Automatic Direction Finder (ADF) — Page 99, Lesson 88

Let's pick this up with the errors and limitations that affect ADF — these are the things that can quietly corrupt your bearing if you don't know what to look for. First, quadrantal error. The loop aerial's theoretical reception pattern — the polar diagram — gets distorted by the airframe itself. The aircraft structure produces a strong electrical field aligned fore and aft, and incoming NDB signals get refracted toward that fore-and-aft axis. The maximum refraction happens in the quadrants — that is, on relative bearings of 045°, 135°, 225°, and 315°. Older ADF systems are regularly "swung" to assess the value of this quadrantal error. In modern aircraft, the manufacturer determines the error and corrections are built into the equipment to reduce the effect to a minimum. Next, angle of bank — dip error. A loop aerial is designed to use vertically polarized waves for direction finding. If the incoming wave has any horizontal component of polarization, it induces currents in the top and bottom horizontal members of the loop, producing a circulating current. That destroys the nulls of the polar diagram — similar to night effect — and reduces bearing accuracy. When you bank during a turn, currents are induced in the horizontal elements of the loop, leading to a bearing error called dip error. The key point: this error is only present when the aircraft is not in level flight. Then there's the lack of failure warning system. False indications due to a system failure are not readily detectable because most ADF instruments have no failure warning. So you must exercise particular care in identifying and monitoring the NDB, and make independent cross-checks with other navigational aids where possible. It's essential that when you use ADF as the primary navigation aid — for example, for a runway approach procedure — you continuously monitor it to detect any failure. Now, the factors affecting ADF range. The major ones are listed. First, NDB transmission power: range is proportional to the square root of the power output. So to double the NDB range, you must quadruple the transmitter power output. Second, NDB range is greater over water — and there's a formula pair for this. Over water, range is 3 × √P, where P is in watts. Over land, it's 2 × √P. But note: using ranges calculated by these formulae does not guarantee the aircraft will be within the DOC — the declared operational coverage. Third, frequency: the lower the frequency, the greater the surface wave — meaning greater diffraction and lower attenuation. Fourth, precipitation: all precipitation, including falling snow, reduces the effective range and accuracy of ADF bearings. Fifth, NDB classification: N0NA1A NDBs have greater ranges than N0NA2A — but note that ICAO Annex 10 recommends the use of N0NA2A for long-range beacons. And finally, receiver quality affects range as well. So when you're planning an ADF leg, you're balancing transmitter power, surface type, frequency, weather, beacon classification, and receiver quality — and even then, the calculated range is no guarantee you'll be inside the declared operational coverage.