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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Short waves in wires. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur Standing current waves Short waves propagate along the wire in a different way than we are accustomed to imagine the propagation of current. We usually assume that the current in any place of the wire has the same strength. With an oscillatory current, this turns out to be incorrect; so-called "standing waves" of current and voltage are formed in the wires, caused by the reflection of electricity from the end of the wire. Strictly speaking, such waves are formed with any alternating current, but we cannot observe them, since very long wires are generally needed for this: it is necessary that the length of the wire or pair of wires exceed at least 1/4 of the wavelength. For short waves this is very easy to do. Let us first analyze what happens in a single wire. Let there be a sufficiently long wire, which has a short-wave generator at one end E, and the other end A is insulated (Fig. 1).
As we have already pointed out, the current in such a wire will not be the same along its length. At the end, the current is 0, and as you move away from the end, it appears and gradually becomes more and more, until at point B, 1/4 wave away from the end, it reaches its maximum value. This means that if we turn on the ammeter at different places on the wire between points A and B, then it will show more and more current as we approach point B, and the current will change along the ABC curve. 1st. Beyond point B, the current gradually drops to point C, where it stops altogether. The distance from C to A is equal to half the wavelength of the shortwave generator. Further, beyond point C, the current increases again, reaching its maximum value in D, and then again drops to zero, after which everything repeats again. Distance AD is equal to 3/4 of a wave, distance AE is the whole wavelength of the generator. At the points of maxima (B and D), the ammeter will show the same current strength, but the current at each given moment at these points flows in opposite directions (as, for example, indicated by arrows). In order to see this in the drawing, we place the current distribution curve CdE down from the EA line, while the first part of it, AbC, is located up from EA. The AbCdE curve has the form of a so-called sinusoidal curve. When we have such an uneven current distribution along the wire, we say that a standing current wave has been established in the wire. The places of the greatest current strength (points B D) are called current antinodes, and those places where it is equal to zero (points A, C, E) are called current nodes. We see that both neighboring nodes and antinodes are at a distance of half a wave from each other. We considered the wire long enough, but if it were shorter, for example, only 1/4 wave (i.e., there would already be a generator at point B), the current distribution would still be uneven. At the same time, since the current at the end of the wire is always 0, then at the end of the wire (A) there will be a node, and at the generator (B) there will be a current antinode. Now it is important to note that if we have a single wire in which standing current waves are established, then it radiates radio waves into space. This means that it consumes energy. The energy consumption for radiation at short waves is very significant and increases with the shortening of the wavelength. If we need the wire to radiate, then this will be a useful expenditure of energy, but sometimes this is just not necessary, and then this expenditure will be a waste of energy. We have such a case, for example, if the EA wire is not an antenna in itself, but only serves to supply energy to the antenna. In this case, the energy lost in it for radiation will not only be wasted for us, but may even be harmful, interfering with the radiation of a real antenna. Lecher's system To supply current without energy loss for radiation, a two-wire line or the so-called Lecherov system is used (Fig. 2). It consists of two wires running at a relatively short distance from each other. Damn it. 2 shows a Lecher system insulated at one end and connected at the other end to a generator. In this system, we also see the formation of standing current waves. But, looking closely at the drawing, you can see that in the same place (for example, cut aa), the current in each wire flows in opposite directions. It is very important. Due to this circumstance, both wires prevent each other from radiating energy and the Lecherov system has no radiation losses.
So far, we have been talking about standing waves of current, but the same waves take place for voltage. Damn it. 3 shows the stress distribution along the Lecher system. We see here the same curve as for the current; knots and antinodes are also observed here. But only voltage antinodes occur exactly where the current has nodes and vice versa. This is easy to see by comparing drawings 2 and 3.
Very often, the Lecherov system with a bridge is used. This is the name of a mobile conductor that short-circuits both wires of the system. This bridge can be made from two thin copper plates screwed together. When the bridge needs to be moved, the screws are loosened, and then screwed again. The Lecherov system with a bridge differs in that at the location of the bridge, the voltage between the wires will always be zero, there will be a voltage node, and, consequently, a current antinode. How the current and voltage curves are arranged in this case is shown in Fig. 4.
