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The millimeter-wave region of the electromagnetic spectrum is usually considered to be the range of wavelengths from 10 millimeters (0.4 inches) to 1 millimeter (0.04 inches). This means they are larger than infrared waves or x-rays, for example, but smaller than radio waves or microwaves. The millimeter-wave region of the electromagnetic spectrum corresponds to radio band frequencies of 30 GHz to 300 GHz and is sometimes called the Extremely High Frequency (EHF) range. The high frequency of millimeters waves as well as their propagation characteristics (that is, the ways they change or interact with the atmosphere as they travel) make them useful for a variety of applications including transmitting large amounts of computer data, cellular communications, and radar.
One of the greatest and most important uses of millimeter waves is in transmitting large amounts of data. Every kind of wireless communication, such as the radio, cell phone, or satellite, uses specific range of wavelengths or frequencies. Each application provider (such as a local television or radio broadcaster) has a unique “channel” assignment, so that they can all communicate at the same time without interfering with each other. These channels have “bandwidths” (also measured in either wavelength or frequency) that must be large enough to pass the information from the broadcaster’s transmitter to the user. For example, a telephone conversation requires only about 6 kHz of bandwidth, while a TV broadcast, which carries much larger amounts of information, requires about 6 MHz. (A kilohertz, is one thousand cycles per second; a megahertz is one million cycles per second). Increases in the amount of information transmitted require the use of higher frequencies. This is where millimeter waves come in. Their high frequency makes them a very efficient way of sending large amounts of data such as computer data, or many simultaneous television or voice channels.
Radar is another important use of millimeter waves, which takes advantage of another important property of millimeter wave propagation called beamwidth. Beamwidth is a measure of how a transmitted beam spreads out as it gets farther from its point of origin. In radar, it is desirable to have a beam that stays narrow, rather than fanning out. Small beamwidths are good in radar because they allow the radar to “see” small distant objects, much like a telescope. A carefully designed antenna allows microwaves to be focused into a narrow beam, just like a magnifying glass focuses sunlight Unfortunately, small beamwidths require large antenna sizes, which can make it difficult to design a good radar set that will fit, for example, inside a cramped airplane cockpit.
A radar sensor used in 2003-model Mercedes S-class automobiles. The circuits that transmit and receive millimeter waves are housed beneath the dome-shaped plastic "radome," which is about 10 cm (4 inches) in diameter. This unit is mounted behind a portion of the car's hood that is made to be transparent to millimeter-wave energy.
But the use of millimeter-length microwaves has allowed engineers to overcome this antenna problem. For a given antenna size, the beamwidth can be made smaller by increasing the frequency, and so the antenna can be made smaller as well. As an example, consider the collision-avoidance radar available in some cars. Specifications developed by auto manufacturers require this system to “see” a bicycle at a distance of 100 meters (330 feet). It must also distinguish the bicycle from other objects such as trees, bridges, roadside signs, and other moving vehicles. At a high frequency such a system uses an antenna approximately 10 cm (4 inches) in diameter, which automobile designers can tuck away behind the car’s grill or fender or design into the front end of the vehicle. At a lower frequency the diameter of the antenna required to achieve the same beamwidth would be 78 cm (31 inches), which would require a pretty silly looking antenna be mounted on the car.
Certain characteristics of the earth’s atmosphere pose both problems and solutions for millimeter wave applications. For example, at 60 GHz (5 mm or 0.2 inches wavelength) oxygen molecules will interact with electromagnetic radiation and absorb the energy. This means 60 GHz is not a good frequency for use in long-range radar or communications, because the oxygen absorbs the electromagnetic radiation—and the signal. On the other hand, since the 60 GHz signal does not travel far before it loses all its energy, this frequency comes in handy for secure short-range communications, such as local wireless area networks used for portable computers, where it is important that hackers do not tap into the data stream. Another use for 60 GHz technology is communications between satellites (called cross-linking) in high earth orbit. Since there is almost no oxygen in space at the geosynchronous altitudes of 43,000 km or 26,000 miles), 60 GHz works just fine for communication between satellites.
Although millimeter waves allow large bandwidth, other frequencies, such as infrared and optical wavelengths, allow the ultimate in high data rates and narrow beamwidths. Unlike millimeter waves, however, these shorter-wavelength signals suffer from absorption by fog, dust, and smoke. The solution, where applicable, is to use optical fiber as a wave guiding medium because it is not affected by fog or other atmospheric conditions. But there are still many situations where optical fibers cannot be used because the transmitters or receivers are mobile (such as cell phones or satellite communication) so radio-wave communications, including millimeter waves, is usually the best choice.
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