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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING radio receiving antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur [an error occurred while processing this directive] The simplest detector or transistor receivers have little gain and require a significant signal level at the input, which is created by the antenna, for normal operation. Such receivers operate in the ranges of long and medium waves (LW and MW), where even one oscillatory circuit is enough to detune from the signals of neighboring, interfering radio stations. Receiving antennas for these ranges will be discussed. Modern radio stations, fortunately, have considerable power and create a large field strength, so they can be received even on a detector receiver with an antenna of moderate size. The antenna wire must be located along the lines of force of the electric field of the received wave, i.e. in the direction of its electric field vector E (Fig. 1a). On the LW and MW radio stations emit waves with vertical polarization, in which the electric field vector E is vertical, and the magnetic field vector H is horizontal. Accordingly, the magnetic antenna must be placed horizontally (Fig. 1, b), and the electric antenna - vertically (Fig. 1, c).
A magnetic antenna is a ferrite rod of rectangular or circular cross section with a coil wound around it, which is also the coil of the input, and perhaps the only one in the receiver of the oscillatory circuit. The ferrite rod, having a high magnetic permeability, concentrates the magnetic field of the received wave in the coil. The antenna is usually located inside the receiver housing, and therefore is very convenient. It is directional and should be approximately perpendicular to the direction of the radio. If the direction is unknown, it can be determined by turning the body of the receiver, and the minimum of reception, when the axis of the magnetic antenna rod looks at the radio station, is more pronounced. From which side the station is located (in the direction found) it is impossible to determine with the help of a magnetic antenna. Unfortunately, the signal voltage developed by the magnetic antenna is completely insufficient for the operation of the detector receiver - one or two transistor radio frequency amplification stages are required in front of the detector. If you begin to master radio engineering with the construction of the simplest detector receiver, you will have to use an electric wire antenna that develops a much higher voltage. Receivers with magnetic antennas will be mastered later. A classic electrical antenna is a dipole, which is a straight piece of wire, open in the middle, with a two-wire line connected at this point, connecting the dipole to the receiver (Fig. 1, c). The dipole is located vertically, it has its own resonant frequency, at which its length is equal to half the wavelength. But in the NE, and even more so the LW, the wavelength is from 200 to 2000 m. And, of course, no one makes receiving dipoles longer than 100 m, especially those located vertically. Shortened dipoles are used, the developed signal voltage of which decreases in proportion to the decrease in length. True, there is a way to shorten the length of the dipole by half without deteriorating its performance - to use grounding (Fig. 1d). The earth will serve as a perfect counterbalance to the top half of the dipole and replace its bottom half. This is done even at transmitting radio centers, where the height of a full-sized antenna-mast should now be a quarter of a wavelength. Further possibilities to reduce the length of the dipole (and hence its height - after all, the dipole is vertical) are to use a capacitive load at its upper end. Current. current through the drop wire, must recharge this capacitance with the frequency of received oscillations. Therefore, the larger the capacitance, the greater the current flowing through the drop wire and entering the receiver. The upper capacitive load is performed in different ways. In the simplest case, a horizontal wire is used, suspended on insulators between two masts or other suitable objects (houses, trees). If it is a continuation of the vertical drop wire, an L-shaped antenna is obtained (Fig. 2, a). It has a weakly pronounced directionality: stations are received a little better from the downside, so it is better to stretch the far, free end of the wire away from the radio station.
