Notes-Class-11-Science-Physics-Chapter-13-Electromagnetic Waves and Communication System-Maharashtra Board

Topics to be Learn : Part-1 Introduction EM wave Electromagnetic Spectrum Propagation of EM Waves Introduction to Communication System Modulation

Introduction :

Electromagnetic wave : Energy in the form of radiation emitted by an accelerated electric charge propagates in space, without requiring a material medium, at the speed of light as a wave of oscillating electric and magnetic fields. This wave, whose associated electric and magnetic fields oscillate in phase at right angles to each other as well as to the direction of propagation, is called an electromagnetic wave. The maximum and the minimum values of the two fields occur at the same instant.

• Information age is largely based on electromagnetic (EM) waves.
• Global connectivity via TV, cellphone, and internet uses EM waves for signal transmission.
• Sun energy, essential for Earth's life, travels through EM waves.
• EM waves also come from light bulbs, automobile engine blocks, x-ray machines, lightning flashes, and radioactive materials.
• EM waves are also emitted by stars and other galaxies.
• Studying EM waves properties is crucial.

EM wave:

Embodied in Maxwell's equations are the following four basic laws that describe all (classical) electromagnetic behaviour. These laws were stated by Gauss (also Coulomb), Faraday and Ampere. Maxwell added one piece of information into Ampere’s law — the displacement current, which made the equation complete.

Four basic laws to describe electromagnetic behaviour :

• Faraday-Lenz's law of electromagnetic induction : It says that a time—varying magnetic flux produces an electric field.
• Ampere-Maxwell's law : It is the analogue of araday’s law and says that an electric field changing with time produces a magnetic field.
• Gauss’s law in electrostatics : It relates the flux of the electric field over a closed surface to the total charge enclosed by the surface.
• Gauss’s law for static magnetic fields : It effectively says that there are no magnetic monopoles

Maxwell’s equations : The set of four equations describing the above four laws is called Maxwell’s equations.

(i) \(∫\vec{E}.\vec{dS}=\frac{Q_{in}}{ε_0}\), (Gauss’ law describes the relation between an electric charge and electric field it produces.)

(ii) \(∫\vec{B}.\vec{dS}\)= 0, (Gauss’ law for magnetism).

(iii)\(∫\vec{E}.\vec{dl}=\frac{dΦ_{m}}{dt}\),  (Faraday’s law with Lenz’s law)

(iv) \(∫\vec{B}.\vec{dl}\) = μ₀ I + ε₀ μ₀ \(\frac{dΦ_{E}}{dt}\) , (Ampere-Maxwell law)

Sources of EM waves:

How EM waves produced :

• According to Maxwell’s theory, "accelerated charges radiate EM waves".
• For example, consider a charge oscillating with some frequency. This produces an oscillating electric field in space, which produces an oscillating magnetic field which in turn is a source of oscillating electric field.
• Thus varying electric and magnetic fields regenerate each other.

Transverse nature of electromagnetic waves :

When an electromagnetic wave propagates through space, its mutually perpendicular electric field and magnetic field oscillations are simultaneous and always transverse, i.e., perpendicular, to the direction of propagation.

As shown in Fig. \(\vec{E}\) is in the xy-plane, \(\vec{B}\) is in the xz-plane and the direction of propagation is along \(\vec{E}\) x \(\vec{B}\), i.e., along the + x-axis. Hence, an electromagnetic wave is a transverse wave.

The magnitudes of both the fields vary sinusoidally with x and time t :

E_y = E₀ sin (ωt — kx)

B_z = B₀ sin (ωt — kx)

where E₀ and B₀ are the respective amplitudes,

k = \(\frac{2π}{λ}\) is the propagation constant and

ω = 2πγ is the angular frequency; λ and γ are the wavelength and frequency of the wave.

Know This : According to quantum theory, an electron, while orbiting around the nucleus in a stable orbit does not emit EM radiation even though it undergoes acceleration. It will emit an EM radiation only when it falls from an orbit of higher energy to one of lower energy. EM waves (such as X-rays) are produced when fast moving electrons hit a target of high atomic number (such as molybdenum, copper, etc.). An electric charge at rest has an electric field in the region around it but has no magnetic field. When the charge moves, it produces both electric and magnetic fields. If the charge moves with a constant velocity, the magnetic field will not change with time and as such it cannot produce an EM wave. But if the charge is accelerated, both the magnetic and electric fields change with space and time and an EM wave is produced. Thus an oscillating charge emits an EM wave which has the same frequency as that of the oscillation of the charge.

