Radio Broadcasting, Transmission and Reception

Radio communication means the radiation of radio waves by the transmitting station, the propagation of these waves through space and their reception by the radio receiver.

Fig. below shows the general principle of radio broadcasting, transmission and reception. It essentially consists of transmitter, transmission of radio waves and radio receiver.

Transmitter-

It essentially consists of microphone, audio amplifiers, oscillator and modulator.

A microphone is a device which converts sound waves into electrical waves. The output of microphone is fed to multistage audio amplifier for raising the strength of weak signal.

The job of amplification is performed by cascaded audio amplifiers. The amplified output from the last audio amplifier is fed to the modulator for rendering the process of modulation.

The function of the oscillation is to produce a high frequency signal called a carrier wave. Usually crystal oscillator is used for the purpose.

The amplified audio signal and carrier waves are fed to the modulator. Here the audio signal is superimposed on the carrier wave in suitable manner. The resultant waves are called modulated waves, and the process is called modulation. The process of modulation permits the transmission of audio signal at the carrier signal (frequency). As the carrier frequency is very high, therefore the audio signal can be transmitted to large distances. The radio waves from the transmitter are fed to the transmitting antenna or aerial from where these are radiated into space.

The transmitting antenna radiates the radio waves in space in all directions. These radio waves travel with the velocity of light 3x10⁸m/sec. The radio waves are electromagnetic waves and possess the same general properties.

Receiver-

On reaching the receiving antenna, the radio waves induce tiny emf in it. This small voltage is fed to the radio receiver. Here the radio waves are first amplified and then signal is extracted from them by the process of demodulation. The signal is amplified by audio amplifiers and then fed to the speaker for reproduction into sound waves.

Need for  modulation

1.      Practical Antenna length-theory shows that in order to transmit a wave effectively the length of the transmitting antenna should be approximately equal to the wavelength of the wave.

As the audio frequencies range from 20 Hz to 20Khz, if they are transmitted directly into space, the length of the transmitting antenna required would be extremely large. For example to radiate a frequency of 20 KHz directly into space we would need an antenna length of 3x10⁸ /20x10³ ≈ 15,000 meters. This is too long to be constructed practically.

1.      Operating Range- The energy of a wave depends upon its frequency. The greater the frequency of the wave, the greater the energy possessed by it. As the audio signal frequencies are small, therefore these cannot be transmitted over large distances if radiated directly into space.

2.      Wireless communication- Radio transmission should be carried out without wires.

Modulation- The process of changing some characteristics (example amplitude, frequency or phase) of a carrier wave in accordance with the intensity of the signal is known as modulation.

Types of modulation-

1. Amplitude modulation
2. Frequency modulation
3. Phase modulation