The first amplitude modulated signal was transmitted in 1901 by a Canadian engineer named Reginald Fessenden. He used a continuous spark transmission and placed a carbon microphone in the antenna lead. This transmission was very crude, signals were audible over a distance of a few hundred metres. The quality of the audio was not good. In amplitude modulation, angular frequency ω and the phase Φ are kept constant and the amplitude A_c of the carrier wave is varied in accordance with the modulating wave. When the amplitude of a high-frequency carrier wave is changed in accordance with the intensity of the signal, it is called amplitude modulation.

Amplitude modulation is done by a circuit called modulator. In amplitude modulation, the amplitude of the carrier is varied in accordance with the information signal. Here we explain the amplitude modulation process using a sinusoidal signal as the modulating signal.

We can see that during the positive cycle of low-frequency signal and the positive cycle of carrier wave the amplitude of carrier wave increased, while in the negative cycle it is decreased.

Features of Amplitude Modulated Wave:

• The amplitude of carrier wave changes according to the intensity of the signal.
• The variation in amplitude of a carrier wave is at the frequency of the signal f_m.
• The frequency of amplitude modulated wave is the same as that of the carrier wave fc.
• The amplitude of amplitude modulated wave is not constant but it has a similar sinusoidal variation as that of the signal wave. Thus the amplitude-modulated wave is loaded with the information contained in the low-frequency signal message.
• As amplitude modulation is simple it is widely used.

Modulation Factor or Modulating Index:

The ratio of change of amplitude of the carrier wave to the amplitude of the normal carrier wave is called the modulation factor. It describes the depth of modulation i.e. the extent in the variation of the amplitude of carrier wave due to the signal.

The value of the modulation factor depends upon the amplitude of the carrier wave and the signal.

Importance of the Modulation Index:

• It is an indicator of the level of modulation.
• If there is too low a level of modulation then the amount of variation in carrier amplitude is small. Thus the audio signal being transmitted will not be very strong. Hence the modulation does not utilize the carrier efficiently.
• If there is a too high level of modulation then the carrier can become over modulated causing sidebands to extend out beyond the allowed bandwidth causing interference to other users. Hence there will be distortion during the reception.

Amplitude modulation with modulation factor 0.5 or 50%

Amplitude modulation with modulation factor 1 or 100%

When the modulation index reaches 1.0, i.e. a modulation depth of 100%, the carrier level falls to zero and rise to twice its non-modulated level.

Amplitude modulation with a modulation factor greater than 1 or greater than 100%

Any increase of the modulation index above 1.0, i.e. 100% modulation depth causes over-modulation.

The carrier experiences 180° phase reversals where the carrier level would try to go below the zero point. Due to this phase reversals, there is a rise in additional sidebands resulting from the phase reversals (phase modulation) that extend out, to infinity theoretically. This may cause serious interference to other users if not filtered.

Broadcast stations, take measures to ensure that the carries of their transmissions never become overmodulated. The transmitters incorporate limiters to prevent more than 100% modulation. They incorporate automatic audio gain controls to keep the audio levels near 100% modulation for most of the time.

Expression For Modulation Index:

The modulation index (μ_a) of an amplitude-modulated wave is defined as the ratio of the amplitude of the modulating signal (Em) to the amplitude of the carrier wave (E_c).

μ_a = (E_m)/(E_c)

To avoid distortion E_m < E_c

For modulated wave

μ_a = (E_max – E_mmin)/(E_max + E_min)

If for AM wave the maximum amplitude is ‘a’ while the minimum amplitude is ‘b’

E_max =  (E_c + E_m) = a …………. (1)

E_min =  (E_c – E_m) = b …………. (2)

Solving equation (1) and (2) we get

E_m = (a – b)/2 and E_c = (a + b)/2

μ_a = (E_m)/(E_c) = (a – b)/(a + b)

Demodulation:

The AM signal received is passed through a demodulator to extract the information being carried by it. The process of separating or extracting the modulation from a signal is called demodulation or detection. Demodulation of AM signals can be done using simple circuits consisting of diodes.

Advantages Of Amplitude Modulation:

• It is simple to implement.
• Demodulation of AM signals can be done using simple circuits consisting of diodes.
• AM transmitters are less complex.
• AM receivers are very cheap as no specialized components are needed.
• AM waves can travel a longer distance.
• AM waves have low bandwidth

Disadvantages of Amplitude Modulation:

• An amplitude modulation signal is not efficient in terms of its power usage. Power wastage takes place in DSB-FC (Double Side Band – Full Carrier ) transmission.
• It is not efficient in terms of its use of bandwidth. It requires a bandwidth equal to twice that of the highest audio frequency. In amplitude modulation sidebands contain the signal. The power in sidebands is the only useful power. For 100 % modulation, the power carried by AM waves is 33.3 %. The power carried by the AM wave decreases with the decrease in the extent of modulation.
• AM detectors are sensitive to noise hence an amplitude modulation signal is prone to high levels of noise.
• Reproduction is not high fidelity. For high fidelity (stereo) transmission bandwidth should be 40000 Hz. To avoid interference the actual bandwidth used by AM transmission is 10000 Hz.

In spite of the disadvantages amplitude modulation is still in widespread use for broadcasting on the long, medium and short wave bands, some mobile or portable communications systems including some aircraft communications.

Applications of Amplitude Modulation:

Broadcast transmissions: 

AM is still widely used for commercial broadcasting on the long, medium and short wave bands because the radio receivers capable of demodulating amplitude modulation are cheap and simple to manufacture. The atmospheric signals like lightening and man-made electrical signals affect this transmission.

Air-band radio:  

VHF transmissions for many airborne applications still use AM. It is used for ground to air and ground to ground radio communications. e.g. television standard broadcasting, aids to navigation, telemetering, radar and, facsimile. etc.

Single sideband:  

Amplitude modulation in the form of single sideband is still used for point to point HF (high frequency) radio links.

Quadrature amplitude modulation: 

AM is widely used for the transmission of data in everything from short-range wireless links such as Wi-Fi to cellular telecommunications and much more. Quadrature amplitude modulation is formed by mixing two carriers that are out of phase by 90°.

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Before the concept of the modulation, the signal used to be sent in form continuous wave with periodic interruption as in morse code. To code, send and decode such message high expertise was required. The direct current is not suitable for transmitting the data because of its steady nature. A continuous AC wave cannot be used for transferring data as all the variations or cycles are alike. Thus continuous AC wave in the form of a series of pulses can be used to carry data effectively.

