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Two aircraft are flying overhead a VOR. Aircraft X is at 1500 ft and aircraft Y is at 15 000 ft. Both are 80 NM away from the destination airfield. Both aircraft have tuned their VHF radio to the ATIS frequency of the destination. The quality and reception of the receiver signal…

  • A

    will be better for aircraft Y because it is flying higher.

  • B

    will be better for aircraft X because it is flying lower.

  • C

    altitude does not influence VHF radio communications.

  • D

    will be the same for both aircraft X and Y because they are both 80 NM away from the destination.

Note: This question is a new version of question 906485.

VHF transmissions are mainly influenced by physical objects, such as terrain and ground cover (clutter), and tropospheric factors, principally refraction. The primary mode of propagation for VHF radio waves is through direct waves. These travel in a straight line, becoming weaker as distance increases. Transmitting and receiving antennas must be able to 'see' each other for communications (Line of sight) to be effective, so antenna height is critical in determining range.

The straight line joining the sender and the receiver must be relatively free of obstacles.

  • Low level flight reduces greatly the range of reception (because of mountains, hills, etc).

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 15 000 ft has more VHF range than one flying at 5000 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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