Principles of Communication

1. Communications: The term communication refers to the transmitting, receiving and processing ofinformation by electronic means.

2. Basic Components of a Communication System:a) Information: The idea/message that is to be conveyed is information.b) Transmitter: The information in the incoming signal is first converted into electrical variations.c) Channel: the term channel refers to the frequency range allocated to a particular service or

transmission.d) Receiver: The signals received by receiver are quite weak; therefore the receiver first amplifies the

signal.3. Modulation: Superposing of electrical audio signal on high frequency carrier wave is called

modulation.4. Amplitude Modulation: When the amplitude of high frequency carrier wave is changed in accordance

with the intensity of the modulating signal, it is called amplitude modulation. The frequency of themodulated wave is equal to carrier frequency.

5. Modulation Factor: The ratio of change of amplitude of carrier wave to the amplitude of normalcarrier wave is called the modulation factor or index of modulation (m).

Amplitude change of carrier waveModulation factor, (m) =Amplitude of normal (unmodulated) carrier wave

6. Frequency modulation: When the frequency of carrier wave is changed in accordance with theinstantaneous value of the (modulating) signal it is called frequency modulation.The modulation index for FM wave is defined as,

c m cf

m m

f f V fmaximum frequency deviationm k

modulating frequency f f

= = =

This implies that modulation index increases with decrease in modulating signal frequency.7. Critical frequency (CF): It is the maximum frequency of radio wave, which gets reflected from

ionosphere and returns to the earth when beamed straight towards the layer of ionosphere. This is themaximum frequency above which rays go through the ionosphere.

8. Maximum Usable Frequency (MUF): It is the limiting frequency of radio waves which when sent atsome angle of incidence towards the ionosphere, get reflected and returns to the earth.

critical frequencyMUFcos

=

9. Skip distance: It is the smallest distance from a transmitter along the earths surface at which a sky

wave of a fixed frequency returns to the earth.10. Height of antenna: For efficient radiation and reception, the height of transmitting and receiving

antennas should be comparable to a quarter wavelength of the frequency used.11. Height of transmitting antenna: d 2hR=

Area covered for T.V. transmission = 2d 2hRpi = piPopulation covered = population density area covered

12. Velocity factor of a transmission line: The velocity factor (V.F) of a transmission line is defined as thevelocity reduction ratio, i.e. v 1VF

c K= = .

13. If the maximum and minimum voltage of an AM wave are Vmax and Vmin respectively, then show that

modulation factor m is given by: max minmax min

V Vm

V V

=

+.