A high-frequency carrier wave is used to carry the audio signal. The audio signal is superimposed over the carrier wave. This process is called modulation.

Modulation may be defined as the process of changing some characteristics like amplitude, frequency or phase of a carrier wave in accordance with the intensity of the signal is known as modulation.

The Need for Modulation in Communication System:

The purpose of a communication system is to transmit information or message signals. Message signals are also called baseband signals, which essentially designate the band of frequencies representing the original signal, as delivered by the source of information. No signal, in general, is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth.

To understand the need for modulation let us suppose that we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than 20 kHz) over a long distance directly. Let us find what factors prevent us from doing so and how we overcome these factors.

Size of the Antenna or Aerial:

For transmitting signals antenna is required whose length should be at least (λ/4), to sense the time variation of the signal.

c = υλ

∴  λ = c/υ

Length of antenna = λ/4 = c/4υ  = 3 x 10⁸ / (4 x 20 x 10³)  =  3.75 x 10³ m = 3.75 km

The antenna size for considered baseband signal frequency is of impracticable length. Hence direct transmission at baseband signal frequency is not practical. If we used a frequency say 1MHz, then

Length of antenna = λ/4 = c/4υ  = 3 x 10⁸ / (4 x 1 x 10⁶)  =  75 m

This is a practical length of the antenna.

Thus we can obtain transmission by use of practical antenna length using high frequency for transmission. Thus there is a need for translating the original low-frequency signals into high frequency before transmission.

Effective Power Radiated by Antenna:

The power radiated is proportional to (l /λ)². Where ‘l’ is the size of the antenna. Thus for the same antenna, the power radiated is inversely proportional to the wavelength of the wave i.e. directly proportional to the frequency of the wave. Thus the effective power radiated by long-wavelength is small.

For good transmission of signal high power is required. Hence direct transmission at baseband signal frequency is not practical. To get more power radiated the high-frequency transmission should be used.

Mixing Up of Signals From Different Transmitters:

For considered baseband frequency there are full chances of mixing up of the signals from the transmitters operating in the same band. It is just like many people talking simultaneously. Thus, all these signals will get mixed up and there is no simple way to distinguish between them. Hence bands are allotted to different types of broadcast and communication systems.

Operating range:

The energy of a wave is directly proportional to the frequency of the wave. For the wave to be propagated over a large distance, the wave should possess more energy. It is only possible using a higher frequency for transmission of the wave.

Wireless Communication:

Wireless communication requires propagation of waves through the atmosphere without wires. The transmission of the wave at audio frequency becomes impractical because its efficiency of radiation is poor. Thus the transmission should be done at a higher frequency for obtaining higher radiation efficiency.

Types of Carrier Waves:

There is a need for translating the original low-frequency baseband message or information signal into the high-frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal. In doing so, we take the help of a high-frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it. The carrier wave may be continuous (sinusoidal) or in the form of pulses.

Continuous or Sinusoidal wave:

Pulse:

Types of Modulation w.r.t. Continuous (sinusoidal) Wave as Carrier Wave:

A sinusoidal carrier wave can be represented as

c(t ) = A_c sin (ωt +Φ)

where c(t) is the signal strength (voltage or current), A_c is the amplitude,

ω  = 2πf_c =  the angular frequency and

Φ is the initial phase of the carrier wave.

During the process of modulation, any of the three parameters, viz amplitude A_c, angular frequency ω, and the phase Φ, of the carrier wave can be varied with baseband signal or message or information signal. This results in three types of modulation.

• Amplitude modulation (AM): In this type of modulation, angular frequency ω and the phase Φ are kept constant and amplitude A_c of the carrier wave is varied in accordance with the modulating wave.
• Frequency modulation (FM): In this type of modulation, amplitude  A_c and the phase Φ are kept constant and angular frequency ω of the carrier wave is varied in accordance with the modulating
• Phase modulation (PM): In this type of modulation, amplitude A_c and angular frequency ω are kept constant and the phase Φ of the carrier wave is varied in accordance with the modulating

Note that the frequency modulation and phase modulation of a carrier wave is collectively called the angle modulation. Analog communication of analog signals is done using continuous wave.

Types of Modulation w.r.t. Pulse as Carrier Wave:

The significant characteristics of a pulse are pulse amplitude, pulse duration or pulse width, and pulse position (denoting the time of the rise or the fall of the pulse amplitude. Hence, different types of pulse modulation are:

• pulse amplitude modulation (PAM),
• pulse duration modulation (PDM) or pulse width modulation (PWM), and
• pulse position modulation (PPM)

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The optical fibre is a device which works on the principle of total internal reflection by which light signals can be transmitted from one place to another with a negligible loss of energy.

Characteristics of Optical Fibre:

• It has a large bandwidth.  The optical frequency of 2 x 10¹⁴ Hz can be used and hence the system has higher bandwidth. Thus optical fibres have greater information-carrying capacity due to greater bandwidth.
• In optical fibre system transmission losses are as low as 0.1 db/km.
• Optical fibres are of small size and have lightweight as compared to electrical cables. They are flexible and very high tensile strength. Thus they can be twisted and bent easily.
• Optical fibres provide a high degree of signal security as it is confined to the inside of fibre and cannot be tapped and tempered easily. Thus it satisfies the need for security which is required in banking and defence.
• Optical fibre communication is free from electromagnetic interference.
• Fibre optic fibres do not carry high voltages or current. Hence, they are safer than electrical cables.

Construction:

It consists of a very thin fibre of silica or glass or plastic of a high refractive index called the core. The core has a diameter of 10 μm to 100 μm. The core is enclosed by a cover of glass or plastic called cladding. The refractive index of the cladding is less than that of the core (which is a must condition for the working of the optical fibre). The difference between the two indices is very small of order 10-3. The core and the cladding are enclosed in an outer protective jacket made of plastic to provide strength to the optical fibre. The refractive index can change from core to cladding abruptly (as in step-index fibre) or gradually (as in graded-index fibre).

Total Internal Reflection of Light and its Explanation:

Let us consider a point source O in an optically denser medium (Water or medium with higher refractive index). Let XY be the boundary separating the optically denser medium (Water or medium with lower refractive index). As the angle of incidence increases, the angle of refraction also increases. At a particular angle of incidence i_C, the angle of refraction is 90° and hence the refracted ray moves along the surface of water i.e. along XY.

