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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING A repeater is a direction-finding attachment to a radio station in the 27 MHz band. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas This set-top box does not require any connections to the radio station, has no light, sound or arrow indicators and contains a minimum of parts. It is used in conjunction with a radio station receiver to determine the direction to 27 MHz transmitters located at a distance of no more than 0,5 km. The author used a set-top box with a single-channel AM radio station "Tom-1", which does not have an antenna connector to which a directional antenna could be connected. The circuit design of its input circuits does not allow the introduction of such a connector. In addition, the AGC system of the radio station receiver would negate the directional properties of the antenna, and the plastic housing does not prevent the signals of direction-finding signals from penetrating the receiver input, bypassing the antenna. The direction finder was required to search for vehicles (bicycles) temporarily left in a secluded place, equipped with radio beacons, and fellow travelers who dispersed through the forest in search of mushrooms. At a distance of more than 200 m, the folk means of communication "au" is no longer valid, especially in dense forests and mountainous areas. A satellite navigator often works reliably only in open areas. Reporting your location by radio is also difficult due to the lack of reliable landmarks. The proposed prefix-direction finder is a repeater of the direction-finding transmitter signal. All RF signals in the passband of the repeater and received by its loop antenna are modulated with a tone frequency signal using a balanced modulator, amplified and re-radiated by an omnidirectional antenna. As a result, at the input of the receiver located near the repeater, two signals are summed up - the one that came directly from the direction-finding transmitter and the rebroadcast one. The sum signal is modulated by the tone signal applied to the input of the modulator in the repeater. The nature of this modulation (AM or FM) depends on the difference in the path of the signal terms from the transmitter to the receiver, therefore, on the relative position of the receiver, repeater and transmitter. The nature of this dependence can be judged from the graphs in Fig. 1. The distances on it are indicated in wavelengths λ of the direction-finding transmitter. In the range of 27 MHz, λ=10,9 m. If the repeater is on one of the red lines, then the modulation of the total signal is amplitude, and if on one of the blue lines, it is frequency. In the gaps between the lines, both types of modulation are present, but in different proportions. As the repeater approaches the blue line, AM falls off, and as the repeater approaches the red line, FM falls.
It must be said that the location of the "pure" modulation lines also depends on the phase shift introduced by the transceiver path of the repeater. For example, if it is 90о, then the red and blue lines are reversed. Distortions and reflections of signals from local objects, including from the body of the operator, are introduced into the picture. Nevertheless, it is always possible to place a repeater near the receiver so that the modulation it introduces into the direction-finding signal will be heard in the best possible way. The direction to the transmitter is determined by rotating the loop antenna of the repeater around the vertical axis. This can be done according to the maximum modulation (the plane of the frame lies in the direction being determined) or according to its minimum (the plane of the frame is perpendicular to the direction being determined). DF at the minimum is usually more accurate. The uncertainty associated with the bidirectionality of a loop antenna can be removed in one of two ways. The first - traditional - is to consistently determine the direction from several points lying on a line approximately perpendicular to it. The bearings found in this way intersect at the location of the transmitter. Of course, during the time between measurements, it should not move. With a relatively small distance to the transmitter, it is usually sufficient to make two serifs from points spaced several meters apart. The second method is based on the nature of the curves in Fig. 1. In the direction from the transmitter, they follow much more often. The goal was to make the device of minimum weight and dimensions, because it must be carried with you through the thickets. Practice has shown that being in the forest, it is enough to have one direction finder per group of tourists or mushroom pickers. Each of the others, equipped with a radio station and a compass, the direction of movement to reach the collection point can be reported by radio. The repeater circuit is shown in fig. 2. It consists of a receiving loop antenna WA1, a ring balanced modulator on diodes VD3-VD6 with transformers T1, T2, a modulating signal generator on a multivibrator of transistors VT1 and VT2, a high-frequency amplifier on a transistor VT3, a transmitting antenna WA2 with an extension coil L3.
