Part 1: Radio Direction Finding – The Foundation

Let's talk about the overhead passage—a particularly useful application of VDF that the exam expects you to know.

Imagine you're flying toward a VDF station, and you want to know exactly when you pass directly over it. You can't always see the station (it might be a small radio antenna), and depending on your altitude and visibility, the ground might be hazy or obscured. So you use VDF: you call for successive bearings as you approach.

When you're far away, the bearing stays fairly constant. As you get closer and closer, the bearing doesn't change much because you're approaching nearly along the same line. But at the moment you pass directly over the station, something remarkable happens: the bearing becomes undefined. Why? Because the station's antenna is pointed at you, but you're directly above it. The bearing changes from whatever it was before to the reciprocal immediately after you pass overhead.

In practice, this shows up as a reversal. Let's say you've been receiving QDM 090 (fly toward the station, which is to the east). As you approach, the QDM stays around 090. Then, within a span of maybe 30 seconds, the bearing changes to approximately 270 (the reciprocal, now pointing away). That swing from 090 to 270 tells you: you just passed over the station.

This is called the "overhead passage indication." It's not a magic trick—it's a geometric reality. The station can't give you a useful bearing when you're directly overhead because the antenna sees you in all directions equally (or in some cases, has a null directly overhead). The sudden reversal is your clue.

How do you confirm this? Request QDM bearings at regular intervals as you approach. As you get closer, the bearing should stabilize. Then, after you report that you've passed the station, request another bearing. It should be roughly the reciprocal of the previous one. This technique is called "homing to the station with sequential bearings."

Sequential Bearings Approaching a Station

Graz Radio · OE-GBG

FREQUENCY: 119.1 MHz

PILOT: Graz Radio, OE-GBG, ten miles south of Graz, REQUEST QDM altitude one thousand five hundred.

ATC OE-GBG, Graz, Class B available. Your QDM is three-three-zero.

PILOT: QDM three-three-zero, OE-GBG, flying north toward you. [5 minutes later]

PILOT: Graz Radio, OE-GBG, now three miles south, REQUEST QDM.

ATC: OE-GBG, your QDM is three-three-zero.

PILOT: QDM three-three-zero, OE-GBG. [5 minutes later, passing overhead]

PILOT: Graz Radio, OE-GBG, over the station, continuing north.

ATC: OE-GBG, ROGER. At your next position report, REQUEST another bearing. [5 minutes later]

PILOT: Graz Radio, OE-GBG, five miles north of Graz, REQUEST QDM.

ATC: OE-GBG, Class B, your QDM is one-five-zero.

PILOT: QDM one-five-zero, OE-GBG.

Notice the pattern: QDM was 330° approaching (fly northwest toward the station), then flipped to about 150° after passing (fly southeast away from the station). The reversal of roughly 180° is your overhead confirmation.