Radio Waves and Propagation

Radio Waves and Their Propagation

Radio-navigation systems (VOR, NDB, ILS, GNSS, etc.) rely on electromagnetic waves between a ground transmitter and an aircraft receiver. Understanding wave propagation is essential for the correct interpretation of cockpit indications.

Source: ITU-R; ICAO Annex 10 Vol I Aeronautical Telecommunications — Radio Navigation Aids; FAA-H-8083-15 §9.

Electromagnetic waves — fundamentals

Speed

The propagation speed of electromagnetic waves is always equal to the speed of light (c = 300 000 km/s) — regardless of frequency or medium (approximately in a vacuum).

Frequency and hertz

The frequency of a radio broadcast is the number of oscillations per second of an electromagnetic wave, expressed in hertz (Hz). In general:

• Frequency = number of oscillations per unit time.
• Units: 1 kHz = 1000 Hz, 1 MHz = 10⁶ Hz, 1 GHz = 10⁹ Hz.
• Relation: c = λ × f (wavelength × frequency = speed of light).

Frequency bands in aviation

Propagation modes

1. Ground wave (surface wave)

• Description: the wave follows the Earth's surface, diffracting around the curvature.
• Strongest: at LF and MF (NDB, AM broadcast).
• Range: NDB typically 25-150 NM depending on power.
• Day-night: stable by day, degraded at night by sky-wave interference.

2. Sky wave / space wave

• Description: the wave is reflected by the sky (ionosphere) and returns to Earth at greater distance.
• The space wave of HF and MF waves propagates as a wave that is transmitted towards space and is reflected by the ionosphere.
• Strongest: at HF (3-30 MHz) — basis for long-range (transoceanic) radio.
• Day-night: at night the ionosphere is higher and reflects better → greater ranges, but interference with ground wave.

3. Direct / line-of-sight wave

• Description: the wave travels in a straight line from transmitter to receiver — like light.
• Radio waves in the VHF range (e.g. VOR) propagate as quasi-optical waves (direct).
• Strongest: at VHF, UHF, SHF — VOR, COM, DME, radar, GNSS.
• Limit: Earth curvature — transmitter must be "visible". Range depends on transmitter and receiver height.

NDB wave propagation

Electromagnetic waves of an NDB propagate from the transmission antenna in all directions, as ground and space waves:

• Ground wave is the primary source for ADF bearings by day.
• Space wave can cause night interference — sky-wave effect → false bearings.

VHF range — height-dependent

The range of a VHF link (VOR, COM) depends on transmitter and receiver height. Rule of thumb:

Range [NM] ≈ 1.23 × √(altitude [ft])

Example: over flat terrain at 5000 ft AGL a range of about 88 NM in VHF (VOR, COM) can be expected.

→ Aircraft altitude and elevation of the transmitter affect the range of VHF waves.

Doppler effect

The Doppler effect describes the apparent frequency change when transmitter and receiver move relative to each other:

• When transmitter and receiver are moving away from each other the perceived frequency decreases and vice versa.
• Aviation use: Doppler VOR (DVOR), Doppler navigation radar.

Disturbance and distortion effects

Absorption (attenuation)

• The attenuation of transmitted energy (conversion of electrical energy to heat) is called absorption.
• Radio waves are absorbed to a higher degree at high frequency and moist air.
• Consequence: VHF and UHF are attenuated in humid air (precipitation, cloud) → reduced range.

Interference

• When two or more radio waves of approximately equal wavelength superimpose, this is called interference.
• Consequence: signal maxima and minima along propagation — fluctuating reception.

Fading

• Fading describes a phenomenon of interference and polarisation errors, e.g. ground- and space-wave superposition.
• Fading in the LF/MF frequency range occurs mainly during the night — when the sky wave starts to interfere with the ground wave.

Reflection and refraction — coast / mountain effects

• Distortions in radio-wave propagation, especially in HF and MF, cause bearing errors — most commonly coast and mountain effects, affecting NDB AND VOR.
• Coast effect / shoreline effect: at the coast NDB waves refract due to land/water conductivity difference → bearing errors up to 10°. NDB is the most affected by the so-called shoreline effect. The ADF can deliver false bearings due to the coast effect.
• Mountain effect: mountains reflect radio waves → multipath produces bearing errors in NDB receivers.

Twilight effect

• Radio communication is affected by the twilight effect — at sunrise/sunset the ionosphere changes rapidly.
• An ADF can operate inaccurately during twilight — sky-wave interference with the ground wave.

Lightning

• Lightning, mountain and coast effect affect the indication of the radio compass (ADF) — lightning produces broadband emission that appears as a spike on the ADF indicator and may disrupt the bearing.

What does NOT affect radio waves

"Extension and amplification" are physical influences that do not affect a radio wave — these are transmission-engineering terms, not intrinsic wave phenomena.

Modulation — how information is impressed on the wave

Modulation = impressing information (audio, data) onto a carrier wave.

Amplitude modulation (AM) is used for radio communication and for the identifier of radio navigation aids (NDB, VOR, ILS etc.).

Polarisation

Radio waves are electromagnetic waves with a defined oscillation plane of the electric field (polarisation):

• Vertically polarised: vertical antenna (e.g. NDB transmit antennas).
• Horizontally polarised: horizontal antenna.
• A receiver antenna with mismatched polarisation reduces signal strength.

Requirements for a good radio link

A prerequisite for flawless radio connection is, amongst others, a sufficient altitude within the possible range of transmission/reception. Further:

• Line of sight to the transmitter (for VHF/UHF).
• No heavy weather between transmitter and receiver.
• Antennas correctly mounted and undamaged.
• Onboard electrical power OK (see Subject 020 ausfallanzeigen).