RADIO THEORY

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RADIO THEORY

• Air Waves in Aviation

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DEFINITIONS

• Radio transmission is an electromagnetic wave
  with the same characteristics as light or heat.
• Wavelength is the linear measurement of the wave.
• Cycle is the interval in which the wave rises and
  falls between its crest and trough.
• Frequency is the number of cycles/second.
• Amplitude is the strength of the signal.

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RADIO BANDS

• Low and Medium Frequency Non Directional Beacons
  and Marker Beacons transmit signals on the LF/MF
  bands of 200 to 415KHz and 510 to 535KHz.
• High Frequency used for air/ground communication
  in remote northern areas and on transoceanic
  flights. (longer range than VHF) 2,500KHz to
  30,000KHz

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• Very High Frequency the most common frequency
  range in aviation, used for voice communication,
  VOR, and ILS. 30MHz to 300MHz
• Ultra High Frequency allocated for military use,
  DME, and glide slope. 300MHz to 3,000MHZ

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PROPERTIES OF RADIO WAVES

• Reflection change of direction occurring at a
  surface separating two different media.
• Refraction bending of wave as it passes from one
  medium to another.
• Diffraction bending of wave as it passes beside
  a non-conductive object.
• Attenuation weakening of wave as it travels
  through a medium.

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PROPAGATION

• LF, MF, and HF transmit in two ways.
• Ground Waves follow the surface of the earth as
  an effect of diffraction and surface attenuation.
• Sky Waves travel into the atmosphere and are
  reflected back to earth by the ionosphere.
• A skip zone exists between the points where the
  ground waves end, and the sky waves strike the
  earth. (erratic or non-existent signals)

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• VHF waves are not affected by the same
  propagation characteristics.
• These waves do not reflect off the ionosphere but
  continue into space.
• They do not follow the curvature of the earth.
• This line-of-sight (space waves) characteristic
  means reception is dependant on altitude.
  1.23vAGLrange(nm)

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ATMOSPHERIC CONDITIONS

• LF, MF, HF are affected by the changing height of
  the ionosphere from day to night.
• At night sky waves travel are reflected by the
  ionosphere which allows for reception at greater
  distances.
• Sunspots and electromagnetic disturbances affect
  the reflectivity of the ionosphere causing signal
  fade out.
• Electrical fields associated with clouds (notably
  thunderstorms) create precipitation static.
• VHF is virtually free from atmospheric and
  precipitation static.

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The ionosphere is a collection of ionized
particles and electrons in the earths atmosphere.
It is responsible for reflecting LF, MF, and HF
radio waves. The D-layer which develops shortly
after sunrise and disappears shortly after sunset
absorbs this radio energy minimizing the
reflection of sky waves. When the D-layer fades
at night the radio waves are reflected back to
earth as sky waves by the E and F-layers.
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HF COMMUNICATION

• Because of HF radios propagation characteristics
  it is very useful for long range communications.
• It is used in remote areas such as Northern
  Canada and during transoceanic flights.
• The higher range of HF travels farther in the day
  while the lower HF signals travel farther at
  night due to the changing ionosphere.
• Sun up, frequency up
• Sun down, frequency down

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ANTENNAS

• Radio antennas operate on the principle of
  resonance when a tuning fork is struck it will
  vibrate at its natural frequency when two tuning
  forks are tuned to the same frequency the first
  tuning fork will induce vibration in the second.
• Radio antennas are tuned to the frequency to be
  received.

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• The ideal antenna length is one wavelength long.
  That means a standard VHF antenna would be 6-30
  feet long. Antennas this length are impractical,
  so sub-multiples are used.
• LF,MF,HF long wavelong antenna (wire or Pod)
• VHF short waveshort antenna (mast)
• UHF very short wavevery short antenna (blade)

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DATA LINK COMMUNICATIONS

• VHF data link used for limited data transfer on
  the ground such as ATIS, taxi instructions,
  clearances, and graphical weather services. The
  information is displayed on a display unit on the
  flight deck.
• Satellite Communications (SATCOM) used for
  communications in oceanic and remote
  environments. Allows virtual global coverage and
  high quality communications. Not subject to
  typical radio reception problems.