Excerpts of HF Management Guidance Material
Informal South Pacific ATS Co-ordination Group
2.2 As a general rule, radio signals travel in straight lines, that is, they follow great circle paths over the surface of the earth. Under certain circumstances, however, the path of a signal may change direction; this change of direction is called refraction. Refraction examples are coastal, atmospheric and ionospheric, and the amount of refraction varies considerably, depending on certain conditions. Those conditions could be a change in direction when a signal crosses a coastline (coastal refraction), a change in direction due to a variation in temperature, pressure and humidity, particularly at low altitude (atmospheric refraction), or a change in direction when the radio wave passes through an ionised layer (ionospheric refraction).
2.2.3 The ionosphere is still under investigation but it is known that several definite ionised layers exist within it. During daytime hours there are four main ionisation layers designated D, E, F1 and F2 in ascending order of height. At night, when the sun’s radiation is absent, ionisation still persists but it is less intense, and fewer layers are found (D and F layers). Factors that affect the ionosphere layers is strength of the sun’s radiation, since it varies with latitude causing that the structure of the ionosphere varies widely over the earth’s surface, and the state of the sun, since sunspots affect the amount of ultra-violet radiation.
22.214.171.124 Maximum Usable Frequency (MUF) at night is much less than by day, because the intensity of ionisation in the layer is less so than lower frequencies have to be used to produce the same amount of refractive bending and give the same critical angle and skip distance as by day. However, the signal attenuation in the ionosphere is also much less at night so the lower frequency needed is still usable. Hence the night frequency for a given path is about half of the day frequency, and shorter distances can be worked at night than by day while still using a single reflection from the F layer.
126.96.36.199 The MUF not only varies with path length and between day and night, but also with season, meteor trails, sunspot state, and sudden ionospheric disturbances produced by eruptions on the sun. Because of the variations of MUF, HF transmitting stations have to use frequencies varying widely between about 2 and 20 MHz.