If the angle of incidence is more than i_C, there is no refracted ray, the incident ray is completely reflected back in the water (or medium with higher refractive index)). This phenomenon is known as total internal reflection.

The critical angle is the minimum angle of incidence when the total internal reflection of light takes place

Working of Optical Fibre:

When a ray of light is incident on the core of the optical fibre at a small angle, it suffers refraction and strikes the core-cladding interface, As the diameter of the fibre is very small hence the angle of incidence is greater than the critical angle. Therefore, the ray suffers total internal reflection at the core-cladding interface and strikes the opposite interface. At this interface also, the angle of incidence is greater than the critical angle, so it again suffers total internal reflection. Thus, the ray of light reaches the other end of the fibre after suffering repeated total internal reflections along the length of the fibre. At the other end, the ray suffers refraction and emerges out of the optical fibre.

We can see that the light travels in the core in a guided manner. Hence the communication through the optical fibre is sometimes referred as an optical waveguide.

Analytical Treatment of Optical Fibres:

Critical Angle:

By Snell’s law, we have

At the start of the total internal reflection, i = i_c and r = 90^o

When i > ic, there will be no refracted ray. All incident rays will get internally reflected

Acceptance Angle or Half Angle of Acceptance Cone:

The maximum angle with the axis of the optical fibre at which the light entering propagates through the fibre by suffering repeated total internal reflections at the core-cladding interfaces is called the acceptance angle or half-angle of acceptance cone.

Let us consider a ray of light PQ incident at the air-core interface at angle θ_i such that it is less than the acceptance angle θ_a. All the angles are measured w.r.t. the axis of the fibre. The face SR is perpendicular to the axis of the fibre. Let μ_air, μ_core and μ_cladding be the refractive indices of air, core and cladding respectively such that μ_core > μ_cladding > μ_air.

Applying Snell’s law at the air-core interface

Where θ_i = angle of incidence at the air-core interface and θ_r = angle of reflection at the air-core interface.

Let ∅ be the angle of incidence at the core-cladding interface such that ∅ is greater than the critical angle θ_c.

From the figure we have

θ_c + ∅ = 90° hence θr = 90o – ∅

substituting in equation (1)

When θ_i = θ_a, then ∅ = θ_C

This is an expression for the acceptance angle of an optical fibre.

Numerical Aperture of Optical Fibre:

The figure (numerical value) of merit which describes the light collecting ability of optical fibre is called its numerical aperture. Thus light collecting capability of an optical fibre is directly proportional to its numerical aperture.

Numerical Aperture for Step Index Fibre:

The largest possible value of NA is unity

Numerical Aperture for Step Index Fibre in Terms of Relative core-cladding index difference (D)

For an optical fibre, the difference between refractive indices of core and cladding is very small

This is an expression for numerical aperture in terms of relative core-cladding index difference. For graded-index fibre NA = sin θ_C.

Fibre Attenuation:

Principle of Fabricating Optical Fibre:

Fabrication of optical fibre involves the following two steps:

Step- 1: A glass rod having a definite refractive index is constantly heated by rotating it on the flame of a burner. Silicon tetrachloride vapours are burnt in the same flame so that an oxidized layer of silicon-di-oxide is uniformly deposited on the outer surface of the glass rod. The glass rod is then gradually cooled from 1400°C to room temperature to form a preformed glass rod having different inner and outer glass compositions.

Step – 2: The performed glass rod is then heated in a fibre drawing furnace. The end of the rod is pulled at a constant rate to form a thin fibre containing the core and the cladding. This fibre is then covered with a protective plastic sheath to obtain a fine optical fibre. A bunch of such optical fibres forms optical fibre cable.

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Line Communication:

Space communication is unguided communication. In this communication channel, there is no physical contact between the transmitting and the receiving antenna. The transmitted signal spreads in all directions. This results in the attenuation of the signal. To avoid the attenuation we use line communication. Inline communication, the signal is transmitted from the transmitter to the receiver through a connecting wire. This communication system requires a solid medium as the communication channel between the transmitter and the receiver. This medium is called a transmission line. This is the oldest method of transmission. The principal type of such communication are:

• two-wire transmission lines
• coaxial cables and
• optical fibres.

Two-wire transmission lines and coaxial cables are used to transmit AF and UHF signals while optical fibres are used to transmit optical signals. At high-frequency two-wire transmission lines are affected by electromagnetic interference and radiation. This disadvantage of the two-wire transmission line is avoided by using a shield in coaxial cables.

Two-wire transmission line:

In this case, the electrical signal is passed through a pair of conducting wires insulated from one another. The most commonly used two-wire transmission lines are i) parallel wireline and twisted wireline

Parallel Wire Lines:

It consists of two metallic wires arranged parallel to each other inside an insulating coating. The metal wires are hard or flexible depending on the power transmitted. Hardwires are used for high power transmission. In this transmission line, it is necessary to match the impedance of the wire with that of the receiver to obtain a maximum transfer of power.

The losses increase with the increase in the length of the wire and the frequency of the transmitted signal. Thus, parallel wire lines are used to send low-frequency electrical signals over small distances. Parallel wires are commonly used to connect the antenna to a T.V. receiver.

Twisted Wire Line:

It consists of two insulated copper wires twisted around each other at regular intervals of distance to minimize electrical interference.
They are used to transmit both digital and analog signals. A twisted pair of lines is inexpensive and easy to install. They are commonly used to connect telephone systems.

Coaxial Cable:

It consists of a Central copper wire surrounded by a PVC (dielectric) insulation and then is covered by a copper wire mesh. A tinned copper wire mesh (braided shield) is covered by an outer shield of thick PVC material. The signal is transmitted through the central copper wire while the outer conductor is connected to the ground. The material like Teflon and polyethene are used as dielectric insulation depending upon frequency and power to be transmitted through the cable. the grounded outer shield provides an electrical shield to the signals carried by the central conductor. The outer PVC jacket prevents inner copper wire or core from radiating signal, power. Thus it reduces losses. The characteristic impedance of coaxial cables is 50 ohms to 75 ohms.

Communication through a coaxial line is more efficient than two-wire lines as the attenuation of the signal is low. However, they are expensive as compared to two-wire lines. If the frequency to be transmitted is more than 20 MHz, there are considerable dielectric losses through the dielectric insulator. Hence this puts a limit to the frequency to be transmitted.

