Cable antenna and UHF converter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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For those wishing to receive programs on decimeter waves (UHF), another version of the indoor antenna and converter is offered for repetition. They have good operational capabilities, are easy to manufacture and adjust.

The UHF antenna shown in fig. 1 is an incomplete zigzag antenna made with a 75 ohm drop cable. At a distance of 240 ... 245 mm from the end of the cable at the bend of section A, the outer sheath and the shielding braid were removed in an interval of 10 mm. In section B, only the outer insulating sheath of the cable was removed in the range of 15 ... 20 mm from the end and at a distance of 480 ... 490 mm from it. The shielding braids in this area are tightly pressed, providing electrical contact.

To fix the antenna web and improve contact in section B, the cables are pulled to the carrier insulating plate of the antenna with brackets made of tinned copper wire with a diameter of 1 mm. The inner conductor at the end of the cable is left free. The entire antenna sheet is also fixed on the plate with staples made of wire with a diameter of 1 mm. The carrier plate is made of organic glass, but can be made of textolite, getinax, dry plywood, etc. 2,5...5 mm thick.

In the case when reflected signals interfere with reception and it is necessary to increase the gain of the antenna, a reflector is added to its web in the form of a rectangular sheet of duralumin or other metal 1,5 ... 2 mm thick and 330x200 mm in size. It is mounted on four posts made of dielectric material (ebonite, getinax, organic glass) with a diameter of 8 and a length of 100 mm to the insulating plate of the antenna with M4 screws (holes for this are provided in the plate and reflector). The antenna is placed on a rack with a height of 300...400 mm with a base.

Converter, schematic diagram of which is shown in fig. 2 provides the conversion of UHF signals into signals on one of the channels (4 or S) MB. It contains an aperiodic RF amplifier (based on transistor VT2), a converter (VT1) and a local oscillator (VT3).

From the antenna, through the XW2 connector and the capacitor C3, the RF signal enters the emitter of the transistor VT2, which is connected according to the OB amplifier circuit

RF. Such an RF amplifier eliminates the passage of the local oscillator signal to the antenna. From the amplifier, the RF signal enters the emitter circuit of the transistor VT1 of the converter. Resistor R3 eliminates the possibility of self-excitation of the converter and RF amplifier, improves the conversion process.

The converter on the transistor VT1 is also made according to the OB circuit so that the setting of the output circuit L1C1 has practically no effect on the frequency of the local oscillator. DC transistors VT1 and VT2 are connected in series.

The local oscillator is assembled on a transistor VT3 according to a capacitive three-point circuit with feedback through a reverse-biased diode VD1, which simultaneously performs the functions of a converter setting element. When moving the slider of the variable resistor R6, the voltage at the base of the transistor VT3, the current through it, and, consequently, the reverse voltage across the diode VD1 and the tuning frequency of the local oscillator resonant circuit, which is an asymmetric strip line L2, smoothly change. The local oscillator signal (voltage drop at the terminals and at the capacitor C5 itself) through the capacitor C 4 comes to the converter.

The MB antenna is connected to connector XW1. The MB signal from it through the contacts of the switch SB1.1 in the "MB" position, the cable and the XW3 plug goes to the input of the TV. When switch SB1 is switched to the "UHF" position, voltage is supplied to the converter through contacts SB1.2 from the power source GB1 (LED HL1 is lit), and from the output of the converter through contacts SB1.1, the received MB signal also passes through the plug XW3 to the TV input.

The converter uses a variable resistor SP-04 (R6) and constant MLT, a tuning capacitor KPK (C1) and constant M750, M1500 or KD-1. Coil L1 is wound on resistor R2 and contains 12 turns of PEL or PEV wire with a diameter of 0,3 mm with a tap from the fourth turn, counting from the output connected to the common wire.

The details of the converter are placed on a printed circuit board made of double-sided foil fiberglass with a thickness of 1,5 ... 2 mm and dimensions of 102X X96 mm. Its drawing together with the body and partitions is shown in fig. 3 (the triangles on it show the places of soldering the leads and wires to the foil of the board or its pads, the dots show the soldering on the hard leads of the elements).

Pic.3

The strip line L2 and mounting pads are cut out on the board with a cutter 1,5 mm thick. The line width is 3 mm when using fiberglass 2 mm thick (if it has a thickness of 1,5 mm, then the line width is 2 mm, and the cutter width is 0,5 ... 1 mm). Pieces of tinned copper wire with a diameter of 1 ... 0,8 mm are inserted into the holes of the board with a diameter of 1 mm and soldered on both sides of the board. Transistors VT1 - VT3 are installed in holes in the board with a diameter of 6 mm. On the other hand, they are covered with circles of foil or a thin copper sheet soldered to the foil of the Board, so as not to push the transistors into the converter case during operation.

The side walls of the housing are made of one-sided fiberglass with a thickness of 1,5...2 mm, foil inside. Two side walls have dimensions of 106X26 mm, the other two - 96X24 mm, a long partition - 102X20 mm, a short one - 28X20 mm. The printed circuit board is located 2 mm above the lower ends of the side walls and is soldered to them on both sides. The places where the side walls, partitions and the board touch each other are carefully soldered.

Switch SB1 - P2K. Its conclusions from the side of the latch are shortened to 1 mm. It is installed with a latch towards the circuit board and fixed through the bushings with screws to the side wall of the converter housing, as seen in Fig. 4. The length of the PK-75-3-31 cable from the switch to the XW3 plug is about 1 m, but it can be of any length, depending on the convenience of using the converter. The cable is attached to the board with a metal bracket and two MOH screws. The converter is closed with a lid made of the same fiberglass with dimensions of 106x96 mm.

When establishing a converter, first measure the current consumed by it. To do this, connect a milliammeter in series with a power source. The current must be 5 mA. Then touch with a metal object to the output of the collector of the transistor VT3. The current should drop to 2,5 ... 4 mA, depending on the position of the variable resistor R6 slider.

After that, by moving the jumper along the strip line L2, a stable image is achieved on the fifth or fourth channel MB (the UHF antenna must be exactly directed to the transmitting station). And, finally, by rotating the rotor of the capacitor C1, they adjust the L1C1 circuit and get the maximum signal level, judging by the image on the TV screen.

Pic.4

The converter can be powered from the TV through a simple power supply, the circuit of which is shown in fig. 4 and the circuit board in fig. 5.

Pic.5

The board is placed inside the TV. Resistors in the source - MLT, capacitors - K50-6. With a stand-alone version of the converter, you can use any small-sized network transformer that has a voltage of 6,3 V on the secondary winding at a load current of 20 mA.

Author: M. Ilaev; Publication: N. Bolshakov, rf.atnn.ru

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