In the first step is to clearly to distinguish between confident and casual reception. Sure called reception transmission a certain transmitter, which is regardless of the weather conditions, solar activity, time of day, season and other factors. Casual reception depends on these factors, and possible only if favourable the current conditions.
Confident the TV reception is provided by distribution direct or, as they say, "the earth" waves along the surface Of the earth. Ultrashort the waves used in television apply straightforward and not nearly reflected by the ionosphere. Therefore, the maximum possible range reception needs to determine the distance line of sight the transmitting antenna of the point where you installed the reception antenna. Based on from the spherical shape the surface of the Earth, distance direct visibility must be equal to
where D-the distance of direct visibility in km; H-the height of the transmitting antenna, in m; h-height reception antenna in m (Fig. 1).
In actually reliable reception TV shows it is possible to a greater distance than distance direct likely, due to the some diffraction spreading the terrestrial signal surface, and account reflections signal different local objects. The area within which is possible reliable reception, can be divided into two zones: the line-of-sight and the area of the penumbra. In the area line of sight reliable reception possible using conventional antennas. In the area of the penumbra the field strength signal small that forces for sure reception use highly effective antenna. When enough high power transmitter on flat location partial shade area limited distance 200...220 km from the transmitter, running on a 1-5-m the channels 120...150 km from transmitter operating 6-12-m channels and for UHF zone partial shade almost does not exist. These the boundaries are not sharp, greatly blurred and very approximately, so as not take into account the actual terrain. When the presence of mountain barriers even near the transmitter reliable reception can to be possible. On smooth same area for the boundary of the zone of penumbra the level of tension field is zero and reliable reception also it is impossible even when using high-performance antennas.
Unlike reliable reception casual reception sometimes observed at distances of several thousands of kilometres and therefore called ultra-long welcome. Ultra long admission is associated with abnormal the state of the ionosphere, very rare, usually only 1-2-m channels. Sessions it short - dated a few minutes to a few hours do not lend themselves prognosis. To navigate for long range reception, no sense.
Main a feature of the TV which determines the possibility of distant reception is the sensitivity. Than lower the value sensitivity, the dalnoboy receiver. However there are several concepts sensitivity that makes confusion, if not to understand the differences between them or not to specify what kind sensitivity is involved.
Sensitivity, limited gain, is minimum voltage signal at the input of the TV which provides the nominal signal level at the modulator tube. The nominal level is adopted sweep voltage, appropriate levels black and white on the screen. Sensitivity, limited synchronization- this is the minimum voltage signal at the input of the TV which still achieved stable sync image. Finally, sensitivity, limited by noise, is minimum voltage signal at the input of the TV which provides the nominal signal level at the modulator of the tube when it is the excess above the noise level 20 dB (i.e., 10 times voltage). In all cases I mean the sensitivity of the channel image.
It is seen that sensitivity, limited gain, characterizes only gain receiving and amplifying tract. The larger the coefficient the gain, the smaller (i.e. better) sensitivity, limited amplification. Here by simply increasing the number of amplifying cascades can be achieved as anything small value sensitivity, limited gain. It leads to the most common the misconception, when in far try taking to improve through the use of various amplifying console. Sensitivity, limited gain, does not ability reception of weak signals television receiver how does not consider the effects self-noise television receiver. Noise each cascade is amplified subsequent cascades along with the signal. The most enhances greatly the noise first cascade, as it increases all cascades. If the level noise at the receiver output divide by its coefficient gain get the level noise, shown to the door that receiver. The most important the noise level of the first cascade receiver, and noise subsequent stages can be neglected.
Obviously voltage noise to the input of the receiver is not affected the number of cascades and from gain reception path. More than the gain of the tract, the lower voltage signal must be submitted to the receiver input, to get the output the nominal signal, and the better (less) sensitivity, limited amplification. However it is clear that when applied to the input signal receiver, a smaller level than the noise voltage, shown to the door, like this a weak signal will be scored noises. On the TV screen this image is not and it will be visible only noise: chaotic shimmering white and black points. In this case we say that the screen visible snow. To to get the image on the screen voltage signal must exceed the voltage noise. The larger the voltage signal at the input on the TV compared to the voltage noise, shown to the door, the the image quality will be better. For the ratio between the signal voltage and voltage noise made take their attitude.
