Let's get acquainted with... KV. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Let's get acquainted with... KV. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Shortwave reception is considered to be the domain of more complex superheterodyne circuits and solid design experience. Isn't that why novice radio amateurs avoid high-frequency bands. And in vain. Recall amateur shortwavers of the early 30s, because they mainly worked with the simplest tube receivers of direct amplification. Of course, the stability of such devices is lower, and their tuning is more "fine". But simplicity and accessibility may well make up for the shortcomings for inexperienced radio amateurs. For the first acquaintance with shortwave broadcasting, it is better to make the receiver in the form of a small desktop structure, and to conduct reception on headphones.

The scheme of such a receiver, capable of operating in the range of approximately 25-41 m, is given in Fig. 1.

Let's get acquainted with ... KV

The receiver has one oscillatory circuit, which allows, if necessary, by changing the number of turns of the L2 coil and the value of the capacitor C2, to shift the range boundaries to the frequency region of interest. Transistor VT1 works in a radio frequency amplifier. To increase the sensitivity from its collector through the coil L1, a positive feedback is fed to the loop coil, regulated by a variable resistor R3. The next transistor detects the received signal and pre-amplifies its low-frequency component. Transistors VT3, VT4 work in an audio frequency amplifier, which is loaded with a sensitive high-resistance telephone BF1.

The details of the receiver can be located on the circuit board as they are placed on the circuit diagram, except for the resistor R3; it is more convenient to move the control knob of the latter to the left of the vernier knob that rotates the rotor of the C3 tuning capacitor. An antenna can be a piece of mounting wire, the length of which is selected empirically. In some cases, satisfactory reception is obtained with a standard telescopic antenna.

The receiver uses fixed resistors of types MLT, MT, variable (R3) - SP-0,4; permanent capacitors - KLS, PM, KPE (C3 any one- or two-section with a maximum capacity of the same order as indicated in the diagram). The phone is "two-eared" with a coil resistance of about 1,5-2 kOhm. For switch S1, a regular toggle switch is suitable. The power source is best made up of two batteries connected in series 336 "Planet".

In addition to the board and case, you yourself will have to make the receiver coils. They are wound on a common plastic frame with a diameter of 6,5-7 mm and a length of about 25 mm. Coil L2 has 23 turns of PEV-0,44 wire; L1 - about 5 turns of PELSHO-0,2 wire. The axis of the tuning knob - it is also the leading axis of the vernier - can be made from an old variable resistor with the rotation limiter removed. This design of the node will allow you to easily fix it with a nut on the board, taking it away from the installation and thereby reducing the influence of hands on the setting. The layout diagram of the receiver is given in fig. 2.

Let's get acquainted with ... KV

After checking the correctness of the assembly and the magnitude of the currents of the transistors (they are specified by the selection of elements R1, R4, R7), make sure that the feedback operates normally within the entire range. Close to the extreme right position of the feedback knob in the phone, a whistle should occur. If this does not happen, increase the number of turns of L1. The generation will be "quenched" by the control knob, but if it fails, reduce the number of turns or move them away from L2. It happens that instead of generating, the signal is attenuated, then you need to swap the L1 pins.

Reception on the generator, which is our receiver, is carried out as follows. Slowly rebuilding the contour, at the same time, using the feedback knob, maintain it at a level close to the breakdown into generation. This ensures the highest sensitivity of the receiver to weak signals. The generation that has begun must be stopped immediately, otherwise the sound quality of the self-excited receiver will deteriorate sharply.

With careful tuning on our receiver, you can catch a lot of radio stations broadcasting on the HF band.

See other articles Section Beginner radio amateur.

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