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

Let’s start with the principle of operation, because that’s the heart of the VDF system. The only equipment you need in the aircraft to get a VDF bearing is your VHF radio. That’s it. The ground side needs specialist gear: a suitable aerial and a display. Here’s the physics behind it. A VHF voice communications radio produces a vertically polarized signal. That means the electric field of the radio wave oscillates in the vertical plane. Because of that, the ground antenna must also be vertically polarized, and it’s built as an array of vertical elements arranged in a circle. So you have a ring of vertical antennas on the ground. The equipment resolves the bearing from the transmissions received at each element within that circular array. In other words, by comparing the signal as it arrives at different points around the circle, the system works out the direction the transmission is coming from. That bearing is then displayed on the display. Now, an important detail: the bearing can be displayed relative to either True North or Magnetic North, at the station. So the reference for the bearing you see depends on how the station is set up. Let’s move to range. Because 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 I want to give you that formula exactly, because it’s a classic one: 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: Range (NM) = 1.23 × (√hTX + √hRX) Here, hTX is the height of the transmitter and hRX is the height of the receiver. Both heights are in feet, and the result is in nautical miles. That’s the standard line-of-sight formula you’ll use again and again in radio navigation. Now, several things limit that range. Intervening high ground will limit range, especially for low-flying aircraft in hilly terrain. The power of both the airborne and the ground transmitters will also limit range. And interestingly, gradients of temperature and humidity can give greater-than-line-of-sight range — that’s a refraction effect, where the signal bends around the curvature of the Earth. Next, factors affecting accuracy. This is where you need to be careful as a pilot, because accuracy is not guaranteed. 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 reflections off hills or buildings distort the apparent direction of arrival. 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 accuracy degrades. Third, poor accuracy is likely in the overhead of a VDF receiver, particularly with the latest Doppler systems. Here’s the mechanism: the reception of both the Direct Wave and the Ground Reflected Wave can cause signal fading or loss. That phenomenon is usually short-lived. Together with other multipath signals, this gives rise to bearing errors. So directly over the station, you get interference between the direct path and the ground-reflected path, and that corrupts the bearing. Fourth, synchronous transmissions by two or more aircraft will cause momentary errors in bearings. If two aircraft transmit at the same time, the VDF gets confused for that instant. Now, 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 the ground, but don’t count on it. Finally, the VDF summary — and this is the part you’ll need to memorise for the exam. There are four bearing codes: QDM — Magnetic TO the station. QDR — Magnetic FROM the station. QUJ — True TO the station. QTE — True FROM the station. So the middle letter tells you the reference: D for Magnetic, U for True. And the last letter tells you the direction: M for TO, R for FROM. QDM is what you’d fly to get to the station on a magnetic heading. QDR is the magnetic bearing from the station to you. QUJ is the true bearing to the station. QTE is the true bearing from the station. That’s the complete VDF picture: how the bearing is derived, what limits its range, what degrades its accuracy, how position fixing works, and the four bearing codes you’ll be tested on.