Communication System Theory for JEE Main 2015 by ednexa.com

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Communication System
Space Communication
There are three main types of space communication.
Ground wave propagation
Ground wave communication is used for low
frequencies (500kHz to 1500kHz). This type of

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communication is used for medium wave radio
transmission, ship communication or radio navigation.
Sky wave propagation
Very long distance
2
0
Ne
n' n 1
m
= -
e w
For very low and high values of frequencies of em
waves, the sky waves are either absorbed or escape
from the ionosphere. Hence, following two terms are
important for sky wave communication.

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Critical Frequency (fc)
It is the maximum value of frequency of the radio
waves, reflected back to the Earth from the ionosphere,
when directed normally to the ionosphere.
It is approximately given as, C maxf g N= where
Nmax is the maximum electron density of the
ionosphere. Depending on the properties of
ionosphere, the critical frequency changes from 5MHz
to 10MHz.

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Space wave propagation
The electromagnetic waves
which travel directly from
transmitting antenna to
receiving antenna without
being influenced by the Earth
are called space waves. In
this propagation, the em waves move in Earth’s
troposphere, within about 15km over the surface of the
Earth. Hence, they are also known as tropospheric
waves.
Their frequency range is in between 30MHz to
300MHz. These waves travel in straight line. Hence,
the transmitting and the receiving antenna must be in

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the line of sight. But, due to the curvature of the Earth,
these waves cannot be received beyond horizon.
Hence, the effective reception
or the range of these waves is
up to line of sight only. Hence,
the communication is also
called line of sight
communication.
The figure shows curved surface of the Earth R1 B R2.
At B there is a transmitting antenna of height h (BA). R
is the radius of the Earth, i.e. distance of R1, B and R2
from centre of the Earth O. C is the midpoint of the line
joining R1 and R2.
Hence, R1 and R2 are at distance d from C. Triangles
OR1A and OCR1 are right angled triangles.

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∴ OA2
= OR1
2
+ AR1
2
But, OA = R + h and
AR1
2
= h
2
+ d
2
∴ (R + h)
2
= R
2
+ h
2
+ d
2
∴ R
2
+ 2hR + h
2
= R
2
+ h
2
+ d
2
∴ 2hR = d
2
=d 2hR
Thus, transmitting antenna is installed at the top of the
mountains to increase height h to increase the range.
For a 100m high antenna, the range is approximately
35km. When these waves are reflected from ground,
there is a phase reversal of 180o. If the direct waves

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and ground reflected waves reach the receiving
antenna in anti phase, they cancel each other.
Satellite Communication
The electromagnetic waves having frequencies beyond
30MHz are very useful in communication because of
their higher band widths. But, these waves cannot
transmitted as ground waves or space waves and also
as sky waves because they escape from the
ionosphere. Hence, satellites are used for
communication by these waves.

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The satellite receives them, amplifies them and sends
them back towards the Earth, which are received by
the receiving antenna.
The process of sending the signal from the Earth to
the satellite is called uplinking and receiving the signals
from the satellite is called downlinking. The uplink and
down link frequencies have atleast 2MHz frequency
difference between them to avoid confusion. The
transmitting and receiving antennae are tuned to the
corresponding frequencies. As the geostationary
satellite doesn’t change its position relative to the
Earth, there is no Doppler’s shift in the downlink
frequencies.

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Global communication
Advantages
1. long distance wide spread communication.
2. A 24 hour communication is possible in remote
and hilly areas with excellent quality
3. As the band width is high a large amount of
information can be send at a faster rate.
4. It is cheaper and maintenance free as compared to
cable communication.
5. It can be used in G.P.S. (Global Positioning
System) to decide position of any object accurately.

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Application of remote sensing
Remote sensing A remote sensing satellite is orbiting
in polar orbit at nearly 1000km from the surface of the
Earth.
Uses
1. In meteorology for weather forecasting, prediction
of storm, snow fall etc.
2. In collection of scientific data such as changes in
Earth’s magnetic field, gravity, ionosphere etc.
3. In geological survey of underground water, oil,
radioactive substances etc.
4. In military operations such as movement of troops,
deployment of tanks etc. and for spying.

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5. Aerial survey of flood, storm, draught affected
areas. Survey of crop yields, crop diseases.
6. For finding fishing zones in sea, to observe
development of forest.
7. In oceanography to study ocean currents,
temperature of the ocean surface.
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