Sensitivity, limited by noise, takes into account the presence of self-noise television receiver and describes the capacity of to receive weak signals, have to work in conditions far admission. Sensitivity, limited by noise, is measured at a certain the signal-to-noise ratio equal to 10 on the modulator tube. In because in the TV in addition to the carrier frequency image is transmitted only one sideband frequencies, and the second side is suppressed, gain end-to-end path for the signal in two times less than for noise. Therefore, to obtain on the receiver output relationship signal/noise ratio equal to 10, at the entrance receiver this relation should to be equal to 20. Specified the signal-to-noise the definition of sensitivity conventionally, as it very bad the image quality is the intelligibility of large parts. To obtain the image good quality attitude signal/noise ratio at the input of the TV must be at least 100. So by the way, if you know that sensitivity, limited by noise, for some TV is, for example, 70 µv the feed to the antenna input of this the TV signal will provide only the legible image poor quality. For obtain a good image, the signal voltage on the input on the TV must be in 5 times more, that is 350 mV.
Comparing the values of sensitivity limited by noise, for different types of TVs, you can choose this type the TV, which is the most suitable for conditions far acceptance, that has the minimum value sensitivity.
For normal operation of the entire scheme the TV it should have reserve strengthening. So usually sensitivity, limited gain, has less important than sensitivity, limited by noise. Sensitivity, limited synchronization represents intermediate value and guarantees only sustainable synchronization without regard the quality of the image. So its value cannot be the basis of determining suitability for TV work in conditions far admission.
Should note that unless specified, what sensitivity TV need to understand the sensitivity, limited amplification. To compare TVs for that characteristic for to determine their suitability for far reception is impossible.
All developed after 1979. black and white and color stationary and portable TVs have sensitivity, limited noise, ranges meter waves - 100 mV, in the range decimetric waves - 140 mV. According to state these values are limit, the actual sensitivity could be better. TVs, developed before 1979, may have other values sensitivity. Worst sensitivity, limited noise - 150 μv in bands MB and 500 mV in ranges UHF possess TV type OPENCT-61, names which include indexes C-201 and C-202. These TVs are less suitable for far admission.
From determining the sensitivity, limited, noise, visible, that it is determined by the level self-noise television receiver, given to its entrance. The noise level is determined in mostly the first design gain stages in the selector channels, type and mode of the lamp or transistor that used in this cascade. For modern selectors channels voltage noise the entrance is approximately 5 mV in the range of MB and 7 mV in the UHF bands. Hence turns out the sensitivity, is $ 100 and 140 mV (in 20 times more the noise level). For this reason the improvement of sensitivity, limited by noise, may been made exclusively by reducing the level of self-noise, refer to the entrance, but not for by increasing the ratio the gain of a receive path replacement lamps, transistors or the use of any intensifying prefixes.
Radical measures to reduce self-noise television receiver without deterioration image currently not available. Used in the first stages of selectors channel microwave transistors GTA have the noise factor when the internal resistance of source 75 Ohm, equal to 7 dB. This is the least noisy of domestic transistors p-n-p structure. If to use in the first stage channel selector overseas the transistor type with AF251 the noise figure is 4.8 dB, the noise level will decrease by 2.2 dB, and sensitivity TV, limited noise, can be improved to 80/110 mV. However, the acquisition low-noise transistors foreign production represents difficult task.
Much easier to solve the issue, if in improve sensitivity to prevent some deterioration-definition image horizontally for by narrowing lanes transmission. In terms of far admission passport the clarity of the image the TV is not implemented, so as low-contrast the image struck intense noise nuisance. As you know, for clarity the horizontal proportional bandwidth receiving and amplifying tract, and the voltage noise proportional to the root square one band transmission. If narrow bandwidth 2 times, clarity will also deteriorate in 2 times, up to 250 items that the far technique can quite acceptable, and the noise floor is reduced by 3 dB, which consistent with the improvement sensitivity to 70/100 mV. The quality of the image subjectively improved by two factors: attenuation noise interference and increase contrast (since the restriction of bandwidth leads to the increase in the coefficient gain tract).