Characteristics of EM waves:

(1) Electromagnetic radiations are produced by an accelerated electric charge, an atomic transition, the combination of positive and negative ions, a nuclear transition, the annihilation of an electron and a positron, and the decay of certain elementary particles.

(2) The electric and magnetic fields, \(\vec{E}\) and \(\vec{B}\) are always perpendicular to each other and also to the direction of propagation of the EM wave. Thus the EM waves are transverse waves

(3) An electromagnetic wave propagates in space at the speed of light as a wave of oscillating electric and magnetic fields.

The cross product \(\vec{E}\) × \(\vec{B}\) gives the direction in which the EM wave travels. \(\vec{E}\) × \(\vec{B}\) also gives the energy carried by EM wave.

(4) The  and  fields vary sinusoidally and are in phase.

(5) EM waves are produced by accelerated electric charges.

(6) EM waves can travel through free space as well as through solids, liquids and gases.

(7) In free space, EM waves travel with velocity c, equal to that of light in free space.

c₀ = λ₀ ν = \(\frac{1}{\sqrt{ε_0μ_0}}\) = 3 x 10⁸ m/s

where λ₀ is the free space wavelength, ν is the frequency, μ₀ (4π ×10^-7 Tm/A) is permeability and ε₀ (8.85×10-¹² C²/Nm²) is permittivity of free space.

(8) In any material medium, the speed of light c is less than c₀, varies with frequency, and depends on the electric and magnetic properties of the medium.

c = λ ν = \(\frac{1}{\sqrt{εμ}}\)  where λ is the wavelength, ν is the frequency, and ε and μ are the permittivity and permeability of the medium, respectively.

(9) Electromagnetic radiations obey the principle of superposition of waves. They exhibit the phenomena of interference and diffraction which show that they propagate in the form of waves.

(10) An electromagnetic wave can be plane polarized which proves its transverse nature.

(11) The electric field (\(\vec{E}\)) of the em waves is responsible for optical effects and is known as the light vector in optics.

(12) The ratio of the electric and magnetic field amplitudes is equal to the speed of the em wave.

c₀ = \(\frac{|E_0|}{|B_0|}=\frac{1}{\sqrt{ε_0μ_0}}\)

(13) The energy of an electromagnetic wave is distributed equally between the constituent electric and magnetic fields.

Since intensity of a wave is proportional to the square of its amplitude, I_E ∝ E₀² and

I_B ∝ B₀² . Energy density in electric field is \(\frac{1}{2}\)ε₀E₀² and that in magnetic field is \(\frac{1}{2}.\frac{B_0^2}{μ_0}\) . The energy of em waves is distributed equally between the electric and magnetic fields.

(14) Electromagnetic waves obey the laws of reflection and refraction.

(15) An electromagnetic wave transports energy and moment-um. If U is the energy absorbed by a surface from a wave, the momentum imparted to the surface is p = U/co, where c₀ is the speed of the electromagnetic wave in free space.

(16) Since an electromagnetic wave imparts momentum, it also exerts pressure on a surface, called radiation pressure.

(17) Electromagnetic waves are influenced by a gravitational field. A gravitational field changes the frequency and path of an electromagnetic wave.

Electromagnetic Spectrum:

The orderly distribution (sequential arrangement) of EM waves according to their wavelengths (or frequencies) in the form of distinct groups having different properties is called the EM spectrum.

The complete electromagnetic spectrum in the order of increasing wavelength (decreasing frequency) extends from very high energy gamma rays (wavelength about 10^—15 m) to long wavelength radio waves (wavelength about 10⁵ m).

The major regions of the spectrum are :

• radio waves.
• microwaves
• gamma radiation
• X-rays
• ultraviolet radiation
• visible light
• infrared radiation

Theoretically, the spectrum has no upper and lower bounds; but there are practical limits on frequencies that are produced and detected. Also, the boundaries between the regions are not sharply defined.

