Reflection, Refraction, and Diffraction

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Reflection, Refraction, and Diffraction

• Observe specular reflection
• Angle of incidence equals angle of reflection
  with both angles measured from a line normal to
  the reflective surface.
• Corner Reflectors
• Parabolic principle of collimation
• Diffuse reflection collective orientation is
  different therefore SCATTERING results.

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Refraction

• When there is a transition from one medium to
  another.
• In optics defined by Snells law.
• When wave enters a region with a higher
  dielectric constant ( a lower propagation
  velocity) it bends toward the normal.
• Critical angle. (large incident angle wave
  travels to area of low dielectric constant.
  Extreme case is total internal reflection)
• An application example of total internal
  reflection is optical fibre.( core and cladding)

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Refraction
Can be used to determine angles of incident and
refraction rays.
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Total internal reflection occurs in optcal
fibres. Light reflects from the boundary between
the core of the fibre and a cladding.
Cladding n2
core
Cladding n2
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Diffraction

• Light appears to go around corners periodically.
• Radio waves as well.
• Assume that each point on a wavefront presents
  itself as an isotropic source.
• Some wavefronts pass beside or above the
  obstruction and radiate in the area beyond.
• Diffraction more pronounced when dimensions of
  obstruction are small compared to wavelength.

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Terrestrial Propagation

• Ground-Wave propagation
• Ionospheric propagation
• Line of sight.
• Tropospheric Scatter
• Tropospheric Ducting

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Ground Waves

• Frequencies up to 2 Mhz.
• Vertically polarized in order to minimize
  currents induced in the ground creating losses.
• Further from transmitter the more horizontal the
  wavefront becomes.
• Ground waves attenuate quickly above 2 Mhz.
• Users Military (15 Khz and 60 Khz)
• Loran (100 Khz)
• AM broadcast.

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Ionospheric Propagation

• Three main regions d, E, anf F layers(F1 and F2)
• Ionization increases with altitude and is greater
  during the day.
• D and E layers diminish at night.
• Follows 11 year sunspot cycle.
• Signal returns by a form of refraction.
• D and E layers absorb low frequencies( 8-10Mhz)
  during the day therefore low frequencies
  propagate better at night.

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Ionospheric Propagation Contnd.

• Frequency Diversity - transmit on multiple
  frequencies over HF band.
• Ionospheric sounding - determines Critical
  Frequency.
• MUF - maximum usable frequency. The highest
  frequency that returns to earth for a given path.
  
• OWF - Optimum usable frequency. ( 0.85 MUF)

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Skip zone
As the angle of elevation increases the distance
covered decreases and MUF becomes lower. For
frequencies above fc there will be a region close
to the transmitter that will not receive the
signal. (skip zone)
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Line of Sight Propagation

• VHF signals and higher are not normally returned
  to eath by ionosphere.
• Space wave, line of sight, and tropospheric
  propagation are all the same.
• For terrestrial application distance is limited
  by the curvature of the earth.
• Height of TX and RX antennas above terrain is
  important in the calculation of distance.
• Subject to signal reflections from the terrain.

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Line of Sight Propagation contd.

• Line of Sight propagation is subject to
  reflections.
• Waves will either be constructive or
  deconstructive.
• Especially a concern where surfaces are flat. If
  signals are 180 degrees out of phase the
  reduction in signal strength is quite high ( 20dB
  or more).
• This contributes to Fading.
• Remedies 1. Locating antennas in order that
  reflections are diffuse in
• nature.
• 2. Frequency Diversity
• 3. Spatial Diversity

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Antenna Heights for LOS

• With Sky wave propagation antenna height is only
  important with regards to the impact of
  reflections on radiation patterns.
• With Space wave propagation antenna elevation is
  important with the higher the better realization.

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Troposcatter 80 to 800 km 250 Mhz to 5 Ghz
Irregularities in the troposphere can cause radio
waves to scatter. Possible causes are water vapor
and temperature variations. Although it is used
as a mechanism for communication systems, the
equipment demands make it inefficient. For
example high power TX,high gain antennas and
sensitive receivers.
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Superrefractive Layers
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