HF – High Frequency

[Ref AVIA4019, ACE The Technical Pilot Interview 149/348, 150/348]

  • 3-30 MHz (application in aviation from 2 to 22 MHz)
  • uses predictable sky wave propagation paths (i.e refracted off the earth’s ionosphere over great distances)
  • for longer-distance communication but degraded quality and for communications between two specific points only
  • usually for oceanic/desert areas which covers a large piece of airspace

Different frequencies have different range capabilities – the higher the frequency, the greater is its range. Therefore, a frequency is chosen by the pilot to meet the range between the transmitting and reception points, and the ground station will monitor a range of frequencies because personnel are unsure of the exact distance to the aircraft. This is also the reason why you can be received by a station 2000 NM away.


Factors that affect the range of HF communications:

  1. Transmitter power
  2. Frequency
  3. Disturbance of the ionosphere
    • Time of day
    • Season
    • Location



Read More –

How are HF communications affected at night (winter)?

At night, due to variation in the ionosphere, half of the HF frequency using at daytime produces the same range (skip distance)

During the day, particularly in the summer, the sun generates ion particles that make up the ionosphere’s D layer at a height of approximately 75 km. This layer has sufficient density to refract HF sky waves.

However, at night or during winter days when the exposure to the sun is less, the D layer disappears. As a result, the HF sky waves are refracted by the higher ionosphere’s E layer (at approximately 125 km). This increases the range (skip distance) of an HF transmission because of the greater vertical distance to the higher E layer before it is refracted. Therefore, because of the higher ionosphere at night, you need a lower frequency to reach the same receiver distance. In general, half the daytime frequency is needed because the signal is refracted more.

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