Reception of low-power radio stations. Scheme, description

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Reception of low-power radio stations. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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It's no secret that at a distance of 100 km or more from Moscow, the ether on the ultrashort waves (VHF) and medium waves (MW) bands noticeably "empty", and at a distance of 300 ... 400 km there is practically no reception (if there is no regional center). This observation corresponds to the conditions for the propagation of radio waves in the indicated ranges. VHF stations are confidently audible in the line of sight, regardless of the time of day.

Broadcasting on SW during the daytime uses surface radio waves, the field strength of which depends on the transmitter power, operating frequency, and parameters of the earth's surface. For example, the strength of the radio wave field over land with a transmitter power of 500 kW and an operating frequency of 1 MHz at a distance of 400 km will not exceed 100 μV/m. At the same time, the sensitivity of even a good radio receiver with a magnetic antenna is usually on the order of 500 µV/m.

Low power radio reception

You can increase the sensitivity of a receiving device with a ferrite antenna using the traditional method by connecting an external antenna. If such an antenna is not available, a simple and small-sized design can be made.

For these purposes, I made the loop antenna shown in the figure with dimensions of 40x40 cm. The oscillatory circuit, composed of the capacitance and inductance of the antenna, an additional KPI, covers the entire medium-wave broadcasting range. The experiments were carried out in a rural area at a distance of 400 km from Moscow in a southeasterly direction.

Having placed the receiver inside the loop antenna as shown in the figure, tune the KPI loop antenna into resonance at the frequency of the received station and orient it in the desired direction. With this simple improvement, it was possible to receive more than a dozen radio stations from Moscow during the daytime at frequencies 549, 612, 693 *. 810*. 873, 963*, 1044, 1116*. 1260*. 1305 and 1440 * kHz with different volume levels and several regional broadcasting stations at frequencies of 774 kHz - Voronezh, 855 kHz - Penza, 567 kHz - Volgograd, 1080 kHz - Saransk, 531 kHz - Cheboksary. The asterisks mark the frequencies of low-power (up to 5 kW) transmitters.

The effectiveness of the applied loop antenna can be estimated quite accurately if, according to the graphs [1] of the dependence of the earth wave field strength on the distance when propagating over land, the field strength of a transmitter with a power of 5 kW at a frequency of 1 MHz at a distance of 400 km is determined. Its value will be 7 μV / m. The sensitivity of the used receivers GRUNDIG OCEAN BOY350 and the domestic Sokol 403 is approximately 500 μV / m.

In the manufacture of a loop antenna, the author was guided by the recommendations set forth in [2]. The antenna is a wooden frame measuring 40x40x5 cm, on which ten cuts must be made in 4 mm increments to wind the wire. It is necessary to wind 10 turns of the PEV-1 winding wire with a diameter of 0,6-0,8 mm in increments of 4 mm onto the frame, connect two KPE sections connected in parallel from the Ocean radio receiver to the ends of the windings.

Literature

Dolukhanov M. Propagation of radio waves. - M.: Communication, 1972, p. 91.
Polyakov V. Technique of radio reception. Simple AM radio receivers. - M.: DMK Press, 2001, p. 146.

Author: V. Tyurin, Moscow

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