Communication
(Important formulae and Concepts)
Length of dipole antena=λ/4
=c/4ν,where symbol has their usual meaning.
Modulation index ma=Em/Ec
= (Emax-Emin) /(Emax+Emin)
Maximum coverage range of antenna d=√2hR
Maximum Los distanced=√2RehT+√2RehR
Very short answer questions
1. What are the thrtee basic units of a communication system?
Ans. the thrtee basic units of a communication system are
(i) Transmitter (ii) Communication Channel nad (iii) rerceiver
2.
Identify the parts X and Y in the following block diagram of a
generalised communication system?
X
Transmitter
Y
Receiver
Ans. Part X is message signal or information source.
Party Y is a transmissions channel.
3.What type ofmodulation is requiredfor televiosion broad cast?
Ans. Frequency modulation
4. why is frequency modulation is prefered over amplitude modulation?
Ans. In F M the noise is minimised.
5. Why short wave s
band are used
for long distance broad cat?
Ans.Short waves are not absorved by earth atmosphere.
6. Name te type of radio wave propagation involved when T V signal,
broad cost b y a tallantena are intercepted directly by the receiver
antena.
Ans. Space wave propagation.
7.Give any one difference between Fax and E mail systems of
communication,
Ans. Fax transmission is achieved by radio , telephone or under cable.
E-mail is transmission of electronicmessage between computers via net
work. E-mail can be stored and processed. A fax system does not have
these facilities.
7. What is the purpose of modulating signal in tyransmission?
Ans . A signal is modulated due to
(i)reduce size of antenna
(ii)
reduce
larger
band width (iii)
avoid mixing up of siugnals.
8.
A modulating signal is a square wave as shown in figure.
1
M (t )
involts
1
2 t (in seconds)
c (t) = 2 sin (8t) volts
(i)
Sketch the amplitude modulated waveform.
(ii)
What i the modulatiron index?
 8

4
2 2
The amplitude modulated wave is shown in fig.
Solution. (i) Frequency of carrier
fc 
(ii) Modulation index
ma 
m0
c0
 Em  1
 or
   0.5
 Ec  2
0
1
2
Short Answer Questions
9. Distinguish between point to point and broadcast communication
modes with example.
Ans.Point to point communication: The communicationover alinke
between a single transmitter and a recever is called poit to pont
communication mode Telephone is an example of such a system.
Broadcast Mode: In such a mode large number of receivers are linked
to a single transmitter. Radio is exmaple of such system.
10. What should ber tye length of dipole antena for a cartrier wave of
frequency 600Mhz ?
Ans. Length of dipole antenna =c/4ν
=3×108/4×6×108=0.125m
10. A message has a band width of 5 Mhz.Suggest a possible
communicationcation channelfor its transmission.
Ans, coxial cable.
11.A Transmitting antenna at the top of the tower has height of 36m
and the height of the receiving atenna is 49 m. What is the maximum
distance between them for satis factory communication in LOPS mode?
Ans : Maximum Los distanced=√2RehT+√2RehR rr
= √2×6.4×106(√36+√49 )
= 46.5km
12. A Carrier wave of peak voltage 20 V is used totransmit amessage
signal. What should be the peak voltage of the modulatig signal in order
to have a modulation indexof 80%?
Ans.
Em=ma ×Ec
=0.80 ×20
=16volt
Some Impotant Questions from Communications systems.
1.Differentiate between analog and digital signals.
2.By What percentage will the transmission rangof a T V tower be
affected when the height of the tower is increased by 21 %.
3.Explain Sky wave, space wave and ground wave propagation with
suitable example.
4. What is the type of communication in which signal signal is discrete
andbinary coded?
5. Name the type of communication systems according to mode of the
transmission.
6. Explain the function of r, transducer,Repeater and amplifier in
communication systems.
7. What is amplitude modulation.Show graphiocally. Write its two
limitations and two advantages.
8. Draw block diagram of a simple amplitude modulation,
explainingbriefly , how amplitude is achieved.
9. What does the process of dectection of amplitude modulated mean .
Explain the function of detecterwith suitable block diagram.
10.What would be the modulation index for an amplitude
modulatedwave for whoich
the maximum amplitude is a and
minimum amplitude is b.
11. Draw a plot of variation of amplitude versus w for an amplitude
modulated wave.Define modulation indwex. State its importace for
effective amplitude modulation.
12. Define the term modulation. Explain the b
Need of modulation. Name three different type modulationused for
message signalusinng carrier w ave. explain the meaning of any one of
them.
13What does the tertm LOS communication mean? Name the types of
wave used for communication. Give typical example s with the help of
suitable figureof communication system for all three wave.
14. a T V tower has a height of 500m at agiven place. If radius of earth
is 6400km. what is its coverage range.
15.A T V tower has a height of 71 km . What is the maximum
distance up to which TV transmission can be received? Assume
radius of earth is 6400km.
15.A schematic arrangement for transmitting a message signal 20 Hz to 20 KHz is
given below
Give two drawbacks from which this arrangement suffers. Describe briefly with
the help ofA block diagram the alternative arrangement for the transmission and reception of
the
Message signal.