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chap5.pdf - Antennas and Propagation Chapter 5 Introduction...

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Antennas and Propagation Chapter 5
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Introduction An antenna is an electrical conductor or system of conductors Transmission - radiates electromagnetic energy into space Reception - collects electromagnetic energy from space In two-way communication, the same antenna can be used for transmission and reception
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Radiation Patterns Radiation pattern Graphical representation of radiation properties of an antenna Depicted as two-dimensional cross section Beam width (or half-power beam width) Measure of directivity of antenna Reception pattern Receiving antenna’s equivalent to radiation pattern
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Types of Antennas Isotropic antenna (idealized) Radiates power equally in all directions Dipole antennas Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi antenna)
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Types of Antennas Parabolic Reflective Antenna
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Antenna Gain Antenna gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) if an antenna has a gain of 3dB, then it improves upon the isotropic antenna in that direction by 3dB, or a factor of 2 At the expense of other directions. Effective area Related to physical size and shape of antenna
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Antenna Gain Relationship between antenna gain and effective area : G = antenna gain A e = effective area f = carrier frequency c = speed of light (≤ 3 ×10 8 m/s) = carrier wavelength 2 2 2 4 4 c A f A G e e
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Propagation Modes 1) Ground-wave propagation 2) Sky-wave propagation 3) Line-of-sight propagation
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Ground Wave Propagation
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Ground Wave Propagation Follows contour of the earth: The electromagnetic wave induces a current in the earth's surface Diffraction Can Propagate considerable distances Frequencies up to 2 MHz Example: AM radio
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Sky Wave Propagation
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Sky Wave Propagation Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops , back and forth between ionosphere and earth’s surface Reflection effect caused by refraction Frequencies up to 30 MHz Examples Amateur radio International Broadcast radio
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Line-of-Sight Propagation
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Line-of-Sight Propagation Transmitting and receiving antennas must be within line of sight : Satellite communication signal above 30 MHz not reflected by ionosphere Ground
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  • Fall '19
  • Hertz, Electromagnetic spectrum, Radio frequency propagation

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