On July 1, 2020, the Extra Class question pool is being updated, making this post obsolete. See the corresponding post from my 2020 version of No Nonsense Extra Class License Study Guide.
E3B – Transequatorial propagation; long path; gray-line; multi-path; ordinary and extraordinary waves; chordal hop, sporadic E mechanisms
There are a number of interesting types of propagation that occur on the HF bands. They include transequatorial propagation, long-path propagation, and gray-line propagation.
Transequatorial propagation is propagation between two mid-latitude points at approximately the same distance north and south of the magnetic equator. (E3B01) The approximate maximum range for signals using transequatorial propagation is 5000 miles. (E3B02) The best time of day for transequatorial propagation is afternoon or early evening. (E3B03)
Long-path propagation is the type of propagation that occurs when the longer of the two direct paths between stations is better for communications than the shorter path. 160 to 10 meters are the amateur bands that typically support long-path propagation. (E3B05) 20 meters is the amateur band that most frequently provides long-path propagation. (E3B06)
Gray-line is the type of HF propagation is probably occurring if radio signals travel along the terminator between daylight and darkness. (E3B08) Gray-line propagation occurs because, at twilight, D-layer absorption drops while E-layer and F-layer propagation remain strong. (E3B10)
Another interesting propagation phenomenon is echoes. While not strictly a type of propagation, echoes are the result of propagation conditions. Receipt of a signal by more than one path is one condition that could account for hearing an echo on the received signal of a distant station. (E3B07)
Sporadic-E propagation occurs when unusally dense patches of ionization form in the E layer of the ionosphere. The time of year that Sporadic E propagation is most likely to occur is around the solstices, especially the summer solstice. (E3B09) Sporadic-E propagation is most likely to occur any time of the day.
Chordal hop propagation occurs when a radio wave is refracted by the ionosphere such that the refracted wave hits the ionosphere again before hitting the ground. The primary characteristic of chordal hop propagation is successive ionospheric reflections without an intermediate reflection from the ground. (E3B12) Chordal hop propagation is desirable because the signal experiences less loss along the path compared to normal skip propagation. (E3B13)
When a radio wave enters the ionosphere, it splits into two waves called the ordinary wave and the extraordinary wave. The terms extraordinary and ordinary waves describe independent waves created in the ionosphere that are elliptically polarized. (E3B04) What happens to linearly polarized radio waves that split into ordinary and extraordinary waves in the ionosphere is that they become elliptically polarized. (E3B14)
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