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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING The role of the ionosphere in long-range radio communications. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Radio transmission over long distances is possible only due to the existence of reflective layers in the upper part of the earth's atmosphere. These layers form because the ultraviolet rays of sunlight split some of the gas molecules into positively charged particles - ions - and into electrons. This (the process is called ionization, and the ionized region of the atmosphere is usually called the ionosphere. Radio waves, penetrating into the ionosphere, are refracted and, with sufficient ionization, can return back to the earth. Figure 1 shows three possible cases of bringing radio waves in the ionosphere, depending on the degree of ionization. In case "a" the ionization is weak, and the waves pass through the layer only slightly bending their path.
In case b, the ionization is sufficient for the waves to be reflected and returned to the ground, and finally, in case c, the ionization is so strong that the waves are completely absorbed.
On fig. 2 shows the path of two radio waves with a length of 20 and 10 meters with some degree of ionization. Waves with a length of 20 meters (solid lines) are reflected from the ionosphere and return to the earth, (waves with a length of 10 meters (dotted lines) are only slightly curved in a layer and go into interplanetary space. All waves longer than 20 meters will also be reflected, and waves shorter than 10 meters will penetrate the ionosphere.The lower the transmitted frequency, the greater the probability of reflection and the stronger the ionization in the layer, the higher the frequency will still be reflected from it. ZONE OF SILENCE The angle at which the radio waves fall on the ionized layer is essential. The zone of silence occurs when the ionization is insufficient to reflect waves incident at "steep angles, however, waves incident at small angles will be reflected. As shown in Fig. 3, all waves radiated from the antenna at an angle greater than some critical angle, pass through the layer, and waves emitted at a smaller angle return to the ground.
Before the silence zone, signals are heard only in the immediate vicinity of the transmitter due to the surface wave. It is often observed that a beam incident on the Earth at point A is reflected from its surface, hits the layer again, is reflected again and returns to the Earth already at point B. Double, triple and multiple reflections of this kind very often occur at transmission at high frequencies, especially over long distances. On fig. 3 shows that the signal can also get to point B after a single reflection. If both signals arriving at point B are approximately the same in strength, then very strong fading due to interference can occur. By the width of the silence zone, one can approximately judge the conditions for the passage of waves of different ranges, listening only in one of them. Let's assume that in the range of 20 meters you can hear stations located at a distance of only 200 km. This indicates that with this ionization, signals at 10 meters are likely to return to earth as well. True, at these frequencies the zone of silence will probably extend up to 2000 km. If there is a very narrow dead zone on waves of 20 meters, then for waves of 40 meters there is no zone of silence. When the zone of silence extends over a long distance, we hear only distant stations. With an increase in ionization, it will narrow, and nearby stations will begin to appear. In this case, we will begin to lose distant stations for two reasons. Firstly, they will be clogged by loud nearby stations and secondly, high ionization causes absorption of signals from distant stations that travel a long way in ionized areas. The wider the dead zone and the higher the operating frequency, the more likely the possibility of long-distance communication. Since ionization in the upper layers of the atmosphere is caused by solar radiation, the conditions for the passage of short waves during the night and day will be sharply different. Consider, for example, a change in the communication condition during a normal winter day. In the early morning hours before sunrise, ionization is very weak. In this case, the 10-meter range will be completely dead, and at 20 meters you can hear only a few very distant stations. However, for lower frequencies, ionization will be sufficient for normal operation. So, on waves of 40 meters there will be good conditions for long-distance communication, waves at 160 meters also pass well. As the sun rises, ionization begins to increase rapidly and reaches its maximum in the afternoon. As noon approaches (the dead zone will narrow on all bands and about two hours after sunrise, ionization is sufficient to reflect waves of the 10-meter band. Around noon, the 20-meter band will be filled with relatively nearby stations, and at 10 meters long-distance communication is possible at this time After sunset, ionization will decrease as the reverse reduction of neutral atoms and molecules will begin. The zone of silence will gradually expand for each range. First, reception of waves of 10 meters will stop, and then 20 meters. MAGNETIC STORMS On some days it can be observed with radio reception that the number of amateur stations in the range is sharply reduced compared to ordinary days, all signals fade very much, many constantly audible stations disappear, and new, mostly distant stations, never received before, appear. These phenomena are caused by magnetic storms, in which the Earth's magnetic field, usually quite stable, undergoes strong changes. Magnetic storms are always accompanied by a decrease in ionization. As a result, the zone of silence expands and nighttime propagation conditions can continue throughout the day. During a magnetic storm, stations on the high frequency bands usually disappear much earlier than on normal days. At 20 meters, there are good conditions for long-distance communication around noon, while on ordinary days during these hours you can only work at distances up to 2000 km. The magnetic storm lasts from one to several days. The disturbances in the ionosphere occurring at this time cause significant fading accompanied by many distortions. Communication at short distances is usually broken and for work it was necessary to switch to longer waves. REFLECTING LAYERS AND ANOMALOUS IONIZATION The ionosphere usually consists of several ionized layers. Of these, the E and F layers play the greatest role in the propagation of radio waves. The height of the E layer above the Earth's surface is about 100 km, and the F layer is 220-240 km. These layers are completely unaffected by the weather near the Earth's surface. Layer F in the daytime breaks up into two layers F1 and F2; the first of them lies somewhat lower than the second. The F2 layer is more strongly ionized than the F1 and E layers, and plays a large role in short-wave transmission. Sufficiently high frequency signals, having penetrated the moderately ionized E and F1 layers, are reflected by the more strongly ionized F2 layer, as shown in Fig. 4 For lower frequencies, the E layer is important, and most communication at 160 meters is due to reflection from this layer.