If the drop wire is connected somewhere closer to the middle of the horizontal part, a T-shaped antenna is obtained (Fig. 2,6). It equally receives radio signals from all directions. The length of the horizontal part can be 10 ... 25 m, it is not advisable to make it too long, since it does not directly participate in the reception of radio waves, but only increases the efficiency of the vertical part. For L- and T-shaped antennas, two supports are needed - this is their disadvantage. If local conditions allow, it is possible to stretch the antenna of the "oblique beam" type from the window where the drop enters to the nearest high object (roof ridge, tree). The free end of the wire must be insulated with one or two porcelain insulators (old rollers from electrical wiring will do). When fixing the antenna on the trees, try not to break the branches and do not wrap the trunks with wire - the trees suffer and die from this, because they have no means to protect themselves from the barbarians! It is best to hang a very loose and in no case tightening loop of hemp or cotton rope on a suitable fork of the branches, and already tie the wire going to the first antenna insulator, or this insulator itself, to it. Keep in mind that trees sway in the wind, so the wire must be hung with a large "sag" so that it does not break. The diameter of the antenna wire is not important and is chosen only for reasons of mechanical strength. A copper winding wire in enamel insulation, wound from old (discarded) transformers, is quite suitable. Even with a diameter of 0,5 mm, its tensile strength reaches 4 kg, increasing in proportion to the square of the diameter. This is quite enough, besides, the antenna turns out to be very light and, by the way, almost invisible from the ground. Two other antennas (Fig. 2, c, d) are mounted on the same mast - a vertical wooden pole, if necessary, reinforced with braces. A small and light pole can be mounted on the roof ridge, a longer and heavier pole is better to install on the ground. Make braces from a synthetic cord or nylon fishing line with a diameter of 0,8 ... 1 mm - it is strong, elastic, and inexpensive. By the way, if, while walking along the river bank, you find a coil of fishing line tangled and thrown away by fishermen, do not be too lazy to pick it up and unravel it. Come in handy. In the antenna shown in Fig. 2c, the upper capacitive load is formed by a wire "wheel" of arbitrary shape and configuration, connected to the drop wire and isolated from the mast by a porcelain insulator. An insulator is needed in case of rainy and wet weather, when a wet mast tree becomes, albeit a bad one, but a conductor, and can degrade the performance of the antenna. The well-known "panicle" antenna is similarly made, in which instead of a "wheel" a bundle of wires is used fan out from the insulator. We do not recommend doing it, because the beam turns out to be heavy, and the antenna works inefficiently, since the wires are too close to each other. It is better to take only 6 or 8 wires about 0.5 m long and spread them apart like knitting needles. This is already enough, but you can still connect the ends of the spokes with a thin copper conductor. The role of the capacitive load in the so-called "umbrella" antenna (Fig. 2, d) is performed by the upper parts of the extensions 2 ... 3 m long, made of wires connected at a central point with a decrease. The ends of the wires are isolated from stretch marks by insulators. If the extensions are made of fishing line, which is a good dielectric, you can do without insulators by connecting the fishing line to the wire. Usually put three or four stretch marks. We talked about antennas, the design of which is suitable for rural residents - they have more options in choosing a place and materials for making antenna masts. We deliberately do not give dimensions, because, within reasonable limits (height no more than 10 ... 15 m, length no more than 20 ... 30 m), the higher and longer the antenna, the louder the detector receiver will work. Those wishing to delve into the theory are advised to read the articles in [D-3]. For more information on antenna design, see [4]. The antenna is useless without grounding - after all, high-frequency currents that go down must flow somewhere! A detector receiver without grounding will not work at all, and a sensitive transistor receiver will "choke" from interference - as experience shows, when grounding is used, the reception of weak stations improves, and the level of interference decreases, and very significantly. In addition, the antenna needs lightning protection, so grounding must be done first. In many cases, you already have grounding if you have plumbing. Water pipes run in the ground and are not isolated from it. Central heating pipes serve as good grounding, although they are isolated, but in modern apartment buildings they are electrically connected to the common ground loop of the house, in any case, an extensive heating network serves as an excellent counterweight to the antenna. It is forbidden to connect to gas pipes, and the electrical network is a source of such powerful interference that it is better to stay away from it - this is also true from a safety point of view. If there is no running water and you live in a wooden rural house with stove heating, carefully go around your household - there will surely be a metal pipe deeply hammered into the ground. It will serve as a ground. The fittings of the water well work perfectly, a fence on metal poles is suitable - you can connect several poles with wire laid along the fence to get both grounding and counterweight at the same time. If this is not the case, you will have to use the option that was repeatedly described back in the twenties: dig a hole in a convenient place, preferably to the level of pound waters or at least to a level where the soil does not freeze through, put an old bucket, a sheet of iron or a trough in it (the area is important, not the name) with a thicker soldered wire, sprinkle salt and charcoal on top (to improve electrical conductivity) and bury, tamping. Grounding is ready. The antenna must be immediately equipped with a lightning switch and a spark gap in order to protect yourself from atmospheric electricity. Once upon a time, lightning switches with a spark gap were produced, made in the form of a small knife switch. They are convenient, although not entirely safe: if you are late to ground the antenna, you can accidentally touch the contact connected to the disconnected antenna during a thunderstorm, which we do not recommend. A toggle switch or electric switch SA1 of any design will serve as a switch (Fig. 3, a).