The repeater is powered by two AG13 disk galvanic cells or batteries of the same design D-0,03. The consumed current does not exceed 4 mA. Since the repeater is usually turned on for the duration of the direction finding, there is no need for high-capacity batteries, and the SB1 button is used to turn on the power. It is possible to increase the number of elements up to three, while the transmission coefficient of the repeater and the depth of the modulation introduced by it into the direction-finding signal will increase, but this can lead to its self-excitation. As a transmitting antenna WA2, a piece of coaxial cable braid 20 ... 30 cm long, hanging down, was used. This antenna can also serve as the electric screen of the loop antenna WA1. To do this, it is necessary to break the connection of the screen with the common wire, as shown in the diagram with a cross, and connect point A to the upper terminal of the L3 coil according to the diagram (instead of the WA2 antenna). Point A should be located between the places where the wires of the frame exit the shield as symmetrically as possible to the cut in its upper part. But it should be borne in mind that with this use of the loop antenna shield, the repeater is more prone to self-excitation. The main reason for self-excitation is the impossibility of achieving perfect isolation between the receiving and transmitting antennas, despite the fact that one of them is magnetic and the other is electrical. The inevitable asymmetry of the frame design and its position relative to the transmitting antenna, as well as the influence of the operator's body, have an effect. The frame screen has the shape of a square with a side of 120 mm. It is made of a copper tube with an outer diameter of 5 mm. An incision about 5 mm wide was made in the center of the upper side of the square. At the end of all settings, this cut must be sealed in any way to prevent moisture from entering the tube. In the center of the lower side of the screen, a cut was made for the exit of the frame winding wires. A copper strip is also soldered here to connect the screen to a common wire or to an L3 coil (if the screen is supposed to be used as a transmitting antenna). The fastening of the frame must be strong enough, as it can serve as a handle for carrying the repeater. Three or four wires in PTFE insulation are threaded into the tube. Their ends on each side are connected in parallel, and the wires form one turn. You can, of course, try to connect the turns in series, but sometimes it is difficult to tune the frame to the desired frequency. The device is assembled on a narrow long board placed in a screen made of a piece of thin-walled aluminum pipe, which simultaneously serves as a counterweight to the WA2 antenna. The parts are assembled "in a line", trying to arrange them symmetrically along the long axis of the board. The furthest from the WA1 antenna should be the coils L1-L3. Their axes should not coincide in direction with the axis of the loop antenna. In addition, the axis of the coil L3 must be perpendicular to the axis of the coils L1 and L2. Diodes VD1, VD2 serve to limit the signal at the input of the balanced modulator. It may be required both when the signal of the direction-finding transmitter is too strong, and during the operation of your own transmitter. Capacitors C2 and C3 suppress interference and signals below the 27 MHz band. Elements R3, C7, R4, C9 determine the oscillation frequency of the multivibrator. With the ratings indicated on the diagram, it is close to 1 kHz. The rectangular signal taken from the multivibrator is smoothed by the R1, C8, R6 circuit, bringing it closer in shape to a sinusoidal one. This is achieved by selecting the capacitor C8. The modulating voltage is supplied to the balanced modulator through the midpoints of the windings of transformers T1 and T2. Capacitor C5 eliminates the DC component of the modulating signal, and capacitors C6 and C10 serve to filter the high-frequency products of the modulator. Transformers T1 and T2 are wound on ring magnetic cores of size 7x4x2 mm made of 400NN ferrite. The winding is carried out with three PEL wires with a diameter of 4 mm twisted with a pitch of 6 ... 0,14 mm. Wires in fluoroplastic, silk or other thick insulation cannot be used. A total of 8 turns are wound, with each wire serving as a separate winding. In transformer T1, the end of winding II is connected to the beginning of winding III. Similarly, windings I and II of transformer T2 are connected. Coil L1 of the output circuit is frameless and consists of 12 turns of varnished wire with a diameter of 0,4 ... 0,5 mm, wound on a mandrel with a diameter of 4 mm and stretched to a length of 10 mm. The communication coil L2 has three turns of the same wire wound over the coil L1, in the middle of it, and stretched by 5 mm. The extension coil L3 is also frameless. Its 36 turns are wound with the same wire on a mandrel with a diameter of 4 mm in two layers. Winding length - about 14 mm. The required number of turns of this coil depends on the size of the WA2 transmitting antenna and the capacitance between the antenna and the operator holding the repeater in his hands. All portable radio stations with a short antenna have a similar disadvantage [1, 2]. The optimal inductance of the coil L3 is selected experimentally according to the maximum field strength emitted by the antenna WA2, and the associated depth of modulation of the direction-finding signal created by the repeater. The repeater circuits are tuned with a loop antenna attached to its board. It is not recommended to use an external power supply, as long wires introduce a significant error. For tuning, you will need a source of a high-frequency test signal, for example, another radio station, a GKCh or a measuring signal generator. You also need a radio station with which the direction finder will work, or the like, and measuring instruments, at least a high-frequency millivoltmeter or oscilloscope. If the oscilloscope has insufficient bandwidth, you will have to make a detector head for it, for example, as described in [3]. Repeating it, you need to reduce the capacitance of the input capacitor C1 of the head to 100 ... 