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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Active antenna MV-DMV. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television antennas Active antennas (a combination of the antenna itself and the amplifier) have already been described on the pages of the magazine. However, the difficulties of receiving television programs in the MB and UHF bands make us return to this topic. In the article placed here, an interesting, in our opinion, variant of the design of the antenna and the combined MB-UHF amplifier is considered. The problem of high-quality reception of television programs has always worried viewers. And today it not only has not lost its relevance, but has become even more important and complex. First, there has been an increase in the number of television programs, primarily commercial ones, which are often transmitted through low power transmitters. Secondly, broadcasters began to actively explore the UHF range. And thirdly, the number of television signal consumers has increased (as a rule, a family has two or more televisions, or even a VCR). Therefore, it becomes necessary to install an UHF antenna, summing the MB and UHF signals for transmission to consumers via one cable, and dividing the television signal between all consumers. The last operation is not without losses, and if the signal level is low, then this leads to a deterioration in reception. It is not uncommon for an antenna that has served well for more than one year to no longer provide the required quality. Installing an antenna for each consumer is a rather troublesome business. The way out of this situation may be to install one combined active MB-UHF antenna. In the general case, it consists of two MB and UHF antennas themselves and an amplifying module that amplifies the signals from each of them, summarizes and transmits to consumers via one drop cable. The module is located directly near the antennas and is powered by a cable. Thanks to this construction, the signals from the antennas are transmitted to the amplifying module with almost no loss, which improves the overall signal-to-noise ratio, and the high signal level in the down cable allows you to connect several loads to such an antenna without fear of degrading the image quality. The schematic diagram of the proposed amplifying module is shown in fig. 1. It contains two independent amplification channels. The signal from the MB antenna has common mode noise. Coils L1, L2 are installed at the input of the cascade, which eliminate the accumulation of static electricity charges on some antennas, as well as diodes VD1 - VD4, which protect the amplifier from lightning discharges. An additional amplifying stage is assembled on the VT5 transistor. The channel transfer coefficient is 15...20 dB.
The MB signals are passed to the drop cable through an L6C19L7 low-pass filter with a cutoff frequency of 250 MHz. Through the same filter and inductor L5, the channel receives the supply voltage from the drop cable. In addition, the filter does not pass UHF signals. The UHF amplification channel consists of two identical amplifying stages connected in series. The first of them is assembled on transistors VT3, VT4 according to a galvanic-coupled circuit, due to which there is an automatic exit to the specified operating mode and its maintenance when the temperature and supply voltage change. At the input of the stage, a C1L3C2 high-pass filter with a cutoff frequency of 450 MHz is installed, which suppresses low-frequency signals and noise. A similar high-pass filter C21L9C22 at the output of the second stage passes UHF signals and does not pass MB signals. Consequently, the filters at the outputs of the channels decouple them mutually. Coil L4 provides coordination between the cascades of the UHF channel and correction of the total frequency response. The total channel gain is 32...36 dB, which allows you to transmit signals of this range through a long cable to several consumers. The UHF channel is fed through the L8 inductor from the drop cable. The amplifying module is assembled on a printed circuit board made of double-sided foil fiberglass. Its sketch is shown in Fig. 2. All parts are located on the side of the printed conductors. The second side is left metallized and plays the role of a screen (below the horizontal dashed line in Fig. 2). Along the contour of the board, strips of copper foil are soldered to it, connecting it to the common wire of the first side. Contacts XT3 - XT6 are made in the form of racks of pieces of copper wire with a diameter of 1 mm, pressed into the holes of the board. Around the contacts XT3 and XT5, the foil on the second side of the board was removed (shown in Fig. 2 by a dashed line). Through holes a and b, the printed tracks of the first side are connected to the printed tracks of the second side, they are shown by a dashed line. Holes are drilled in these tracks, which are necessary when placing the amplifying module directly at the MB antenna vibrator. The module is connected to the UHF antenna with a piece of coaxial cable.
When located near antennas, the module must be reliably protected from atmospheric influences. To do this, all parts and printed conductors are coated with a layer of moisture-resistant radio varnish (for example, UR-231) or liquid epoxy glue, and then with waterproof paint. After that, it is desirable to close the parts with a protective cover, which is made of food tin and soldered along the contour of the board. Places for soldering cables, as well as the cables themselves up to the outer protective layer and the place where the module is attached to the antenna, must also be painted over with moisture-resistant paint. In the device, in addition to those indicated in the diagram, it is permissible to use transistors KT363A (VT5) and KT382A (the rest). Capacitors - KM, KLS, KD or unpackaged. Resistors - MLT, S2-33, R1-4. When using case capacitors in the UHF channel, preference should be given to capacitors with minimal dimensions, and the length of their leads should not exceed 3 mm. Coils L3, L4, L9 are wound with wire PEV-2 0,2 on a mandrel with a diameter of 2 mm and contain, respectively, 2,5; 4 and 2,5 turns. Inductors L5, L8 - DM-0,1 with an inductance of 20 ... 100 μH. The remaining coils are wound with PEV-2 0,8 wire on a mandrel with a diameter of 4 mm. Coils L1, L2 contain 15, and L6, L7 - 4,5 turns. Establishing an amplifying module comes down to setting its frequency response. In the MB channel, this is done by selecting capacitor C13, and in the UHF channel - capacitors C12, C20, setting the maximum and uniform gain over the entire frequency range. By changing the inductance of the coil L4 (shifting or pushing the turns or changing their number), the frequency response unevenness in the upper part of the UHF range is corrected. If only one or two television channels operate in the UHF range, then it makes no sense to obtain uniform gain, it is more expedient to achieve the greatest gain at the frequencies of these channels. In the MB range, it makes sense to get the maximum gain at the frequency of the least powerful channel. The amplifying module is powered by a 12 V power supply at a current of at least 70 mA through an adapter module, the circuit of which is shown in fig. 3. The adapter is included in the drop cable break. It is assembled on a small printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in Fig. 4. On one side, only three contact pads are cut out on it and one hole is drilled, the second side is left completely metallized and serves as a common wire. If the parts on it are filled with epoxy glue, then you can do without a case.
