VHF direction finding and the Q codes

Navigation · Chapter 37 · DGCA CPL and ATPL

How a ground station takes a bearing on your own call, the four Q codes and how to convert between them, and what VDF is good for: accuracy, range, errors and fixes.

Remember

VDF: an Adcock ring on the ground takes a bearing on your VHF call; you need only a VHF radio.
FROM the station: QTE (true), QDR (magnetic, radial). TO: QUJ (true), QDM (magnetic, no-wind heading).
Reverse: ± 180. True ↔ magnetic: variation west, magnetic best; east, magnetic least.
Classes A ± 2°, B ± 5°, C ± 10°, D worse; no class = B. Doppler VDF ± 0.5°.
Range 1.23 (√h₁ + √h₂) NM. Errors: site, attitude, overhead, fading, two at once. 121.5: auto-triangulation.

What is VDF, and why would a pilot use it?

A bearing on your voice

Ask the ground • You are not sure where you are, or which way the airfield is • You talk on the radio; a station on the ground works out which way your voice came from and tells you • That is vdf
Nothing extra on board • Every aircraft already carries a VHF radio to talk to ATC • VDF listens to that same call, on the VHF voice band 118–137 MHz (AM voice, A3E) • All the special kit is on the ground: the aerials and a display
The answer comes by voice • The controller reads the bearing off the display and says it: the code you asked for, the bearing and its class
Why it is rarely used • VOR, DME and GNSS give you a bearing or a position by themselves • VDF needs a controller and a call every time, so it is a backup that needs nothing but a radio

How does a ring of aerials know which way you are?

How the ring hears direction

Ripples • Your call spreads out from the aircraft in circles, like ripples on a pond • Watch the ring: each aerial lights as a ripple reaches it
Nearest first • The aerial nearest you hears each ripple first, the far side last • Opposite pairs: the pair pointing at you hears the biggest difference, the pair side-on hears none • From those differences the receiver works out your direction: an adcock array
Why upright aerials • An aircraft's VHF aerial stands upright, so its wave is polarised • The ground aerials stand upright too: matched, they pick it up best
Move the aircraft • Drag the slider: the lit side follows you round, and so does the trace
The display • A rose with a trace pointing at the caller • A selector chooses QDR, QDM or QTE; the station can show bearings from true north or from magnetic north
Doppler VDF • Newer sets switch round the ring so fast that it acts like one spinning aerial • Spinning towards you the signal's pitch rises, away it falls; where it peaks is your direction • A digital read-out good to about ± 0.5°: doppler