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How does a radio receiver work .. Electronics

How does a radio receiver work

137 - Extended antenna.
138 - Frame antenna.
139 - Iron core antenna.

Radio waves or radio waves spread at a speed of 300,000 meters per second and are characterized by their electromagnetic field being perpendicular to their electric field, and that they have a wavelength or "frequency". If the frequency of the radio wave is 300,000 Hz, this means that the wave changes from a positive wave to a negative wave and then from a negative wave to a positive wave 300,000 Hz per second. This frequency corresponds to a wavelength of 1,000 meters, which represents the distance between one peak of the wave and the next peak. Therefore, it can be said that a wavelength of 1000 meters corresponds to a frequency of 300,000 oscillations per second (Z/s).

1000 Z/s = 1 kilo-oscillation/second or kilohertz.
1000 m corresponds to 300 kilo-oscillation/second.
1000 kilo-oscillation/second = 1 megahertz or 1 megacycle/second.

Any wire, if placed in the path of radio waves, can cut these waves, and currents of different intensities (137) are generated in it via coil L1, which are generated by induction, in coil L2, a voltage whose intensity is proportional to the voltage generated in coil L1, and it is tuned by a variable capacitor that can select the required frequency. This group is called an antenna. It is known that the antenna is replaced in small receivers by a wire wrapped around a frame inside the device itself (138). In this type of device, the wave generates an induced voltage in the coil itself that is tuned to the station to be heard by means of a variable capacitor SMA. As for small receivers, they include an antenna with an iron core (139), but the tuning process is also done by means of a variable capacitor S M1.

140 - Dipole antenna for television and very high frequencies.

In addition, the S-S connection coils that feed the transistors (or valves) are wound on the iron mold itself. It is known that the antenna, whether wrapped around the frame or around the iron core, is a directed antenna (directional), meaning that it is placed so that the best position for receiving signals is when the axis of the coil is perpendicular to the direction of propagation of the waves to be heard. In contrast, the reception is minimal when the axis of the coil is parallel to the direction of propagation of the waves (see page 122). Television and Very High Frequencies.

Television waves and very high frequency signals are characterized by a very short wavelength, which requires the use of a special antenna called a "dipole" (140) to receive them. The length of this type of antenna is equal to half the wavelength, and it is cut in the middle, where a coaxial wire with a resistance of 75 ohms is connected, and it transmits the required signals to the primary coil L1.

It is possible to place another rod, called the "reflector" behind the antenna, which is useful in strengthening the signal intensity. In addition, some "guides" are placed in front of the dipole, and both the reflector and the guides are useful in reducing the central or average impedance of the dipole. Therefore, this part is bent and connected to the primary coil through its two ends S - S by a specific wire, and the antenna components are arranged vertically and horizontally in a way that allows the polarization of television signals or very high frequencies to be heard.