Therefore, by installing a bridge somewhere on the system, we thereby determine the place of the current antinode. This is very convenient when the system is designed to work with different wavelengths, as it allows you to easily change the system settings. The fact is that in order to obtain distinct standing waves, the Lecherov system cannot be connected to the generator somehow. It is imperative that the generator is located in a certain place, for example, in the antinode of the current. It's shown in hell. 2, where the system is connected to the generator coil so that a current antinode passes through the coil. If we now change the wave of the generator, then exactly 3/4 of the wave will not fit on the wire. Since there will always be a current node at the end of the system, our generator will leave the antinode and the standing waves in this case will turn out to be very weak. If we have a bridge, then we can always move it so that the generator again falls into the antinode of the current. Experiments with Lecher's system It is not difficult to carry out an experiment that makes it possible to visually verify what has been said. To do this, you need to have a short-wave generator, a Lecherov system and several light bulbs from a flashlight. The generator must be of sufficient power - from two ten-watts; with two amplifying or micro-tubes, satisfactory results can only be obtained with a very good generator. Wave range: 30 meters and below. Lecher's system must be taken from two wires with a diameter of about 1 mm (telephone bronze wire is very good) and pull these wires at a distance of 5-10 cm from one another, taking care that this distance does not change between the wires. To do this, ebonite or glass spacers must be placed between them at a distance of 3-4 meters from one another. It is better to take the system as long as possible, preferably 25-30 meters. The ends of the wires must be insulated, especially the ends closest to the generator. Here the wire must be intercepted before reaching the generator, as shown in hell. 5, leaving the end free to connect to the generator.
Insulators should be nut-shaped - a chain of 4-5 pieces, necessarily connected with a rope, not wire, - or glass - tubular or whole. Taking a light bulb from a flashlight, solder two hard bare conductors to it and take them in opposite directions. The ends of the conductors must be bent so that they wrap around the wires of the Lecher system, as shown in Fig. 6, allowing, however, to move the resulting bridge with a light bulb along the system. The ends of the system are connected to the generator or as shown in Fig. 2, or coupled inductively (Fig. 7). In both cases, the most advantageous connection must be selected by experience.
Having tuned the generator to some wave, for example, 20 meters, then they move the bridge, moving away from the generator. The light bulb in the bridge, which initially glows, gradually goes out; but if you move about half a wave away, it lights up again, and when it shines the most, Lecher's system will be tuned. Then a standing half-wave with current antinodes at the light bulb and at the generator will fit on the system. If you move the light bulb further, it will go out again and light up again when two half-waves fit from the generator to the bridge, etc.
When the Lecher system is set up, we can also detect nodes at the voltage antinode. Voltage nodes can be found by touching the wire with some conductor held in the hand. Usually, with such a touch, the system setting is disturbed and the light in the bridge goes out. But if we touch the wire in the voltage node, then we will not violate the settings and everything will remain unchanged. This happens because the wire has no voltage in the node and therefore, by connecting the node to the ground, we cannot divert the current to the ground. The voltage nodes are located in the same place as the current antinode. To find the antinodes, you need to hang a light bulb from a flashlight to one of the wires as shown in Fig. 7. Sheet A can be of any metal (except iron) 10x10 cm or larger. The light bulb will glow most strongly in the antinode of the voltage, because here the current will flow most strongly from the wire through the light bulb and the capacitance of the metal sheet. If the generator has a significant power, then by hanging an ordinary electric light bulb (without a sheet) in the antinode of the voltage, we will be able to observe the bluish glow of the rarefied air contained in it. If you leave the antinode of tension, the described phenomena disappear. About measuring wavelength The reader from what has been said, by the way, can conclude that it is convenient to apply the Lecher system to determine the wavelength of the generator. Indeed, by measuring the distance between two adjacent current antinodes, we will have exactly half the wavelength. However, it should be noted that the measurement of the wave using the described setup will not give completely accurate results. The light bulb located in the bridge absorbs energy and, as a result, the measured wave will be somewhat shorter than the actual one. The measurement error reaches 1-2%. To avoid this error, in laboratory installations, instead of a light bulb, sensitive devices are used, which, moreover, are not included in the bridge, but are connected to it inductively. The method itself remains the same and is used to calibrate shortwave wavemeters. Let us now get acquainted with some more properties of the Lecher system, which, by the way, will allow us to further describe another more accurate way of measuring the wavelength. Lecher's system as a wattless resistance Self-induction and capacitance encountered on the path of alternating current represent for it the so-called wattless resistance - inductive or capacitive. The Lecher system can also be used as such a resistance, moreover, it sometimes has advantages over conventional self-induction coils and capacitors. To see why this is so, let us turn to figure 8. Here are the current and voltage curves along the Lecher system ending at A. We know that the undulating distribution of current and voltage is due to reflection from the end of the conductor. But you can look at things a little differently. Let's take two sections a and b on the system and note that the current in a is greater than in b, and the voltage is vice versa. If so, then we can say that the resistance of the Lecher system at a is less than at b. By resistance we mean the resistance of a section of the system with a length from the end to a and from the end to c.