470 pF and add a smoothing capacitor with a capacity of up to 1 ... 470 pF after the resistor R4700. You can use similar device nodes described in [4] or [5]. The output of the head must be connected with a twisted pair of wires about a meter long to the input of the oscilloscope, after putting on each end of the pair ferrite rings measuring approximately 25x12x6 mm (for example, from switching power supplies) and winding them with twisted wires of 6 turns. This is necessary for high frequency decoupling from the oscilloscope. If a signal generator or GKCh is used, then a round frame with a diameter of about 51 cm from a wire with a diameter of 30 ... 1 mm should be connected to its output through a 5 Ohm resistor and placed at a distance of several centimeters parallel to the WA1 antenna. The signal level can be adjusted not only by the generator attenuator, but also by changing the distance between the frames. Frame screen WA1 must be connected at point A to the common wire of the repeater. You should start by tuning the WA1 antenna to the selected frequency by selecting the capacitor C1 according to the maximum reading of a millivoltmeter or an oscilloscope with a detector head connected to any of the windings II or III of the transformer T1. In this case, it should be borne in mind that limiting diodes VD1 and VD2 are connected in parallel to the antenna, so tuning must be performed with a signal amplitude on them of no more than 0,6 V. You should not turn off the diodes, since their capacitance is included in the total capacitance of the tuned circuit. In addition, balanced modulator diodes can open with a large signal, which will also interfere with proper tuning. Coupling capacitors C2, C3 also affect the setting. When using a radio station transmitter as a test signal source, its level is adjusted by changing the distance between the repeater and this radio station. You may need outside help to do this. But first you have to make sure that the meter is not receiving the test signal directly. To do this, you need to temporarily connect the terminals of the WA1 antenna winding with a short jumper. The readings of the millivoltmeter or oscilloscope connected to the transformer T1 should become zero. Having set up the loop antenna, they proceed to setting the output circuit of the amplifier on the transistor VT3. The HL1 LED serves as a bias voltage regulator for this transistor. To configure, you need to temporarily disconnect resistors R1 and R6 from the balanced modulator and install temporary jumpers in parallel with the diodes VD3 and VD6 (or VD4 and VD5). The transmitting antenna WA2 must be disconnected, and it is desirable to disconnect the lower output of the L2 coil from the common wire. In parallel with this coil, a load resistor with a resistance of about 50 ohms is connected, and in parallel with it, the input of a millivoltmeter or the detector head of an oscilloscope. Having turned on the power of the repeater, you should first of all make sure that in the absence of a test signal from the generator or transmitter, the voltage at the load of coil L2 is zero. If this is not the case, then the repeater is self-excited. To eliminate self-excitation, you can take the following measures: - connect in parallel blocking capacitors C4, C11, C12 high-frequency ceramic capacitors with a capacity of about 1000 pF; If self-excitation is not eliminated by the described measures, then its cause should be sought in the amplifier based on the VT3 transistor. To eliminate it, you can try to shunt the L1 coil with a resistor R11 with a resistance of 470 Ohm to 4,7 kOhm, connect a capacitor with a fraction or unit of picofarad capacity between the collector and the base of the VT3 transistor, increase the number of turns of the L2 communication coil, replace the VT3 transistor with a lower frequency. Sometimes it helps to introduce a decoupling filter into the power supply circuit of a multivibrator based on transistors VT1 and VT2. The filter consists of a choke connected in series in this circuit and a blocking capacitor in parallel with the multivibrator. The inductor can be wound on the same magnetic circuit as transformers T1 and T2, filling the stacked turn to turn with a PEL wire with a diameter of 0,12 ... 0,14 mm from half to two thirds of the circumference of the ferrite ring. Resistor R8 must be selected according to the highest gain, as it increases, the absence of self-excitation should be checked, and in order to avoid limitation, reduce the level of the test signal. The limitation is manifested in the fact that the readings of a millivoltmeter or an oscilloscope cease to depend on the level of this signal. When self-excited, their readings are maximum even in the absence of a test signal. The L1C14 circuit, like all other repeater circuits, is tuned to the frequency of the direction-finding radio station. In this case, it must be taken into account that changing the operating mode of the transistor also changes the capacitance introduced by it into the circuit. Therefore, it is recommended to select the resistor R8 and configure the circuit at the same time. Practice has shown that its setting is also affected by a change in the capacitance of the capacitor C15. The circuit is tuned by selecting the capacitor C14, changing the pitch and number of turns of the L1 coil, or screwing an aluminum trimmer from the PTK of an old TV into the coil (it reduces the inductance). At the end of the setup, remove temporary jumpers and reattach resistors R1 and R6. Briefly touch on the selection of capacitor C8. With its low capacitance, the shape of the modulating signal is close to the initial shape of the pulses at the output of the multivibrator, and its amplitude is maximum (Fig. 3a). But when modulating with a square wave, there are too many sidebands. As a result, when several transmitters operate at close frequencies, the spectra of their signals modulated in the repeater may overlap, which will create mutual interference and make direction finding difficult.
