Microwaves are wavelengths longer than those of terahertz (THz) frequencies, but relatively short for radio waves. Microwaves have wavelengths approximately in the range of 30 cm (frequency = 1 GHz) to 1 mm (300 GHz). This range of wavelengths has led many to question the naming convention used for microwaves as the name suggests a micrometer wavelength. However, the boundaries between far Infrared light, Terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. The same equations of electromagnetic theory apply at all frequencies. Apparatus and techniques may be described as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory is no longer accurate. The term microwave generally refers to "alternating current signals with frequencies between 300 MHz (3×108 Hz) and 300 GHz (3×1011 Hz)."
Microwaves Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the entire gigahertz range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated).
Discovery of microwaves.
The existence of electromagnetic waves, of which microwaves are part of the frequency spectrum, was predicted by James Clerk Maxwell in 1864 from his equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a horse trough, a wrought iron point spark, Leyden jars, and a length of zinc gutter whose parabolic cross-section worked as a reflection antenna. In 1894 J. C. Bose publicly demonstrated radio control of a bell using millimetre wavelengths, and conducted research into the propagation of microwaves.
Microwaves frequency range.
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.
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. These devices work in the density modulated mode, rather than the current modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream.
- A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through the food. water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwave beam. This molecular movement creates Heat as the rotating molecules hit other molecules and put them into motion. Microwave heating is most efficient on liquid water, and much less so on fats and sugars (which have less molecular dipole moment), and frozen water (where the molecules are not free to rotate). Microwave heating is sometimes incorrectly explained as a rotational resonance of water molecules: such resonance only occurs at much higher frequencies, in the tens of gigahertz. Moreover, large industrial/commercial microwave ovens operating in the 900 MHz range also heat water and food perfectly well.
- A common misconception is that microwave ovens cook food from the "inside out". In reality, microwaves are absorbed in the outer layers of food in a manner somewhat similar to heat from other methods. The misconception arises because microwaves penetrate dry nonconductive substances at the surfaces of many common foods, and thus often deposit initial heat more deeply than other methods. Depending on water content the depth of initial heat deposition may be several centimeters or more with microwave ovens, in contrast to grilling ("broiling" in American English), which relies on infrared radiation, or the thermal convection of a convection oven, which deposit heat shallowly at the food surface. Depth of penetration of microwaves is dependent on food composition and the frequency, with lower microwave frequencies being more penetrating.
AT&T Long Lines Microwave Relay Tower, Utah. The lower horn antennas are for TD Radio, 3.7-4.2 GHz, and can simultaneously carry signals in the 6 and 11 GHz bands.
- Microwave radio is used in broadcasting and telecommunication transmissions because, due to their short wavelength, highly directive antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van.
- Before the advent of fiber optic transmission, most long distance telephone calls were carried via microwave point-to-point links through sites like the AT&T Long Lines facility shown in the photograph. Starting in the early 1950's, frequency division multiplex was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the hop to the next site, up to 70 km away.
- 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 802.11 specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM band and UNII frequencies 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.
- Metropolitan Area Networks: MAN protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) based in the IEEE 802.16 specification. The IEEE 802.16 specification was designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.5 GHz, 3.5 GHz and 5.8 GHz ranges.
- Wide Area Mobile Broadband Wireless Access: MBWA protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (e.g. iBurst) are designed to operate between 1.6 and 2.3 GHz to give mobility and in-building penetration characteristics similar to mobile phones but with vastly greater spectral efficiency.
- Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies. Some mobile phone networks, like GSM, also use the lower microwave frequencies.
- Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD).
- 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.
- A maser is a device similar to a laser, except that it works at microwave frequencies.
- Most radio astronomy uses 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 as defined by the Radio Society of Great Britain in the table below:
Microwave frequency bands.
|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 26.5 GHz|
|Ka band ||26.5 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 Ku Band. For other definitions see Letter Designations of Microwave Bands
Microwaves health effects.
First, it is imperative to understand that the word "radiation" applies to different things. One kind is ionizing radiation or particle radiation, which can (for example) damage organic molecules by impacting them and imparting energy. Another is nonionizing wave radiation, like microwaves, which cannot. It is common for the terms to be crossed, so that people get the impression a microwave oven might make food "radioactive", which is literally impossible no matter how much microwave energy is used. Similarly, microwave radiation could not cause cancer in the same way that uranium could. The word "radiation" refers to the fact that energy can radiate, and not to the different nature and effects of different kinds of energy.
The health effects of microwaves are highly controversial. A great number of studies have been undertaken in the last two decades, some concluding that microwaves pose a hazard to health, and others concluding they are safe. It is understood that microwave radiation of a level that causes even minute heating of living tissue is hazardous, and most countries have standards limiting exposure, such as the Federal Communications Commission RF safety regulations. Still at issue is whether lower levels of microwave energy have bioeffects.
Synthetic reviews of literature indicate the predominance of their safety of utilisation. The motivations of each "side" in this debate are held in suspect by the other.
For example, in a paper published in January 2007, Panagopoulos et al. showed that exposing flies to a cellular phone in similar conditions to those to which a mobile phone user is exposed resulted in cell death. There has also been research showing damage in human sperm cells , and DNA damage .
History and research of microwaves.
Perhaps the first, documented, formal 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:
- Hans Christian Ørsted.
- 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.
- Julius Lange.
Specific significant areas of research and work developing microwaves and their applications:
Specific work on microwaves.
|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|
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