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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING 144 MHz converter for CB radio. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception You can expand the functionality of the CB radio station by supplementing it with a converter of the appropriate range. Radio stations are best suited for this, having an indication of not only the channel, but also the operating frequency. If there is a need to receive SSB stations, then, of course, the CB radio station should have this kind of work. It can be, for example, the Dragon SS-485 radio station and similar ones. An important feature of these radios is the presence of manual smooth frequency adjustment. A converter is offered to the attention of readers, which, together with the indicated radio stations, allows receiving signals from amateur radio stations with FM and SSB in the frequency band 144 ... 146 MHz. The converter circuit is shown in fig. 1. UHF is assembled on transistor VT1. Its resonant circuits suppress the signals of the mirror reception channels and CB signals by at least 40 dB. This is especially important as it reduces the chance of interference from nearby CB radios. UHF uses a low-noise transistor. UHF transmission coefficient - 15 ... 20 dB. The mixer is assembled on a VT3 transistor, the load of which is a C13L4C14 low-pass filter with a cutoff frequency of 35 MHz. The local oscillator (on the transistor VT2) has a quartz frequency stabilization.
The input circuit C 1C2L1 is set to the middle frequency of the range (145 MHz). Trimmer capacitor C1 ensures optimal matching of the circuit with the antenna-feeder path, and C2 - tuning the circuit to the operating frequency. Coordination of the circuit with the transistor is provided by a tap from the coil L1. Transistor VT1 is loaded on the L2C5 circuit, which is also tuned to the middle frequency of the range. Resistor R2 increases the stability of the UHF. The amplified signals of the 144 MHz range enter the mixer base circuit (VT3 transistor). The voltage from the local oscillator is fed into the emitter circuit of this transistor. If the radio station used covers the frequency band 25,16 ... 29,66 MHz (for example, it is Dragon SS-485), then in order for the converted signals from the 144 MHz range to fall into this frequency band, the local oscillator frequency must be in the range from 116,34 to 118,84 MHz or 171,16 to 173,66 MHz. In doing so, the following should be taken into account. The first option is simpler to implement, and in this case it is more convenient to count the frequency of the received signal on the scale of the radio station. However, in this case, the mirror reception channels will be in the 86,68 ... 93,68 MHz band, i.e., they will fall into the FM television or radio broadcasting range. If your city has powerful radio transmitters on these frequencies, then there may be significant interference to reception. For the second variant of the local oscillator frequencies, the mirror reception channels will be within 196,32 ... 203,32 MHz. In this case, interference with reception from television transmitters of the 9th channel is possible. For any of these options, it is desirable that the local oscillator frequency be a multiple of 1 MHz. This will make it easier to convert the tuning frequencies of the radio to the frequency of the received signal. In the author's version of the converter, the local oscillator frequency was chosen to be 118 MHz. The quartz resonator is excited at the third harmonic (59 MHz). The L2C3 circuit, tuned to 9 MHz, is included in the collector circuit of the transistor VT118. All parts of the converter are placed on a printed circuit board made of double-sided foil fiberglass. The sketch of the board is shown in fig. 2. There is foil on one side. It is connected along the edges of the board with a common wire of the second side, on which the parts are located. To increase the stability of the converter, screens are installed on the board - partitions about 12 mm high (shown by a dashed line), made of thick foil or thin foil fiberglass. The same screens are installed along the edge of the board. The following parts can be used in the converter: transistor VT1 - KT3132A-2 or KT3101A-2; VT2 - KT315A(B-G); VT3 - KT368A(B) or KT399A; tuning capacitors - KT4-25, the rest - KLS, KM, KD; resistors - MLT, S2-33. Coils L1 and L2 are wound with PEV-2 wire with a diameter of 0,8 mm on a mandrel with a diameter of 3,5 mm. They have 4,5 turns with a tap in the middle. L3 is wound on a frame with a diameter of 4 mm with a carbonyl iron trimmer with a diameter of 3 mm and has 4,5 turns of PEV-2 wire 0,4 mm (outlet from the 1,5th turn). L4 is wound with the same wire on a 3,5 mm mandrel and has ten turns. Establishment begins with setting the mode of the transistor VT1 for direct current. To do this, by selecting the resistor R1, they ensure that the collector has about half the supply voltage. Then the UHF LOOP is tuned to a frequency of 145 MHz. Resistor R2 is selected so that the UHF is not excited at any position of the trimmer capacitors. By selecting capacitors C8, C10, they achieve stable operation of the local oscillator at a frequency of 59 MHz, and with the coil trimmer L3, tune the local oscillator circuit to 118 MHz. After that, the converter can be connected to the radio station and apply a signal to it in the frequency band 144 ... 