Radio waves :

• Radio waves are produced by low-frequency, electronic oscillator circuits containing a capacitor and an inductor.
• The frequency of the waves produced depend upon the capacitance and inductance. Astronomical objects that have a changing magnetic field also produce radio waves.
• Radio waves are detected by the oscillating currents they induce in an antenna coupled with a tuned circuit (radio).

Properties :

• Radio waves are electromagnetic radiations with wavelengths in the range from about 10 cm to 100 km or a frequency range of about 3 kHz to 300 GHZ.
• They have very long wavelengths ranging from a few centimetres to a few hundreds of kilometres.
• They obey the laws of reflection and refraction but the components must be physically larger than optical mirrors and lenses since radio waves have much longer wavelength than light waves.
• They can penetrate through rain, snow and clouds.

Uses :

• Radio waves are used for wireless communication purpose.
• They are used for radio broadcasting and transmission of TV signals.
• Cellular phones use radio waves to transmit voice communication in the ultra high frequency (UHF) band.

Microwaves :

• Microwaves are produced by oscillator electric circuits containing a capacitor and an inductor. They can be produced by special vacuum tubes called klystrons, magnetrons and Gunn diodes.
• Microwaves are electromagnetic radiations whose wavelengths extend from about 1 mm to 30 cm. Their discovery is lost in the secrets of World War II where they were used for RADAR.
• Faint cosmic microwave background (CMB) radiation in the gigahertz range, believed to be the remnant of the Big Bang, comes from every direction in space.
• Microwaves are detected by the oscillating currents they induce in an antenna coupled with a tuned circuit.

Properties

• They heat certain substances on which they are incident.
• They can be detected by crystal detectors.

Uses

• Used for the transmission of TV signals.
• Used for long distance telephone communication.
• Microwave ovens are used for cooking.
• Used in radar systems for the location of distant objects like ships, aeroplanes etc,
• They are used in the study of atomic and molecular structure.

Infrared waves :

• IR radiations were discovered in 1800 by Sir Frederick William Herschel (1738— 1822) German-born British astronomer.
• Infrared radiation arises due to transitions between vibrational and rotational energy states of molecules.
• The wavelengths in this region extend from about 800 nm (0.8 pm) to 0.3 mm, beyond the visible red.
• Every object emits infrared radiations; the warmer the object, the more infrared radiation it emits.
• The Earth, stars (e.g., the Sun) and galaxies also emit IR radiations. They are also produced by gas lasers and infrared light-emitting diodes.
• IR radiation is detected by a thermal imaging camera using a semiconductor photodetector and thermocouple, thermopile and bolometer.

Properties

• When infrared rays are incident on any object, the object gets heated.
• These rays are strongly absorbed by glass.
• They can penetrate through thick columns of fog, mist and cloud cover.

Uses

• Used in remote sensing.
• Used in diagnosis of superficial tumours and varicose veins.
• Used to cure infantile paralysis and to treat sprains, dislocations and fractures.
• They are used in Solar water heaters and cookers.
• Special infrared photographs of the body called thermograms, can reveal diseased organs because these parts radiate less heat than the healthy organs.
• Infrared binoculars and thermal imaging cameras are used in military applications for night vision.
• Used to keep green house warm.
• Used in remote controls of TV, VCR, etc.

Visible light :

• This extremely narrow band of electromagnetic spectrum arises due to electronic transitions in atoms.
• The human eye is sensitive to radiations of the visible spectrum in which colours vary continuously from violet to red.
• Under bright light conditions, the human eye can distinguish about 1500 colour hues but conventionally the visible spectrum is divided into seven wavelength ranges with colour names. They are violet, indigo, blue, green, yellow, orange mid red.
• The sensitivity of the human eye has a peak near 530 nm (green colour) and falls off for extreme colours.
• Visible light can also be detected or measured by a photographic plate, photomultiplier and photodiode.

Properties :

• Different wavelengths give rise to different colours.

Colour	Wavelength
violet	380-450 nm
blue	450-495 nm
green	495-570 nm
yellow	570-590 nm
orange	590-620 nm
red	620-750 nm

The wavelengths in this region range from about 380 nm, at violet end, to about 750 nm, at red end. These are given in Table given in below table.