Due to higher bandwidth, the coaxial cable can transmit digital data at a much higher rate of up to 20 Mbps. but the rate of 10 Mbps is standard Co-axial Cables are used to transmit microwave and ultra high-frequency signals. They are used in local area networks (LAN) and cable television systems. The cable television system also known as Community antenna television (CATV) transmits audio, video, and data through coaxial cables. The central originating unit where all signals are processed is called head end.

Optical Communication:

The mode of communication which uses light waves for transmission of information from one place to another is called optical communication. The ability of a wave to transmit information depends upon its frequency. Higher the frequency of the carrier wave, the greater is the amount of information (bandwidth) transmitted in a given time. Radio communication systems use electromagnetic waves of frequencies of about 10⁶Hz to 10⁶¹⁴ Hz, whereas satellite communication systems use microwaves of frequency 10¹¹Hz. The frequency of light waves ranges between 10¹²Hz to 10¹⁶Hz which are very large as compared to that of radio and microwaves. Hence, light waves are better substitutes for communication of large information in a short interval of time.

Advantages of Optical Communication:

• The optical communication system has greater information-carrying capacity due to greater bandwidth.
• In an optical communication channel, the transmission losses per kilometre are very small as compared to electrical cables.
• The optical communication channel (fibre cables) are of small size and have lightweight as compared to electrical cables.
• The optical communication channel provides a high degree of signal security as it is confined to the inside of fibre and cannot be tapped and tempered easily. Thus it satisfies the need for security which is required in banking and defence.
• Optical fibre communication is free from electromagnetic interference.
• Fibre optic cables do not carry high voltages or current. Hence, they are safer than electrical cables.

Basic Optical Communication System:

An optical communication system consists of three main blocks a) Optical source b) Optical fibre and c) Optical signal detector

Optical Source:

A Light Emitting Diode (LED) or a semiconductor laser diode is used as an optical source. The analog signal to be transmitted is first converted into digital signal pulses by using an encoder circuit. The digital signal pulses then drive the optical source to modulate light waves.

Optical Fiber:

The modulated light waves are transmitted from one place to another by using an optical fibre. The optical fibres work on the principle of total internal reflection. The optical signal is made incident on the core of fibre with such an angle that every interaction with cladding the angle of incidence is greater than the critical angle for the interface and thus the signal travels through the fibre to the receiver end.

Optical Signal Detector:

The optical signal detector converts light signals into electrical signals by using a photocell or a photodiode. A decoder circuit then converts the digital signal back into its analog form which is then processed.

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Radio Communication uses ground wave, skywave, and space wave propagation. Ground wave and space wave communication are restricted to small distances, whereas skywave propagation uses frequencies ranging from 3 MHz to 30 MHz. Very high frequency (> 30 MHz) and microwave communication over long distances use a Satellite. Satellite communication is useful in sending a large amount of information in a small-time over a large distance.

The Principle of satellite communication:

A communication satellite carrying microwave transmitting and receiving equipment called radio transponders is placed in the geostationary orbit of the earth. A beam of modulated microwaves carrying the Signal is directed towards the satellite. This is known as uplinking. The satellite receives the signal; amplifies and redirects it after re-modulation to a receiving station on earth by using a different carrier wave. This is known as downlink. To avoid confusion the uplink and downlink frequencies are kept different. Frequency modulation is used for both up channel and down channel transmission. Frequency modulation is preferred because it offers good immunity from interference and it requires less power in the transmitter of the satellite. Satellites are generally powered by solar batteries.

Geostationary Satellite:

A communication satellite or geostationary satellite is an artificial satellite which revolves in a circular orbit around the earth in the equatorial plane such that, a) its direction of motion is the same as the direction of rotation of the ‘earth about its axis. and b) its period is the same as the period of rotation of the earth, i.e. 24 hours.

When observed from the earth’s surface, this satellite appears stationary. Therefore, it is called a geostationary satellite. As its motion is synchronous with the rotational motion of the earth, it is called a geosynchronous satellite. The height of the communication satellite above the surface of the earth is about 36,000 km. The angle made by the orbit of geostationary orbit with the equatorial plane is 0°.

Notes:

• The orbit of the geosynchronous satellite is called geosynchronous orbit.
• It lies in an equatorial plane. i.e. the angle between geosynchronous orbit and the equatorial plane is 0°.
• The radius of the geosynchronous orbit is about 42400 km.
• The satellite parking strip is an area over the equator is becoming congested with several hundreds of communication, weather, military, and transmission satellites.

Uses of Communication Satellites:

• The communication satellites are used for sending microwave and TV signals from one place to another.
• The communication satellite is used for telephony.
• The communication satellites are used for weather forecasting.
• The communication satellites are used for detecting water resource -locations and areas rich in ores.
• The communication satellites are used for spying In enemy countries i.e. It can be used for military purposes

Polar or Sun-synchronous Satellite:

A polar satellite is a low altitude satellite orbit around the earth in north-south orbit passing over the north pole and south pole. The orbit of the polar satellite is called polar orbit. The polar orbit makes an angle of inclination of 90° with the equatorial plane. Polar satellites cross the equatorial plane at the same time daily The height of the polar satellite above the earth is about 500-1000 km.

To understand the Earth’s atmosphere and changes in it, the whole planet must be scanned periodically and most effectively. To this polar satellites are used. The information gathered from polar satellites is extremely useful for remote sensing, meteorology as well as for environmental studies of the earth. These satellites are not used for communication.

Advantages of Polar Satellite:

Geostationary satellites are fixed at one position w.r.t. the earth at height 36000 km above the Earth. Its long-range helps meteorologists to understand and analyze the weather. But to understand Earth’s atmosphere and changes in the atmosphere, the whole planet must be scanned periodically and most effectively. To this polar satellites are used.

Since its time period is about 100 minutes it crosses any altitude many times a day and its height h above the earth is about 500-800 km, a camera fixed on it can view only small strips of the earth in one orbit. Adjacent strips are viewed in the next orbit so that in effect the whole earth can be viewed strip by strip during the entire day. From the path shown in the figure, we can see that it covers almost all geographical areas.

Satellite in Highly Elliptical Orbit:

These satellites are used for communication in high latitude regions. The preferred inclination of the orbit with the equatorial plane is 63°.

Global Communication:

A single communication satellite covers about one-third of the earth’s surface. Therefore in order to achieve communication link over the entire globe, we need a minimum of three communication satellites which are 120° apart. Microwave signals are transmitted from one satellite to another with each satellite covering one-third of the globe. Thus, interlinking between these satellites covers the entire earth’s surface.

