VHF Direction FInder (VDF) — Page 80, Lesson 69

Let’s start with the principle of operation, because that’s the heart of the whole VDF system. The only equipment you need in the aircraft to get a VDF bearing is a VHF radio. That’s it. The clever stuff is on the ground. On the ground you need specialist equipment: a suitable aerial and a display. Now, why does the aerial matter so much? A VHF voice communications radio produces a vertically polarized signal. That means the electric field of the radio wave oscillates vertically. So the ground antenna must be vertically polarized to match it, and it’s built as an array of vertical elements arranged in a circle. That circular arrangement is what lets the system work out where the signal is coming from. Here’s how it works. The equipment resolves the bearing from the transmissions received at each element within the array. In other words, each vertical element in that circle picks up the signal slightly differently, and by comparing what each element receives, the system can work out the direction the transmission came from. That bearing is then displayed on the display. And importantly, the bearing can be displayed relative to either True North or Magnetic North at the station. So the reference is the station’s own north, not yours. Now let’s talk about range, because VDF has some very specific limitations. Since VDF uses the VHF band — or UHF as required — the range obeys the line of sight formula. The higher the transmitters, the greater the reception range. And here’s the actual formula: line of sight range in nautical miles equals 1.23 times the square root of the height of the transmitter plus the square root of the height of the receiver. So you’ve got hTX for the transmitter height and hRX for the receiver height, both in feet, and the result is in nautical miles. That’s why a high-flying aircraft gets a much longer VDF range than one down low. But there are other factors that limit range. Intervening high ground will limit range, especially for low flying aircraft in hilly terrain. The power of the airborne and ground transmitters will also limit ranges. And interestingly, gradients of temperature and humidity can give greater than line of sight range — so under certain atmospheric conditions you can actually get a bearing from beyond the geometric horizon. Now, accuracy. This is where things get interesting, because several things can degrade the bearing. First, propagation error and site error. These are caused by the aircraft’s transmissions being reflected from terrain as they travel to the site, or being reflected from buildings at the site. So the signal bounces off hills or buildings and arrives at the aerial from a slightly wrong direction, and that corrupts the bearing. Second, aircraft attitude. The VDF system and VHF communications are vertically polarized, so best reception and results are obtained when the aircraft flies straight and level. If you’re banking, the polarization changes and the bearing quality suffers. Third, poor accuracy is likely in the overhead of a VDF receiver, particularly with the latest Doppler systems. When you’re directly above the station, the reception of both the Direct Wave and the Ground Reflected Wave can cause signal fading or loss. That phenomenon is usually short-lived, but together with other multi path signals it gives rise to bearing errors. So being directly overhead is a bad place to ask for a bearing. Fourth, synchronous transmissions by two or more aircraft will cause momentary errors in bearings. If two aircraft transmit at the same time, the system gets confused for a moment. Finally, determination of position. If there are sufficient ground stations linked to an ATCC — that’s the Air Traffic Control Centre — the aircraft’s position can be fixed using auto-triangulation, and the position transmitted to the pilot. This facility may be available to Distress and Diversion Cells, but it cannot be guaranteed. So in an emergency, you might get a position fix from multiple VDF stations, but don’t rely on it. Now let me give you the VDF summary, because these four Q-codes are the ones you’ll actually use in the air. QDM is Magnetic TO the station. QDR is Magnetic FROM the station. QUJ is True TO the station. And QTE is True FROM the station. So the pattern is: D means TO, R means FROM, M means Magnetic, and UJ or TE means True. QDM and QDR are magnetic, QUJ and QTE are true. That’s the whole summary — those four bearings and their uses.