Easiest narrow bandwidth by increasing resistance load the video detector and amplifier. In black and white TVs ULPT-61-II-22 and ULPT-61-II-28 increase the resistors 3-R42 and 3-R47, TV STL-50-III-2 and SWLPT-50-III-1 - 2-R13 and 2-R22, TVs UPIT-61-II-1/2 and MOUTH-61-3/4-P25 and R26. In TVs color image narrowing bandwidth can lead to dropouts color and the image will be reproduced in black and white. You should not strive to excessive resistances of these resistors, especially in the amplifier cascades, in avoid violations normal modes electron tubes and transistors. Can be considered allowable increase resistance load detector about 2 times and load resistances amplifier 1.2 times. When this mode change is within tolerance and bandwidth narrows about 2 times.
Obviously, to get on the screen the TV image on it the antenna input is needed signal, level which should be higher the sensitivity of this television receiver, limited noise. From as far as the level of the signal exceed the sensitivity depends the quality of the image. If not the ability to affect the the sensitivity for her significant improvements need to try to increase the level the signal on the antenna input the TV that he was more value sensitivity Than.. signal level is determined at the entrance of a television receiver? In the first place the level of tension the electromagnetic field in the point of space, in which is reception antenna, the gain of this antenna, its actual length finally, attenuation of the signal in the feeder, which antenna connected to the TV. Of course, the antenna must be well matched with the feeder, and feeder - with TV, otherwise there will be additional attenuation due to its reflection and radiation back in space.
Tensions field at the point of reception depends on transmitter power, the distance to this transmitter, terrain on the track, signal attenuation in atmosphere. Radically affect the level of the field strength at the point reception is not possible. But usually there is a choice of location the location of the antenna, and having done some experiments to choose the optimum antenna position on the roof of the building and its height location corresponding to the maximum signal level at the input of the TV. The effective length of the antenna depends solely on the length wave of the received signal, is the channel number: the shorter wavelength (more the channel number), the less the effective length of the antenna. Thus, in order increase the signal level on the input on the TV remains the possibility of exposure to the antenna gain and signal attenuation in the feeder.
The coefficient antenna gain shows many times the voltage the signal at the output of the antenna exceeds voltage the signal at the output a half-wave vibrator, placed in the same point of the electromagnetic field. The gain can also be expressed in decibels. The larger the coefficient antenna gain, the more will the voltage signal at the input of the TV when other equal conditions. Therefore, in the far reception you must use antenna with large gain. It is characteristic that an increase the antenna gain does not increase noise. If the improvement sensitivity television receiver, limited noise, and the choice the optimal location antennas allow only small steps to improve receiving, using high-performance antennas may lead to increased the level of the signal many times.
Thus, the choice of antenna the determinant of far admission. And more than high-frequency signal need to take (what more the channel number), the greater should be the ratio the antenna gain. This is due to the fact that the current length the antenna is proportional to the length wave signal. Therefore, when the same field strength two signals, for example the 1st and 12-th channels, and the use of similar antenna with the same gain the signal voltage at the output antenna 12-channel will be in 4.3 times less than the output antenna 1 channel. Only for this reason, to obtain the same voltage signal at the input of the TV the antenna gain The 12th channel must be greater than the antenna gain The 1st channel is 4.3 times the voltage that corresponds to Of 12.7 dB. In the UHF range the need to use antennas with high gain by this the cause is even greater.
In the frequency range, which reserved for TV use different types high-performance antennas. In professional equipment (radio communication, radiolocation and so p.) preference usually given multi-element antennas type "Wave channel. In the same Amateur the conditions of the use of such antennas impractical for the following reasons. Multi-element antenna need to be careful the setting that produced by modifying the size of each element antennas and the distances between them. The setting is made in field conditions devices for control of a shape pattern antenna, the size and nature its input impedance. To make the adjustment antenna Amateur radio operator is not in condition.
