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

Let’s start with the big picture. The Automatic Direction Finder, or ADF, is the airborne equipment, and it works with a ground-based transmitter called a Non-directional Beacon, or NDB. Together they give you a bearing to the beacon, which you can use for navigation and for non-precision approaches to airfields. I want to be clear on the status of this system: it was due to be phased out in 2005, but it still continues in use. In fact, many UK aerodromes still have NDB instrument approach procedures, and at some aerodromes it is the only instrument approach procedure available. So you will still see this in the real world. Now let’s look at the NDB itself. The Non-directional Beacon is a ground-based transmitter. It transmits vertically polarized radio signals, and it transmits in all directions — that’s why it’s called non-directional. It operates in the Low Frequency and Medium Frequency bands, which we abbreviate as LF and MF. When your aircraft’s ADF is tuned to the NDB’s frequency and you have identified its call sign, the direction of the NDB will be indicated on your instrument. There’s one important operational quirk you need to know about: directly overhead the NDB transmitter there is a ‘cone of silence’. During the time the aircraft is inside that cone, it does not receive any signals from the beacon. And here’s the key relationship — the diameter of that cone increases with aircraft height. So the higher you are, the wider the silent zone overhead the beacon. Now let’s get into the principle of operation, because this is where the physics matters. The ADF measures the bearing of an NDB relative to the fore/aft axis of the aircraft. That means it tells you the direction to the beacon measured from the aircraft’s nose-to-tail line. The sensing element is a loop aerial. Here’s the physics: 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. That happens because of the phase difference of the wave in each of the vertical elements. In plain terms, the radio wave reaches the two vertical sides of the loop at slightly different times, and that phase difference creates a voltage. As you rotate the loop, the induced voltage decreases until it becomes zero when the loop is perpendicular to the radio wave. If you keep rotating past that point, a voltage is induced in the opposite sense. That behaviour produces a polar diagram shaped like a figure of eight, and I want you to look at that carefully. There are two null positions on that figure-of-eight pattern. A null is where the received signal drops to zero. So by rotating the loop until you reach a null, you can determine the direction of the beacon. That works fine if you already know roughly which way the beacon is. But if you don’t know the approximate direction, you have a problem — because there are two possible choices. The figure of eight is symmetrical, so a null could mean the beacon is in one direction or exactly the opposite direction. And here’s the deeper issue: if the equipment is to automatically determine position, then with only the single loop it would have an insoluble problem. It simply cannot resolve which of the two directions is correct. To resolve this ambiguity, a simple dipole aerial is added, and it’s called a sense aerial. The polar diagram of the sense aerial is circular — it receives equally well from all directions. The currents from the loop and the sense aerial are combined electronically, as if the sense aerial were placed in the middle of the loop aerial. The relative signal strengths of the two signals determine the result. That combination is what breaks the left-right ambiguity and lets the system decide which of the two null directions is the true bearing to the beacon. So to summarise the chain: the NDB transmits vertically polarised LF/MF signals in all directions. The ADF’s loop aerial produces a figure-of-eight pattern with two nulls, which gives you a direction but with a 180-degree ambiguity. The sense aerial, with its circular pattern, is combined electronically with the loop signal to resolve that ambiguity. And remember the cone of silence overhead the beacon, which widens as you climb. That’s the complete picture of how ADF and NDB work together.