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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Power amplifier for a single-channel "portable". Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications The regional rescue services that have appeared in recent years with their operators working around the clock on the air are doing a job whose importance can hardly be overestimated: residents of remote towns and villages that had no connection with their "centers" can now seek emergency medical help, call the police, firefighters and so on. However, it is unlikely that foreign-made multi-channel CB radio stations can be recommended to this far from the richest category of our citizens. And not only because they are still expensive. Worse another. The annual fee that the current "masters of the air" demand from every radio station that buys CBS is large and increasing (the annual "air lease" set by them may exceed the cost of the radio station itself). Even if it only works once a year. Or never. This fee will be assigned based only on the potential capabilities of the purchased radio, primarily on the number of working channels it has. So the annual payment for a foreign-made multi-channel radio station (others "there" have not been made for a long time) may turn out to be 5 ... 10 times higher than the minimum possible. Although such requisitions are quite immoral in and of themselves (the sale of the appropriated, and in its original form), the desire to "make money" on people's troubles (rescue services themselves do not take anything for their work) causes particular indignation ... After the citizens of our country were finally allowed to work in the civil band, the domestic industry began to produce portable single-channel low-power CB radio stations such as Ural-R, Laspi, etc. Having such a station, it is already possible to contact the rescue service *, but, unfortunately, only being close to it. This connection, of course, does not make much sense. However, the "range" of such a station can be increased to 30...40 km if it is supplemented with a 10-watt power amplifier **, and the standard antenna is replaced with a stationary one ***. A schematic diagram of an amplifier for a low-power CB radio station is shown in fig. 12. The level of the signal entering the preamplifier (transistor VT2, etc.) is set with a trimming resistor R3. One or another of its position will depend on the output power of the "portable". The amplified signal is taken from the capacitive divider included in the L1C6C7 oscillatory circuit, autotransformer connected to the collector of the transistor VT2.
The power amplifier is made on the transistor VT3. Its output is consistent with the antenna load Ra = 50 Ohm by a two-section C8C9L5C10C11L6C12 P-loop, which attenuates out-of-band radiation. Both transistors operate without initial bias (in class C), so the current drawn from the power supply by the unexcited amplifier is close to zero. The amplifier turns on automatically: when a signal appears at its input from the "portable" switched on for transmission, a voltage appears at the output of the detector, made on the elements VD1, VD2, C1 and C2, and in the base of the normally closed transistor VT1, a current opens it up to saturation. And relay K1, the winding of which is included in the collector circuit VT1 (it is useful to shunt it with a damping diode), will connect the input of the power amplifier to the output of the "portable". In the receiving mode, relay K1 will obviously be de-energized and its contact "three" K 1.1 will remain in the one shown in Fig. 12 position, providing reception of the signal of the correspondent. The amplifier elements, excluding the VT3 transistor, are mounted on a printed circuit board made of double-sided foil fiberglass 2 mm thick. The mounted board is installed on a metal heat sink plate (the size of the board, the material is duralumin 3 mm thick), to which they are attached, having smeared the junction with heat-conducting paste, and the VT3 transistor. The contour coil L1 is wound on a frame with a diameter of 5 mm, having a hole with an M11 thread for a carbonyl core. It contains 2 turns wound in a row with PEV-0,62 4,5 wire. Retraction - from the 1th turn (counting from the upper, "cold" end of the coil LXNUMX). Coils L5 and L6 are frameless. They are wound with PEV-2 0,82 wire on a blank with a diameter of 6 mm. Each coil has 7 turns. They are stretched so that the length of the coil is approximately 11 mm. Coils L5 and L6 are placed on the board so that their inductive coupling is minimal. Other details of the amplifier. Chokes: L4 - type DPM2-2.4; L2 and L3- DMO,4. Resistors: R3 - SPZ-386; R1, R2, R4-R6 - MLTO, 125. Capacitors: C4 - any oxide, C3, C5 and C7 - KM-6 or K10-176; C2 - CD; the rest KD, KM, KCO-1, etc., having a small TKE, low losses at high frequencies and an operating voltage of at least 50 V. Relay K1 - RES-55A (for 12 V, for example, RS4.569.600-06). The oscillatory circuits of the amplifier are tuned to the operating frequency of the radio station. A 50-ohm antenna equivalent (four 200-ohm MLT-2 type resistors connected in parallel) and a high-frequency voltmeter head (see its description in this book) are connected to the antenna output of the amplifier. The tuning of the circuits (L1C6C7 is adjusted by moving the trimmer in the L1 coil, and L4L5L6C8-C12 - shifting-pushing the turns in the coils L5 and L6) is carried out to the maximum readings of the RF voltmeter. The power developed by the amplifier at the load Ra: Pout \u2d U ^ 10 / Ra, where Pout is in watts, U is in volts (effective value) and Ra is in ohms. Accordingly, the voltage generated by a 50-watt amplifier into a XNUMX-ohm load should be: U =ЦPout*Ra \u22,4d 3 V. If it is less and cannot be increased by adjusting R5, reduce the resistance of resistor RXNUMX. The amplifier can be powered by any 12-volt source capable of delivering a current of 2 ... 2,5 A. It can even be an old car battery that has lost a significant part of its capacity. In this case, the radio station will be independent of the power supply of the village, the absence of which, by the way, in itself may be the reason for contacting the rescue service. *) If its frequency channel coincides with the radio channel of the local rescue service. It is not difficult to rebuild such a radio station to work in another channel - you just need to replace the quartz resonators of the master oscillator and local oscillator in it. **) The maximum radio transmitter power permitted today by Gossvyaznadzor to work in CBS is 10 W (in security systems - 2 W). ***) For example, the "Simple CB Antenna" described in this book is suitable. Publication: cxem.net
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