Automatic Direction Finder (ADF) — Page 90, Lesson 76

Let's pick this up with the sense aerial and the loop aerial working together, because that's the heart of how an ADF actually resolves a bearing. I want you to picture the loop aerial first. A loop aerial on its own has a figure-of-eight polar diagram — two lobes, one positive, one negative — and it produces two nulls, two points of minimum signal, 180 degrees apart. That's ambiguous. You can't tell which null is the true bearing and which is the reciprocal. That's the fundamental problem the ADF has to solve. Now we bring in the sense aerial. The sense aerial is non-directional — it receives equally from all directions. The clever bit is this: the field from the sense aerial is arranged to be in phase with one element of the loop aerial — specifically the left-hand element, as shown in the diagram. When you combine the sense aerial's omnidirectional pattern with the loop's figure-of-eight, you get a new polar diagram, and that shape is called a cardioid. A cardioid has a single null — only one point of minimum signal — so it removes the 180-degree ambiguity. But here's the catch: that single null is ill-defined. It's broad and shallow, so on its own it would not give you an accurate bearing. It tells you the general direction, but not precisely enough. So how do we sharpen it? We introduce a logic circuit. The logic circuit defines the correct null as being that null in the loop aerial which, when the loop aerial is rotated clockwise, produces an increase in signal strength in the cardioid. Think about that for a moment. As you rotate the loop, the signal in the cardioid changes. The logic circuit watches which way the signal moves as the loop turns clockwise. That tells it which null is the true one — the correct null — as opposed to the reciprocal. But even with that logic circuit, the single cardioid's null is still not precise enough. The ICAO accuracy requirement for ADF is plus or minus 5 degrees. A single cardioid can't meet that. So the system does something elegant: it reverses the polarity of the sense aerial to produce a right-hand cardioid — a mirror image of the first. Then it rapidly switches between the two cardioids, at about 120 Hz, which is 120 times per second. By comparing the two cardioid patterns as they switch, the null becomes much more sharply defined, and that's how the accuracy is improved to meet the ICAO requirement. Now, here's a practical problem. In reality, you can't have a rotating loop sticking out of the aircraft — it's not feasible aerodynamically or mechanically. So the loop is fixed, and it has four elements: two aligned with the fore-aft axis of the aircraft, and the other two perpendicular to the fore-aft axis. So you have one pair along the aircraft's longitudinal axis and one pair across it. The electrical fields from these four elements are transmitted to a similar set of four elements in a device called a goniometer. The goniometer reproduces the electromagnetic field that the aerial detected — it's essentially a remote, stationary replica of the rotating loop. The signal from the sense aerial is also fed into the goniometer. Inside, a search coil rotates and detects the unambiguous direction. The principle inside the goniometer is exactly the same as the rotating loop principle I described earlier — the search coil plays the role of the rotating loop, and the fixed four elements reproduce the field it would have seen. Let me now move to the other side of the system: the ground stations, the NDBs — Non-Directional Beacons. The allocated frequencies for NDBs are 190 to 1750 kHz, spanning the LF — Low Frequency — and MF — Medium Frequency — bands. Because the propagation mode used is surface wave, meaning the signal follows the curvature of the Earth, most NDBs are found between about 250 and 450 kHz. That's the sweet spot for practical surface-wave propagation. There are two types of NDB in current use. The first is the Locator, designated with the letter L. These are low-powered NDBs used for airfield or runway approach procedures, or they are co-located with — and supplement — the outer and middle markers of an ILS system, the Instrument Landing System. Locators normally have ranges of 10 to 25 nautical miles, and they may only be available during an aerodrome's published hours of operation — so they might not be on all the time. The second type is the En route NDB. These have a range of 50 nautical miles or more, and where they serve oceanic areas, they may have ranges of several hundred miles. They're used for homing, holding, en route and airways navigation — the everyday workhorses of NDB navigation. So to tie it together: the sense aerial resolves the loop's ambiguity into a cardioid, the logic circuit and the 120 Hz switching between left and right cardioids sharpen the null to meet the ICAO plus-or-minus 5 degree requirement, the fixed four-element loop and goniometer make it physically practical, and on the ground, NDBs come in two flavours — Locators for approach work and En route NDBs for the bigger picture.