Chelmsford Amateur Radio Society Advanced Course (6) Propagation Part-1: Electromagnetic Waves

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Chelmsford Amateur Radio Society Advanced
Course(6) Propagation Part-1 Electromagnetic
Waves
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Electromagnetic Waves

• Electromagnetic radiation comprises both an
  Electric and a Magnetic Field
• The two fields are at right-angles to each other,
  and the direction of propagation is at
  right-angles to both fields
• The Plane of the Electric Field defines the
  Polarisation of the wave
• The relationship between the magnitudes of the E
  and H fields is constant

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EM Wave Fronts head-on
Electric Field (E)
Magnetic Field (H)
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EM Wave Fronts head-on
Electric Field (E)
Magnetic Field (H)
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Electromagnetic Radiation

• In free space conditions Electro-Magnetic (EM)
  waves travelin straight lines and spread out
• Both Power Flux Density (PFD) in W/m2 and Field
  Strength in V/m, drop with distance
• Double the distance from an antenna and you
  willhalve the field strength, but also quarter
  the Power Flux Density (PFD), according to an
  inverse square law

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EM Field vs Distance
Rx
Tx
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Inverse Square Law forDistance and Intensity

• Relationship between distance and intensity of
  electromagnetic radiation (rf or light)
• Inverse square gives a 1/d2 reduction
• Example
• D 1m - Intensity 1
• D 2m - Intensity ¼
• Doubling the distance spreads same amount of
  energy over four times the area

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Another PFD Example
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Velocity Of Propagation

• Propagation velocity depends on the medium in
  which EM waves travel
• Free Space ? Maximum velocity
• Air ? slightly less than free space and dependent
  on water vapour
• Coax ? 0.66 Times that of Free Space
• Water ? far less, and lossy
• v f . ?
• v is the speed of radio waves light - v
  300,000,000 m/s
• f is the frequency in Hz.
• ? is the wavelength in metres
• For l /4 for 2m
• 300 /145.5 x 0.66 / 4 Coax length 340.2mm

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Linear Polarisation

• Transmit and receive antennas should have the
  same polarisation
• To be efficient...
• Otherwise, signal strength may be reduced by up
  to 32dB
• Polarisation losses can be up to 5 S-points
• Note that after travelling through the
  ionosphere, the polarisation may have changed
  significantly

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Circular Polarisation

• The E-field of the wave rotates as it propagates,
  with either
• Left-handed polarisation (LHCP) (anticlockwise
  from behind) as shown, or
• Right-Handed polarisation (RHCP) (clockwise from
  behind)

Electric Field, E
Magnetic Field, H
Direction of Propagation

• Used for satellite comms where the orientation
  of the EM field from the satellite is
  indeterminate
• Make sure you receive correct circular
  orientation or high polarisation losses will
  result

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Helical Antennas for Circular Polarisation

• 2.4 GHz Helical Antenna
• Designed for OSCAR 40

• Omni-directional Quadrifilar Helical RHCP
  antenna