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

I want to walk you through the Automatic Direction Finder — the ADF — starting with the aircraft equipment itself. This is the kit that lets you home in on Non-Directional Beacons, the NDBs, and it's a classic piece of radio navigation you'll see on many aircraft. The aircraft equipment comprises five main items. First, a loop aerial — that's the directional antenna that senses the bearing to the beacon. Second, a sense aerial — this is the non-directional antenna that resolves the 180-degree ambiguity of the loop, so the needle knows which way is actually toward the beacon rather than the opposite direction. Third, a control unit — that's your panel where you tune and select the ADF. Fourth, a receiver, which processes the signals. And fifth, a display — the instrument that shows you the bearing information. Now, before we get to the display, I want to cover the emission characteristics and the Beat Frequency Oscillator, the BFO. The NDBs have a 2 or 3 letter identification — that's the Morse code identifier you hear. And there are two types of emission: N0NA1A and N0NA2A. Let me break down that emission designator. The N0N part is the transmission of an unmodulated carrier wave. Now, an unmodulated carrier would not be detectable on a normal receiver — you'd hear nothing. So the ADF equipment provides a BFO, the Beat Frequency Oscillator. 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. That's the beat you hear. Now the next part of the designator. A1A is the emission of an interrupted unmodulated carrier wave — so the carrier is keyed on and off to send the Morse identification. Because it's unmodulated, it requires the BFO to be ON for aural reception — you need that beat tone to hear anything. A2A, on the other hand, is the emission of an amplitude modulated signal — that one 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. When using N0NA2A beacons, the BFO should be ON for manual tuning, but OFF for identification and monitoring — because the signal is already amplitude modulated and audible. And note, the BFO may be labelled TONE or TONE/VOICE on some equipment. Now let's move to the presentation of information — how the bearing is actually shown to you. The information may be presented on a Relative Bearing Indicator, the RBI, or a Radio Magnetic Indicator, the RMI. In either case, the information being presented is relative bearing — that's the bearing of the beacon relative to the aircraft's heading. The RBI has a standard compass rose where 360° is aligned with the fore-aft axis of the aircraft. So the needle shows the bearing relative to the nose. With some RBIs, it's possible to manually set the heading to directly read the magnetic bearing. Let me give you the worked example from the diagram. The aircraft is heading 300°(M) — that's 300 degrees magnetic. The RBI is showing a relative bearing of 136°. So the magnetic bearing is 300° plus 136° minus 360°, which equals 076°. That's the magnetic bearing to the beacon. Now the RMI — 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 directly — you don't have to do the arithmetic. And here's the key relationship to remember. The needle always points to the beacon — that's the QDM, the magnetic bearing to the station. And the tail of the needle gives the QDR — the magnetic bearing from the station. So the head points toward, the tail points from. Let me just tie that together. The loop and sense aerials feed the receiver, the control unit tunes it, and the display — RBI or RMI — shows you relative bearing. The BFO lets you hear the unmodulated N0N signals. And remember the needle head is QDM, the tail is QDR. That's the ADF system in a nutshell.