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Encyclopedia of radio electronics and electrical engineering / RF power amplifiers

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Since 1987, a network of UKB FM amateur radio stations has been operating in Moscow at a frequency of 145,6 MHz (repeater channel RO). After the repeater was turned off at the request of the GIE, it turned out that the power of 50 watt of my portable radio station was insufficient for successful operation in the radio network at a distance of up to 1 km. Even an external whip antenna does not help - the energy losses in the cable 60 meters long, and even made up of several pieces, are too great.

Everything changed radically after a transceiver antenna amplifier (AU) was installed on the roof. Power, RF buildup and control of the reception-transmission mode is supplied and carried out via 14 cable. When supplying +12,6 V (automobile version), it was necessary to apply a relay switching circuit, RES-60 in AC (+24 V) at reduced voltage.

(click to enlarge)

Design

The AU scheme is assembled on "piglets" (Fig. 1). Structurally, its body is made of one-sided fiberglass 1,5 mm thick, and the installation is made on the bottom, reinforced with a duralumin plate 2 ... 3 mm thick, which is used as a heat sink. The cover is fixed between two short sides, so one M18 screw and a threaded bushing 3 mm high are enough to fix it. At the lowest point of the LU, it is necessary to drill a drainage hole with a diameter of XNUMX mm to remove condensate from the box.

Before setting up the circuit, it is necessary to cover the installation with a thin moisture-proof layer of BF-2 glue diluted with alcohol, followed by drying for 24 hours. Before installation on the roof, AC should be thickly coated on the outside with window putty or a sealant replacing it (plasticine can be used). On the body, it is necessary to provide for the installation of the attachment point to the mast or to the roof railing.

The amplifier circuit does not claim to be original. Its main merit. perhaps, but high repeatability and stability of parameters (10 copies were made). The contour data are shown in the table:

 Dr1,2,5,6 PEL-0,27 wire 15 mm long on a mandrel 2 mm. DrZ, 4 6 vit. PEL-0,8 turn to turn on a mandrel 6 mm. L1,2,3,43 vit. PEL-0,8 turn to turn on a mandrel 6 mm. L5,7,134 vit. PEL-0.8 turn to turn on the mandrel 6 mm. L6,11 (L13 - branch from 1 and 2 turns). L8,9 2,5 vit. PEL-0,5 on a mandrel 4 mm. L10 4.5 vit. PEL-0,8 on a mandrel 6 mm. L12 3,5 vit. PEL-0,8 on a mandrel 6 mm.

As trimmer capacitors, it is necessary to use capacitors with an air dielectric or new types of KPK 5-20, the rotors of which must be coated from the end with a layer of thick lubricant of the TsIA-TIM type. so that they are not "grabbed" with BF-2 glue when the installation is covered with a moisture-proof layer.

First, the mechanical part of the structure, the board, the radiator are made. holes are drilled, then the box is soldered, not forgetting to install a threaded sleeve at point B for attaching the lid. Permanent capacitors - small-sized, ceramic. Leads should be kept as short as possible - this is a general rule for VHF mounting.

Setting

Before applying the supply voltage through the RF connector, you should check with a tester that the +12,6 V circuit is not shorted to the case. It must be borne in mind that in the FM mode, the transmitter transistors operate in class C, and there is no need to turn off their power in the "receive" mode. The sensitivity of an AU receiver is usually so high that it may be necessary to place the VHF FM GSS at a sufficient distance from the structure, since a weak signal in the noise is needed to fine-tune the contours. Collector current V7 is set by resistor R4 within 35...40 mA.

Before setting up the transmitter circuits, turn off the power to transistors VI and V2 and, turning on the FM transceiver for transmission, make sure that when the RF buildup arrives through the cable in the AU, relays P1 and P2 are triggered. Having restored the connection, tune the transmitter to maximum power. As an indicator, you can use the simplest RF probe (Fig. 2).

Ris.2

Since there is no protection device for the output transistor in the AU circuit in case of mismatch with the load (antenna), then during tuning it is necessary to control the Ik of the V2 transistor with a 1,5 A ammeter, and, gradually “pumping up” the output stage, adjust it to the maximum RF voltage. To adjust the circuit L3C9C10 and L5C12, you must use a screwdriver made of a dielectric material (for example, from a PK-50 cable with a diameter of 9 mm, into the end of which you can melt a metal plate with a soldering iron - a screwdriver sting). The RF probe should be located at some distance from the antenna with an amplifier mounted on the mast and a cable of the required length that will go to the roof. You cannot change the cable length after setting. Since the dimensions of the GP antenna and the 145 MHz amplifier are small, it is possible to test the performance of the device indoors.

With the correct setting of the output stage, the MN-3 neon light should be brightly lit, brought to the end of the quarter-wave whip antenna. The brightness of the glow gradually decreases when approaching the base of the pin. Current consumption in transmission mode: VI - 400...500 mA; V2 - 1,2 ... 1,3 A.

This antenna amplifier has proven itself well in working with directional antennas. In this case, the effect, of course, is even more significant. The maximum communication range between two radio stations equipped with AU can reach 80 km or more, depending on the specific topographic conditions.

Author: A. Koval (UA3AFO), Moscow; Publication: N. Bolshakov, rf.atnn.ru

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