The high-frequency coaxial socket XW1 (ordinary television) is soldered with a common lobe to the metallized surface of the second side, and with the central lobe - to one printed area according to fig. 4. A coaxial cable with a plug at the end is soldered with a braid to a common metallized surface, and the central conductor is soldered to another site according to fig. 4. The power supply is connected in parallel with the capacitor C2. If you use a source with a voltage regulated within 6 ... 12 V, then it will be possible to smoothly adjust the gain of the module, i.e., select its optimal value.
In addition to the amplifying module, the active antenna must also contain the antenna itself. Here you can go in different ways. If the radio amateur already has an MB antenna (for example, a wave channel or any other), which does not work very well, and it is planned to install an UHF antenna, then do so. The drop cable is unsoldered from the old MB antenna and the amplifying module is installed directly on it (on the active loop vibrator for the "wave channel" antenna). Install the UHF antenna and connect it with a cable to the module. If reception is carried out in the zone of reliable reception or close to its border, then, as a rule, there is no need to make complex antennas. Simple broadband antennas, both MB and UHF, are suitable here. The construction of just such an active antenna is described in more detail below. In the MB band, it is convenient to use a "fan vibrator" antenna, sometimes called a "butterfly". It represents (Fig. 5) four segments of a metal pipe or strip 1 108...115 cm long, which are securely fixed on a dielectric plate 2. The angle between the vibrators (in the vertical plane) of each arm is within 35...45 °, and the vibrator arms themselves are placed at an angle of 120° in the horizontal plane, as shown in Fig. 5, which makes it possible to eliminate dips in the radiation pattern. Sometimes the vibrators in each arm are made different in length, for example, the lower one is 30 ... 40% shorter than the upper one. The material and diameter of the pipes or the thickness of the strips are selected based on obtaining mechanical strength. Too thin will not withstand wind loads, and the use of thick ones will lead to a significant mass of the antenna. The most suitable are pipes with a diameter of 6 ... 8 mm. An amplifying module is attached directly to this antenna.
In the UHF range, it is convenient to use a zigzag antenna, which is easy to manufacture and fairly broadband. Its design is shown in Fig. 6. It consists of four strips 1 made of aluminum or copper 300 mm long, 15 ... 20 mm wide and 1,5 ... 2 mm thick, as well as four strips 2 145 ... 150 mm long. All strips are fastened together at points a with rivets or screws. A gap of about 10 mm is provided between the short strips. To obtain mechanical strength, a plate 3 of fiberglass with a thickness of 1,5 ... 2 mm is installed between them (Fig. 6,b) and fixed at points σ1 and σ2. If the strips are copper, then they must be tinned at the attachment points, and if they are aluminum, then petals are placed under the rivets or screws, to which the coaxial cable is subsequently soldered. It is laid as shown in Fig. 6,a with a dashed line, and from the side of the gasket, a braid is soldered to the point σ2, and the central conductor is soldered to the point σ1. After installation, all soldering and fastening points are covered with moisture-resistant paint.
After the manufacture of the MB and UHF antennas, a general assembly is carried out. If the active antenna is intended for installation on the roof, then they can be mounted on a metal pipe, as shown in fig. 7, which must then be grounded. In this case, the UHF antenna is attached to the pipe with metal screws through small spacers at two points a (the lowest and the highest in Fig. 6) so that the cable soldering points do not touch the pipe. Antenna MB, to which the amplifying module is screwed (contacts ХТ1 and ХТ2 in Fig. 2), is also fixed to the pipe with screws through a dielectric plate, and so that its metal parts do not touch the pipe. If the antenna is placed on a wooden stand, it must be dried well and painted with waterproof paint. However, you will have to run the ground bus on the side free from antennas. When installing the antenna on a balcony, loggia, open veranda, the requirements for mechanical strength are reduced, and grounding in these cases is not necessary. The appearance of the mounted structure is shown in the photo at the beginning of the article.
If you plan to use an active antenna in an area of poor reception, then you should use more efficient antennas. For the MB range, this can be a "wave channel", zigzag, on the UHF - a frame, consisting of three frames, or a "wave channel" (however, it is narrow-band). Author: I.Nechaev
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Comments on the article: R'RёS,R ° F "RёR№ For the completeness of the article, it is necessary to supplement the SSTF scheme of the antennas used.
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