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A pilot of an aircraft has difficulties hearing a VHF radio transmission due to its distance from the transmitter. What action is most likely to help the pilot to improve the reception?
  • A
    Initiate a descent.
  • B
    Initiate a turn.
  • C
    Set the transponder on stand-by.
  • D
    Initiate a climb.

VHF is amplitude modulated direct (line of sight) waves - practically straight-line propagation. Therefore, the VHF propagation characteristics are ideal for short-distance terrestrial communication. When, due to shadowing (e.g. by a hill), VHF communication is not possible, it is still possible to communicate using the upper part of the HF band where the surface wave is of little use.

The range of VHF propagation is the function of transmitter power:
R = 2 x √P

Where:

  • R is the range [NM]
  • P is the power [W]
The maximum distance at which one may expect solid VHF contact:

R = 1.23 x (√H + √h)
Where:

  • R is the range [NM]
  • H is the height over terrain [ft]
  • h is the elevation of the station]
  • The actual range depends on the height of the antenna. Thus, an aircraft at FL400 has more VHF range than one flying at 15 000 ft. Therefore, in order to improve your reception, one techinique is to initiate a climb. Another point to note is that transmission reception is highly dependant upon line of sight. When we are flying around at 15 000 feet, there are very few obstructions that our transmissions have to deal with.

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