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Thermal noise due to agitation of electrons.Free space loss accounting for gain of other antennas can be recast as.Ar = effective area of receiving antenna.At = effective area of transmitting antenna.Free space loss accounting for gain of other antennas.Free space loss equation can be recast:.c = speed of light (» 3 ´ 10 8 m/s) where d and are in the same units (e.g., meters).d = propagation distance between antennas.Pr = signal power at receiving antenna.Pt = signal power at transmitting antenna.Free space loss, ideal isotropic antenna.Attenuation is greater at higher frequencies, causing distortion.Signal must maintain a level sufficiently higher than noise to be received without error.Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal.Attenuation factors for unguided media:.Strength of signal falls off with distance over transmission medium.Attenuation and attenuation distortion.Maximum distance between two antennas for LOS propagation:.K = adjustment factor to account for refraction, rule of thumb K = 4/3.d = distance between antenna and horizon (km).Wave bends at the boundary between mediums.When wave changes medium, speed changes.Velocity of electromagnetic wave is a function of the density of the medium.Refraction – bending of microwaves by the atmosphere.Ground communication – antennas within effective line of site due to refraction.Satellite communication – signal above 30 MHz not reflected by ionosphere.Transmitting and receiving antennas must be within line of sight.Reflection effect caused by refraction.Signal can travel a number of hops, back and forth between ionosphere and earth’s surface.Signal reflected from ionized layer of atmosphere back down to earth.Relationship between antenna gain and effective area.Related to physical size and shape of antenna.
Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna).
Quarter-wave vertical antenna (or Marconi antenna).Half-wave dipole antenna (or Hertz antenna).Radiates power equally in all directions.Receiving antenna’s equivalent to radiation pattern.Depicted as two-dimensional cross section.Graphical representation of radiation properties of an antenna.In two-way communication, the same antenna can be used for transmission and reception.Reception - collects electromagnetic energy from space.Transmission - radiates electromagnetic energy into space.An antenna is an electrical conductor or system of conductors.Antenna and Wave Propagation Unit-IV Special Antennas Prepared by, Dr.S.Senthil kumar Principal Tagore Institute of Engineering and Technology