نقل الصوت بموجات الراديو - الدارة المتجاوبة - الالكترونيات .. الفيزياء

Radio Wave Transmission - Resonant Circuit - Electronics .. Physics

Radio Wave Transmission

Feeding an antenna from the output of an audio frequency amplifier produces an electromagnetic wave with an audio frequency, which, due to its long length (3,000,000 meters for a wave with a frequency of 1 kilohertz), cannot travel long distances unless the antenna designated for its transmission is very large. The physicist Henry Hertz (1857-1894) discovered that feeding the antenna with radio frequencies (ranging between 1,000 and 100 million hertz) is sufficient to transmit it to long distances and in all directions. These frequencies are characterized by being of a much higher value than the value of the audio frequencies themselves corresponding to the sound. However, it is possible to combine these two types with each other and send them together. This happens in two different ways. The first, called amplitude modulation, is the modification of the amplitude of the radio wave (known as the carrier wave) with the audio wave, and this requires primarily generating it inside the transmitter before modifying it. As for the process of separating the two waves from each other - that is, separating the negative oscillation of the wave - it is done inside the receiver itself. Then the remaining part of the wave feeds an audio frequency amplifier that recognizes the shape of the audio frequency, extracts it from the carrier wave, and then converts the amplified audio wave into sound inside the loudspeaker. The second method of transmitting sound is frequency modulation, that is, modifying the frequency of the carrier wave with the audio wave itself.

A radio transmitter, whether based on amplitude modulation or frequency modulation, is identified by a specific frequency that characterizes its carrier wave. When one listens to Studio 4 at the British Broadcasting Corporation, for example, this means that one is tuning one's receiver to pick up a wave with a frequency of 903 kilohertz. Here we can ask how the carrier wave is generated and how the tuned radio is able to select one specific frequency from among all the frequencies broadcast by all the world's radio stations. It should be noted that these two processes are directly related to the resonant circuit.

The principle of resonance is very familiar in the world of acoustics and it is known that inducing an object to vibrate by a sound wave with a frequency similar to its own is a common practice. For example, if we place a small piece of paper on a piano string, then play a string close to the first string, we find that the paper starts to vibrate on its own, resulting from the resonance of the string that carries it.

Frequency modulation (above)
and capacitive modulation (below)

The same process occurs in electronics, within resonant circuits, and in the field of radio frequencies. The resonant circuit consists of an induction coil and a capacitor connected in parallel or in series. In the case of parallel connection, when the capacitor, which is fully charged with electricity, begins to discharge its charge into the induction coil, a magnetic field is generated around the latter that soon disappears as soon as the charge is empty In its entirety. But, due to its change, it generates a secondary voltage in the coil itself, in the opposite direction to the first, whose task is to charge the capacitor again with a charge opposite to the first charge, which is soon discharged into the coil again, completing a full cycle of oscillation cycles. The process continues in this way, from one oscillation to another, until the circuit resistance consumes all the energy present in the initial charge. It is worth noting that the oscillation frequency is related to the inductance L and the capacitance of the capacitor S, and that in the case of resonance it equals N = 2/1 T root L S, where T = 3.1416 0

The resonant circuit cannot oscillate infinitely unless it has the energy necessary to overcome the resistance of the wires in it for the flow of electrons in both directions. To secure this energy, a valve is used in a triode whose network is fed from the current of the resonant circuit itself and gives the anode circuit a current similar to it but greater in intensity. If we allow this current to bounce back and feed the resonant circuit through a coupling coil, the whole device can oscillate infinitely, as it derives its energy from the source that powers the valve. This principle is used in transmitters, and we see in the drawing the oscillating triode (i.e. the tuning network circuit) and the modulating valve (the network modulation). The modulating output must be amplified before feeding it to the antenna.

(Above) The resonant circuit.
(Below) A simplified transmitter.