The lightning switch is mounted on a board made of any insulating material fixed on a wall or window frame near the drop inlet. The arrester F1 is served by two metal plates with teeth, between which a gap of about 1 mm is left. In parallel with the arrester, we advise you to connect any neon lamp VL1 - its flashes will signal the electrification of the antenna. You will be surprised to find that this can happen not only during a thunderstorm, but also in a bitter frost during a snow blizzard. And what will happen if the lightning switch is not made, and the end of the antenna drop is not connected anywhere and is thrown, say, on the windowsill? The antenna will accumulate a large charge, its potential can rise to tens and hundreds of thousands of volts (we're not kidding!). Then touching the drop will become deadly (this is how Richman, an associate of Lomonosov, was killed), large sparks can jump from the drop wire, causing a fire. So be sure to ground! The most radical method of lightning protection (not excluding the "neon" and the arrester!) Will be the construction of a detector receiver according to the simplest scheme with inductance tuning (Fig. 0.3.b). It is better to wind the coil, which "forever" galvanically connects the antenna to ground, with a fairly thick wire (0.5 ... 20 mm in diameter) on a frame made of any insulating material. A piece of plastic pipe used in plumbing, a plastic shampoo or cream bottle, etc. will do. The winding is carried out in one layer turn to turn. With a frame diameter of 40 ... 100 mm, about 300 turns are needed to receive radio stations in the CB range, and about 100 in the DV range. In the latter version, you can make a tap from the XNUMXth coil turn and install a range switch. For tuning, a ferrite rod from the magnetic antenna of any transistor receiver is inserted into the coil. Better selectivity, i.e. detuning from the signals of interfering stations is provided by detector receivers with a tuned antenna circuit, the circuits of which are shown in fig. 4. If the antenna is large and the coupling coil L1 has a large inductance, it is better to use a serial tuning scheme (Fig. 4a). and with a short antenna and low inductance L1 - parallel (Fig. 4,6). The coils are wound on separate frames and tuned with separate variable capacitors (KPI) C1 and C2. You can tune coils and ferrite rods from magnetic antennas. The tuning turns out to be quite complicated - three parameters need to be adjusted: two frequencies for tuning the circuits with capacitors and the connection between the coils, bringing together and pushing their frames apart. But on the other hand, you can achieve good results in terms of volume and reception quality.
