|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Long range TV reception. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Телевидение First of all, it is necessary to clearly distinguish between confident and random reception. Confident is the reception of transmissions of a certain transmitter, which is carried out regardless of weather conditions, solar activity, time of year, day and other factors. Random reception depends on these factors and is possible only under favorable conditions. Confident television reception is ensured by the propagation of a direct or, as they say, "earth" wave along the surface of the Earth. The ultrashort waves used in television propagate in a straight line and are almost not reflected by the ionosphere. Therefore, the maximum possible receiving range should be determined by the line-of-sight distance of the transmitting antenna from the point where the receiving antenna is installed. Based on the spherical shape of the Earth's surface, the line-of-sight distance should be equal to
where D is the line-of-sight distance in km; H is the height of the transmitting antenna in m; h is the height of the receiving antenna in m (Fig. 1).
In reality, reliable reception of television broadcasts is possible at a greater distance than the direct line of sight, due to some rounding of the earth's surface by the propagating signal, as well as due to signal re-reflection by various local objects. The area within which reliable reception is possible can be divided into two zones: the line-of-sight zone and the penumbra zone. In the line-of-sight zone, reliable reception is possible using conventional antennas. In the penumbra zone, the signal field strength is low, which forces the use of highly efficient antennas for reliable reception. With a sufficiently high transmitter power on flat terrain, the penumbra zone is limited by a distance of 200 ... 220 km from the transmitter operating on channels 1-5, 120 ... 150 km from the transmitter operating on channels 6-12, and for the decimeter range of the penumbra zone practically does not exist. The indicated boundaries are not sharp, are significantly blurred and very approximate, since they do not take into account the actual terrain. In the presence of mountain obstacles, even near the transmitter, reliable reception may not be possible. On a flat terrain outside the penumbra zone, the field strength level is zero and reliable reception is also impossible even when using highly efficient antennas. In contrast to strong reception, random reception is sometimes observed at distances of several thousand kilometers and is therefore called ultra-long reception. Ultra-long-range reception is associated with anomalous states of the ionosphere, it is observed extremely rarely, as a rule, only on channels 1-2. His sessions are short - from a few minutes to several hours - and completely unpredictable. It makes no sense to focus on ultra-long reception. The main characteristic of the TV, which determines the possibility of long-range reception, is the sensitivity. The lower the sensitivity value, the longer the range of the receiver. However, there are several concepts of sensitivity, which can be confusing if one does not understand the difference between them or indicate which sensitivity is being referred to. Gain-limited sensitivity is the minimum signal voltage at the TV input, which ensures the nominal signal level at the kinescope modulator. The nominal level is the voltage swing corresponding to the levels of white and black on the screen. Sync-Limited Sensitivity is the minimum signal voltage at the TV input that still achieves stable picture sync. Finally, noise-limited sensitivity is the minimum signal voltage at the TV input, at which the nominal signal level on the kinescope modulator is provided when it exceeds the noise level by 20 dB (i.e., 10 times the voltage). In all cases, the sensitivity of the image channel is meant. It can be seen that the gain-limited sensitivity characterizes only the gain of the receiving-amplifying path. The larger the gain, the less (i.e., better) the gain-limited sensitivity. Hence, by simply increasing the number of amplifying stages, it is possible to achieve an arbitrarily small gain-limited sensitivity. This leads to the most common misconception when, in long-range reception conditions, they try to improve it by using various amplifying attachments. Gain-limited sensitivity does not at all characterize the possibility of receiving weak signals by a television receiver, since it does not take into account the influence of the television receiver's own noise. The noise of each stage is amplified by subsequent stages along with the signal. The noise of the first stage is most strongly amplified, since it is amplified by all stages. Dividing the noise level at the output of a receiver by its gain gives the noise level normalized to the input of that receiver. The noise level of the first stage of the receiver is most important, and the noise of subsequent stages can be neglected. It is obvious that the noise voltage reduced to the receiver input does not depend on the number of stages and on the gain of the receiving path. The greater the gain of the path, the lower the signal voltage must be applied to the input of the receiver in order to obtain a nominal signal at the output, and the better (less) the sensitivity limited by the gain. However, it is clear that when a signal is applied to the input of the receiver that is lower in level than the voltage of the noises brought to the input, such a weak signal will be clogged with noise. In this case, the image will not work on the TV