Automatic Direction Finder (ADF) — Page 90, Lesson 74

Let's start with the Automatic Direction Finder — the ADF. This is a piece of equipment that's been around for a long time, and it's still relevant today, so I want you to understand it properly. First, the big picture. The ADF in the aircraft works together with a simple ground-based transmitter called a Non-Directional Beacon — the NDB. The NDB is a ground transmitter that sends out vertically polarized radio signals in all directions — that's why it's called "non-directional." It operates in the Low Frequency and Medium Frequency bands — LF and MF. So the beacon itself doesn't point anywhere; it just broadcasts. The ADF is used for navigation and for non-precision approaches to airfields. Now, here's an interesting bit of history: this system was due to be phased out in 2005, but it's still in use. In fact, many UK aerodromes still have NDB instrument approach procedures, and at some aerodromes it's the only instrument approach procedure available. So don't dismiss it as obsolete — you need to know it. Now, how does the aircraft use it? When the ADF is tuned to an NDB's frequency and the call sign is identified, the direction of the NDB will be indicated. So the ADF measures the bearing of the NDB relative to the fore/aft axis of the aircraft — that's the aircraft's longitudinal axis, the nose-to-tail line. So it tells you where the beacon is relative to which way the aircraft is pointing. There's also something called a "cone of silence" overhead the NDB transmitter. During that cone, the aircraft doesn't receive any signals. And the diameter of that cone increases with aircraft height — the higher you are, the wider the silent zone directly above the beacon. Now let's get into the principle of operation, because this is where the physics comes in. The ADF uses a loop aerial. If a loop aerial is placed in the plane of the transmitted radio frequency, a voltage will be generated in the vertical elements of the loop. Why? Because of the phase difference of the wave in each of the vertical elements. The wave reaches one side of the loop slightly before the other, so there's a phase difference, and that generates a voltage. As the loop is rotated, the induced voltage decreases until it becomes zero when the loop is perpendicular to the radio wave. That zero point is called the null. As the loop continues to rotate, a voltage is induced in the opposite sense — so the signal reverses polarity. The polar diagram formed by this loop is a figure of eight. Look at Figure 7.1 and Figure 7.2 — you'll see the loop aerial and the figure-of-eight pattern. There are two null positions in that figure of eight. So by rotating the loop until you reach a null, you can determine the direction of the beacon. But here's the problem. If you only have the single loop, you get two possible choices — two nulls, 180 degrees apart. If you know the approximate direction of the beacon, you can pick the right one. But if you don't, you have an ambiguity — you can't tell which null is the true bearing. And if the equipment is to automatically determine position, then with only the single loop it would have an insoluble problem. It can't resolve which of the two directions is correct. To resolve this ambiguity, a simple dipole aerial is added — and this is called the sense aerial. The polar diagram of the sense aerial is circular — it receives equally from all directions. The currents generated by the loop and the sense aerial are combined electronically, as if the sense aerial were in the middle of the loop aerial. Look at Figure 7.3 — you can see the relative signal strengths of the two signals. The sense aerial's circular pattern, combined with the loop's figure of eight, breaks the symmetry. One of the two nulls becomes a true null, and the other becomes a weaker, non-null response. That's how the ambiguity is resolved — the sense aerial tells you which of the two directions is the real bearing. So to sum up the key points: the NDB is a ground transmitter in LF and MF, vertically polarized, broadcasting in all directions. The ADF measures bearing relative to the aircraft's fore/aft axis. The loop aerial gives you a figure-of-eight pattern with two nulls — that's the ambiguity. The sense aerial, with its circular pattern, resolves that ambiguity. And remember the cone of silence overhead the beacon, which widens with altitude. That's the foundation of ADF.