Satellite Communication is Costly:

The satellite communication is costly due to the following reasons

• The life of the satellite depends on its fuel capacity. Hence the life of the satellite is limited. It is required to be replaced after some time.
• The cost of making satellite and launching is more.
• A precise control system is required for monitoring satellite.

Note:

Satellite communication is costly but it costs very less compared to laying of the line network.

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An antenna or aerial is a system of elevated conductors which couples the transmitter or receiver to the communication channel. Thus it is required at both ends i.e. transmitter end and receiver end. The same antenna can be used for transmitting and receiving functions.

Expression for Coverage Area of Transmission Antenna:

Let us consider a TV transmission antenna ST of height ‘h’ situated at point S on the surface of the earth. Let O be the centre of the earth and R_e be the radius of the earth. Let P be the point on the surface of the earth at a distance of ‘d’ from S beyond which the signal emitted from transmitter T cannot be received. TP is tangent to the earth’s surface. The height of the tower is negligible compared to range. Hence SP = PT = d

Δ OPT is right angled triangle. By Pythagoras theorem

OT² = OP² + PT²

∴ (R_e + h)² = R_e² + d²

∴ R_e² +2 R_eh + h² = R_e² + d²

∴ d² = 2 R_eh + h²

Now h is small compared to the radius of the earth. Hence h² can be neglected.

∴ d² = 2 R_eh

Using the formula A = πd², the coverage area is calculated

Using the formula Population covered = A x population density of that area, the viewership is calculated.

Numerical Problems:

Example – 1:

A TV tower has a height of 100 m. What is the maximum distance up to which the TV transmission can be received? The radius of the earth is 6400 km. If the population density of the region is 500 per square kilometre, find the population reach of the transmission.

Given: Height of tower = h = 100 m, Radius of earth = R = 6400 km = 6.4 x 10⁶ m, Population density = 500 per square kilometre

To Find: Maximum range = d =? Population reach =?

Solution:

D = 35.77 km

Area of reach A = pd² = 3.142 x (35.77)² = 4020 square kilometer

Population reach = Area of reach x population density

Population reach = 4020 x 500 = 2.01 x 10⁶ or 2.01 million

Ans: Coverage range = 35.77 km, Population reach = 2.01 million

Example – 2:

A TV tower has a height of 160 m. What is the maximum distance up to which the TV transmission can be received? The radius of the earth is 6400 km. If the population density of the region is 1200 per square kilometre, find the population reach of the transmission.

Given: Height of tower = h = 160 m, Radius of earth = R = 6400 km = 6.4 x 10⁶ m, Population density = 500 per square kilometre

To Find: Maximum range = d =? Population reach =?

Solution:

D = 45.25 km

Area of reach A = pd² = 3.142 x (45.25)² = 6430 square kilometer

Population reach = Area of reach x population density

Population reach = 6430 x 1200 = 7.72 x 10⁶ or 7.72 million

Ans: Coverage range = 45.25 km, Population reach = 7.72 million

Example – 3:

A TV tower has a height of 160 m. What should be the increase in the height of the tower so that the coverage range is doubled? Also, find percentage increase in the height of the tower.

Given: Height of tower = h₁ = 160 m, d₂ = 2d₁.

To Find: Increase in height of tower = h₂ – h₁ =?, % increase in height =?

Solution:

h₂ = 4 x 160 = 640 m

The increase in height of tower = 640 m – 160 m = 480 m

Ans: The increase in height of the tower is 480 m and % increase is 300 %

Example – 4:

If a height of a transmitting tower is increased by 21%. By what percentage the range of the tower is affected.

Given:  % change in height of tower = 21%

To Find: Percentage change in the range of tower =?

Solution:

% increase in the height of the tower

h₂ = h₁ + 21% h₁ = h₁ + 0.21 h₁ = 1.21 h₁

Ans: Percentage increase in coverage range is 10%

Example – 5:

A transmitting antenna at the top of the tower has a height of 50 m and that on receiving antenna is 32 m. What is the maximum possible distance between them for satisfactory communication in the line of sight mode? The radius of the earth is 6400 km.

Given: Height of transmitting tower = h_T  50 m = 0.050 km, Height of receiving tower = h_R  32 m = 0.032 km, Radius of earth = R = 6400 km

To Find: Maximum distance between the tower  =?

Solution:

Ans: Maximum distance between towers is 45.53 km

Example – 6:

A fax message is to be sent from Delhi to Washington using a geostationary satellite, If the height of geostationary height above the surface of the earth is 36000 km, find the time delay between the dispatch and being received. The radius of the Earth is 6400 km.

Solution:

Given: Height of satellite above the surface of the earth = h = 36000 km, Radius of earth = R = 6400 km

To Find: time delay between the dispatch and being received =?

Assuming the maximum range = distance between the two cities

The motion of the electromagnetic wave is a uniform motion

Thus time delay = d/c = 21500 x 10³ / 3 x 10⁸ =7.17 x 10^-2 s

Ans: Thus the time delay between the dispatch and being received is 7.17 x 10^-2 s

Example – 7:

A radar has a power of 1kW is operating at a frequency 10 GHz is located at a mountaintop of 500 m. Find the maximum distance up to which it can detect an object located on the surface of the earth. The radius of the earth is 6400 km.

Solution:

Given: height of the radar above the surface = h = 500 m = 0.5 km, Radius of earth = R = 6400 km

To Find: Maximum distance of reach = d = ?

The height h is negligible w.r.t. the radius of the earth

Ans: The maximum distance up to which it can detect an object is 80 km

Maximum Line of Sight Distance:

At the frequency of 40 MHz, the communication is limited to line of sight paths. At these frequencies the size of the antenna is relatively smaller than the radius of the earth, the curvature of the earth blocks the direct transmission of the wave from transmitter t the receiver. Let C be such a point. Let T be the position of transmitting antenna and R be the position of the receiving antenna. Let hT be the height of the transmitting antenna and h_R be the height of receiving antenna. Let d_T be the maximum range of transmitting antenna and d_R be the maximum range of receiving antenna. Now AB is tangent to the earth’s surface at C. AB is the maximum line of sight distance

AB  =  d_T  +  d_RT

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Science > Physics > Communication > Coverage Area of Antenna

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A communication channel is a link connecting a transmitter and a receiver. It is the physical medium which carries the signal from the transmitter to the receiver ideally without any noise or distortion. The atmosphere, optical fibres, parallel wires, coaxial cables, etc. are used as communication channels. In this article, we shall study communication wave propagation in the system.

