Automatic Direction Finder (ADF) — Page 90, Lesson 78

We’re starting the Automatic Direction Finder, or ADF, chapter. Let’s begin with the aircraft equipment, because everything else builds on it. The aircraft equipment comprises five items: a loop aerial, a sense aerial, a control unit, a receiver, and a display. The loop aerial is the key directional element — it’s what lets the system sense the direction of the incoming signal. The sense aerial resolves the 180-degree ambiguity that a loop alone would leave you with, so you know which side the beacon is actually on. The control unit is your interface for selecting frequencies and modes, the receiver processes the signals, and the display presents the resulting bearing to you. Now, the emission characteristics and the Beat Frequency Oscillator, or BFO. NDBs — non-directional beacons — have a two or three letter identification, and there are two types of emission: N0NA1A and N0NA2A. Let me break that down. The N0N part of the emission is the transmission of an unmodulated carrier wave. That means the signal has no audio information on it at all — so on a normal receiver, you wouldn’t hear anything. That’s why the ADF equipment provides a BFO. When you select the BFO, it produces an offset frequency within the receiver. When that offset frequency combines with the received frequency, it produces a tone — say 400 or 1020 Hz. So the BFO essentially creates an audible tone from a signal that would otherwise be silent. Now the A1A and A2A parts. A1A is the emission of an interrupted unmodulated carrier wave — the carrier is keyed on and off, but still unmodulated — so it requires the BFO to be ON for aural reception. A2A is the emission of an amplitude modulated signal, which can be heard on a normal receiver without the BFO. So here’s the practical rule. When using N0NA1A beacons, the BFO should be selected ON for manual tuning, identification, and monitoring. For N0NA2A beacons, you need the BFO ON for manual tuning, but OFF for identification and monitoring. And note — the BFO may be labelled TONE or TONE/VOICE on some equipment. Now let’s look at presentation of information. The information may be presented on a relative bearing indicator, or RBI, or a radio magnetic indicator, the RMI. In either case, the information being presented is relative bearing. The RBI has a standard compass rose where 360° is aligned with the fore-aft axis of the aircraft. With some RBIs, you can manually set the heading to directly read the magnetic bearing. Here’s the worked example from the diagram: the aircraft is heading 300°(M), the RBI is showing a relative bearing of 136°. So the magnetic bearing is 300° + 136° − 360° = 076°. The information from the ADF to the RMI is still relative, but the RMI compass card is fed with magnetic heading, so the bearing shown is the magnetic bearing of the NDB. And here’s the key relationship: the needle always points to the beacon — that’s the QDM — and the tail of the needle gives the QDR. So to tie it together: the loop and sense aerials feed the receiver, the BFO lets you hear unmodulated signals for tuning and identification, and the display — RBI or RMI — turns that into a bearing you can fly. The RBI gives you relative bearing, and the RMI gives you magnetic bearing directly.