We know a person speaks with another person i.e., the first person communicates some thing with the second person. Now a days communication is studied as a special branch of engineering.

 Let us discuss the elements of communication system. A communication system has mainly three parts...        

1) Transmitter      2) Channel and       3) Receiver

Here the generation of information may be speech or message. Let the microphone converts speech signals into electrical signals, such device which converts into electrical signals are called transducers. Here the signal is a single-valued function of time. Which conveys the information.

This electrical signal containing information is transmitted into channel by a device called Transmitter.

The physical medium that connects transmitter and receiver is called channel. Air, connecting cable etc. are examples of channel.

The signals present in the channel are received by a device called Receiver. Radio, Cell Phones etc. are receivers.

During the Transmission of signal through a channel there is a loss of strength of the signal called attenuation. The unwanted signals that disturb the original signal are called Noise. The original weak signal is strengthened by a device is called Amplifier.

Modulation: The process of combining the audio frequency signal with high frequency carrier wave is called modulation.

The need for the modulation is explained below

1) Size of the antenna aerial: Antenna is used to transmit a signal into the channel. The approximate height of antenna is λ/4 of the transmitting signal. For audio frequency range 20 Hz, 20 KHz, corresponding minimum wave length is λ = c/ = 15 km.

Hence required antenna height will be λ/4 ≈ 4 km. Which is impractical. To over come this problem modulation with high frequency is necessary.

2) Effective power radiated by the antenna: The power radiated by an antenna is directly proportional to (l/λ). Hence for high power transmission there is the need of high frequency. This can be achieved by modulation.

3) Mixing up of signal from different transmitters: If many transmitters are transmitting base band (audio frequency range 20 Hz to 20 KHz) information signals simultaneously all the signals will be mixed up and there is no way to distinguish them. Hence modulation with high frequency carrier waves of different frequencies will solve this problem.

        The types of modulations are

         i) Amplitude Modulation (AM)

        ii) Frequency Modulation (FM) 

        iii) Phase Modulation

In amplitude modulation the amplitude of the carrier wave varies in accordance with the message signal and the frequency and the phase of carrier wave unchanged. Similarly in frequency modulation frequency of the carrier wave changes and in phase modulation phase of the carrier wave changes in according to the modulating signal.

Band Width: The communicated signal is not a single frequency, but spreads over a range of frequencies called signal band width.

Different types of transmission have different band widths.

Some important wireless communication frequency bands are given below

        In wireless communication electromagnetic waves propages through the atmosphere in three modes. They are

         1) Ground wave propagation

         2) Space wave propagation

         3) Sky wave propagation

Ground wave Propagation: The waves that glide over the surface of the earth are called ground waves. This process is useful for low frequencies (0.5 MHz to 1.5 MHz)because for high frequencies at tenuation increases rapidly. It is useful for very short distances.

        The ground waves are vertically polarized because the horizontal component of electric field in contact with the earth is short circuited by the earth.

Sky wave propagation: Long-distance, short-wave communication by means of ionospheric reflection is called sky wave propagation. This process is useful for the frequency range from a few MHz to about 30 MHz. The electromagnetic waves of frequencies above 30 MHz penetrate the ionosphere and escape and for low frequency noise and attenuation increases due to ionosphere absorption.

         The sky wave propagation is due to total internal reflection (TIR).

Space wave Propagation: The propagation of electromagnetic wave between transmitting and receiving antennas for greater than 30 MHz frequency is called space wave propagation.

        The space wave used for line-of-sight (LOS) communication and satellite communication. The LOS waves may get blocked at some point by the curvature of the earth.

Line of sight communication by space waves

        If hT and hR are the height of transmitting and receiving antennas then, if R is the radius of the earth

        i) The distance between transmitting antenna to the horizon is 

        ii) The distance of receiving antenna to the horizon is 

        iii) The maximum distance between the two antennas is    

        Derivation of 

        from ΔOPQ   

 (R + hT)² = R² + h²T + d²T

  R² + h²T + 2RhT = R² + h²T + d²T

      

     The space wave communication used in TV, radio, aircraft radar etc.