RadioWave Propagation

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Radio-Wave Propagation

• Wave model vs. light model(photons)
• Electromagnetic Radiation
• Transverse Electromagnetic Waves (TEM)
• Charges of electrons moving in a conductor(s)
  called an antenna.
• Travel in free space after launched.

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Electromagnetic Waves

• E field and H field are perpendicular to each
  other and to the direction of travel.
• The two fields vary together in time and space
  and develop an apparent ratio between E field
  intensity and H field intensity.
• The ratio is the characteristic impedance of the
  medium it is traveling in expressed in ohms.

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For a lossless medium
Where u permeability of medium in henrys/m

permitivity of medium in farads/m
Free Space
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Electric and Magnetic Fields Contd.

• For the most part EM waves travel in a media
  where the permeability is the same as free space
  however the permitivity will be given as a
  dielectric constant of the relative medium.

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Plane and Spherical Waves

• Picture a wavefront- surface where all waves have
  same phase such as a sphere.
• Isotropic Radiator
• Point source approximation

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Polarization

• Polarization of plane wave is determined by the
  direction of the E field vector.
• Polarization of RX antenna must match that of
  source for optimum reception.
• Circular polarization axis rotates 360 degrees
  for each wavelength and field strength equal at
  all angles.
• Elliptical polarization if field strength varies
• Right hand and left hand circular polarization

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Free Space Propagation and Attenuation of Free
Space

• Radio Waves travel at C in free space.
• Radio wave energy is not absorbed by free space.
• The energy is however spread over a larger
  surface as distance from the source
  increases.(Square law attenuation)
• Amplitude of field intensity decreases with
  distance however the identity remains the same as
  generated by the source.(in phase)
• ISOTROPIC Radiator- antenna that radiates well in
  all directions. It would produce spherical waves.
  Therefore watts per square meter would be given
  by

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Combine both equations for
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Transmitting and Receiving Antenna Gains

• Important What is receiver signal strength?
• Factors 1. Transmitter power.
• 2. Distance from transmitter.
• 3. TX antenna gain
• 4. RX antenna gain.
• Effective Area
• Antenna Apeture
  

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Path Loss

• CONSIDERATIONS TX antenna gain
• RX antenna
  gain
• wavelength
• distance

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Path Loss Contnd.

• Attenuation of free space
• More convenient expression includes unit
  conversion
• Expressed as a loss