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Ionospheric sky-wave propagation
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Sky-wave propagation occurs because signals in the ionosphere are refracted so much that they are bent back toward the earth s surface To observers on the surface, it looks like the signal was reflected from a radio mirror at the virtual height of the ionosphere The skip distance is the surface distance between the transmitter point (A in Fig 2-31) and the point where it returns to earth (point C in Fig 2-31) The ground-wave zone is the distance from the transmitter site (A in Fig 2-31) to where the ground wave fades to a low level, below usefulness (point B in Fig 2-31) The skip zone is the distance from the outer edge of the ground-wave zone to the skip distance, or the distance from B to C in Fig 2-31 It is possible for the sky wave and the ground wave to interfere with each other at some frequencies, under some circumstances When this happens, the sky wave has a relative phase that depends on its path length (among other
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* Now the National Institute of Standards and Technology
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48 Radio-wave propagation
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Skip distance 2-31 Relationship between ground wave and different cases of sky wave
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things), so it will arrive at some seemingly random phase relative to the ground wave Thus, the sky wave can selectively strengthen or cancel the ground wave, giving rise to a type of fading Incident angle One of the factors that affects the length of the skip distance is the incident angle of the radio wave This angle is partially a function of the frequency, and partially a function of the natural radiation angle of the antenna ( r) The frequency effects are seen as a function of the ionization level and how much a given frequency is refracted The antenna radiation angle is the angle of the main vertical lobe with respect to the earth s surface; it is partially a function of its design, and partially of its installation configuration For example, a 5 8-wavelength vertical antenna tends to have a lower angle of radiation than a 1 4-wave vertical antenna Similarly, the dipole s angle of radiation is a function of its height above ground Figure 2-32 shows how the angle of radiation affects skip distance Low-angle-ofradiation signals tend to travel farther with respect to the earth s surface before refracting, so they produce the longest skip distances Higher angles of radiation have shorter skip distances because they tend to return to earth more rapidly In summertime, some high-frequency bands (eg, the 11-m 27 MHz Citizens Band) offer highangle short skip during the summer, and longer skip during the other months At higher angles of radiation, there will be a critical angle wave and escape angle waves that are not returned to earth These waves are not used for terrestrial communications or broadcasting Figure 2-33 shows the difference between single-hop skip and multihop skip, as a function of incident angle It is generally true that a multihop transmission is more subject to fading, and is weaker, than a single-hop transmission
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The refraction of high-frequency and some medium-wave radio signals back to earth via the ionosphere gives rise to intercontinental HF radio communications This phenomenon becomes possible during daylight hours and for a while after sunset when the ionosphere is ionized Figure 2-34 reiterates the mechanism of long-distance skip communications The transmitter is located at point T, while receiving stations
EM wave propagation phenomena 49
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2-32 Sky-wave propagation as a function of antenna radiation angle
Ionosphere
Single-hop transmission Transmitter
Two-hop transmission Receiver
2-33 Single-skip and multihop-skip communications
are located at sites R1 and R2 Signals 1 and 2 are not refracted sufficiently to be refracted back to earth, so they are lost in space Signal 3, however, is refracted enough to return to earth, so it is heard at station Rl The skip distance for signal 3 is the distance from T to R1 At points between T and R1, signal 3 is inaudible, except within ground-wave distance of the transmitter site (T) This is the reason why two stations 40 mi apart hear each other only weakly, or not at all, while both stations can communicate with a third station 2000 mi away In amateur radio circles, it is common for South American stations to relay between two US stations only a few miles apart For an example of this problem, listen to the Inter-American and Halo Missionary
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