Communication Channels : Atmosphere

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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 transmitter to the receiver ideally without any noise or distortion. The atmosphere, optical fibres, parallel wires, coaxial cables, etc. are used as communication channels.
  • 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 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 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 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 103m 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 diofferent 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 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 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 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 Sky Wave Propagation:

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

Wave Propagation

  • The sky wave 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 fc = critical frequency and Nmax = 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 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.

Calculation of Coverage Area of 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 Re 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 earth’s surface. The height of tower is negligible compared to range. Hence SP = PT = d

Δ OPT is right angled triangle. By Pythagoras theorem

OT2 = OP2 + PT2

∴  (Re + h)2 = Re2 + d2

∴  Re+2 Reh + h2 = Re2 + d2

∴ d2 = 2 Reh + h2

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

∴ d2 = 2 Reh

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.

Example – 2:

  • 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 population density of the region is 500 per square kilometre, find the population reach of the transmission.
  • Solution:
  • Given: Height of tower = h = 100 m, Radius of earth = R = 6400 km = 6.4 x 10m, Population density = 500 per square kilometre
  • To Find: Maximum range = d =? Population reach =?

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 – 3:

  • 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 population density of the region is 1200 per square kilometre, find the population reach of the transmission.
  • Solution:
  • Given: Height of tower = h = 160 m, Radius of earth = R = 6400 km = 6.4 x 10m, Population density = 500 per square kilometre
  • To Find: Maximum range = d =? Population reach =?

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 – 4:

  • A TV tower has a height of 160 m. What should be the increase in the height of tower so that the coverage range is doubled? Also, find percentage increase in the height of the tower.
  • Solution:
  • Given: Height of tower = h1 = 160 m, d2 = 2d1.
  • To Find: Increase in height of tower = h2 – h1 =?, % increase in height =?

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 – 5:

  • If a height of a transmitting tower is increased by 21%,, by what percentage the range of the tower is affected.
  • Solution:
  • Given:  % change in height of tower = 21%
  • To Find: Percentage change in the range of tower =?

% increase in the height of tower

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

Ans: Percentage increase in coverage range is 10%

Example – 6:

  • 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.
  • Solution:
  • Given: Height of transmitting tower = hT  50 m = 0.050 km, Height of receiving tower = hR  32 m = 0.032 km, Radius of earth = R = 6400 km
  • To Find: Maximum distance between the tower  =?

Ans: Maximum distance between towers is 45.53 km

Example – 7:

  • 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 electromagnetic wave is 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

The motion of electromagnetic wave is 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 – 8:

  • 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 point. Let T be the position of transmiting antenna and R be the position of the receiving antenna. Let hT be the height of transmitting antenna and hR be the height of receiving antenna. Let dT be the maximum range of transmitting antenna and dR 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  =  dT  +  dRT

Satellite Communication:

  • Radio Communication uses ground wave, sky wave, and space wave propagation. Ground wave and space wave communication are restricted to small distances, whereas sky wave propagation uses frequencies ranging from 3 MHz to 30 MHz.
  • Very high frequency (> 30 MHz) and microwave communication over long distances use a Satellite. a 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.

Wave Propagation

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 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°.
  • Uses of Communication Satellites:
    1. The communication satellites are used for sending microwave and TV signals from one place to another.
    2. The communication satellite is used for telephony.

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 polar satellite above the earth is about 500-1000 km.
  • To understand 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.

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.
Science > Physics > Communication > You are Here
Physics Chemistry Biology Mathematics

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