Explain which layer does what, why the D region blocks low bands by day, and how scatter and NVIS fill the skip zone.
Syllabus: Ionospheric regions; critical angle and frequency; HF scatter; near vertical incidence skywave (NVIS)
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
The D region is closest to Earth. F2 is the highest, so a hop off it lands the farthest. Height, not density, is the reason.
RememberD lowest, F2 highest, F2 skips farthest.
The D region is a daytime layer that absorbs signals below 10 MHz, which includes the 160, 80, 60 and 40 m bands. Long-distance contacts there are harder by day. At night D fades and the F region returns them.
RememberDaytime, D absorbs. Not the E or F region.
Each layer can refract only so hard. The critical frequency at a given incidence angle is the highest frequency refracted back to Earth. The critical angle is the highest takeoff angle that still returns the wave at a given frequency.
RememberHighest frequency, highest angle that still return. Beyond either: gone.
Skywave skips over a skip zone. Scatter is the way you can still hear a signal there: a small part of the energy is scattered back down.
Only a small part of the energy goes that way, so scatter signals are weak. Energy arrives over several different paths, so the audio is distorted with a fluttering sound.
RememberWeak and fluttery: scatter into the skip zone.
Near vertical incidence skywave is short-distance MF or HF propagation at high elevation angles. The signal goes almost straight up and comes back nearby, so there is no skip zone.
RememberNVIS = high angle, short distance, no gap.