Chapter:9 Telecommunication

Telecommunication is concerned with sending and receiving information over a

distance. In the broadest sense, information can be in the form written or spoken
words, numbers, diagrams, pictures, music or computer data.
Early history
Line-of-sight method
(sender and receiver can see which each other) involves sending by
• Smoke signals from hill top
• Flag signaling by semaphore
• Ship to ship signaling by Aldis lamp

The mid-19th century saw the invention of the electric telegraph by Wheatstone or Samuel Morse
The term telegraph comes from the Greek words “tele” which means distance and “graph” which
means write
Alexander Graham Bell “1847 – 1922” developed the telephone, in 1876, making it possible to
transmit speech electrically
In 1867, Maxwell predicted the existence of radio waves or predicted by mathematically form In
1889, Hertz has shown how to produce the electromagnetic waves and produced radio wave in
laboratory
In 1890, Marconi succeeded in transmitting them over a mile or used radio wave by developing first
radio wave transmitter and receiver over mile
In 1930, John Logie Baird developed the television. The development of telecommunication
continued up to today where the world becomes global village.
Representing Information
Information can be represented electrically in two ways.
1. Digital Method
In this method electricity is switched on and off and the information is in the form of
electrical pulses. For example, in the simple circuit of Figure 3.2 a, data can be sent by the
‘dots’ and ‘dashes’ of Morse code by closing the switch for a short or a longer
In Figure 3.2 b, the letter A (.-) is shown.
Computers use the simpler binary code with 1 and 0 represented by ‘high’ or ‘low’
voltages respectively. They can only handle numbers (i.e. 1 and 0) and so there is a pattern
of 1s and 0s for each of the 26 letters of the alphabet and for other symbols

2. Analogue Method
In this case the information is changed to a voltage or current that varies continuously and smoothly over a
range of values. The waveform of the voltage or current, i.e. its variation with time, represents the information
and is an analogue of it, Figure 3.3. For example, the loudness and frequency of a sound determines the
amplitude and frequency of the waveform produced by a microphone on which the sound falls
 Basic communication System
Electrical signals representing ‘information’ from a microphone, a television camera, a
computer, etc., can be sent from a place to place using either cables or radio waves.
While some signals can be sent directly through cables, in general, and certainly in radio
and television, as we will see later, a ‘carrier’ wave is required to transport them.
The basic building blocks of any communication system are shown in Figure 3.4.
Signals from the input transducer are added to the carrier in the modulator or encoder by the
process of modulation

The modulated signal is then sent by the transmitter into the propagating medium
(i.e. cable or radio wave).
At the receiving end, the receiver may have to select and perhaps amplify the modulated signal before the
demodulator or decoder extracts from it the information signal for delivery to the output transducer.
Basically, the transmitting and receiving ends each consists of an electronic system comprising an input transducer, a
processor consisting of several subsystems, and an output transducer.

Figure 3.4.

Radio waves

Radio waves are members of the family of electromagnetic radiation they have the
longest wavelength and the lowest frequency in the electromagnetic spectrum
They are energy carriers which travel through the space at the speed of light c, their
frequency f and wavelength being related, as for any wave motion, by the equation

The table below shows how they are grouped into different frequency bands
 Radio waves can travel from a transmitting aerial in one or more of three different ways as shown in
the figure below:

A. Surface waves (Ground waves)

These follow the earth’s surface and have a limited range, being greatest (1500
km) for long waves but much less for VHF.
B. Sky waves
These travel skywards and if they are below a certain critical frequency (typically 30 MHz)
are returned to Earth by bouncing off the ionosphere. This consists of layers of air molecules,
stretching from about 80 km above the Earth to 500 km, which have become positively
charged (i.e. ionized) by the removal of electrons due to the Sun’s ultraviolet radiation. The
critical frequency varies with the time of day and the seasons.
C. Space waves
These give straight-line transmission over 100 km or so on Earth in the absence of intervening obstacles such as hills and
buildings, and are the way VHF, UHF and microwaves travel. They can also penetrate the ionosphere so satellite
communication uses this type of transmission

Radio system

Radio waves are emitted by aerials when a.c. flows in them but the length of the aerial must be
comparable with the wavelength of the wave produced for the radiation to be appreciable. A
50 Hz a.c. corresponds to a wavelength of 6 × 106 m (since v = f λ) where v = 3 × 108 m/s and
f = 50 Hz).

Alternating currents with frequencies below about 20 kHz are called audio frequency (a.f) currents; those with
frequencies greater than this are radio frequency (r.f) currents. Therefore, so that aerials are no too large, they are
supplied with r.f. currents. However, speech and music generate a.f. currents and so some way of combining a.f with r.f is
required if they are to be sent over a distance
a) Transmitter
A collection of devices which convert the information signals is called transmitter.

In this an oscillator produces an r.f. current which would cause an aerial connected to it to send out an
electromagnetic wave, called a carrier wave,
The process of adding a.r and carrier wave called modulation
This done by two ways

amplitude modulation (AM) the amplitude of the r.f. carries from r.f oscillator is varied at the
frequency of the a.f signal from microphone
amplitude modulation is used in medium, short and long wave broad costing
In frequency modulation (FM) in this method the frequency of the carrier wave is varied at rate equal to the frequency a.f
but the amplitude remains constant

Receiver
A block diagram of a simple AM receiver is shown in Figure 3.7b. This tuning circuit selects the wanted signal from the
aerial. The detector (or demodulator) separates the a.f. (speech or music) from the r.f. carrier. The amplifier then boosts
the a.f. which produces sound in the loudspeaker

Electrical oscillations
The production of r.f. currents is impossible with mechanical generators but is readily achieved electrically by using an
oscillatory circuit. This contains a capacitor C and a coil L, Figure 3.8a

Figure 3.8a

Tuning circuit
Different transmitting stations send out radio waves of different frequencies. Each induces a
signal in the aerial of a radio receiver but if the aerial is connected to an L-C circuit, only the
signal whose frequency equals the natural frequency of the L-C circuit is selected. If the
value of C is varied different stations can be ‘tuned in’. This is an example of electrical
resonance.

Detection or demodulation
In detection or demodulation, the a.f which was ‘added’ to the r.f. in the
transmitter is recovered in the receiver.