Let's pick this up right where the ionosphere leaves off — Page 99, Lesson 86

Let's pick this up right where the ionosphere leaves off. I want to walk you through what happens to your ADF at night, and then the three big error sources that can corrupt your bearings: station interference, mountain effect, and coastal refraction. First, the night-time problem. During the day, the D-region of the ionosphere absorbs signals in the LF and MF bands — that's the low frequency and medium frequency bands where NDBs operate. That absorption is actually your friend, because it kills the sky wave. At night, the D-region disappears. And when it goes, the sky wave — the signal that bounces off the ionosphere — comes back down and contaminates the surface wave you're actually using for your bearing. Now, why does that contamination happen? Two reasons. First, phase interference: the sky wave and the surface wave travel different paths, so they arrive out of phase and interfere with each other. Second, the sky wave induces currents in the horizontal elements of the loop aerial — the very part of your antenna that's trying to sense direction. The aerial design helps a little: very short vertical elements and screening the aerial above and below reduce the effect. But I want to be clear — the contamination is not eliminated. It's reduced, not cured. When does this start to bite? The effect first becomes significant at 70 to 100 nautical miles from the NDB. And how do you see it? The audio signal fades, and the needle "hunts" — it swings around instead of sitting steady. It's worst around dawn and dusk, when the ionosphere is in transition, changing from day to night state. So if you're going to use ADF at night, here's the drill. Positively identify the NDB call sign. Keep checking the tuning and the identification throughout. Avoid using the equipment within 1 hour of sunrise or sunset. Use NDBs within their promulgated range — and note that range is valid during daytime only. Treat bearings with caution if the needle wanders and the signal fades. And cross-check your NDB bearing information against other navigation aids. That last one is your safety net. Now, station interference. The LF and MF bands are congested — lots of stations packed in. So there's always the possibility of interference from stations on or near the same frequency, and that causes bearing errors. By day, using an NDB within its DOC — that's the Documented Operating Coverage, the published range — normally protects you from interference. But at night, expect interference even within the DOC, because of sky wave contamination from stations that are out of range by day. Their sky waves reach you at night. That's exactly why positive identification of the NDB at night should always be carried out — you have to be sure you're tracking the station you think you are. Next, mountain effect. Mountainous areas cause reflections and diffraction of the transmitted radio waves, and both of those produce errors in ADF systems. Here's the key relationship: these errors increase at low altitude. So the fix is to fly higher — that minimizes the error. Finally, coastal refraction. This one's about what happens when the signal crosses a coastline. Radio waves speed up over water, because there's reduced absorption of energy — reduced attenuation — compared to over land. That speeding up bends the wave front — it refracts — away from its normal path and pulls it towards the coast. Now, the geometry matters: refraction is negligible at 90° to the coast, but it increases as the angle of incidence increases. So the more obliquely your signal crosses the coast, the worse the bending. What does that do to you? For an aircraft flying over the sea, the error puts your aircraft position closer to the coast than its actual position. So you think you're further out to sea than you really are — that's a dangerous bias. You minimise it three ways: use NDBs on or near the coast, fly higher, and use signals that cross the coast at or near 90°. So to tie it together: night brings sky wave contamination you can't eliminate, only manage with identification and cross-checking. And regardless of time of day, mountains and coastlines bend your bearings in predictable ways — and you have specific techniques to counter each one.