• Visible light emitted or reflected from objects around us provides us information about those objects and hence about the surroundings.

Ultraviolet rays :

UV radiation was discovered by Johann Wilhelm Ritter (1776 — 1810), German scientist.

• UV radiation is electromagnetic radiation emitted in atomic transitions of orbital electrons. It is produced when the substance is at very high temperature.
• The wavelengths extend from long X-rays up to visible violet light : about 10 nm to 390 nm.
• Stars, e.g., the Sun, are the main sources of UV radiation, but this radiation is mostly absorbed by the ozone layer in the Earth's atmosphere. It is also produced by gas discharge tubes (e.g., deuterium, argon), mercury vapour lamps and UV laser diodes.
• UV radiation is detected by a photographic plate, photoelectric effect (UV photomultipliers) and silicon photodiode.

Properties :

• They produce fluorescence in certain materials, such as 'phosphors'.
• They cause photoelectric effect.
• They cannot pass through glass but pass through quartz, fluorite, rock salt etc.
• They possess the property of synthesizing vitamin D, when skin is exposed to them.

Uses :

• Ultraviolet rays destroy germs and bacteria and hence they are used for sterilizing surgical instruments and for purification of water.
• Used in burglar alarms and security systems.
• Used to distinguish real and fake gems.
• Used in analysis of chemical compounds.
• Used to detect forgery.
• UV astronomy reveals newly formed stars in distant galaxies allowing astrophysicists to study formation of stars.

X-rays:

German physicist W. C. Rontgen (1845-1923) discovered X-rays in 1895 while studying cathode rays

• X-rays are produced when high energy electrons bombard a metal target of high atomic number. Deceleration of some of these electrons in the target produces X-rays.
• Also, some incident electrons knock inner orbital electrons out of an atom. Then, a transition of an outer electron into the inner shell is accompanied by the emission of an X-ray photon (packet of radiation).
• X-rays correspond to a wavelength range of about 10 pm (1 pm = 10^-12 m) to 30 nm (1 nm = 10^-9 m).
• Some nuclear transitions result in emission of electromagnetic radiation that lies in the X-ray region.
• X-rays for medical and industrial purposes are produced by an X-ray tube. Also, stars emit X-rays.
• X-rays are detected by ionization chamber (Geiger counter), photographic plate, scintillation counters, semiconductor detector, etc.

Properties

• They are high energy EM waves.
• They are not deflected by electric and magnetic fields.
• X-rays ionize the gases through which they pass.
• They have high penetrating power.
• Their over dose can kill living plant and animal overdose tissues and hence are

Uses

• Useful in the study of the structure of crystals.
• X-ray photographs are useful to detect bone fracture. X-rays have many other medical uses such as CT scan.
• X-rays are used to detect flaws or cracks in metals.
• These are used for detection of explosives, opium etc.

Gamma Rays (γ-rays)

Gamma rays were discovered in 1900 by Ernest Rutherford (1871 — 1937) New Zealand-British physicist and independently by Paul Villard.

• Gamma radiation are electromagnetic radiations emitted by excited atomic nuclei during their transition from a higher energy state to a lower energy state produced in the annihilation of an electron and a positron.
• They are shortest wavelength (highest frequency) electromagnetic radiations, corresponding to a wavelength range of about 1 fm (10^-15 m) to 0.1 nm (10^-10 m).
• Consequently, gamma rays are the most energetic among all electromagnetic radiations.
• Terrestrial sources of gamma radiations include radioactive nuclei (e.g., uranium, radium, etc.), nuclear reactions and nuclear explosions.
• Most abundant sources of gamma rays are the stars and violent celestial events like supernova explosions, but these are mostly absorbed by the Earth's atmosphere.
• Gamma rays are detected by ionization chamber (Geiger counter), photographic plate, fluorescence.

Properties

• They are highest energy EM waves. (energy range keV - GeV)
• They are highly penetrating.
• They have a small ionising power.
• They kill living cells.

Uses

• Used as insecticide disinfection for wheat and flour.
• Used for food preservation.
• Used in radiotherapy for the treatment of cancer and tumour.
• They are used to produce nuclear reactions.

