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

All right, let's pick up with the Automatic Direction Finder, or ADF. We've already set the scene with the loop aerial and the sense aerial, so now I want to walk you through how they work together to actually give you a bearing. The key idea here is that the field from the sense aerial is arranged to be in phase with one element of the loop aerial—specifically, the left-hand element shown in the diagram. When you combine those two fields, the resultant polar diagram is known as a CARDIOID. That's a heart-shaped pattern, and it's important because it has a single null—a single point of minimum signal. Now, here's the catch. That null in the cardioid is ill-defined. It's not sharp enough to give you an accurate bearing on its own. So the system needs a way to find the correct null in the loop aerial. That's done by introducing a logic circuit. The logic circuit defines the correct null as the one in the loop aerial which, when the loop is rotated clockwise, produces an increase in signal strength in the cardioid. So it's a directional check—it tells you which null is the right one. But even with that, the resultant null from a single cardioid isn't precise enough to meet the ICAO accuracy requirement of ±5°. That's a strict tolerance, and a single cardioid just doesn't cut it. So to improve the accuracy, the polarity of the sense aerial is reversed to produce a right-hand cardioid. Then, by rapidly switching between the two cardioids—at about 120 Hz—the null becomes much more precisely defined. That switching sharpens the null, and that's how the accuracy is improved to meet the standard. Now, in reality, you can't have a rotating loop outside the aircraft. It's just not feasible. So the loop is fixed, and it has four elements. Two are aligned with the fore-aft axis of the aircraft, and the other two are perpendicular to the fore-aft axis. The electrical fields from these elements are transmitted to a similar set of four elements in a goniometer. The goniometer reproduces the electromagnetic field detected by the aerial. The signal from the sense aerial is also fed to the goniometer, where a search coil detects the unambiguous direction. The principle employed within the goniometer is exactly as I described above—the same cardioid and null logic, just reproduced inside the aircraft. Let's move on to the frequencies and types of NDBs. The allocated frequencies for NDBs are 190 to 1750 kHz, spanning the LF and MF bands. Since the mode of propagation used is surface wave, most NDBs will be found between about 250 and 450 kHz. That's where they typically operate. There are two types of NDB in current use. First, the Locator, designated L. These are low-powered NDBs used for airfield or runway approach procedures, or they're co-located with, and supplement, the outer and middle markers of an ILS system. They normally have ranges of 10 to 25 NM, and they may only be available during an aerodrome's published hours of operation. Second, there are En route NDBs. These have a range of 50 NM 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. So to tie it all together: the sense aerial and loop aerial combine to form a cardioid, the logic circuit and rapid switching between two cardioids sharpen the null to meet ICAO accuracy, the fixed loop and goniometer reproduce that field inside the aircraft, and the NDBs themselves operate in specific frequency bands with two distinct types for different operational roles.