There are two types of communication a) space communication and b) line communication.

• Space Communication: In this method, the signal is freely transmitted in space using transmitter antenna and it is received by intercepting the signal with the help of a receiver antenna. This is a non-directional form of communication.
• Line Communication: In this method, a signal is guided along conducting a physical path like cables or optical fibre (a line) to the receiver. This is directed communication.

Earth’s Atmosphere:

Earth is surrounded by an envelope of gases called the atmosphere. It extends to about 400 km above the surface of the earth. Its composition is not the same everywhere. The atmosphere plays an important role in the transmission of electromagnetic waves. The earth’s atmosphere is broadly divided into four different layers.

Troposphere:

The layer of the atmosphere extending up to a height of 12 km from the surface of the earth is called the troposphere. This layer mostly contains water vapour which leads to the formation of clouds. The local weather changes in the earth’s atmosphere occur in this layer. The density of air at the surface of the earth is about 1.29 kg/m³ it decreases gradually and at the top of the troposphere, it is about 0.129 kg/m³. The temperature falls from about 15 °C to – 50 °C

Stratosphere:

The region of the earth’s atmosphere lying between 12 km to 50 km is called the stratosphere. The ozone layer is part of the stratosphere extending from 15 km to 30 km; which absorbs the harmful ultraviolet radiations from the sun. Ultraviolet rays are very harmful to living cells. The density of air at the bottom of stratosphere 1.29 kg/m³, it decreases gradually and at the top of the stratosphere, it is about 1.29 x 10^-3 kg/m³. The temperature increases from – 50 °C to 10 °C.

Mesosphere:

The region of the earth’s atmosphere lying between 50 km to 80 km is called the mesosphere. The density of air at the bottom of mesosphere 1.29 x 10^-3 kg/m³, it decreases gradually and at the top of the mesosphere, it is about 1.29 x 10^-5 kg/m³. The temperature falls from 10 °C to – 90 °C.

Ionosphere:

The outermost layer of the earth’s atmosphere is called the ionosphere extending from 80 km to 400 km. The Ionosphere contains charged particles and plays an important role in space communication. The density of air at the bottom of ionosphere 1.29 x 10^-5 kg/m³, it decreases gradually and at the top of the ionosphere, it is about 1.29 x 10^-10 kg/m³. The temperature rises from -90 °C to 400 °C to a height of 110 km from the surface of the earth. This region is called thermosphere.

The ultraviolet rays and x-rays coming from sun ionize the gases in the upper layer to produce electrons and positive ions. The ionosphere is not uniform due to the varying composition of the atmosphere at different heights. The free electron density is found to be very high in a layer between 100 km to 125 km from the surface of the earth. This layer is called E-layer or Kennelly Heaviside layer. Instead of attenuating radio communications signals this layer chiefly refracts them, often to a degree where they are returned to earth. As such they appear to have been reflected by this layer.

Beyond E-layer up to 250 km the electron density decreases considerably. Again from 250 km to 350 km, there is high electron density. This region is known as the Appleton layer or F-layer. This layer is useful for long-distance transmission of high-frequency waves.

Importance of Radio Waves in Communication:

Radio waves are electromagnetic waves of wavelength 10^-3m and higher. Their frequency range is from a few kHz to nearly a few hundred MHz. The propagation of radio waves through the atmosphere is relevant in all modern forms of communication like radio, television, microwaves etc. The different bands of radio wave frequencies are as follows

Space Communication:

The process of sending, receiving and processing of information through space without any special communication channel is called space communication. Waves travel in straight lines until the earth and the atmosphere alter their path. HF waves travel in straight (due to a change in density of atmosphere) line except for refraction

The information to be transmitted is superimposed on a high-frequency electromagnetic wave called the carrier wave. After superimposition, the resultant wave called the modulated wave is transmitted from one place to another by using an antenna. The electromagnetic waves emitted by a transmitting antenna can reach the receiver antenna by the following three modes:

A Ground Wave or Surface Wave Propagation:

The electromagnetic waves emitted from the transmitting antenna propagate along the surface of the earth are called ground waves or surface waves and this type of propagation is called ground wave propagation or surface wave propagation. This type of propagation can take place when the transmitting and receiving antenna are close to the surface of the earth.

When a ground wave propagates over the surface of the earth, eddy currents are induced in the surface of the earth which causes attenuation of these waves. Moreover, as they travel over the earth’s surface they bend along the curvature of the earth which results in energy losses. Thus, ground wave propagation is restricted to small distances. The maximum range of coverage depends upon the transmitted power and frequency because the high-frequency waves suffer more absorption of energy in the earth’s atmosphere. Hence it cannot be used for high-frequency TV and frequency modulated (FM) broadcasts.

The ground wave transmission becomes weaker as the frequency of the electromagnetic wave increases hence this mode of transmission is restricted to frequencies below 1500 kHz. Hence it is used in amplitude modulated (AM) medium wave and long-wave radio broadcast and radio navigational support. All broadcast below 1500 kHz radio signals received in daytime propagate by means of the surface wave.

A Skywave Propagation:

The electromagnetic waves emitted by transmitting antenna are received after being reflected from the ionosphere are called sky waves and this type of propagation is called Sky wave Propagation.

The skywave propagation occurs due to the total internal reflection of the electromagnetic waves by the ionosphere. The ionosphere consists of free positive and negative ions produced due to the ionization of atoms and molecules present in the atmosphere. The charged density of the ionosphere increases with height which results in the decrease in its refractive index.

The electromagnets waves having frequencies less than 2 MHz are absorbed by the ionosphere whereas those having frequencies greater than 30 MHz pass through it. Hence the waves with a frequency range from 2 MHz to 30 MHz can be propagated by this method.

Waves in HF range and around are beamed at the sky and reflected by the ionosphere layers of the atmosphere and are received as sky waves. For further transmission, they are reflected by ground towards the sky again. Due to these repetitive reflections, the wave travels long distances. For skywave propagation gaseous medium is required. Hence this type of communication is not possible ins space or on the moon due to the absence of the atmosphere.

As the angle of emission of waves from the transmitter with respect to earth’s surface increase, a stage will be reached when the ionosphere layers do not reflect the waves towards the earth but transmit through it.