screen, but only noise in the form of chaotic flickering white and black dots will be visible. In this case, they say that snow is visible on the screen. To get an image on the screen, the signal voltage must exceed the noise voltage. The higher the voltage of the signal at the input of the TV compared to the voltage of the noise brought to the input, the better the picture quality will be. To assess the relationship between signal voltage and noise voltage, it is customary to take their ratio. Noise-limited sensitivity takes into account the presence of the inherent noise of a television receiver and characterizes its ability to receive weak signals, that is, to work in long-range reception conditions. Noise-limited sensitivity is measured at a specific signal-to-noise ratio of 10 on the kinescope modulator. Due to the fact that in television, in addition to the carrier frequency of the image, only one sideband is transmitted, and the second sideband is suppressed, the gain of the end-to-end path for the signal is two times less than for noise. Therefore, to obtain a signal-to-noise ratio of 10 at the receiver output, this ratio must be equal to 20 at the receiver input. The specified signal-to-noise ratio when determining the sensitivity was taken conditionally, since it corresponds to a very poor image quality, only legibility of large details is provided. To obtain a good quality image, the signal-to-noise ratio at the TV input must be at least 100. Thus, if it is known that the noise-limited sensitivity for a TV is, for example, 70 μV, applying such a signal to the antenna input of this TV will only provide a legible image of poor quality. To obtain a good image, the signal voltage at the TV input must be 5 times greater, that is, 350 μV. By comparing the noise-limited sensitivity values for different types of TVs, you can choose the type of TV that is most suitable for long-range reception conditions, that is, it has the lowest sensitivity value. For normal operation of the entire TV circuit, it must have a gain margin. Therefore, gain-limited sensitivity is usually less important than noise-limited sensitivity. Sync-limited sensitivity is an intermediate value and guarantees only stable synchronization without regard to image quality. Therefore, its value cannot be taken as the basis for determining the suitability of the TV set for operation in long-range reception conditions. It should be noted that if it is not indicated what sensitivity of the TV in question, you need to understand the sensitivity limited by the gain. It is impossible to compare TV sets according to this characteristic to determine their suitability for long-range reception. All black-and-white and color stationary and portable televisions developed after 1979 have a sensitivity limited by noise, in the meter wave bands - 100 μV, and in the decimeter wave bands - 140 μV. According to GOST, these values are limiting, the actual sensitivity may be better. Televisions designed before 1979 may have other sensitivity values. The worst sensitivity, limited by noise - 150 μV in the MB bands and 500 μV in the UHF bands - is possessed by TVs of the UPIMTST-61 type, whose names include the indices Ts-201 and Ts-202. These TVs are less suitable for long-distance reception. From the definition of noise-limited sensitivity, it can be seen that it is determined by the level of the television receiver's own noise, given to its input. The noise level is determined mainly by the design of the first gain stage in the channel selector, the type and mode of the lamp or transistor used in this stage. For modern channel selectors, the noise voltage at the input is approximately 5 μV in the MB bands and 7 μV in the UHF bands. Hence the sensitivity is obtained, equal to 100 and 140 μV (20 times the noise level). For this reason, improving noise-limited sensitivity can only be achieved by lowering the input noise floor, but not by increasing the gain of the receiving path by replacing tubes, transistors, or using any amplifying attachments. There are currently no radical measures to reduce the level of intrinsic noise of a television receiver without degrading the image quality. The GT346A transistors used in the first stages of the channel selectors have a noise figure of 75 dB with an internal resistance of the signal source of 7 Ω. These are the least noisy pnp structures of domestic transistors. If you use a foreign type AF251 transistor with a noise figure of 4,8 dB in the first stage of the channel selector, the noise level will decrease by 2,2 dB, and the noise-limited TV sensitivity can be improved to 80/110 μV. However, the acquisition of foreign-made low-noise transistors is a difficult task. The issue is much easier to solve if, in order to improve the sensitivity, some deterioration in the horizontal clarity of the image is allowed due to the narrowing of the bandwidth. In conditions of long-range reception, the passport clarity of the TV image is not realized, since the low-contrast image is affected by intense noise interference. As is known, the horizontal clarity is proportional to the passband of the receiving-amplifying path, and the intrinsic noise voltage is proportional to the square root of the passband. If the bandwidth is narrowed by 2 times, the clarity will also deteriorate by 2 times, up to 250 elements, which can be considered quite acceptable under long-range reception conditions, and the level of intrinsic noise will decrease by 3 dB, which corresponds to an improvement in sensitivity up to 70/100 μV. In this case, the image quality is subjectively improved due to two factors: the attenuation of noise interference