As the capacitance of the capacitor C8 increases, the signal is smoothed out (Fig. 3, b), more and more approaching a triangular one (Fig. 3, c). Its amplitude decreases, therefore it is not recommended to bring the shape to a triangular one, since the supply voltage of the multivibrator is small and the modulating signal may become too weak to open the balanced modulator diodes, although they are germanium. Precise balancing of the modulator is not required and no means are provided for this. You can read about the selection of diodes for the modulator in [6]. After performing all the described operations, it becomes possible to listen to the operation of the repeater together with the radio station receiver. To do this, position the radio so that the input circuits of its receiver are in close proximity to the L1d4 loop of the repeater. When the repeater is on, the test signal must be heard with modulation by a tone of 1 kHz (corresponding to the frequency of the multivibrator), and with it turned off, without this tone. If a tone is heard when the test tone is off, the repeater is self-exciting. The most difficult step is tuning the WA2 antenna with the L3 extension coil. It is recommended to make it in a fully assembled repeater in order to take into account the influence of all elements, including the case. You must first disconnect all measuring devices from the repeater, remove the load connected to the L2 coil, and connect the lower output of this coil according to the diagram to the common wire of the repeater, and the upper output through the L3 coil to the WA2 antenna. As a signal source, it is recommended to use a radio station remote at some distance, replacing the direction-finding one, with a conventional antenna. The repeater must be held in the hands, as its body and the body of the operator serve as a counterweight to the WA2 antenna. The receiver of "your" radio station must be turned on and be at a distance of about half a meter from the repeater. As already mentioned, as an antenna WA2, a segment of a shielding braid 5 ... 8 mm wide in a smoothed form was used. The initial length of the segment is 30 cm. Its free end should be tucked up to a length of 25 cm and secured with an insulating tube. You should not make a longer antenna, it will interfere when carrying the repeater. To adjust the L3 coil, you will need an aluminum rod that fits inside the coil and is attached to the end of a wooden stick to exclude the influence of the operator's hands. Having tuned the receiver to the signals of the transmitter and repeater and making sure that there is modulation, we introduce an aluminum rod into the L3 coil. If the modulation depth (tone volume 1 kHz) is maximum when the rod is inserted approximately half the length of the coil, then the goal is achieved, the rod can be removed and an aluminum trimmer inserted into the coil instead. Its exact position is found by the maximum volume of the tone. If the maximum is reached with the full introduction of the aluminum rod, then it is necessary to reduce the inductance of the coil L3 by stretching its turns or reducing their number, and then repeat the test by inserting the rod. If the introduction of an aluminum rod only reduces the volume, then the number of turns of the coil should be increased. It is not recommended to use a ferromagnetic trimmer to increase its inductance. By changing the length of the WA2 antenna by more or less twisting its free end, you can tune it to the desired frequency more accurately. The L3 coil is set up in the same way if the WA2 antenna shield is used as the WA1 antenna. When tuning the transmitting antenna, the excitation of the repeater as a whole can be observed. This manifests itself in the loss of the signal in the receiver or the appearance of interference. If the excitation occurs at the frequency of the DF transmitter, then the continuous tone does not disappear when the transmitter is turned off. To eliminate excitation, you will have to reduce the gain by shunting the L1C14 circuit with resistor R11, selecting resistor R8, or installing a capacitor between the collector and base of the VT3 transistor, as previously recommended. In this case, naturally, the depth of modulation of the direction-finding signal will also decrease. If everything is set up correctly, then the bandwidth of the repeater turns out to be wide enough for direction finding of a radio station operating not only in the frequency channel on which the tuning was performed, but also in several neighboring ones. Capacitors in high-frequency circuits C1-C3, C6, C10, C13-C15 must be ceramic, and C5, C7-C9 - ceramic or film. Capacitor C4 - oxide. Diodes KD512A can be replaced by KD510A, KD520A. The use of germanium diodes D311 in a balanced modulator is due to the low supply voltage of the device. If it is increased, then silicon high-frequency diodes, for example KD503A, can also be used. The HL1 LED should be red, as it serves as a 1,8 V voltage regulator. Instead of KT361B transistors, you can install KT209B or replace them with KT315B (npn). The high-frequency transistor KT3128A is replaced by KT3127A, which can be found in the channel selector SK-M-24-2. You can also install lower-frequency KT326B (pnp) or KT368A (npn). Please note that replacing pnp transistors with npn transistors should only be done at the same time. In this case, it is also necessary to change the polarity of the power supply, capacitor C4 and LED HL1. Literature
Author: G. Safronov
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