The maximum frequency at which total internal reflection from ionosphere takes place is called critical frequency. Mathematically critical frequency is given by

Where f_c = critical frequency and N_max = Maximum electron density of the ionosphere

Space Wave Propagation:

The electromagnetic waves emitted by transmitter antenna travel directly from the transmitting antenna to the receiving antenna are called space waves and this type of propagation is called space wave propagation. It is used for line of sight (LOS) communication and satellite communication.

High-frequency electromagnetic waves cannot be transmitted as ground waves due to high energy losses. Moreover, these waves are absorbed by the ionosphere hence they cannot be transmitted via skywave propagation. Therefore, such high-frequency electromagnetic waves are directly transmitted throng Earth atmosphere using a transmitting antenna As these waves travel in a straight line, the receiving antenna must be in the line of sight of the transmitting antenna.

This method is used for transmission of waves in very high frequency (VHF) band, the ultra-high-frequency band (UHF), microwaves, etc. The TV signals having frequency band 54-806 MHz can propagate neither via ground waves (due to high absorption in the atmosphere) nor via sky waves (due to non-reflection of the ionosphere). Hence TV signals can only be propagated through space wave only.

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Science > Physics > Communication > Communication Channel: Atmosphere

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In this article, we are going to study the terms used in the communication system and the concept of the bandwidth of a signal.

Transducer:

Any device that converts one form of energy into another can be termed as a transducer.

Example: A microphone is a transducer which converts sound energy into electrical energy (electrical signals). A loudspeaker is a transducer which converts electrical energy (electrical signals) into sound energy.

An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature,
etc) into corresponding variations in the electrical signal at its output.

Signal:

Information converted into electrical form and suitable for transmission is called a signal. Signals can be either analog or digital. A message is defined as a physical manifestation of information as produced by the source. The terms signal and message are used interchangeably.

In an electronic communication system, a signal means a time-varying electrical signal obtained from the original signal using a transducer. These signals have their own nature, frequency, and amplitude. There are two types of electrical signals

Analog Signals: 

An analog signals are continuous variations of voltage or current. They are essentially single-valued functions of time. The sine wave is a fundamental analog signal. All other analog signals can be fully understood in terms of their sine wave components. Sound and picture signals in TV are analog in nature.

The general equation of such signal is Q = Q_o sin ωt here ω = 2π/T

Digital Signals:

Digital signals are those which can take only discrete stepwise values. A binary system that is extensively used in digital electronics employs just two levels of a signal. ‘0’ corresponds to a low level and ‘1’ corresponds to a high level of voltage/ current. Generally, such signals are in the form of pulses. There are several coding schemes used for digital communication. The output of a computer, transmission of documents through the internet is done using digital signals. The digits 0 and 1 are called bits. A group of bits is called a byte or binary word. There are several coding schemes used for digital communication. They employ suitable combinations of number systems such as the binary coded decimal (BCD). American Standard Code for Information Interchange (ASCII) is a universally popular digital code to represent numbers, letters, and certain characters.

Noise:

Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system. The source generating the noise may be located inside or outside the system. Lightning, turning on or off electrical appliances may cause noise.

Transmitter:

A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception. The important component of a transmitter in radio communication are microphone, audio amplifiers, oscillator, modulator, and antenna.

Receiver:

A receiver extracts the desired message signals from the received signals at the channel output. The important components of a receiver in radio communication are an antenna, demodulator, amplifier, loudspeaker.

Communication Channel:

It is a link connecting a transmitter and a receiver. It is the physical medium which carries the signal from transmitter to the receiver ideally without any noise or distortion. The atmosphere, optical fibres, parallel wires, coaxial cables, etc. are used as communication channels. Communication channels are divided into two types.

• Guided communication channel: This is generally used in point to point or in-line communication. Parallel wires, coaxial cables are used for this type of communication.
• Non-guided communication channel: This channel is used for space and satellite communication. Free space is an example of a non-guided communication channel.

Attenuation:

The loss of strength of a signal while propagating through the communication channel is known as attenuation.

Amplification:

It is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier. Amplification is necessary to compensate for the attenuation of the signal in communication systems. The energy needed for additional signal strength is obtained from a DC power source. Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength.

Range:

It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.

Repeater:

A repeater is a combination of a receiver and a transmitter. A repeater picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency. Thus repeaters serve as amplifiers. Repeaters are used to extend the range of a communications system keeping original form intact. A communication satellite is essentially a repeater station in space. Mobile towers are the repeaters.

Modulation:

The original low-frequency message/ information signal cannot be transmitted to long distances. Therefore, at the transmitter, the information contained in the low-frequency message signal is superimposed on a high-frequency wave, which acts as a carrier of the information. This process is known as modulation. There are several types of modulation, abbreviated as amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM).

Demodulation:

The process of retrieval of information from the carrier wave at the receiver is termed demodulation. This is the reverse process of modulation. Thus at the end of the process low-frequency message is retrieved again.

Antenna:

An antenna or aerial is a system of elevated conductors which couples the transmitter or receiver to the communication channel. Thus it is required at both ends i.e. transmitter end and receiver end. The same antenna can be used for transmitting and receiving functions.

Bandwidth:

Bandwidth refers to the frequency range over which equipment operates or the portion of the spectrum occupied by the signal.

A Bandwidth of a Signal:

In a communication system, the message signal can be voice, music, and picture or computer data. Each of these signals has different ranges of frequencies. The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for the communication process.

The important thing to be noted that irrespective of where the band is located in the frequency spectrum, it will carry the same amount of information. For example, Signal will carry the same amount of information in a frequency range from 1 MHz to 2 MHz (bandwidth 1 MHz) or the frequency range from 5 MHz to 6 MHz (Bandwidth 1 MHz)

Bandwidth of Transmission Medium:

Similar to message signals, different types of transmission media offer different bandwidths. The commonly used transmission media are wire, free space, and fibre optic cable. Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz. Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz. This range of frequencies is further subdivided and allocated for various services. Optical communication using fibres is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fibre can offer a transmission bandwidth in excess of 100 GHz.

Spectrum allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.

Frequency allocation to different services is given below.

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Science > Physics > Communication > Terminology of Communication System

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Communication is the process of exchange of information between two entities. It involves sending, receiving and processing information. Thus there is a transmission of information from a source at one place to a receiver located at another place. The communication system is a device or setup used in the transmission of the information from one place to another.

