Wednesday, December 10, 2014

AWP Important 2 marks Short questions and answers

Posted by vishnu vardhan reddy boda at 11:10 AM

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1. Define an antenna.

transducer between a guided wave and a free space wave or

vice versa. Antenna is also said to be an impedance

transforming device.

2. What is meant by radiation pattern?

Antenna is a transition device or a

Radiation pattern is the relative distribution

of radiated power as a function of distance in space .It is a

graph which shows the variation in actual field strength of

the EM wave at all points which are at equal distance from

the antenna. The energy radiated in a particular direction by

an antenna is measured in terms of FIELD STRENGTH.(E

Volts/m)

3. Define Radiation intensity?

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The power radiated from an antenna per unit

solid angle is called the radiation intensity U (watts per

steradian or per square degree). The radiation intensity is

independent of distance.

4. Define Beam efficiency?

The total beam area ( ΩA) consists of the

main beam area ( ΩM ) plus the minor lobe area ( Ωm) .

Thus ΩA = ΩM+ Ωm .

beam area is called beam efficiency.

The ratio of the main beam area to the total

Beam efficiency = ΣM = ΩM / ΩA.

5.Define Directivity?

The directivity of an antenna is equal to the

ratio of the maximum power density P(θ,φ)max to its

average value over a sphere as observed in the far field

of an antenna.

D = P(θ,φ)max / P(θ,φ)av. Directivity from

Pattern.

D = 4π / ΩA. . Directivity from beam

area(ΩA ).

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6.What are the different types of aperture.?

i) Effective aperture.

ii). Scattering aperture.

iii) Loss aperture.

iv) collecting aperture.

v). Physical aperture.

7.Define different types of aperture.?

Effective aperture(Ae).

It is the area over which the power is

extrated from the incident wave and delivered to the

load is called effective aperture.

Scattering aperture(As

.)

It is the ratio of the reradiated power to

the power density of the incident wave.

Loss aperture. (Ae).

It is the area of the antenna which

dissipates power as heat.

Collecting aperture. (Ae).

It is the addition of above three

apertures.

Physical aperture. (Ap).

This aperture is a measure of the

physical size of the antenna.

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8. Define Aperture efficiency?

The ratio of the effective aperture to

the physical aperture is the aperture efficiency. i.e

Aperture efficiency = ηap = Ae / Ap

(dimensionless).

9. What is meant by effective height?

The effective height h of an antenna is the

parameter related to the aperture.It may be defined as

the ratio of the induced voltage to the incident field.i.e

H= V / E.

10. What are the field zone?

The fields around an antenna ay be divided into

two principal regions.

i. Near field zone (Fresnel zone)

ii. Far field zone (Fraunhofer zone)

11.What is meant by Polarization.?

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The polarization of the radio wave can be defined by

direction in which the electric vector E is aligned during

the passage of atleast one full cycle.Also polarization can

also be defined the physical orientation of the radiated

electromagnetic waves in space.

The polarization are three types. They are

Elliptical polarization ,circular polarization and linear

polarization.

12. What is meant by front to back ratio.?

It is defined as the ratio of the power radiated in

desired direction to the power radiated in the opposite

direction. i.e

FBR = Power radiated in desired direction / power

radiated in the opposite direction.

13. Define antenna efficiency.?

The efficiency of an antenna is defined as the ratio

of power radiated to the total input power supplied to the

antenna.

Antenna efficiency = Power radiated / Total input

power

14. What is radiation resistance ?

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The antenna is a radiating device in which power

is radiated into space in the form of electromagnetic

wave.

W‟

= I

Rr = W‟/ I

2 R

2

Where Rr is a fictitious resistance called called as

radiation resistance.

15 What is meant by antenna beam width?

Antenna beamwidth is a measure of directivity

of an antenna. Antenna beam width is an angular width in

degrees, measured on the radiation pattern (major lobe)

between points where the radiated power has fallen to

half its maximum value .This is called as “beam width”

between half power points or half power beam

width.(HPBW).