Reasoning in this way, we can define the resistance for a Lecher system of any length. It turns out that, depending on the length, it can be either inductive (equivalent to the resistance of the self-induction coil) or capacitive. Damn it. Figure 9 shows the curves of this resistance for the bridged Lecher system. The curves refer to a system of 1 mm diameter wires 8 cm apart, but will be about the same for all similarly sized systems. In the drawing, inductive resistance in Ohms is plotted upwards from the horizontal axis, capacitive resistance downwards. The length of the Lecher system is plotted along the horizontal axis in fractions of a wave. Suppose we want to have such a system that its resistance is inductive and equal to 1000 ohms. It is easy to determine from the curves that for this the system must have a length equal to 0,16 wavelengths.
The wattless resistance curves of the Lecher system allow, among other things, to understand what the system tuning process actually consists of. In order to obtain the greatest current, and, consequently, the most noticeable standing waves, it is necessary that the system connected to the Lecherov generator should not have much resistance; least of all, this resistance will be just when the length of the system is equal to a half-wave or a multiple of it; in this case, the generator will be in the antinode of the current. It makes sense to use the Lecher system instead of self-induction coils and capacitors for very short waves, especially for waves of the order of several meters. The advantages here are that the Lecherov system has very low losses, which in coils and capacitors increase greatly with wave shortening. It is more convenient to use the Lecher system instead of chokes or blocking capacitors; it is more difficult to use it in oscillatory circuits *. Of course, it must be remembered that the Lecherov system presents a certain wattless resistance only for a given wave; as soon as we change the wave, the resistance changes. It should also be noted that for capacitors (if they must not pass direct current), a system without a bridge should be taken. Capacitance curves for such a system are given in Fig. 10. In this case, the ends of the wires must be well insulated.
More about wave measurement Having become acquainted with the resistance of the Lecher system, we can describe another method for measuring the wavelength, which, however, requires a powerful generator if possible. To do this, it is necessary to have a symmetrical oscillatory circuit connected inductively with the generator (Fig. 11).
Capacitors should have a capacity of approximately 8 to 100 cm, coils of 4-10 turns with a diameter of about 8 cm. In the circuit, a flashlight bulb is included as an indicator. The connection should be possibly weak, which is why a more powerful generator is desirable. The oscillatory circuit is broken at points a and b, where the Lecherov system with a bridge is connected. The bridge is first installed not far from the circuit (about 1/8 of the wave) and the circuit is tuned to resonance: at the same time, the light bulb flashes. Then. without touching the circuit, move the bridge away until the light bulb lights up again the brightest. The distance between the first position and the last will be just half a wave. This method is based on the fact that the same resistance values of the system are repeated along the length of the system strictly after half a wave, unless the system has large energy losses. In conclusion, we point out that the Lecher system is of particular importance for supplying energy to antennas and, in particular, to complex directional antennas. We will not dwell on this issue, which requires a special essay. As the reader sees, in the technique of short waves Lecherov's system has been widely used; it has every reason to take its rightful place in the practice of our shortwave radio amateurs. * It is useful to remember that the inductive reactance of the self-inductive coil L is 6,28 fL ohms, the capacitance for the capacitor C is 1/(6,28fC) ohms, where f is the oscillation frequency = 3 * 108/Lambda, where Lambda is the wavelength in meters. L and C must be expressed in henries and farads. According to these formulas, it is possible to determine which coil and which capacitor are equivalent to the Lecherov system of one or another length. Author: A.Pistohlkors
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