Antenna amplifier for VHF FM radio. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / radio reception

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VHF FM radio has become an indispensable attribute of the house, in the car and in the country. And one of the problems that arises during its operation is to ensure the required quality of reception at a great distance from broadcasting stations. Unfortunately, not all radios have good sensitivity, so when you are several tens of kilometers away from radio stations, for example, while staying in a country house or traveling out of town, the reception quality deteriorates significantly. Installing effective receiving antennas on a car or portable equipment is very difficult or simply impossible.

The way out of this situation may be to use a low-noise antenna amplifier at the input of the radio receiver. If the radio is multi-band and has one antenna input (for example, a car), then the amplifier must amplify the signals of this particular range and pass the signals of others without loss. In addition, in urban conditions, when the level of radio station signals and interference is significant, such an amplifier must be turned off to avoid crosstalk. If reception is carried out far outside the city, then the amplifier can be made non-switchable.

The amplifier circuit is shown in fig. 1. It is made on a low-noise bipolar transistor. The gain is about 20 dB. At the input of the amplifier, a low-pass filter (LPF) C1L1C2 with a cutoff frequency of 115 ... 120 MHz and a high-pass filter (HPF) with a cutoff frequency of 60 ... 65 MHz are installed in series. Thus, the amplifier provides amplification of the signals of radio stations operating in both VHF broadcasting bands.

If the amplifier is planned to be placed in a radio receiver where one antenna is used for all ranges (car version), then its circuit must be finalized and two relays should be introduced, as well as an additional power filter. The scheme of the device of this variant is shown in fig. 2. When the supply voltage is applied, relays K1 and K2 connect amplifier A1 (shown in the diagram) between the antenna and the receiver, and when the power is off, the radio input is connected directly to the antenna. If the amplifier will be installed in a car, then it must be placed in a metal case and power must be supplied through a feed-through capacitor (C9).

The elements of the amplifier are placed on a printed circuit board made of double-sided foil fiberglass, the pattern of printed tracks and the location of the elements are shown in Fig. 3. The second side of the printed circuit board is left metallized and connected by foil along the contour with a common conductor of the first side. For the automotive version, the PCB must be extended to accommodate the relay and power filter.

It is desirable to use a low-noise transistor with a normalized noise figure in the amplifier, the KT3120A indicated on the circuit is best suited, as well as KT382A, KT382B, KT399A, KT3101A-2, KT3106A9, it is possible to use the KT368A transistor, but the results may be somewhat worse. Capacitors - KD, K10-17 and similar imported ones, C9 - K10P-4, K10-51, KTP, B23. Resistors - MLT, S2-33, R1-4. Coils L1 and L2 are wound with PEV-2 0.4 wire on a mandrel with a diameter of 4 mm and contain 3,5 and 4,5 turns, respectively. Coil L3 is wound on a ring with a diameter of 8-10 mm from ferrite grade 2000NN and contains 20-30 turns of PEV-2 0.2 wire.

The amplifier is connected between the antenna socket and the radio input. In this case, the connection from the amplifier to the receiver input must be made with a short shielded cable. When installed in a vehicle, all connections must be made with shielded cable and the amplifier placed close to the radio in a shielded compartment. If the amplifier is switched (car version), then a power switch is needed, which is placed in any convenient place. The filters are designed to work on a cable with a wave impedance of 50 ohms; when working on a 75 ohm cable, it is necessary to reduce the capacitances of the capacitors C1-C4 and increase the inductance of the coils L1 and L2 by one and a half times.

Setting up the amplifier comes down to setting the required DC mode. By selecting the resistor R4, the optimal collector current of the transistor is set, at which the noise figure is minimal. Such modes are usually indicated in the directory. Then, the frequency response of the input filters is checked, and, if necessary, adjustments are shifted and pushed apart by the turns of the inductors.

An experimental test of the amplifier was carried out in the city of Fatezh at a distance of 40 ... 45 km from the broadcasting stations of the city of Kursk. It was installed in a receiver with a VHF band with frequencies of 65,8 ... 74 MHz. Reception was carried out on an external antenna. Without an amplifier, two government radio stations could be reliably received, and one commercial radio station was received with poor quality. After installing the amplifier, it turned out to be possible to confidently receive six radio stations operating in this range in the city of Kursk

Author: N.Lukyanchikov

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