In the E layer, there are occasionally areas of very intense ionization, which are called the anomalous E layer. Anomalous ionization of the E layer can occur at any time, and the cause is unknown. In the case of anomalous ionization, the E layer can cause waves to be reflected at 5 and 10 meters. Another anomalous phenomenon, called the Delinger effect, consists in the complete disruption of shortwave communication in the illuminated part of the globe. The cause of the Delinger effect seems to be eruptions in the sun, which cause a very large increase in ionization in the lower part of the ionosphere. As a result, short radio waves are absorbed. At this time, long-distance communication on ultrashort waves is sometimes possible. The Delinger effect can last for minutes or even hours. SEASONAL CHANGES The ionization of the F2 layer reaches its greatest value in winter, with a daily maximum occurring in the afternoon. This means that the narrowest dead zone will be after noon on a winter day, at which time reliable communication is possible at very high frequencies, for example, on waves of 10 meters. In summer, ionization is less significant than in winter, and the daily maximum for the layer moves towards sunset. Thus, for waves of 10 meters in summer, the silence zone will be wider, and communication on these waves can often be impossible. Due to the increase in the silence zone in summer at waves of 20 and 40 meters, one can expect improved conditions for long-distance communications, but at distances of many thousands of kilometers the picture is complicated by the ratio of illuminated and dark places on the globe. When transmitting across the equator, summer conditions may prevail at one end of the link and winter conditions at the other. The best conditions for long-distance communication are in spring and early autumn. During the spring and summer months, there are significantly more cases of anomalous reflections from the E layer. These reflections can give good conditions for long-range communications at 5 and 10 meters for several hours. The transition from winter conditions to summer conditions, and vice versa, does not occur smoothly. The spring and autumn months are characterized by an unstable state of the ionosphere. This is especially noticeable to amateurs who regularly work on the 10-meter band. CRITICAL FREQUENCIES The critical frequency is the highest frequency that is still reflected from a given layer when the signal is incident on the layer at right angles. If the signal is reflected at right angles, it will also be reflected at all other angles, and thus there will be no zone of silence at all frequencies below the critical one. Critical frequencies indicate the degree of ionization of the layers and can be used to predict "radio weather", select the most favorable waves for communication, calculate the length of the silence zone, etc. Critical frequencies are measured at ionospheric stations. There are several such stations in the Soviet Union, one of them in Tikhaya Bay, on Franz Josef Land, is the northernmost ionospheric station in the world. Over the past 3-4 years there have been many more long-distance communications on 10 and 5 meters than before. This is explained, on the one hand, by a sharp increase in the number of radio amateurs operating in these bands, and, on the other hand, by the effect of the 11-year cycle of sunspot activity. The ionization of the atmosphere is closely related to the number of sunspots; the more spots are observed during the year, the greater the degree of ionization. Sunspots have long been an object (observations by astronomers, and records of their number have been kept regularly since 1750. These records show that the number of sunspots usually reaches a maximum every 11 years. The last maximum was in 1939 and 1940. The average level of ionization over the past five years increased from year to year, as a result, ever higher frequencies were able to be reflected.Conditions for communication on the waves 10 and 5 meters in the winter of 1940/41 are already somewhat worse than they were in 1939/40. available hours on these waves will decrease and activity on these bands will come to a trough in 1944 or 1945. By that time, conditions on the 20m band will be similar to those seen last year on 10 meters, and the 40-meter band will again be suitable for long-distance communications. LONG COMMUNICATION ON VHF The frequency of ultrashort waves is too high to be reflected from the F2 layer. If such reflections are observed, they occur during periods of very high ionization, such as during sunspot maximum, and occur during long distance transmission when signals enter the layer at a very obtuse angle. Numerous links in the 5-meter band that have been observed during the summer months in the US in past years are explained by anomalous ionization of the E layer. Most of these connections took place in the evening. Ionospheric measurements show that in summer the anomalous E layer often forms in the morning before sunrise and in the evening, and its area is sometimes only a few square kilometers. Due to this, communication on VHF is possible only between a very limited number of points. However, if there are many such sites at the same time in different areas, the conditions of communication on VHF can be quite good. Author: B. Khitrov
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