Receiving KB to the detector. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Using the simplest loopless detector receiver, the circuit of which is shown in Figure [1], I sometimes manage to listen to the transmissions of powerful remote KB broadcasting stations. If the transmission is satisfactory, then, as a rule, several radio stations are heard at the same time. Signals are subject to severe fading. German and Swedish radio stations predominate, although others can be heard.

The pipe of the central heating system of a nine-story building is used as an antenna, telephone wires with a certain capacitance Sp serve as a counterweight. I note that electrical contact with the pipe is not at all necessary, a good capacitive coupling is sufficient. To do this, wind the pipe with several turns of insulated wire, and connect the other end of this wire to the receiver.

Even better results can be obtained from an antenna in the form of a long wire, the length of which is not less than half the wavelength. A particularly strong signal at the end of the antenna is obtained if its length is close to a multiple of half the wavelength. How close it should be to it depends on the ratio of the diameter of the antenna wire and the wavelength.

In such an antenna, which is an electric vibrator, resonant phenomena occur. When exposed to an electromagnetic wave, voltage antinodes appear at its ends. Antennas with a length of more than one wavelength have complex radiation patterns [2, 3], and when creating such antennas, this must be taken into account - the direction of wave arrival can coincide with one of the maxima of the diagram, and the signal will be heard loudly, or it can fall into a minimum, and the signal will be weakened.

At a distance of 450 km from Moscow, where my receiver is located, there are no powerful signals from LW or MW radio stations. Even on a transistor receiver, they are not heard very well. But if there are such signals at the receiving place, then they can interfere with listening to KB band stations. In this case, a choke with an inductance of 2 ... 50 μH should be connected between the antenna and one of the terminals of the blocking capacitor C100, which will at least partially suppress the signals of low-frequency radio stations and the background of the alternating current. In this case, it should be taken into account that the choke itself, together with its parasitic capacitances, the capacitance of the antenna and the counterweight, has a resonant frequency at which signal reception will be amplified.

You can, of course, assemble the detector receiver according to the "classical" scheme with a coil, a variable capacitor and one diode, as in Fig. 2.1 [1]. Grounding in the presence of a long antenna is not required, telephone wires are quite enough as a counterweight. However, since signals with field strengths greater than 3,5 mV/m are relatively rare in the HF band, there is no need to tune in to any specific HF radio station.

This simplest receiver is a good indicator of the passage of KB signals, which can be used to judge the state of the ionosphere. For more confident listening to weak signals, the output of the detector receiver can be connected to the input of a sensitive ULF by loading the detector with a resistor of 10 ... 22 kOhm. To reduce background interference from the AC mains, it is desirable to power the amplifier from an independent power source.

If a silicon diode is used as a detector, opening it with a small bias current, then the sensitivity of the receiver to weak signals will increase. Since a good ULF is capable of amplifying signals with a level of hundreds of microvolts or less, the sensitivity of such a receiver from the antenna input is about 1,5 mV, and with a long antenna it is able to capture fields with a strength of about 50 μV / m.

When listening to this more sensitive receiver, its very poor selectivity will become apparent, and the next step may be to create a regenerative detector.

Literature

1. Polyakov V. T. Radio reception technique: simple AM ​​signal receivers. - M.: DMK Press, 2001, p. 56,86.
2. Kocherzhevsky G. N. Antenna-feeder devices. Chapter 5. Some questions of the theory of receiving antennas. - M.: Communication, 1972, p. 97-108.
3. Rothammel K. Antennas. - M.: Energy, 1969.

Author: S. Kovalenko, Kstovo, Nizhny Novgorod Region

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