and the increase in contrast (since the narrowing of the bandwidth leads to an increase in the gain of the path). The easiest way to narrow the bandwidth is to increase the load resistances of the video detector and video amplifier. In black and white TVs ULPT-61-II-22 and ULPT-61-II-28 increase the resistance of resistors 3-R42 and 3-R47, in TVs ULT-50-III-2 and ZULPT-50-III-1 - 2 -P13 and 2-R22, in TVs 2UPIT-61-II-1/2 and UST-61-3/4-P25 and R26. On color televisions, narrowing the bandwidth can cause color to drop out and the picture to be displayed in black and white. One should not strive to excessively increase the resistances of these resistors, especially in the stages of a video amplifier, in order to avoid disrupting the normal modes of electronic tubes and transistors. It can be considered acceptable to increase the load resistance of the video detector by about 2 times and the load resistance of the video amplifier by 1,2 times. In this case, the mode change is within the tolerance, and the bandwidth is narrowed by about 2 times. Obviously, in order to receive an image on the TV screen, it is necessary to apply a signal to its antenna input, the level of which must be higher than the sensitivity of this television receiver, limited by noise. The quality of the image depends on how much the signal level exceeds the sensitivity. If there is no way to influence the sensitivity to significantly improve it, you need to try to increase the signal level at the antenna input of the TV so that it is greater than the sensitivity value .. What determines the signal level at the input of the television receiver? First of all, the level of electromagnetic field strength at the point in space where the receiving antenna is located, the gain of this antenna, its effective length, and, finally, the attenuation of the signal in the feeder that connects the antenna to the TV. Of course, the antenna must be well matched with the feeder, and the feeder with the TV, otherwise there will be additional attenuation of the signal due to its reflection and radiation back into space. The field strength at the receiving point depends on the power of the transmitter, the distance to this transmitter, the terrain on the path, and the attenuation of the signal in the atmosphere. It is not possible to radically influence the level of field strength at the receiving point. But usually there is a choice of the location of the antenna, and after doing a few experiments, you can choose the optimal position of the antenna on the roof of the building and its height, corresponding to the maximum signal level at the TV input. The effective length of the antenna depends solely on the wavelength of the received signal, that is, on the channel number: the shorter the wavelength (the larger the channel number), the smaller the effective length of the antenna. Thus, in order to increase the signal level at the TV input, it remains possible to influence the antenna gain and signal attenuation in the feeder. The antenna gain shows how many times the signal voltage at the output of a given antenna exceeds the signal voltage at the output of a half-wave vibrator placed at the same point in the electromagnetic field. The gain can also be expressed in decibels. The greater the antenna gain, the greater the signal voltage at the input of the TV, all other things being equal. Therefore, in conditions of long-range reception, it is necessary to use antennas with a high gain. Characteristically, an increase in the antenna gain does not lead to an increase in the noise level. If improving the noise-limited sensitivity of a television receiver and choosing the optimal antenna location can only improve reception to a small extent, then using a high-performance antenna can lead to an increase in signal level many times over. Thus, the choice of antenna is a decisive factor in long-range reception. And the higher the frequency signal needs to be received (the higher the channel number), the higher the antenna gain should be. This is because the effective length of the antenna is proportional to the wavelength of the signal. Therefore, with the same field strength of two signals, for example, the 1st and 12th channels, and the use of the same type of antennas with the same gain, the signal voltage at the output of the antenna of the 12th channel will be 4,3 times less than at the output of the antenna of the 1st channel. For this reason alone, in order to obtain the same signal voltage at the TV input, the antenna gain of the 12th channel must be 1 times higher than the antenna gain of the 4,3st channel in terms of voltage, which corresponds to 12,7 dB. In the decimeter range, the need to use antennas with increased gain for this reason increases even more. In the frequency range reserved for television, various types of high-performance antennas are used. In professional equipment (radio communication, radar, etc.), preference is usually given to multi-element antennas of the Wave Channel type. In amateur conditions, the use of such antennas is impractical for the following reasons. Multi-element antennas need careful tuning, which is done by changing the dimensions of each antenna element and the distances between them. Tuning is performed in polygon conditions using instruments while controlling the shape of the antenna pattern, the magnitude and nature of its input impedance. The radio amateur is not able to make such an antenna adjustment. A multi-element antenna, even if it is made exactly according to the drawings, turns out to be detuned, just as a multi-circuit radio receiver is detuned immediately after assembly. As a result of such