VHF Airband Propagation and Frequency Allocation: General Principles

090-06-01Pablo Asensio Martínez2026-04-012 min

Frequency Bands

The radio spectrum is divided into specific bands based on frequency. Aviation uses several of these:

Band Name Frequency Range Aviation Use
VLF Very Low Frequency 3 – 30 kHz Long range nav
LF Low Frequency 30 – 300 kHz NDB
MF Medium Frequency 300 – 3,000 kHz NDB, Commercial AM Radio
HF High Frequency 3 – 30 MHz Long Range Comms (Oceanic)
VHF Very High Frequency 30 – 300 MHz Standard Comms & Nav
UHF Ultra High Frequency 300 – 3,000 MHz Military, ILS Glideslope, DME
SHF Super High Frequency 3 – 30 GHz Radar, Radalt
EHF Extremely High Frequency 30 – 300 GHz -

The Airband (VHF)

The "Airband" is the VHF spectrum allocated for civil aviation, ranging from 108.000 MHz to 137.000 MHz.

Allocation

  1. Navigation (NAV): 108.000 – 117.975 MHz.
    • Used for: VOR, ILS (Localizer), ATIS (sometimes on VOR voice).
    • Note: Frequencies like 116.30 MHz are NAV frequencies and cannot be selected on a standard COM radio for voice transmission.
  2. Communication (COM): 118.000 – 136.975 MHz.
    • Used for: ATC, Air-to-Air, Operations.
    • Note: 121.500 MHz is the International Air Distress frequency.

Channel Spacing (8.33 kHz)

Traditionally, channels were separated by 25 kHz. To combat frequency congestion in Europe, channels were subdivided into 8.33 kHz spacing.

  • This creates 2 extra channels for every 25 kHz block.
  • Mandatory carriage of 8.33 kHz capable radios is required in the ICAO European region (e.g., above FL195).

VHF Propagation Characteristics

VHF radio waves propagate as Space Waves (Direct Waves).

Line of Sight

  • Transmission is "quasi-optical" or Line of Sight.
  • The waves travel in a straight line and do not bounce off the ionosphere (unlike HF).
  • Range is limited by the curvature of the Earth and terrain/obstacles.

Range Calculation

The theoretical maximum range depends on the height of the transmitter ($h_{TX}$) and the receiver ($h_{RX}$). $ Range (NM) \approx 1.23 \times (\sqrt{h_{TX}} + \sqrt{h_{RX}}) $ (Where heights are in feet).

  • Conclusion: Climbing to a higher altitude significantly increases VHF range and reception.

Factors Affecting Reception

  1. Altitude: Higher is better.
  2. Obstacles: Mountains/terrain block signals (Shadowing).
  3. Atmospheric Ducting: Under certain conditions (Temperature Inversion), VHF signals can be trapped in a duct and travel further than the normal line of sight.
  4. Attenuation: Signal strength weakens with distance due to atmospheric absorption.