Communication System:

Elements of Communications System:

Primarily communication system consists of three main parts: i) a transmitter ii) a communication channel and iii) a receiver. The transmitter is located at one place, the receiver is located at some other place (far or near) separate from the transmitter, while the channel is the physical medium that connects them. Depending upon the type of communications system, a channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless.

Example: When a person talks with another person directly, then the speaker is the transmitter conveying the information in the form of sound waves through a communication channel, the intervening air and listener is the receiver. If the distance between the speaker and the listener is large sound waves cannot reach from the speaker to the listener directly and communication is not possible. Now to make communication possible we have to change the communication channel and method.

General Working of Communication System:

The devices such as microphones, photodetectors and piezoelectric sensors which convert non-electrical signals into electrical signals are called transducers and vice versa. Microphones convert the sound signals into electrical signals. Photodetectors convert light signals into electrical signals, and piezoelectric sensors convert pressure variations in electrical signals.

The purpose of the transmitter is to convert the message signal produced by the source of information into a form suitable for transmission (generally electric) through the channel. If the output of the information source is a non-electrical signal like a voice signal, a microphone (transducer) is used.

Next step is the modulation. The information signals in the form of electrical signals cannot be transmitted over large distances, so they are superimposed on a high-frequency wave called the carrier wave. This superimposition of the electrical signal on the high-frequency wave (a carrier wave) is called modulation. Thus modulator converts it to transmittable form.

Then the signal is amplified and fed as an input to the transmitter. Which is projected through communication channels using the antenna.

Then this signal propagates through the communication channel. Wires, cables, optical fibres or earth’s atmosphere are used as a communication channel. When a transmitted signal propagates along the channel there is a possibility that the signal may get distorted due to channel imperfection, noise, etc.  Thus the receiver receives a corrupted or low powered version of the transmitted signal.

The receiver has the task of operating on the received signal. Antenna retrieves the signal from a communication channel. At the receiver end, the signal is converted back into a recognizable form which can be interpreted by the receiver. The demodulator separates the low-frequency audio signal from the modulated signal. The audio amplifier boosts up the signal to compensate for the loss of power due to attenuation. the amplified signal is fed to a transducer (loudspeaker) which converts the electrical signals into audio signals.

Types of Communication System:

• Point-to-Point Communication: In this mode, communication takes place over a link between a single transmitter and a receiver. Example: Telephony.
• Broadcast Communication: In this mode, there are a large number of receivers corresponding to a single transmitter. Example: radio and television broadcast

Undesirable Effects on Signal During Transmission:

Attenuation:

The loss of strength of a signal while propagating through the communication channel is known as attenuation. Though there are losses at the transmitter end and at the receiver end still it is standard practice to assume transmitter and receiver as ideal. The losses due to attenuation can be compensated with amplification. Hence it is not a serious problem.

Distortion:

It is a waveform perturbation caused by the imperfect response of the system to the desired signal itself. Distortion disappears when the signal causing distortion is turned off or disappears. If the channel has a linear but distorting response, then distortion may be corrected or at least reduced using filter devices called equalizers.

Interference:

Interference is contamination due to extraneous signals from other transmitters, machinery, power lines, switching circuits, and human sources. It is mainly at the receiver end due to intercepting several signals at the same time by the antenna at the receiver end. Proper filtering circuits remove the interference by making the interfering signals to occupy different frequency band than the desired signal.

Noise:

Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system. These are random and unpredictable electrical signals produced by natural processes. The noise may be internal or external. When such random variations in the electrical signal are superimposed on the information-bearing symbol it gets partially or fully corrupted. Using filters noise can be reduced but cannot be eliminated completely.

Antenna:

An antenna or aerial is a system of elevated conductors which couples the transmitter or receiver to the communication channel. Thus it is required at both ends i.e. transmitter end and receiver end. At the transmitter end, the antenna converts electrical signals into electromagnetic waves while at the receiver end the antenna converts electromagnetic waves into electrical signals.

A vertically held transmitting antenna produces vertically polarized electromagnetic waves. The design of an antenna mainly depends on the frequency of the carrier wave and the directivity of the beam.

Types of Antenna:

Hertz Antenna: It is a straight conductor antenna whose length is equal to the half of the wavelength of the radio signals to be transmitted or received.

Marconi Antenna: 

It is a straight conductor antenna whose length is equal to the quarter of the wavelength of the radio signals to be transmitted or received. The lower end of the Marconi antenna is grounded.

Doublet or Bipolar Antenna: 

It is a bilobed (Two rounded structures) conductor antenna whose length is equal to the half of the wavelength of the radio signals to be transmitted or received. The Number of lobes can be increased to obtain longer antenna.

Omnidirectional Dipole Antenna:

It is used for transmission of radio waves

Dish Type Antenna:

Dish antenna is a directional antenna because it can receive only those waves which are directed towards it and send waves in a particular direction only. The main component of this antenna is dipole or horn feed. In the receiver form, the dish which is in a shape of parabolic reflector collects waves directed towards it and focusses them on the horn feed. In horn feed the electromagnetic waves are converted into electrical signals and are then fed to the amplifier using cables. In the transmission mode, the electrical signals are converted into electromagnetic waves in horn feed and projected on the dish reflector. From reflector, waves are transmitted in the form of a parallel beam.

Size of Antenna or Aerial: 

An antenna or aerial is a system of elevated conductors which couples the transmitter or receiver to the communication channel. Thus it is required at both ends i.e. transmitter end and receiver end. At a transmitter end, it converts electrical signals into electromagnetic waves, while at a receiver end it converts electromagnetic waves into electrical signals.

For the efficient transmission and reception, the length of the antenna or aerial is such that it acts as a resonant circuit at the frequency of operation. If λ is a wavelength of the RF signal employed, then the length of the antenna is generally λ/4. As the wavelength is inversely proportional to the wavelength, the length of the antenna decreases with the increase in the carrier wave frequency.

Example- 1:

Calculate the required length of dipole antennas for following carrier waves

40 MHz

c = νλ

∴ λ = c/ν = 3 x 10⁸ / 40 x 10⁶ = 7.5 m

Length of antenna = λ / 4 = 7.5/4 = 1.875 m

400 MHz

c = νλ

∴ λ = c/ν = 3 x 10⁸ / 400 x 10⁶ = 0.75 m

Length of antenna = λ / 4  = 0.75/4 = 0.1875 m

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