Properties of different types of EM waves :

Propagation of EM Waves:

Several factors influence the propagation of EM waves and the path they follow. The composition of the Earth’s atmosphere plays a vital role in the propagation of EM waves.

Earth’s atmosphere :

Figure shows the structure of the atmosphere, in which there are four distinct layers as defined by reversals of temperature, although the boundaries are not very sharp.

These are the troposphere, stratosphere, mesosphere and thermosphere.

• The atmosphere of the Earth is not physically uniform but has significant variations in pressure, density, temperature and composition with altitude.
• As we go up, the atmosphere gradually thins out and the air pressure decreases to zero. Almost 99% of the mass of the atmosphere lies within 30 km from the surface.

Different layers of Earth’s atmosphere :

Space communication : Space communication is the wireless or broadcast type of transmission system in a telecommunications network that uses the physical space surrounding us to transmit information-carrying signals via radio waves.

In space communication, basic information carrying signals, such as the output from a microphone or video camera, are superimposed on a radio frequency electromagnetic carrier wave for transmission.

Different modes of propagation of EM waves :

In space communication, unguided radio waves propagate from the transmitting antenna to the receiving antenna in three modes : (1) ground wave (2) space wave (3) sky wave.

Ground (surface) wave:

• When the radio waves propagate along the ground, the wave propagation is called ground wave propagation or surface wave propagation.
• For this mode of propagation, the transmitting and the receiving antenna are very low or close to the ground.
• The electric field of the radio wave induces charges in the ground. As the wave propagates, the induced charges in the ground also travel along with it and constitute a current in the Earth’s surface and loses energy by absorption by the Earth.
• Hence, signals cannot be transmitted over large distance and is suitable for local broadcasting only. This mode of propagation cannot be used for TV or FM signals.
• Suitable frequency range : Medium frequencies of the AM band (300 kHz to 3 MHZ).

Space wave:

• When the radio waves reach the receiver travelling through the troposphere, the wave propagation is called space wave propagation.
• Radio waves reach the receiving antenna, which is in the line-of-sight of the transmitting antenna, in the following three ways : as a direct wave, as a wave reflected off the ground and as a tropospheric wave totally internally reflected at the tropopause (the upper boundary of the troposphere).
• Suitable frequency range : VHF (30 MHZ to 300 MHZ) and UHF (300 MHz to 3 GHZ) ranges of the FM band, i.e., at frequencies used for TV, FM radio, radar, etc.

Space wave range: It is the farthest distance from a transmitting antenna that radio waves can reach by space wave propagation. OR It is the straight line distance from the transmitting antenna to the horizon where the direct wave will hit the Earth.

Sky wave propagation:

• In space communication, when the radio waves from a transmitting antenna reach a receiving antenna after reflection in the ionosphere, the mode of propagation is called sky wave or ionospheric propagation.
• The ionosphere is an inhomogeneous ionized gas with the electron concentration increasing with altitude and thus the refractive index decreases with altitude.
• Hence, radio waves from a ground station undergo total internal reflection towards the Earth.
• The two layers of the ionosphere that are important for sky wave propagation are the E-layer and the F-layer. The F-layer is the most useful for long distance transmission.
• Suitable frequency range : HF (2 MHZ to 20 MHZ) range of the AM band

Critical frequency : It is the maximum value of the frequency of radio wave which can be reflected back to the Earth from the ionosphere when the waves are directed normally to ionosphere.

Skip distance (zone) : It is the shortest distance from a transmitter measured along the surface of the Earth at which a sky wave of fixed frequency (if greater than critical frequency) will be returned to the Earth so that no sky waves can be received within the skip distance.

Important layers for space communication :

The propagation of radio waves in the atmosphere depends on the nature of various layers of the atmosphere. The two layers which are important for space communication are :

• the troposphere, that extends up to about 20 km from the surface of the Earth.
• the ionosphere that extends from an altitude of about 80 km to 400 km.