Multi-element antenna, even if she made to exactly drawings, is upset, like is upset a multichannel radio immediately after Assembly. In the result of this mismatch is the antenna parameters are obtained much worse passport, and the positive effect of such the antenna does not. The upset the antenna shape is distorted and widens the main lobe the directivity, increase side and rear its petals, which leads to reduction ratio gain. Maximum main petal chart deviates from the geometric the axis of the antenna. In addition to the antenna was aligned with feeder, input resistance should be purely resistive and equal to wave resistance feeder. The upset of the antenna the input resistance is comprehensive and contains reactive component, and the active component significantly is different from the nominal values.
Professional the instrument usually contains special blocks for control for alignment of the antenna with feeder. Television the receiver of such blocks is not contains. As a result the error further lost part of the energy the signal that leads to the attenuation at the antenna output and equal to a reduction of its gain. Than more elements contains antenna type "Wave channel, the sharper rises the question about the necessity of its settings. Practice shows that satisfactory can to work without setting only three-element antenna of type "Wave channel". However gain three-element antenna for the voltage does not exceed 2.2 (about 6.8 dB), which is too low for long distance reception. Five element antenna has a gain of 2.8 (about 9 dB), but because of the inevitable the detuning in practice it gives the same result as three-element antenna. Theoretically, the coefficient the voltage gain of 11-element antenna of type "Wave channel" is 4 (about 12 dB). But this increased matches only configured and coordinated with feeder antenna. Due to the large the number of elements detuning of the antenna after her build is significant, which leads significant deterioration of its efficiency as a thanks to the fall the actual factor gain, and due strong mismatch antenna with feeder. These the reasons are explained frequent failure hams, tried through the use of multi-element antennas to improve television reception in conditions of weak signal. It is regrettable that, despite repeated the publication stated, many the authors of articles and books continue to recommend hams use multi-element antennas in the far reception television, apparently based solely on theoretical assumptions.
In connection with the fact that at the present time a significant part of the territory countries covered two - and even techprogram television broadcasting, when choosing reception the antenna is very tempting the opportunity use wideband antenna that would one antenna to ensure the acceptance of two or three TV programs on different channels. Such antennas there are, for example zigzag and automatic process of the antenna. However, their use possible only in the zone of direct likely, as the coefficient of the gain is relatively small. If the transmitters located in different directions a wideband antenna have to be installed on turning the mast and every time when moving from one program to another to refocus. Thus for the expense of inaccurate orientation the antenna signal is further is attenuated. In the zone of penumbra if you need to take several programmes for different channels must to install separate narrowband antenna. Two separate antennas can be connected to a common feeder with by using the separation filter. If the number of more than two antennas, additional switching can be done the contacts of the electromagnetic relays mounted near antenna management are made remotely, the toggle switch set at TV. Power relay coils can do from the TV on the same feeder without using additional wires.
In Amateur radio terms for long distance reception television well proven - phase system comprising several relatively simple antennas. Two antennas, arranged one above the other, form a two-storey system, which is characterized by narrower chart orientation to vertical plane. The four antennas can form a two-storey double-row system with constricted the chart in both vertical and horizontal planes. Narrowing charts orientation corresponds to the increase in the coefficient gain. Each doubling the number of antennas in common the system complies with the increase in the coefficient gain 3 dB (at 1.41 times voltage) only through summation of signals, adopted by each antenna. Additionally, by narrowing the directivity, gain increases approximately by 1 dB every time you double the the number of antennas in the system.
Use in the structure-phase system relatively simple antennas allows you to get a large gain without setup needed antennas. It is only necessary to ensure consistency system with feeder that easily occur because the values of the input the resistance of a simple antenna known and not depend on adjustment of the antenna. So manner, increasing the number antennas in the system can infinitely increase gain. It it is often necessary to the UHF range where, under other under equal conditions the voltage the signal at the antenna output significantly less than in the range of MB, due to the decrease the wavelength. However, due to the small size antennas of this range the increase in their number in the system is easily doable and not leads to excessive the dimensions of the system.