146 MHz. The CB radio station is tuned to the appropriate frequency, and then by adjusting the UHF circuits and selecting the resistor R10, maximum sensitivity is achieved. The final stage is the adjustment of the UHF capacitors C1 and C2 to the maximum according to the real signal when using a specific antenna. The converter is connected to the radio station with a cable with the appropriate connector. In this case, it should be remembered that turning on the radio station for transmission will lead to its failure. To exclude such a situation, it is necessary to block the "Transmit" button on the PTT using a special bracket or remove the PTT to a hard-to-reach place. A good way out of this situation is to replace the PTT with a plug with jumpers that ensure the operation of the radio in the normal mode for reception only. In this case, the power supply of the converter and switching the input of the radio station from the converter to the CB antenna can be carried out from the radio station through this plug. A variant of such a connection for the Dragon SS-485 radio station is shown in Fig. 3. In this case, the converter is made in the form of a high-frequency adapter module, connected between the radio station and the CB antenna. The board needs to be slightly increased or an additional small board should be made, diodes VD1, VD2 should be installed to protect the converter from the signal of the radio station transmitter and the relay, which has closed contacts in the de-energized state.
When the tangent is connected to the radio station, the relay and the converter are de-energized and the signal from the antenna through the XS2 socket and the closed relay contacts is fed to the radio station input through the XP1 plug, i.e. it is operating normally. Having connected the XS1 plug into the PTT socket, we put it into a permanent receive mode and supply power to the converter. In this case, the relay is activated and the CB antenna is disconnected from the input of the radio station, and instead of it, the output of the converter is connected. The radio station turns into a 144 MHz radio receiver. The relay can be small-sized, types RES-49, RES-60 and others similar with an operating voltage of 10 ... 12 V, but it is better to use special high-frequency relays. This will increase the suppression of CB radio signals, which can cause interference. The converter board together with the relay must be placed in a metal case with high-frequency connectors. If the converter is planned to be used frequently, then it is better to make a small low-frequency adapter between the PTT and the radio station. This version of the circuit for the same radio station is shown in Fig. 4.
The adapter has two connectors, the XS1 socket, which is connected to the PTT socket, and the XP1 plug, to which the PTT is connected. The adapter has a switch SA1. In the "Radio" mode, the XS1 contacts are directly connected to the XP1 contacts, and the radio station, together with the PTT, works in the normal mode. When the switch SA1 is switched to the "Converter" position, the supply voltage is disconnected from the PTT and is supplied to the converter and relay K1. The RF input of the radio will be connected to the output of the converter. At the same time, the circuit for turning on the radio station for transmission (pin 3) is opened - it will constantly work in the "Receive" mode. In addition, the dynamic loudspeaker circuit (pin 2) is closed to the common wire, ensuring its normal operation. The SA1 switch can be any small-sized switch with two positions and three directions. All connections must be made with wires of minimum length and preferably shielded, and the body of the adapter must be made of metal. If it is necessary to indicate the "Converter" mode, then the resistor R1 and the LED HL1 are introduced into the circuit, installing it on the adapter body in a visible place. Author: Igor Nechaev, Kursk; Publication: N. Bolshakov, rf.atnn.ru
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