern microwave applications see the following figures:
Specific significant areas of research and work developing microwaves and their applications: The Microwave integrated devices which are called MMIC (Monolithic Microwave Integrated Circuit) are manufactured by using mostly Gallium Arsenide (GaAs) wafers. Microwave are electromagnetic waves with wavelengths longer than those of infrared light, but relatively short for radio waves.-Gayla Davis of Commerce,TX References
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