Antennas And Wave Propagation

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Code No: R05310403
Set No. 1
III B.Tech I Semester Regular Examinations, November 2007
ANTENNAS AND WAVE PROPAGATION
( Common to Electronics & Communication Engineering and Electronics &
Telematics)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks

1. (a) Draw the Dual characteristics of an antenna.
(b) Explain the Radiation from Two wire. [8+8]

2. (a) Derive the expressions for electric eld in case of short current element and
hence obtain the conditions for the eld to be in Franhofer region.
(b) Find the distance from a radiating element with 60Hz current such that radi-
ation and induction elds are equal. [10+6]

3. (a) A linear broadside array consist of 4 identical equal in phase point source with
/3 spacing. Calculate and plot the eld pattern. Also nd the directivity
and beam width.
(b) What is optimum spacing used in parasitic array? Why. [12+4]

4. (a) Write short notes on “Helical Antenna”.
(b) Derive an expression for radiated electric eld strength of a traveling wave
radiation of length ‘l’. [8+8]

5. (a) Sketch the current distribution of folded dipole and nd out input impedance
when two legs have unequal diameters.
(b) A Parabolic dish provides a power gain of 50dB at 10GHz, with 70% e ciency.
Find out
i. HPBW.
ii. BWFN, diameter. [8+8]

6. (a) Describe the metho d of measuring the gain and radiation pattern of an an-
tenna.
(b) A standard gain horn antenna with a power gain of 12.5, is used to measure the
gain of a large directional antenna by comparison metho d. The test antenna
is connected to the receiver and an attenuator adjusted to 23dB in order to
have the same receiver output. Find out the gain of the large antenna. [8+8]

7. (a) Describe the following:
i. Space wave propagation.
ii. Duct propagation.

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Code No: R05310403
Set No. 1
(b) VHF communication is to be established with a 50watt transmitter at 100MHz.
Calculate the LOS distance if the heights of transmitting and receiving an-
tennas are respectively 50m and 10m. Assuming the capture area of the
transmitting antenna is 25sqmts, calculate the eld strength at the receiving
neglecting ground re ected wave. [8+8]

8. (a) Discuss the characteristics of F1 and F2 layers.
(b) Discuss the reasons for reduction of eld strength in sky wave propagation.
[8+8]

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Code No: R05310403
Set No. 2
Telematics)

1. (a) Distinguish between Directive Gain and Power Gain.
(b) An antenna has a radiation resistance of 73 ohms and a lossy resistance of 7
ohms. If the power gain is 20, calculate the directivity and the e ciency of
the antenna. [8+8]

2. (a) Derive an expression for radiance resistance of current element starting from
the expression for radiation elds.
(b) Prove that the impedance of an isolated antenna when used for receiving is
same as when used for transmitting. [8+8]

3. (a) In order to scan the beam of a linear array to 300 o broadside. Calculate the
inner element phase shift required if the elements are spaced at 3 cms and the
frequency is 64 KHz.
(b) What are linear arrays. Compare Broadside array and End re array. [8+8]

4. (a) Derive an expression for electric eld of a Non Resonant antenna of length ‘l’
carrying current.
(b) Sketch and explain the constructional features of a Helical Antenna. [10+6]

5. (a) What is a Parasitic Element? Describe the use of di erent types of parasites
in TV receiving antennas.
(b) Derive an expression for aperture eld distribution of a parabolidal re ector.
[8+8]

tenna.

7. (a) De ne MUF and Critical frequency. Derive the expressions for the same.
What is Secant law?
(b) Describe the Ground wave propagation. [8+8]

8. (a) Derive the eld strength of tropospheric wave.

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Code No: R05310403
Set No. 2
(b) Give an account of e ect of earths imperfections and roughness. [8+8]

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Code No: R05310403
Set No. 3
Telematics)

1. (a) De ne the terms:
i. Beam Width
ii. Side Lobe Level
iii. Polarization
iv. E ective Aperture Area.
(b) What is the e ective length of an antenna. Determine the e ective length of
a half wave dipole antenna. [8+8]

2. (a) Show that the radiation resistance of a small loop is equal to 320 4(A/ 2)
ohms where A is loop area.
(b) What is Folded Dipole ? Find its Radiation Resistance. [8+8]

3. (a) Explain the pro cedure for measuring the radiation pattern of a half wave
dipole.
(b) What is the requirement for tapering of arrays.
(c) State the applications of arrays. [8+4+4]

4. (a) State the advantages and disadvantages of Rhombic Antenna.
(b) Draw the radiation pattern for traveling wave antenna for L= /2, ,2 ,4 and
8. [8+8]

5. (a) What is a Parasitic Element? Describe the use of di erent types of parasites
in TV receiving antennas.
(b) Derive an expression for aperture eld distribution of a parabolidal re ector.
[8+8]

tenna.

7. It is de ned to establish short wave communication between two points on earths
surface(assumed at) separated by 1200km. Calculate MUF and angle of take o

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Code No: R05310403
Set No. 3
of the transmitted wave from the following: Highest signal frequency returned to
earth after vertically upward propagation=7.2MHz. Virtual height of the ionized
layer=200kms. Deduce the working formula. [16]

8. (a) Distinguish between Radio and Optical horizons. Give the reasons.
(b) Write short notes on M-Curves. [8+8]

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Code No: R05310403
Set No. 4
Telematics)

1. (a) De ne antenna beam width and directivity and obtain the relation between
them.
(b) Calculate the electric eld (Erms) due to an isotropic radiator radiating 3KW
power at a distance of 2 Km from it. [8+8]

2. (a) State the following antenna theorems and bring out their importance in an-
tenna measurements :
i. Reciprocity theorem.
ii. Maximum power transfer theorem.
(b) A half wave transmitting antenna radiates 10KW of power at 100MHz.If the
heights of transmitting and receiving antennas are 100m and 9m, calculate the
power received at a distance of 10kms from the transmitting antenna. [8+8]

3. (a) Calculate
i. HPBW
ii. Solid Angle if a linear array having 10 isotropic point source with /2
spacing and phase di erence =900.
(b) Write short notes on Hansen-Wood yard end re array. [8+8]

4. (a) Distinguish between Traveling wave and Standing wave antennas.
(b) Compare Resonant and Non Resonant antennas.
(c) Explain the working of Rhombic antenna. [4+4+8]

5. (a) Compute the gain, principle beam width and HPBW of a 10m diameter par-
abolic dish with a half wave length dipole feed in focus at 10GHz.
(b) Explain the Cassegrain mechanism in transmission mode. List out the advan-
tages and disadvantages of Cassegrain feed. [8+8]

6. (a) What is an electromagnetic horn antenna? What are its applications? The
length of an E-plane sectoral horn is 15cms. Design the horn dimensions such
that it is optimum at 10GHz.
(b) Calculate the minimum distance required to measure the eld pattern of an
antenna of diameter 2m at a frequency of 3GHz. Derive the necessary equa-
tion. [8+8]

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Code No: R05310403
Set No. 4
7. (a) Show that Ionosphere act as a variable refractive index medium.
(b) A radio link has to be established between two earth station at a distance of
25000kms. If the height of ionosphere is 200kms and its critical frequency is
5MHz. Calculate the MUF for the given path. Also calculate the electron
density in the ionosphere layer. [8+8]

8. (a) Describe the troposphere and explain how tropospheric ducts can be used for
microwave propagation.
(b) Derive the expression for space wave electric eld produced by an antenna at
a distance point, assuming a at earth. [8+8]

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Antennas And Wave Propagation

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