TV transmission by space wave propagation :

• TV transmission uses the VHF (30 MHz to 300 MHz) and UHF (300 MHz to 3 GHZ) ranges of the FM band.
• Ground wave propagation occurs when radio waves travel along the ground. The electric field of a radio wave causes charges in the ground, which move with the wave and form a current on the Earth's surface. Energy is lost owing to absorption by the Earth as well as diffraction around barriers in the wave's passage. Both losses are significant at the very high frequencies required for TV transmission.
• Ground wave propagation cannot support long-distance communication. The frequencies involved are above the crucial frequency for ionospheric reflection.
• Sky wave propagation cannot support short-wave communication. As a result, TV transmission must be accomplished via space wave propagation.

Relation between space wave range and antenna heights :

Space wave range is the straight line distance from the point of transmission (the top of the antenna) to the point on Earth where the wave will hit while travelling along a straight line.

Range is shown by d in Fig. Let the height of the transmitting antenna (AA') situated at A be h. B represents the point on the surface of the Earth at which the space wave hits the Earth. The triangle OA'B is a right angled triangle.

From Δ OA' B we can write

OA'² = A'B² + OB²

(R+h)² = d² + R²

or R² + h² + 2Rh = d² + R²

∴ h² + 2Rh = d²

As h << R, we can ignore h²

∴ d ≅ \(\sqrt{2Rh}\)

The range can be increased by mounting the receiver at a height h' say at a point C on the surface of the Earth. The range increases to d + d' where d' is \(\sqrt{2Rh'}\) Thus

Total range = d + d ' = \(\sqrt{2Rh}\)+\(\sqrt{2Rh'}\)

Q. Why does TV transmission require tall transmitting antennas?

Answer :

Space waves cover a longer distance than ground waves. But due to the finite curvature of the Earth, the receiving antenna must be in the line of sight of the transmitting antenna.

Such waves cannot be propagated beyond the line-of-sight distance or space wave range, d ≅ \(\sqrt{2Rh}\) where h is the height of the transmitting antenna and R is the radius of the Earth.

For greater space wave range, h must be increased. Hence, the transmitting antennas are made very tall or located on the top of hills or high buildings.

Introduction to Communication System:

The inventions of the 19th and early 20^th centuries which revolutionized long distance communication are :

• Telegraph in 1835 and the Morse code in 1838, by Samuel Morse (1791 — 1872) US inventor; Sir Charles Wheatstone’s contribution to cable telegraphy and invention of printing telegraph;
• Telephone (in 1876), by Alexander Graham Bell (1847— 1922) British American speech therapist, and Antonio Santi Giuseppe Meucci (1808-89) Italian-American inventor.
• Wireless or radio telegraphy in 1895—99, by Indian physicist and plant physiologist Sir Jagadish Chandra Bose (1858—1937), Italian physicist and engineer Guglielmo Marquis Marconi (1874— 1937) and Russian physicist Aleksandr Stepanovich Popov (1859 — 1906);
• Television (1926), commercial success pioneered by Iohn Logie Baird (1888-1946) British electrical engineer; television has no single inventor.

In the 20th century we could send messages over large distances using analogue signals, cables and radio waves. With the advancements of digitization technologies, we can now communicate with the entire world almost in real time.

Elements of a communication system:

There are three basic (essential) elements of every communication system: (i) Transmitter, (ii) Communication channel and (iii) Receiver.

• Electrical transducers, such as a voice coder or microphone and video camera, which generates the basic information-carrying signal.
• A transmitter first modulates the signal on a radio frequency carrier wave and then transmits it.
• A communication channel is the path over which the signal is transmitted.
• A receiver demodulates and recovers the information/message signal. This
• information signal is then passed to appropriate transducers for conversion into audio or video information for use by a user.

From the transducers at the transmitting end to those at the receiving end, noise can distort original information. The challenge is to keep the noise to the minimum.

The two basic modes of long distance communication :

(1) Point-to-point or guided or line communication : Line communication is a transmission system in a fixed (land-based) telecommunication network that uses transmission lines as communication channel.

(2) Unguided or space communication, called broadcast : Space communication is a transmission system in a fixed (land-based) as well as mobile (maritime, aeronautical and land-based) telecommunications network that uses the physical space surrounding us as the communication channel. Space communication is via radio waves in certain internationally agreed frequency bands.

