Power amplifier for CB radio. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / RF power amplifiers

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Most portable radios on the 27 MHz band have a transmitter with an output power of no more than 1-2 watts. The sensitivity of the receivers of such radio stations usually lies in the range of 3-0,5 μV / m. As a result, even with the use of a high-quality outdoor antenna, the communication range at which reliable reception is provided is not very high. There is no point in increasing the receiver sensitivity further than 0,5 μV/m, since the noise level increases, both atmospheric and the noise of the receiver input elements. So the real way to increase the radio communication range on the CB is only to increase the output power of the radio station transmitter.

This power amplifier is designed to operate a portable CB radio station in stationary mode, while the signal from the output of its transmitter is fed to the input of this amplifier through a coaxial cable. The input impedance of the amplifier is 50 ohms, with an input power of about 1-2 watts, a power of about 30-40 watts develops at the output of this amplifier. The output is designed for a 75-ohm antenna.

A schematic diagram of the amplifier and its power supply is shown in the figure. The signal from the output of the transmitter of the radio station through the X2 connector is fed to the input of the amplifier on the double lamp VL1 GU-29, the signal is fed to the control grids of this lamp. Resistor R7 brings the input impedance of the amplifier to the level of 50 ohms. The anode load of the lamp is a choke L2, from which the signal is taken and through a tunable "P-shaped filter L1C3C4 enters the antenna. The output stage of the transmitter is equipped with an SWR meter that allows you to measure the direct and reflected SWR (switch S1), this makes it possible to optimally adjust the output circuit using variable capacitors C3 and C4.

The transformer power supply contains two rectifiers and three parametric stabilizers.

The power amplifier coils have the following data: L1 is wound with a bare copper wire with a diameter of 2 mm, without a frame, the winding diameter is 25 mm, the winding length is 22 mm, the number of turns is 8. The L2 coil is wound on a frame with a diameter of 20 mm, it contains 150 turns of PELSHO 0,25 wire, 80, winding length 3 mm. Coils L4 and L2 are wound on resistors R4 and R5, they contain 1,0 turns of PEV 5 wire. Coils L6 and L0,5 - DM-1 chokes. Transformer T6 is 0,31 turns of PEV 1, with a tap from the middle, wound on the inner core of the coaxial cable that goes from LXNUMX to the output connector (the shielding braid of the cable is removed at the winding point).

Transformer T2 is wound on a magnetic circuit Ш25Х32, winding 1 contains 1030 turns of PEV 0,55 wire, winding 2 - 1300 turns of PEV 0,25, winding 3 - 60 turns of PEV 1,0 with a tap from the middle, winding 4 contains 175 turns of PEV 0,2, XNUMX.

The power amplifier is mounted on a box-shaped metal chassis in a three-dimensional manner using the leads of large parts, a lamp panel and contact combs and petals, which are usually used for mounting such equipment. It is necessary to ensure high-quality heat removal from the lamp, it is most convenient to do this with the help of a small electric motor with an impeller (such fans are used in computer power supplies), which will blow the lamp during operation.

Resistor R8 sets the quiescent current of the lamp in the range of 15-17 mA. The alternating control voltage supplied to the lamp grids (voltage at R7) should be about 10V, and not exceed 15V.

The amplifier was tested with a homemade radio station with an output power of 2 W into a load of 50 ohms.

Author: Andreev S.; Publication: N. Bolshakov, rf.atnn.ru

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