detuning, the antenna parameters are much worse than the passport ones, and such an antenna does not give a positive effect. In a detuned antenna, the shape is distorted and the main lobe of the radiation pattern expands, its side and rear lobes increase, which leads to a decrease in the gain. The maxima of the main lobe of the diagram deviates from the geometric axis of the antenna. In addition, for the antenna to be matched to the feeder, its input impedance must be purely active and equal to the characteristic impedance of the feeder. For a detuned antenna, the input impedance is complex and contains a reactive component, and the active component differs significantly from the nominal value. Professional equipment usually contains special blocks to control the matching of the antenna with the feeder. The television receiver does not contain such blocks. As a result of the mismatch, part of the signal energy is additionally lost, which leads to a decrease in the signal voltage at the antenna output and is equivalent to a decrease in its gain. The more elements an antenna of the "Wave channel" type contains, the more acute the question arises of the need to tune it. Practice shows that only three-element antennas of the "Wave channel" type can work satisfactorily without tuning. However, the voltage gain of a three-element antenna does not exceed 2,2 (about 6,8 dB), which is too low for long-range reception. A five-element antenna has a gain of 2,8 (about 9 dB), but due to the inevitable detuning in practice, it gives the same result as a three-element antenna. Theoretically, the voltage gain of an 11-element Wave Channel antenna is 4 (about 12 dB). But such amplification corresponds only to an antenna tuned and matched to the feeder. Due to the large number of elements, the detuning of such an antenna after its assembly turns out to be significant, which also leads to a significant deterioration in its efficiency, both due to a drop in the actual gain and due to a strong mismatch between the antenna and the feeder. These reasons explain the frequent failures of radio amateurs who tried to improve television reception in weak signal conditions through the use of multi-element antennas. It is regrettable that, despite repeated publications of the above, many authors of articles and books continue to recommend to radio amateurs the use of multi-element antennas in conditions of long-range television reception, apparently based solely on theoretical premises. Due to the fact that at present a significant part of the country's territory is covered by two- and even three-program television broadcasting, when choosing a receiving antenna, it seems very tempting to use a wide-range antenna, which would allow one antenna to receive two or three television programs on different channels. Such antennas exist, for example, zigzag and log-periodic antennas. However, their use is possible only in the line of sight, since the gain is relatively small. If the transmitters are located in different directions, the wide-range antenna has to be installed on a rotary mast and reoriented each time you switch from receiving one program to another. In this case, due to the inaccurate orientation of the antenna, the signal is further weakened. In the penumbra zone, if it is necessary to receive several programs on different channels, it is necessary to install separate narrow-band antennas. Two separate antennas can be connected to a common feeder using a crossover filter. If the number of antennas is more than two, additional switching can be carried out by the contacts of an electromagnetic relay installed near the antennas, which is controlled remotely, by a toggle switch installed by the TV. In this case, the relay winding can be powered from the TV through the same feeder without using additional wires. In amateur radio conditions for long-distance reception of television transmissions, in-phase systems, consisting of several relatively simple antennas, have proven themselves well. Two antennas, located one above the other, form a two-story system, which is characterized by a narrowed radiation pattern in the vertical plane. Four antennas can form a two-story two-row system with a narrowed pattern in the vertical and horizontal planes. The narrowing of the radiation pattern corresponds to an increase in the gain. Each doubling of the number of antennas in an in-phase system corresponds to a gain of 3 dB (1,41 times the voltage) due to the sum of the signals received by each antenna alone. Additionally, by narrowing the beam pattern, the gain increases by about another 1 dB for each doubling of the number of antennas in the system. The use of relatively simple antennas in an in-phase system makes it possible to obtain a large gain without the need to tune the antennas. It is only necessary to ensure the coordination of the system with the feeder, which is easily done, since the values of the input impedance of simple antennas are known and depend little on the antenna tuning. Thus, by increasing the number of antennas in the system, it is possible to increase the gain indefinitely. This is often necessary in the UHF band, where, ceteris paribus, the signal voltage at the antenna output is much less than in the MB band, due to a decrease in wavelength. At the same time, due to the small size of antennas in this range, an increase in their number in the system is easily feasible and does not lead to excessive system dimensions. Common-mode systems assembled from two-element and three-element loop antennas "Double Square" and "Triple Square" have found the greatest distribution among fans of long-range television reception. Two-element loop antennas are usually used in the MB bands, and three-element loop antennas in the UHF bands. According to some authors, a two-story two-row in-phase system assembled from four two-element loop antennas has a voltage gain of the order of 6-8 (16 ... 