WEBVTT NOTE The Ionospheric Regions, Scatter, and NVIS 1 00:00:00.000 --> 00:00:18.515 The four regions. There is a diagram here. Four stacked bands above the Earth's surface. The D region sits from about 60 to 90 kilometres and absorbs signals below 10 megahertz during daylight, vanishing at night. 2 00:00:18.515 --> 00:00:35.640 The E region from about 90 to 150 kilometres refracts and supports sporadic E, weakening at night. The F1 region from about 150 to 250 kilometres exists in daytime only and merges into F2 at night. 3 00:00:35.640 --> 00:00:46.940 The F2 region from about 250 to 400 kilometres is the highest and supports the longest single-hop paths, persisting day and night. 4 00:00:46.940 --> 00:00:53.420 Question. Which ionospheric region is closest to the surface of Earth? 5 00:00:54.320 --> 00:00:57.360 Answer. The D region. 6 00:00:57.960 --> 00:01:20.840 From the bottom up: D, E, F1, F2. They are not sharply bounded layers but regions of differing ionisation, and their behaviour differs in a way that explains almost everything about HF. The D region only absorbs. 7 00:01:20.840 --> 00:01:31.600 Question. Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours? 8 00:01:32.500 --> 00:01:35.540 Answer. The D region. 9 00:01:36.140 --> 00:01:46.500 The D region refracts essentially nothing useful. What it does is absorb, and it absorbs low frequencies most. 10 00:01:46.500 --> 00:01:59.300 Question. Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day? 11 00:02:00.200 --> 00:02:07.680 Answer. The D region absorbs signals at these frequencies during daylight hours. 12 00:02:08.280 --> 00:02:25.406 That is the single most useful propagation fact at General level. The low bands are local by day and worldwide by night, and the reason is that the D region forms in sunlight and disappears after dark. 13 00:02:25.406 --> 00:02:44.578 It is why 80 metre nets run in the evening, why 160 metres is a winter-night band, and why an 80 metre contact at noon reaches the next county and at midnight reaches another continent, on the same antenna, at the same power. 14 00:02:44.578 --> 00:02:47.560 The F2 region carries the distance. 15 00:02:47.560 --> 00:02:56.577 Question. Why is skip propagation via the F2 region longer than that via the other ionospheric regions? 16 00:02:57.477 --> 00:03:01.077 Answer. Because it is the highest. 17 00:03:01.677 --> 00:03:22.118 Pure geometry: reflect off a higher point and the wave travels further before it comes back down. F2 at about 250 to 400 km gives roughly a 2,500-mile hop; the E region at 90 to 150 km gives about 1,200. 18 00:03:22.118 --> 00:03:39.539 F2 also persists overnight, while F1 merges into it and E weakens. The night-time high-band DX you occasionally hear is F2 hanging on. Critical frequency and critical angle. 19 00:03:39.539 --> 00:03:45.077 Two terms that sound similar and mean different things. 20 00:03:45.077 --> 00:03:51.517 Question. What is meant by the term "critical frequency" at a given incidence angle? 21 00:03:52.417 --> 00:03:58.057 Answer. The highest frequency which is refracted back to Earth. 22 00:03:58.657 --> 00:04:07.497 A property of the ionosphere at that moment, for that angle. Above it, signals at that angle escape. 23 00:04:07.497 --> 00:04:14.617 Question. What does the term "critical angle" mean, as applied to radio wave propagation? 24 00:04:15.517 --> 00:04:24.828 Answer. The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions. 25 00:04:25.428 --> 00:04:34.903 A property of the geometry, for a given frequency. Radiate steeper than the critical angle and the wave punches through. 26 00:04:34.903 --> 00:04:48.246 The two are two views of the same threshold: for any frequency there is a steepest angle that comes back, and for any angle there is a highest frequency that comes back. 27 00:04:48.246 --> 00:04:59.300 This is why antenna takeoff angle matters as much as gain on HF. A low antenna radiates at high angles, good for short hops, useless for DX. 28 00:04:59.300 --> 00:05:06.880 A high antenna, or one over good ground, radiates at low angles and reaches further. HF scatter. 29 00:05:06.880 --> 00:05:19.750 Between the ground wave's reach and the first skip landing point there is a skip zone, a ring where, in theory, nothing is heard. In practice something usually is. 30 00:05:19.750 --> 00:05:28.018 Question. What type of propagation allows signals to be heard in the transmitting station's skip zone? 31 00:05:28.918 --> 00:05:32.198 Answer. Scatter. 32 00:05:32.798 --> 00:05:47.598 Small amounts of energy are scattered out of the main path by irregularities in the ionosphere and by the ground, and some of it lands in the skip zone. Three characteristics, all asked: 33 00:05:47.598 --> 00:05:52.638 Question. What is a characteristic of HF scatter? 34 00:05:53.538 --> 00:05:57.618 Answer. Signals have a fluttering sound. 35 00:05:58.218 --> 00:06:03.671 Question. What makes HF scatter signals often sound distorted? 36 00:06:04.571 --> 00:06:10.971 Answer. Energy is scattered into the skip zone through several different paths. 37 00:06:11.571 --> 00:06:18.331 Question. Why are HF scatter signals in the skip zone usually weak? 38 00:06:19.231 --> 00:06:26.431 Answer. Only a small part of the signal energy is scattered into the skip zone. 39 00:06:27.031 --> 00:06:41.311 Weak, fluttering, and distorted, because it is a small fraction of the energy arriving by several routes at once. Recognisable immediately once described. NVIS. 40 00:06:41.311 --> 00:06:48.751 Question. What is near vertical incidence skywave (NVIS) propagation? 41 00:06:49.651 --> 00:06:57.131 Answer. Short distance MF or HF propagation at high elevation angles. 42 00:06:57.731 --> 00:07:10.605 Deliberately radiating almost straight up, so the signal comes back down within a few hundred miles, filling exactly the gap that ground wave cannot reach and skip overshoots. 43 00:07:10.605 --> 00:07:19.506 NVIS wants the opposite of a DX antenna: a low horizontal wire, a quarter wavelength or less above ground, on a low band. 44 00:07:19.506 --> 00:07:30.173 It is the standard technique for regional emergency communications, where the traffic is 50 to 300 miles away and a repeater network may be down. 45 00:07:30.173 --> 00:07:46.284 The takeoff-angle discussion above is why it works: a low antenna's high-angle radiation, usually a defect, is the whole point here. Check yourself. Why does an 80 metre net meet in the evening rather than at lunchtime? 46 00:07:46.284 --> 00:07:56.951 You hear a weak, fluttering, distorted signal from a station 400 miles away while the band is otherwise carrying 2,000-mile contacts. What is it? 47 00:07:56.951 --> 00:08:05.705 You need reliable contact with stations 150 miles away when repeaters are down. What propagation mode and what antenna? 48 00:08:11.705 --> 00:08:23.131 The D region absorbs the low bands during daylight and disappears after dark, so 80 metres goes from local to long-distance at sunset. 49 00:08:23.131 --> 00:08:35.665 HF scatter into the skip zone, weak because only a small part of the energy scatters, fluttering and distorted because it arrives by several paths. 50 00:08:35.665 --> 00:08:45.385 NVIS, high-angle radiation on a low band, from a low horizontal antenna a quarter wavelength or less above ground.