Traffic Congestion Alert System Using Gsm

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  • Topic: Polarization, Optics, Wave
  • Pages : 1 (333 words )
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  • Published : March 26, 2013
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Lens Antenna 
an antenna whose directivity pattern is a result of the difference between the phase velocity of propagation of an electromagnetic wave in air and that in the lens material. Lens antennas are used in radar and metering equipment that operates in the centimeter wavelength range. A lens antenna consists of a lens proper and of a feed. The shape of the lens depends on the refractive index n (the ratio of the phase velocity of propagation of a radio wave in a vacuum to that in the lens). A decelerating lens antenna, as in optics, is one for which n > 1. An accelerating lens antenna (without an optical analogy) is one for which n < 1. The feed is usually a horn antenna that generates a spherical wave front or an antenna array that produces a cylindrical wave front. Decelerating lens antennas are made of high-quality low-loss homogeneous dielectrics (polystyrene, fluoroplastics) or synthetic dielectrics (systems of variously shaped metallic particles suspended in air or in a homogeneous dielectric with a relative dielectric constant approaching unity). The refractive index can change within wide limits with extremely low losses. Accelerating lens antennas are made of shaped metal sheets. There is no analogy in optics. The principle of operation derives from the fact that the phase velocity of an electromagnetic wave propagating between parallel metal sheets depends on the distance between them if the electric field vector is parallel to the sheets. In such a case the phase velocity is greater than the speed of light, and the refractive index is less than unity. Zoning the surfaces of a lens antenna reduces its weight and dimensions. The shape and height in profile of the individual zones are chosen such that the electromagnetic waves refracted by adjacent zones leave the lens with a phase shift of 360°. The field at the aperture remains cophasal. With aplanatic and Lüneberg lens antennas it is possible to control the directivity pattern virtually...
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