Coil data is the same as in the previous case. KPI can be of any type, with a maximum capacitance of at least 150 ... 200 pF (suitable for any old radios). If two-section KPE blocks are used. it is better to connect both sections in parallel to expand the tuning range. With the described receivers, you can use only high-resistance telephones with a resistance (both capsules are connected in series) 3600 ... 4400 Ohm Blocking capacitor C1 in fig. 3, b and C3 in fig. 4 serves to close high-frequency currents after the detector and can have a capacitance from 2000 to 10 pF. But what about a city dweller who would like to experiment with detector receivers, but does not have the opportunity to penetrate the roof of a house and install a large antenna? By the way, there is no need to climb the roof, since lowering the antenna along the wall of a reinforced concrete house will not work effectively. her, i.e. in the horizontal direction. Under these conditions, an antenna with a capacitive load (Fig. 2, c), placed on a horizontal pole two meters long from the balcony, can be much more effective. If there is a tree or some kind of towering object in front of the window, you can extend an "oblique beam" towards it. Get smart and see if there are any "surrogate antennas" near the window, such as a drainpipe or a flagpole. It is quite possible to attach the antenna wire to them. Even if the pipe is grounded somewhere or connected to the roof, the antenna will still work, but good or bad depends on the specific local conditions. Just try not to fall out of the window when you reach for such antennas. Safety first! However, it is not at all necessary to take the antenna outside, since the field of LW and MW stations penetrates well into buildings. Use an indoor antenna! The configuration of the field inside the buildings is unpredictable, so you need to experiment. Pick up any insulated wire 5 ... 10 meters long, connect it to the receiver (do not forget about grounding) and move the wire around the room and near the window, while setting up the receiver and watching the reception volume. It is not necessary that the wire be under the ceiling, sometimes, thrown on the floor, it works better! Having chosen the optimal position of the wire, fix it by hiding it behind a curtain, under a carpet, behind a sofa, stretching it along the junction of the wall and ceiling, etc. Since the room is dry, there are no special requirements for antenna insulation, the wire can even be nailed to the baseboard with small nails. If the antenna wire is routed along the wires of the telephone or broadcasting network without being in contact with them, reception may be improved due to the capacitive coupling of the network and the antenna. There is another interesting possibility. Sometimes various metal pipes are laid in the room (or in the wall of the room), for example, central heating. Try to place the indoor antenna wire near them, because in these pipes, as in any antenna, currents are also induced by the electromagnetic field of radio stations. If the reception volume increases, wind a few turns of insulated wire around the pipe and connect the end of the wire to the antenna jack (or clamp) on the receiver. A capacitor will be obtained that provides capacitive coupling of this surrogate antenna with the receiver.
Of course, if the same pipe is used as grounding, success is unlikely, but it is possible - in the case of spacing the connection points of the "antenna" and grounding away from each other. Very good results can be obtained if the ground is connected, for example, to water pipes, and the antenna is capacitively connected to heating pipes. What to do if you don’t want to entangle the whole apartment with wires or your parents don’t allow it, and there is only one pipe in the room, for example, central heating? And there is a way out: the author obtained very good results using a magnetic antenna placed close to the pipe and perpendicular to it. You don’t even need to connect to the latter and strip the paint on it! A schematic of an experimental receiver with such an antenna is shown in Fig. 5. The oscillatory circuit of the receiver is formed by the coil of the magnetic antenna L1 and KPE (of any type) C1. The detector, as in previous receivers, will be any point germanium diode of the D2 series. D9, D18, D311, GD507, etc. The blocking capacitor C2 and telephones were discussed above. The magnetic antenna can be used ready-made (together with the coil) from any transistor receiver or wound by yourself. The coil of the CB range on the rod of the magnetic antenna contains 60 .. 80 turns of any thin insulated wire, the DV range is 180 ... 240 turns. The core can serve as half of the magnetic circuit of the horizontal transformer from an old discarded TV or half of the ring from the deflecting system. The number of turns of the coil in this case is approximately halved, since the magnetic permeability and the cross section of such magnetic circuits are larger. The method of placing the resulting antenna near the pipe is clear from Fig. 5, where the cross section of the pipe is shown by a dashed line. All parts of the receiver are placed on a small plate of getinax (textolite, plywood, cardboard, etc.). Avoid only short-circuited turns around the magnetic circuit of the coil when fixing it to the board. By bringing the magnetic circuit to various extended metal objects, you can find the best place. In one administrative building, this turned out to be the metal frame of the window and, oddly enough, the corner of the elevator shaft. How does such a system work? high frequency current. induced by the field of the radio station and flowing through the pipe, creates a magnetic field, the lines of force of which look like concentric rings worn on the pipe. This magnetic field is concentrated in the ferrite magnetic circuit and induces an EMF in the circuit coil. Everything is very simple and effective. The connection with the pipe will increase if, on the other side of it, you bring the second half of the magnetic circuit of the horizontal transformer, forming a closed magnetic system around the pipe. In this case, the inductance of the circuit increases, which has to be compensated by a decrease in the capacitance of the capacitor C1. It is generally possible to replace the capacitor with a constant one, and to carry out the adjustment by mutual movement of the halves of the magnetic circuit. We wish you success in your experiments! Literature
Author: V.Polyakov, Moscow
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