Most dissemination fans of long-distance reception television found in phase systems assembled from two-element and three-element framework antennas "Double square and a Triple square". Two-element framework antennas are typically used in bands MB and a three - in ranges UHF. According to some the authors, two-story DIL-phase system, collected from four two-element framework antennas, has a coefficient of the voltage gain of the order 6-8 (16... 18 dB), and the same system from three-element framework antennas-11-13 (21...23 dB). To achieve such gain using multiple-element antenna of type "Wave channel" impossible as even a factor gain 16-element antenna "Wave channel" does not exceed 14 dB, and even then, if it is carefully configured and coordinated with feeder.
Should to warn against frequent attempts the build-phase systems a few broadband antennas. In this way, trying to achieve a high coefficient gain the high bandwidth the antenna, in order to the far to have a reception the possibility of a single antenna system to make the transfer several programs in different channels. Such attempts, as are unsuccessful, as not able to negotiate the antenna frequency range. Elements approval usually contain resonant nodes in the form half-wave and quarter-wave segments cable, performing their functions only in a certain frequency. In a wide range frequency they can't work can. Do not give also success attempts to build common-mode systems from multiple multi-element antennas "Volovoi channel", what antenna are upset differently, the phase voltages signal at their outputs also to be different, and phase addition they are not turns out, sometimes instead of addition is subtraction.
When far the reception antenna set on high the mast and is connected to TV long feeder. The longer the feeder, the more attenuation it introduces and the less than the voltage signal at the input on the TV. For feeder the most common cable stamps of the Republic of Kazakhstan-75-4-11 with linear attenuation of 0.07 dB/m at 1-5-m channels of 0.13 dB/m at 6-12 m channels, 0,25-0,37 dB/m at 21-60-m channels. Graphs of linear attenuation of different brands of cable shown in Fig. 2.
If the length of the feeder 50 m attenuation signal on a 1-5-m channels small (3.5 dB), at the 33rd channel it reaches 15 dB, corresponds to a reduction the voltage signal is almost 6 time. To compensate for the attenuation signal in the feeder use antenna amplifier, mounted on the mast about antenna. This allows to ensure the receipt of the input antenna signal amplifier, which is not weakened by passing through a long feeder. This preserves high level relationships signal/noise ratio at the input of the antenna amplifier and antenna input television receiver. In this is also a key in contrast to the case when antenna amplifier set about the TV and no useful effect. Antenna amplifier therefore called antenna that should be installed around antenna and not about the TV. Gain antenna amplifier needs to be at least the same as the amount of signal attenuation in the feeder, better - for 5... 10 dB more. Then level self-noise television receiver you can neglect, and the image quality will be be determined solely signal-to-noise ratio at the input antenna amplifier,
The need in the long feeder sometimes occurs in the conditions of the enclosed space, when the TV is the hollow. If the antenna set on the top of the middle hill, a strong signal will be provided, but the length the connecting feeder will be about 100...200 m. Even at the frequency of the 1st channel with a length of feeder 200 m signal attenuation in it will be 14 dB. And in this if installing the antenna amp .about antenna will allow you to compensate signal attenuation. If the gain of one amplifier not enough, you can enable two amplifiers sequentially one by one, placing them evenly along the length of the feeder.
Need also to pay attention to the ability to use the quality of the coaxial feeder cables of different brands. Cable RK-75-9-13 has less linear attenuation than cable RK-75-4-11. This is especially noticeable in the UHF ranges: frequency 60 channel cable RK-75-9-13 has a loss about three times smaller in voltage than cable RK-75-4-11. Thus, due to the use the best cable if the length is too long can to raise the level of the signal at the input of the TV in a few time.
As buying a cable usually no the opportunity to determine its mark, can be guided the fact that the larger the diameter cable, the less attenuation he is making. As the feeder always use a cable with a characteristic impedance of 75 Ohms. If the brand of the cable and its impedance unknown, its easy to determine if there calipers, if the cable has a solid polyethylene insulation. The ratio of outer the diameter of the inner polyethylene insulation to the diameter of the Central core at cables with wave 75 Ohm should to be in the range of from 6.5 to 6,9.
Literature
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