antwave key.doc

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PRIST UNIVERSITY

PUDUCHERRY CAMPUS

ANTENNAS AND WAVES PROPAGATION

ANSWER KEY FOR QUESTION PAPER SET-I

PART-A

1. Define antenna efficiency.

It is defined as the ratio of power radiated by the antenna to the total input power

supplied to the antenna.

2. Differentiate radian and steradian.

Measure of phase angle is radian&solid angle is steradian.1 steradian=1 radian2

.

3. What is uniform linear array?

Group of antennas are fed with a current of equal magnitude with uniform phase shift

along the line.

4. What is parasitic array?

Multielement arrays having number of parasitic elements are called parasistic array. They

have one or more parasitic elements and atleast one driven element.

5. What are the applications of loop antennas?

Used in radio receiver,aircraft receiver,direction finding&UHF transmitters.

6. What are travelling wave antennas?

Antennas in which there is no reflected wave is called travelling wave antennas.

7. What are the various modes of propagation?

i)ground wave propagation ii) sky wave propagation iii) space wave propagation.

8. Write short notes on F region.

F1&F2 combine to form one layer called F region.

It lies between 140-400km from earth surface.

It is the top most ionized layer.

9.What are the types of antenna measurement?


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Impedance measurement, gain measurement, bandwidth measurement and radiation

pattern measurement.

10.Define VSWR.

Voltage wave standing ratio is defined as the ratio of maximum to minimum

voltage.

PART-B

11.(A) Explain retarded vector potential in detail.

Diagram-(4)

Explanation-(4)

Expressions-(8)

(b) Explain the reciprocity theorem in detail.

Statement-(2)-If a current at terminals of antenna no.1 induces an emf at open

terminals of antenna no.2 and current at terminals of antenna no.2 induces an emf at openterminals of antenna no.1 provided both currents are equal.

Diagram-(4)

Explanation-(4)

Proof-(6)

12. (a)Write notes on various form of arrays.

i) broadside array- diagram(2) statement(2)-direction of maximum radiation is

perpendicular to array axis.

ii) end-fire array- diagram(2) statement(2)-direction of maximum radiation coincides

with array axis.

iii) collinear array- diagram(2) statement(2)-radiation pattern has circular symmetry.

iv) parasistic arrays- Diagram(2) Statement(2)

(b) Explain the array of N-sources of equal amplitude and spacing-broadside casei)

direction of pattern maxima ii)direction of pattern minima iii) beamwidth of major

lobe.

Diagram-(2)

Definition-(2)

Expression of field strength-(6)

Pattern maxima(2)-90,270


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Pattern minima(2)-0,180

Beam width of major lobe(2)

13.(a) Derive the expressions for fields radiated due to TWA.

Diagram(6)

Explanation(08)

Expression(4)

(b)Write short notes on i)parabolic reflectors ii)Log-periodic antennas

i) Parabolic reflectors- diagram(4),Explanation(4)

ii) Log periodic antennas-Diagram(4),Explanation(4)

14.(a)Explain briefly the various modes of propagation.

i) Ground wave propagation(4)

Diagram(4)

iii) Sky wave propagation(4)

iv) Space wave propagation(4)

(b)Explain maximum usable frequency and how to calculate MUF.

MUF definition(2),Explanation(6),Calculation of MUF(8)

15.(a) Describe the methods for measuring gain and beamwidth of antenna.

Gain measurement- Diagram(4),Procedure for measurement(4)

Beamwidth of antenna- diagram(4),Procedure(4)

(b)Describe briefly about polarization measurement.

Diagram(6), Procedure(4),polarization pattern method(2),Linearly component

method(2),Circular component method(2).

ANSWER KEY FOR QUESTION PAPER SET-II

PART-A

1. Define antenna efficiency.

It is defined as the ratio of power radiated by the antenna to the total input powersupplied.


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2. Explain the directivity of an antenna.

It is defined as the ratio of maximum intensity to the average radiation intensity.

3. What is pattern multiplication concept.

The total field pattern is the addition of phase pattern of individual sources multipliedwith multiplication of individual source pattern.

4. What is tapering of array?

The reduction of sidelobe level is called tapering which means current in source of

linear array is non- uniform.Normally it is done from end to end.

5. Draw the structure of log-periodic antenna.

6. What are the applications of Yagi-uda antenna?

High frequency communication, TV reception.

7. What is critical frequency?

It is defined as the highest frequency which can be reflected back by a particular

layer at vertical incidence.

8. Define skip distance.

It is defined as the minimum distance from the transmitter at which a sky wave of

given frequency is returned to the earth by ionosphere.

9. Define gain of an antenna.

Gain of an antenna is defined as the ability to concentrate the radiated power in a

given direction or to absorb effectively the incident power from that direction.

10. Define power gain.


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It compares the radiated power density of the actual antenna and that of an isotropic

antenna on the basis that both are fed with same input power.

PART-B

11. (a)Derive an expression for the power radiated by the current element and

calculate the radiation resistance.

Diagram(4),Explanation(4),Expressions(8)-Rr=80 2( dl/)2 .

11.(b)Derive an expression for electric and magnetic field due to a current element atdistance point in free space.