Commonly used terms in electronic communication system:

(1) Signal : Information in the form of sound, light, etc. converted into electrical form suitable for transmission is called a signal.

• An analog signal is a continuous variation of voltage or current as a single-valued function of time.
• A digital signal is a sequence of electrical pulses in which the voltage takes only discrete stepwise values, switching between two values.
• The lower value of the voltage is labelled as LOW or O and the higher value as HIGH or 1.

(2) Transmitter :- A transmitter converts the signal produced by a source of information into a form suitable for transmission through a channel and subsequent reception.

(3) Transducer :- A device that converts one form of energy into another form of energy is called a transducer.

• For example, a microphone converts sound energy into electrical energy. Therefore, a microphone is a transducer. Similarly, a loudspeaker is a transducer which converts electrical energy into sound energy.

(4) Receiver :- The receiver receives the message signal at the channel output, reconstructs it in recognizable form of the original message for delivering it to the user of information.

(5) Noise :- A random unwanted signal is called noise. The source generating the noise may be located inside or outside the system. Efforts should be made to minimise the noise level in a communication system.

(6) Attenuation :- The loss of strength of the signal while propagating through the channel is known as attenuation. It occurs because the channel distorts, reflects and refracts the signals as it passes through it.

(7) Amplification :- Amplification is the process of raising the strength of a signal, using an electronic circuit called amplifier.

(8) Range :- The maximum (largest) distance between a source and a destination up to which the signal can be received with sufficient strength is termed as range.

(9) Bandwidth :- The bandwidth of an electronic circuit is the range of frequencies over which it operates efficiently. It is measured in hertz.

• The information communicated by a signal in a communication system can be of various types, such as voice, music, picture or binary data.
• Depending on the type of information, a signal has different range of frequencies. For example, voice or human speech ranges from about 300 Hz to about 3100 Hz. The audible range of frequencies in humans is from about 20 Hz to about 20000 Hz. Video signals to transmit visual or picture information require a bandwidth of about 4.2 MHz.

(10) Modulation :- The signals in communication system (e.g. music, speech etc.) are low frequency signals and cannot be transmitted over large distances. In order to transmit the signal to large distances, it is superimposed on a high frequency wave (called carrier wave). This process is called modulation. Modulation is done at the transmitter and is an important part of a communication system.

(11) Demodulation :- The process of regaining signal from a modulated wave is called demodulation. This is the reverse process of modulation.

(12) Repeater :- It is a combination of a transmitter and a receiver. The receiver receives the signal from the transmitter, amplifies it and transmits it to the next repeater. Repeaters are used to increase the range of a communication system.

Modulation:

Modulation can be done by modifying the (i) amplitude (amplitude modulation)

(ii) frequency (frequency modulation), and (iii) phase (phase modulation) of the carrier wave in proportion to the amplitude or intensity of the signal wave keeping the other two properties same.

Amplitude modulation (AM) : It is a type of modulation in which the amplitude of a radio frequency carrier wave is varied by an amount that is proportional to the amplitude of a baseband or basic information-carrying signal at a frequency equal to that of the modulating baseband signal. The frequency of the modulated wave is the same as that of the carrier wave.

• Advantages : (AM) is simple to implement and has large range. It is also cheaper.
• Disadvantages : (i) it is not very efficient as far as power usage is concerned (ii) it is prone to noise and (iii) the reproduced signal may not exactly match the original signal.
• Use : Used for commercial broadcasting in the long, medium and short wave bands.

Frequency modulation (FM) : It is a type of modulation in which the frequency of a radio frequency carrier wave is varied by an amount that is proportional to the amplitude of a baseband or basic information-carrying signal at a frequency equal to that of the modulating baseband signal; the amplitude of the carrier wave remains the same.

• Its main advantage is that it reproduces the original signal closely and is less susceptible to noise.
• This modulation is used for high quality broadcast transmission.

Phase modulation : It is a type of modulation in which the phase of a carrier wave is varied by an amount that is proportional to the amplitude of the baseband or basic information-carrying signal; the amplitude and frequency of the carrier wave remain the same.

• Phase modulation (PM) is easier than frequency modulation.
• It is used in determining the velocity of a moving target which cannot be done using frequency modulation.