Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The article provides a description of amplifiers for the 144 and 430 MHz bands, with a maximum output power of 80 and 50 W, respectively. It should be noted that these values ​​of output powers exceed the values ​​allowed by the "Instructions on the procedure for registration and operation of amateur radio stations". Before building such an amplifier, it is necessary to obtain permission from the local GRFC for experiments at increased power.

The proposed power amplifiers are made on a dual transistor VT7, VT8 type 2T985AC (KT985AC), built according to almost identical circuits (Fig. 1). In the diagram, the element ratings shown without brackets correspond to the 144 MHz range, in brackets - 430 MHz. If only one denomination is specified, then it is used in both options, and in the absence of one or another denomination, this element is completely absent in this range.

(click to enlarge)

The amplifiers are complemented by a useful service - an LED output power indicator and a high-frequency VOX circuit (designed by YU1AW). On fig. 2 shows the appearance of the 430 MHz band amplifier.

Of all the VOX circuits tested by the author, this one provides the highest response speed, which protects against trouble when switching the contacts of the antenna relay. Indeed, if you look at the VOX circuit, there are practically no capacitances in the +T signal conditioning circuit. The electrolytic capacitor C38 provides the VOX release delay. Its value is selected such that the release time of the VOX amplifier approximately corresponds to the time for switching the receive-transmit modes set in the base transceiver. In the author's version, with the value of the specified capacitance of 4,7 μF, the VOX delay time is 0,2 s. If desired, the VOX circuit can not be used, classically connecting the switching circuits with the transceiver. Switch SA1 "QRO" allows you to turn off VOX ("Bypass" mode).

In amplifiers, any non-wire carbon resistors are applicable. Trimmer capacitors are used of the KT4-21 type, constant (up to 1000 pF) - KM with soldered leads (used as leadless), and any electrolytic ones (in the author's version of K52-1) for a voltage of 25 V. All LEDs available in the amplifier are for voltage 1,5 V (for example, AL307).

Inductors L1, L2 and L17 have 4 (2) turns of PEV-0,8 wire on a mandrel with a diameter of 4mm; L3, L4, L15, L16 are made with the same wire and mandrel and have 2(1) turns. When removing all of the above coils from the mandrels, it is necessary to stretch them so that the distance between the turns is approximately 0,6 ... 1,0 mm. Chokes L9 - L12 are pieces of wire PEV-0,47 25 mm long. Coils L7, L8, IZ, L14 - strip lines in the form of a semicircular bracket made of copper (preferably silver-plated) wire with a diameter of 1,0 mm, a length of 15 (10) mm. Chokes L5 and L6 type DM. Relays K1 and K2 are used type RPV-2/7 (12 V). RES-34 are also applicable. Connectors ХР1 and ХР2 - bayonet, type СР-50.

The assembly is carried out by surface mounting on support pads cut out in the printed circuit board foil from fiberglass. The arrangement of the elements of the HF part is in a line. It repeats the drawing of the circuit. Installation requirements are classic for VHF structures - the minimum length of the leads of the parts. The printed circuit board is installed in a heatsink housing with dimensions of 50x120x150 mm. I would especially like to emphasize the method of soldering trimmer capacitors KT4-21. The conclusions of the latter before soldering are bent upwards, almost along the body, then they clean the bending points with sandpaper and tin them with a soldering iron. When soldering, press the capacitor with tweezers against the foil (board pads) and quickly heat up the foil (previously tinned) near the output with a soldering iron. Thus, the capacitor is soldered with a minimum lead length. Soldering such a capacitor is also simple - grasping the output with tweezers, again heat the foil. Relays K1 and K2 are installed directly on the XP1 and XP2 connectors.

To reduce the thermal drift of the collector current of the output transistors, the VD9 diode in the stabilizer can be replaced with a KT814 type transistor, as shown in Fig. 3, and through the heat-conducting mastic, install it on top of KT985A.

Capacitors C18, C19, C27 and C28 are soldered directly to the terminals of transistors VT7 and VT8 at the minimum possible distance from their cases.

It should be noted that in the collector circuit, the values ​​​​of the RF currents are quite large, so the use of other elements than indicated does not guarantee stable operation (and operation in general) of the amplifiers. An incorrect tuning procedure with full power input can also lead to the same result. When setting up, the following must be observed:

• apply power to 13.13,8 V and, by selecting resistor R18, set the quiescent current of the transistors (0,1 ... 0,2 A);
• smoothly increase the input power until the high-frequency VOX is triggered, this level should be 0,5 ... 1 W;
• apply a power of 1 W to the input, adjust in series with a dielectric screwdriver all trimmer capacitors in the base circuits according to the maximum current of the collectors;
• adjust the tuning capacitors in series in the collector circuits according to the maximum output power;
• doubling the input power, repeat steps 3 and 4;
• measure the SWR at the input, it should not exceed 1,5, if it does not match, repeat the adjustment of the base circuits, monitoring the SWR value;
• measure the input and output powers and collector currents, they should approximately correspond to those given in the table.

Capacitor C23 is involved in the reception and "Bypass" modes (amplifier power off) and is designed to compensate for the inductance of the relay outputs K1.1 and K2.1. Its use improves the end-to-end SWR of the amplifier. In the 144 MHz amplifier version, it is absent, and there is a jumper in its place. By installing a tuning capacitor instead of C23 at the setup stage, you can change this parameter of the amplifier.

Resistor R1 is used to correct the calibration of the LED power indicator. The diagram (see Fig. 1) shows the value of this resistor for the 430 MHz range.

With the correct setting of the output circuit of the amplifier, successively touching the "hot" terminals of the capacitors C18 (C19), C14 (C15), C7, C2, C1 with a neon bulb leads to an increase in the brightness of its glow, which indicates a transformation of the output resistance in matching circuits.

Author: N.Myasnikov (UA3DJG), Ramenskoye, Moscow Region

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