Diagram(4)

Electric field components (E,Er)-(6)

Magnetic field components(H)-(6)

12. (a)Derive the expression for the far field pattern of an array of 2-isotropic point

sources i) equal amplitude and phaseii)equal amplitude and opposite phase iii)unequal amplitude and any phase.

i) equal amplitude and phase

Maxima, minima and half power point direction (4)

Radiation pattern(2)

ii) equal amplitude and opposite phase

Maxima,minima and half power point dtrection(4)

Radiation pattern(2)

iii)Unequal amplitude and any phase(4)

(b)Write short notes on binomial arrays ii)phased arrays

i)Binomial arrays ii) phased arrays

Diagram(2) Diagram(2)

Explanation(2) Explanation(2)

Advantages(2) Advantages(2)

Disadvantages(2) Disadvantages(2)


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13. (a)Obtain the expression for the EMF due to loop antenna and explain its use asdirection finder.

Diagram(4)

EMF of loop antenna(6)

Application as direction finder(6)

(b)Write short notes on i) slot radiators ii) Loop antennas

Slot radiators- Diagram(4),Explanation(4)

Loop antennas- Diagram(2),EMF of loop antenna(2),Radiation resistance of loop

antenna(4).

14. (a)Explain Ground wave propagation in detail.

Diagram(4)

Calculation of field strength(6)

Attenuation characteristics(6)

(b)Explain briefly about ionspheric abnormalities.

Normal and abnormal variations(2)

Mongel-Dellinger effect(2)

Ionospheric storms(2)

Sun spot cycles(2)

Tides and winds(2)

Fadings(6)-Selective,interference,polarization,skip.

15. (a)Describe the methods for measuring power gain , Radiation efficiency and

transmission bandwidth of an antenna.

i) Measurement of gain-direct comparison method

Diagram(3)

Explanation(3)

ii) Measurement of radiation efficiency

Diagram(3)

Explanation(3)


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iii) Transmission bandwidth measurement

Diagram(2)

Explanation(2)

(b)Derive an expression to determine antenna efficiency and antenna aperture.

i) Antenna efficiency

Expressions(5),Definition(3)

ii) Antenna Aperture

Definition(3),Expression(5).

ANSWER KEY FOR QUESTION SET-III

PART-A

1. Calculate directivity of an isotropic antenna.

Directivity= maximum radiation intensity of test antenna

---------------------------------------------------------

Average radiation intensity of test antenna

For an isotropic antenna G0=KD.

2. What is radiation intensity in terms of pointing vector?It is defined as power per unit solid angle. In terms of pointing vector

P=EH.

3. What is an array and mention various forms of arrays?

Antenna array is a group of isotropic radiators arranged in a similar manner.

The various forms of arrays are i) broadside array ii) End-fire array iii) Parasitic array iv)

collinear array.

4. Write notes on binomial arrays.

*It is a kind of non-uniform amplitude array.

* Sidelobes are completely eliminated.

*The amplitudes of radiating sources are proportional to the coefficient of binomialseries.

5. Give the reason for poor radiation of loop antenna.


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Loop antenna of single turn have very small radiation resistance and to improve the

radiation resistance no.of turns may be increased but efficiency is still poor due to the loop

design.

6. What is folded dipole?What are its important properties?

A folded dipole is one in which two half wave dipoles one continuous and the othersplit at the centre have been folded and joined together in parallel at the ends.

Properties:

It has built in impedance transforming properly.

The directivity is bi-directional.

7. Define virtual height.

It is defined as the height to which a short pulse of energy sent vertically upward and

traveling with speed of light would reach taking the same two ways travel time as does theactual pulse reflected from the layer.

8. Define MUF.

Maximum Usable Frequency is defined as the frequency which can be reflected back to

earth for some specific angle of incidence.

9. Define antenna aperture efficiency.

It is related to the directive gain of large aperture antenna of the type horn,

parabolic reflector,etc. D= A ( 4/2) A- Effective aperture.

10. Define vertical incidence measurement.

It is defined as the transmission of a radio wave vertically to the ionosphere and

back.PART-B

11.(a) Derive the total power radiated by half wave dipole.

Diagram(4)

Explanation(4)

Derivation(8) W=60 Irms2 .I

(b) Derive radiation resistance,directivity and effective aperture of a half wave

dipole.

Diagram(4)

Radiation resistance(4) Rr=W/ Irms2


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Effective aperture(4)

Directivity(4)

12(a) Explain the principle of pattern multiplication.

Definition-(4)

Radiation pattern of 4 isotropic elements fed in phase, spaced /2 apart-(6)Radiation pattern of 8 isotropic elements fed in phase, spaced /2 apart (6)

(b) Derive the expression for electric field sources of n-isotropic point sources

and also find maxima, minima and half power point direction.

Expression for electric field-(4)

Pattern maxima(4)

Pattern minima-(4)

Half power point direction-(4)

13.(a) With suitable diagram discuss the principle and operation of yagi-uda

antenna.

Construction-(6)

Diagram-(4)

Working-(6)

(b) Explain the principle of operation and application of loop antenna.

Diagram-(2)

Working principle-(6)

EMF equation-(2)

Application - (6)

14.(a) Explain the effect of earth magnetic field on radio wave propagation.

Diagram-(4)

Explanation-(12)

(b) Determine the effective earth radius in space wave propagation.

Diagram-(4)

Explanation-(4)


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Expressions-(8)

15. (a) Write short notes on vertical incidence measurement of the ionosphere.

Diagram-(6)

Explanation-(10)

(b) Derive an expression to determine the directivity of an antenna.

Definition-(4)

Expressions-(12)

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