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| Basic Principles of Wireless Radio Communication |
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| Let us now discuss the basic principles of wireless radio communications. We shall mainly concentrate on the principle of amplitude modulation and demodulation. |
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| The simplest scheme of wireless communication would be to convert the speech or music to be transmitted to electric signals using a microphone, boost up the power of the signal using amplifiers and radiate the signal in space with the air of an antenna. This would constitute the transmitter. At the receiver end, one could have a pick-up antenna feeding the speech or music signal to an amplifier and a loud speaker. (below figure) |
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| The above scheme suffers from the following drawbacks: |
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| (i) EM waves in the frequency range of 20 Hz - 20 kHz (audio-frequency range) cannot be efficiently radiated and do not propagate well in space. |
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| (ii) Simultaneous transmission of different signals by different transmitters would lead to confusion at the receiver. |
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| In order to solve these problems; we need to devise methods to convert or translate the audio signals to the radio-frequency range before transmission and recover the audio-frequency signals back at the receiver. Different transmitting stations can then be allotted slots in the radio-frequency range and a single receiver can then tune into these transmitters without confusion. The frequency range 500 kHz to 20 MHz is reserved for amplitude-modulated broadcast, which is the range covered by most three band transistor radios. The process of frequency translation at the transmitter is called modulation. The process of recovering the audio-signal at the receiver is called demodulation. A simplified block diagram of such a system is shown in the below figure. |
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| Why microwaves are preferred over other waves to beam signals in a particular direction? |
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| Microwaves are radio waves with frequencies higher than television signals. The wavelengths of microwaves are of the order of a few millimeters. We know that sound waves spread and bend around the corner of an obstacle. This is because the wavelength of sound wave is generally comparable to the size of the obstacle. Unlike a sound wave, a light wave keeps itself along a straight path. Moreover, the light waves bend by only a small amount at the corners of the obstacles. This is because the wavelength of light waves is smaller as compared to the wavelength of sound waves. Thus, lesser the wavelength of a wave, smaller is its bending at the corners of ordinary obstacles and greater the ability of the wave to follow a straight path. The wavelength of microwaves is very small as compared to the wavelength of radio waves. So, microwaves are better suited to beam signals in a particular direction. |
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