The essential elements of a communication system are:
Commonly used signals in communication are either audio or visual. These are characterized by amplitude, frequency, phase and polarization. For example for sound signals, we confine to audible range (20 Hz – 20 kHz), whereas for normal telephony, the range is limited to 4 kHz only.
A communication which depends on the range of frequencies is called bandwidth limited communication.
An input signal (bearing information) is transmitted to a distant point by a transmitter. A receiver intercepts such signals and transforms them in such a way that the information hidden therein can be converted into usable form.
In the case of A radio transmission the input signal is usually in the form of voice or music and the transmitter transforms it (by a process called modulation) into electrical signal (by superposing over electromagnetic waves in the frequency range 30 kHz – 300 MHz). These radio signals are broadcast by means of aerials or antennas either in all directions or in some specified direction.
An antenna or aerial is essentially a system of conductors, which effectively radiates and absorb electromagnetic waves. The antenna can be in the form of a long, stiff wire (as AM/FM radio antennas on most cars) or a huge dish (for far away satellites). In a radio transmitter the antenna launches the radio waves into space. In a receiver, the idea is to pick up maximum transmitted power and supply it to the tuner.
The optimum size of a radio antenna is related to the frequency of signal that the antenna is trying to transmit or receive. The size of these conductors has to be comparable to the wavelength λ of the signal (at least λ/4 in dimension), so that they can detect the time-variation of the signal properly.
In the case of radio receivers, the signals picked up by the receiving antenna may be extremely weak, often only a small fraction of a micro watt. Such signals are amplified before being analysed.
The important characteristics of a receiver are: sensitivity to input signal, amplitude range of the input signal which can be received and converted to output, linearity, between the input and output signals, and frequency response or fidelity, which refers to the degree of faithfulness to which input signals can be reproduced.
Sensitivity signifies the minimum input voltage required to produce a standard output signal voltage. The greater the amplification of the receiver, the greater is its sensitivity.
Selectivity is the capability of a receiver to differentiate between a desired signal of a particular frequency and all other unwanted signals of nearby frequencies.
Fidelity represents the variation of the output of a receiver with the modulation frequency and denotes the ability of the receiver to reproduce the waveform of the modulating signal.
A signal is communicated from the transmitter to the receiver through a medium. The carrier is in the form of a wave.