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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Your first transmitter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications Amateur radio communication on short and ultra-short waves is one of the most interesting areas of amateur radio, combining both traveling on the air and designing (receivers and transmitters, measuring equipment, antennas). With this article, we open the publication of a series of articles on the basics of amateur radio communications for those who have decided to become a shortwave operator. Usually, the path to short waves begins with building a receiver for a range in which beginner shortwaves are allowed to work (range 160 meters), listening to signals from amateur stations and learning the telegraph alphabet. A description of a simple radio receiver for this range was in the May 2001 issue of the Radio magazine. Now we give a description of a simple transmitter that will allow the radio amateur not only to listen, but also to go on the air himself. It should be remembered that going on the air is possible only after obtaining permission to operate an amateur radio station. Beginning radio amateurs (radio stations of the 4th category) are allowed to work in the KB range of 160 meters. With a maximum allowable power of 5 W, interesting radio communications on this range can be made by working by telegraph (CW). This article describes a simple telegraph transmitter available for beginners to manufacture. The transmitter consists of an oscillator with quartz frequency stabilization and a field-effect transistor power amplifier. This provides an excellent CW tone. Its disadvantages include the impossibility of smooth frequency tuning, but at first this is not so significant. The schematic diagram of the transmitter is shown in fig. one.
The master oscillator is assembled on a medium power bipolar transistor VT1 according to the capacitive "three-point" scheme. A capacitive divider is formed by capacitor C1 and the input capacitance of a powerful transistor VT2, connected between the emitter of transistor VT1 and a common wire through a separating capacitor C2. The frequency of generation in the telegraph section of the range of 160 meters sets the quartz resonator ZQ1. The telegraph key is installed in the gap of the emitter circuit of the transistor VT1. When the switch is open, there is no current through the transistor and the master oscillator is not energized. The transmitter power amplifier is assembled on a field-effect transistor VT2. The initial offset to its gate is supplied from the divider R3R4. The resonant circuit L2C6 is turned on at the output of the power amplifier. Coil L3 is used to communicate with the antenna. Power is supplied to the amplifier in parallel through inductor L1. The operation of the transmitter is controlled by a double switch (tumbler) S1. In the position shown in the diagram (reception), power is not supplied to the transmitter, and the antenna is connected to the receiver of the radio station. In the other position of the switch (transmission), the transmitter is powered, and the antenna is connected to the output of the power amplifier. The transmitter is powered by a DC source with a voltage of 12...13,5 V. There is no current consumption in the receive mode. In transmit mode, when the key is released, it will be about 100 mA, and when pressed, it will be about 400 mA. Transmitter output power 2...3 W. The following parts were used in the transmitter: a quartz resonator of any type, for a frequency from 1830 to 1930 kHz, but it must be borne in mind that at frequencies above 1880 kHz, stations with single-sideband modulations (telephone) usually work and it is rather difficult to find correspondents for telegraph work. All capacitors are of the KM type, except for oxide C4 and mica C5 (500 V). Capacitor C6 is a dual KPI block with an air dielectric from an old radio. All resistors are MLT type. Inductor L1 is used standard, for a current of 2 A with an inductance of at least 4 μH. Coil L2 is wound with wire PEV-2 0,35 on a frame with a diameter of 16 mm and contains 60 turns, the winding is carried out "turn to turn". The communication coil L3 is wound over the turns L2 with an MGTF wire with a cross section of 0,1 mm2, its number of turns is selected for a specific antenna. Most of the transmitter parts are mounted on a board made of one-sided foil-coated getinax or fiberglass (Fig. 2).
The areas to which the leads of the parts are soldered are separated from each other by grooves made by the cutter until the insulating material is exposed. Thus, galvanic work in the manufacture of the board is not required. The transmission indicator HL1 and the current limiting resistor R5 are located on the front panel. The design of the transmitter is shown in fig. 3.
The front panel made of duralumin and the back panel of getinax or fiberglass are attached to the board with the help of an angular profile 10 ... 12 mm wide. The front panel has high-frequency coaxial connectors for connecting an antenna and a receiver, an arrow indicator RA1 (at 200 μA, from a tape recorder), a switch S1 and an output circuit tuning knob. The rear panel contains terminals or connectors for connecting a telegraph key and a power source. The transmitter box housing is made of any sheet metal, it must have a reliable connection to the common wire. The establishment of the transmitter begins before the installation of the quartz resonator ZQ1 (there will be no generation when the key is pressed) from the selection of the resistor R1 until a voltage of 1 ... 5 V is obtained on the emitter of the transistor VT7 (the key is pressed). The mode of the transistor VT2 is selected by the resistor R3 until a drain current of about 80 mA is obtained (the key is pressed). With the quartz resonator installed and the key pressed, the high-frequency voltage at the VT1 emitter or, which is the same, at the VT2 gate should be 3 ... 4 V, and the drain current VT2 should increase to 0,3 ... 0,4 A. By connecting the antenna and tuning the output circuit to resonance with the capacitor C6, the number of turns of the coupling coil L3 is selected according to the maximum voltage at the transmitter output. The number of turns at the input impedance of the antenna feeder is 50 ... 75 Ohm - about 10. The resonance of the output circuit should be obtained when the capacitance of the capacitor C6 is about 70% of the maximum. Beware of tuning to the harmonics of the microwave voltage of the master oscillator with a small capacitance C6! In conclusion, the resistor R6 is selected so that the pointer of the PA1 device deviates by about three quarters of the full scale. We will give some recommendations for choosing an antenna. The transmitter works well for a dipole with sides 40 ... 42 m long and powered in the center by a coaxial cable with a characteristic impedance of 75 ohms. You can use a vertical or inclined wire 40 ... 42 m long, connected at one end to the antenna socket. But in this case, a good ground must be connected to the transmitter housing. In any case, the antenna tuning is carried out by the capacitor C6 according to the maximum readings of the indicator PA1. Operation on the air, since the frequency of our transmitter is fixed, is "on a general call" - a little patience, and an experienced radio amateur will answer you. In this case, you need to listen to the frequencies adjacent to your working one. It is advisable to use a good professional receiver with the described transmitter, but you can also make a homemade one, the one with which you started radio observations on the air. Author: Ya.Lapovok (UA1FA)
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