18 dB), and the same system of three-element loop antennas-11-13 (21. ..23 dB). It is impossible to achieve such a gain using a multi-element Wave Channel antenna, since even the gain of a 16-element Wave Channel antenna does not exceed 14 dB, and even then, if it is carefully tuned and matched with the feeder. Caution should be taken against frequent attempts to assemble in-phase systems from several wide-range antennas. In this way, an attempt is made to achieve a high gain with a broadband antenna in order to be able to receive transmissions of several programs on different channels under long-range reception conditions with one antenna system. Such attempts, as a rule, are unsuccessful, since it is not possible to match the antenna in the frequency range. Matching elements usually contain resonant nodes in the form of half-wave and quarter-wave cable segments that perform their functions only at a certain frequency. They can no longer work in a wide frequency range. Attempts to assemble in-phase systems from several multi-element "wave channel" antennas also do not give success, due to the fact that the antennas are detuned in different ways, the phases of the signal voltages at their outputs also turn out to be different, and they cannot be combined in-phase, and sometimes instead of adding subtraction takes place. For long-range reception, the antenna is installed on a high mast and connected to the TV with a long feeder. The longer the feeder, the more attenuation it introduces and the lower the signal voltage at the TV input. For the feeder, the most common cable brand is RK-75-4-11, which has a linear attenuation of 0,07 dB / m on channels 1-5, 0,13 dB / m on channels 6-12, 0,25-0,37 .21 dB/m on channels 60-2. Graphs of per unit attenuation of different brands of cable are shown in fig. XNUMX.
If, with a feeder length of 50 m, the signal attenuation on channels 1-5 is small (3,5 dB), then on channel 33 it reaches 15 dB, which corresponds to a decrease in signal voltage by almost 6 times. To compensate for signal attenuation in the feeder, an antenna amplifier is used, mounted on a mast near the antenna. This makes it possible to ensure that a signal is received at the input of the antenna amplifier, which has not yet been attenuated due to passing through a long feeder. At the same time, a high signal-to-noise ratio is maintained at the input of the antenna amplifier and at the antenna input of the television receiver. This is the fundamental difference from the case when the antenna amplifier is installed near the TV and does not give any useful effect. The antenna amplifier is called an antenna amplifier because it should be installed near the antenna, and not near the TV. The gain of the antenna amplifier should be at least the same as the signal attenuation in the feeder, better - 5 ... 10 dB more. Then the level of intrinsic noise of the television receiver can be neglected, and the image quality will be determined solely by the signal-to-noise ratio at the input of the antenna amplifier, The need to use a long feeder sometimes arises in closed areas when the TV is located in a hollow. If the antenna is installed on the top of a nearby hill, reliable reception will be provided, but the length of the connecting feeder will be about 100 ... 200 m. Even at the frequency of the 1st channel with a feeder length of 200 m, the signal attenuation in it will be 14 dB. And in this case, the installation of an antenna amplifier near the antenna will compensate for signal attenuation. If the gain of one amplifier is not enough, you can turn on two amplifiers in series one after the other, placing them evenly along the length of the feeder. It is also necessary to pay attention to the possibility of using coaxial cables of various brands as a feeder. The RK-75-9-13 cable has a lower attenuation per unit length than the RK-75-4-11 cable. This is especially noticeable in the UHF ranges: at the frequency of the 60th channel, the RK-75-9-13 cable introduces attenuation approximately three times less in voltage than the RK-75-4-11 cable. Thus, by using the best cable with its long length, you can raise the signal level at the TV input several times. Since when buying a cable it is usually not possible to determine its brand, you can be guided by the fact that the larger the diameter of the cable, the less attenuation it introduces. A cable with a characteristic impedance of 75 ohms is always used as a feeder. If the brand of the cable and its characteristic impedance are unknown, it is easy to determine it with a caliper if the cable has a continuous polyethylene insulation. The ratio of the outer diameter of the internal polyethylene insulation to the diameter of the central core for cables with a characteristic impedance of 75 ohms should be in the range from 6,5 to 6,9. Literature
Publication: cxem.net
Alcohol content of warm beer
07.05.2024 Major risk factor for gambling addiction
07.05.2024 Traffic noise delays the growth of chicks
06.05.2024
▪ The admixture of carbon improves the electrical conductivity of copper ▪ Cold is more dangerous than heat ▪ Again about the Shroud of Turin ▪ Strong emotions bring people together
▪ section of the site Instructions for use. Article selection ▪ article Get rich! Popular expression ▪ article Through a drop of water. Children's Science Lab
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page