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Antennas & Propagation

Antennas & PropagationCS 6710 Spring 2010 Rajmohan RajaramanIntroduction An antenna is an electrical conductor orsystem of conductorsoTransmission - radiates electromagnetic energyinto spaceoReception - collects electromagnetic energyfrom space In two-way communication, the sameantenna can be used for transmission andreceptionRadiation Patterns Radiation patternoGraphical representation of radiation properties of anantennaoDepicted as two-dimensional cross section Beam width (or half-power beam width)oMeasure of directivity of antennaoAngle within which power radiated is at least half of thatin most preferred direction Reception patternoReceiving antenna s equivalent to radiation pattern Omnidirectional vs. directional antennaTypes of Antennas Isotropic antenna (idealized)oRadiates power equally in all directions Dipole antennasoHalf-wave dipole antenna (or Hertz antenna )oQuarter-wave vertical antenna (or Marconi antenna ) Parabolic Reflective AntennaoUsed for terrestrial microwave and satellite applicationsoLarger the diameter, the more tightly directional is thebeamAntenna Gain antenna gainoPower output, in a particular direction,compared to that produced in any direction bya perfect om

reflected by ionosphere oGround communication – antennas within effective line of site due to refraction Refraction – bending of microwaves by the atmosphere oVelocity of electromagnetic wave is a function of the density of the medium oWhen wave changes medium, speed changes oWave bends at the boundary between mediums

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