Lecture 3: Very High Frequency Direction Finding (VDF)

Introduction to VDF

• VDF stands for Very High Frequency Direction Finding.
• Utilizes VHF signals to determine direction.
• Relies on communication signals to find direction.

Radio Communication Basics

• Antennas:
  • Ideal length is half the wavelength of the radio wave.
  • Quarter wavelength antennas also used but less efficient.
• Frequency Bands:
  • Low-frequency signals (e.g., 300 kHz) are impractical due to long wavelengths.
  • High-frequency (HF) and very high frequency (VHF) bands are used in aviation.

High Frequency (HF) Communications

• Suitable for long-range communication due to propagation characteristics.
• Frequency range: 2 to 22 MHz.
• Wavelengths: 150 m to 13.6 m.
• Ideal antenna length is around 7 m, typically placed in the tail of aircraft.
• HF signals face challenges with antenna size and signal clarity.

Very High Frequency (VHF) Communications

• Frequency range: 118 MHz to 137 MHz.
• Wavelengths: 2.5 m to 2 m.
• Ideal antenna length: about 1.25 m, easily fits in aircraft tail.
• Propagates via line-of-sight (space waves only).
• Range dictated by max theoretical range formula.

VDF (Very High Frequency Direction Finding)

• Utilizes a circular array of dipole antennas, known as Adcock aerial.
• Determines direction based on phase differences in received signals.
• Provides air traffic control with the direction of incoming signals.
• The direction is usually a magnetic direction.

Q Codes in VDF

• QDR: Magnetic bearing from station to aircraft.
• QDM: Magnetic bearing to station from aircraft (reciprocal of QDR).
• Accuracy classes:
  • Class A: ±2°
  • Class B: ±5°
  • Class C: ±10°
  • Class D: Less than ±10°

Usage and Errors in VDF

• Used infrequently due to high workload on air traffic controllers.
• Common in quiet airports and in situations like cloud cover or getting lost.
• Errors can degrade bearing accuracy:
  • Multiple Signal Error: Difficulty differentiating signals if multiple broadcasts on the same frequency.
  • Terrain Error: VHF signals blocked by mountains or terrain.
  • Refraction Error: Signal bending around obstacles, altering direction.
• Errors can downgrade accuracy, making it less reliable for navigation.