Radio Wave Propagation in Aviation

062-01-03Pablo Asensio Martínez2026-04-012 min

The way radio waves travel from the transmitter to the receiver depends on their frequency and the environment.

Propagation Modes

1. Space Waves

These are Line-of-Sight waves. They travel in a straight line through the atmosphere.

  • It is the primary mode for VHF, UHF, SHF, and EHF.
  • Range is limited by the curvature of the Earth and obstacles.
  • Theoretical Range Formula (VHF): $Distance (NM) = 1.23 \times (\sqrt{Height_{Tx}(ft)} + \sqrt{Height_{Rx}(ft)})$

2. Ground/Surface Waves

These waves follow the curvature of the Earth due to diffraction.

  • Effective at low frequencies (VLF, LF, MF).
  • Range depends on power and surface type (greater range over sea than over land).
  • Used by NDBs.

3. Sky Waves

These are waves that refract in the ionosphere and return to Earth, allowing communications beyond the horizon.

  • Primary propagation mode for HF (and MF at night).
  • Allows transoceanic and global communications.

The Ionosphere

A layer of the atmosphere ionized by solar radiation. It mainly affects HF waves.

  • Layers: D, E, F1, F2.
  • D Layer: Exists only during the day and absorbs low-frequency waves (LF/MF), preventing their propagation as sky waves.
  • Night: The D layer disappears, allowing MF and HF waves to reach the upper layers (E and F) and refract, drastically increasing range (and interference).

Key HF Propagation Concepts

  • Skip Distance: The minimum distance from the transmitter to where the first sky wave returns to Earth.
    • Increases with frequency.
    • Increases if the ionosphere is higher (night).
  • Skip Zone (Dead Zone): An annular area where no signal is received: it is too far for the ground wave but too close for the first sky wave.
  • Fading: Signal fluctuation due to changes in the ionosphere or interference between multiple paths (e.g., sky wave vs. ground wave).

Phenomena and Errors

Doppler Effect

Change in perceived frequency due to relative motion between source and receiver.

  • Frequency increases if approaching, decreases if receding.
  • Used in: DVOR (to create the variable phase signal), GNSS (to determine velocity), MTI Radars, and Weather Radars (to detect turbulence).

ADF Errors (LF/MF Waves)

  • Night Effect: Interference between ground wave and sky wave at night. Causes needle oscillation.
  • Coastal Refraction: The wave bends towards the coast when passing from land to sea (change in speed).
  • Mountain Effect: Reflections in mountainous terrain.
  • Thunderstorms: Lightning emits signals in LF/MF bands that attract the ADF needle.
  • Quadrantal Error: Deviation caused by the metal of the aircraft itself. Maximum at relative bearings 045°, 135°, 225°, 315°.

Physical Phenomena

  • Refraction: Change of direction when passing into a medium with different density (e.g., ionosphere).
  • Diffraction: Bending of the wave around obstacles (allows ground wave).
  • Reflection: Bouncing off surfaces (ground, buildings).
  • Absorption: Loss of energy (e.g., D layer).