16. What is meant by reciprocity Theorem.?

If an e.m.f is applied to the terminals of an antenna

no.1 and the current measured at the terminals of the

another antenna no.2, then an equal current both in

amplitude and phase will be obtained at the terminal of

the antenna no.1 if the same emf is applied to the

terminals of antenna no.2.

17.What is meant by isotropic radiator?

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A isotropic radiator is a fictitious radiator and is

defined as a radiator which radiates fields uniformly in all

directions. It is also called as isotropic source or omni

directional radiator or simply unipole.

18. Define gain

The ratio of maximum radiation intensity in

given direction to the maximum radiation intensity from a

reference antenna produced in the same direction with

same input power. i.e

Maximum radiation intensity

from test antenna

Gain (G) = --------------------------------------------------------

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

Maximum radiation intensity from the reference antenna

with same input power

19. Define self impedance

Self impedance of an antenna is defined as its

input impedance with all other antennas are completely

removed i.e away from it.

20 . Define mutual impedance

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The presence of near by antenna no.2 induces a

current in the antenna no.1 indicates that presence of

antenna no.2 changes the impedance of the antenna

no.1.This effect is called mutual coupling and results in

mutual impedance.

21. What is meant by cross field.?

Normally the electric field E is perpendicular to the

direction of wave propagation. In some situation the

electric field E is parallel to the wave propagation that

condition is called Cross field.

22.Define axial ratio

The ratio of the major to the minor axes of the

polarization ellipse is called the Axial Ratio. (AR).

23. What is meant by Beam Area.?

The beam area or beam solid angle or ΩA of an

antenna is given by the normalized power pattern over a

sphere.

ΩA = ∫ ∫4π Pn ( θ,φ ) dΩ

Where dΩ = Sin θ d θ .dφ

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24. What is duality of antenna.?

It is defined as an antenna is a circuit device with

a resistance and temperature on the one hand and the

space device on the other with radiation patterns, beam

angle ,directivity gain and aperture.

25.State Poynting theorem.

intensity vector E and the magnetic filed intensity vector

H at any point is a measure of the rate of energy flow per

unit area at that point.The direction of power flow is

perpendicular to both the electric field and magnetic field

components.

It states that the vector product of electric field

26.What is point source?

emitter source at the center „O‟ of the observation circle.

It is the waves originate at a fictitious volumeless

27.What is meant by array.?

An antenna is a system of similar antennas oriented

similarly to get greater directivity in a desired direction.

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28.What is meant by uniform linear array.?

An array is linear when the elements of the array are

spaced equally along the straight line. If the elements are

fed with currents of equal magnitude and having a uniform

progressive phase shift along the line, then it is called

uniform linear array .

29.What are the types of array.?

a. Broad side array.

b. End fire array

c. Collinear array.

d. Parasitic array.

30.What is Broad side array.?

which the principal direction of radiation is perpendicular

to the array axis and also the plane containing the array

element. For Broad side array the phase difference

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31.Define End fire array.?

which the principal direction of radiation is coincides with

the array axis.

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:KHUH $QG G GLVWDQFH EHWZHHQ WKH HOHPHQWV

32. What is collinear array.?

Broad side array is defined as an arrangement in

End fire array is defined as an arrangement in

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In this array the antenna elements are arranged

coaxially by mounting the elements end to end in straight

line or stacking them one over the other with radiation

pattern circular symmetry. Eg. Omnidirectional antenna.

33. What is Parasitic array.?

In this array the elements are fed parasitically to

reduce the problem of feed line. The power is given to one

element from that other elements get by electro magnetic

coupling.

Eg. Yagi uda antenna.

34. What is the condition on phase for the end fire array

with increased directivity.?

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 = 0 but does not give the maximum directivity. It has

been shown by Hansen and woodyard that a large

directivity is obtained by increasing the phase change

EHWZHHQ WKH VRXUFHV VR WKDW - G Q

This condition will be referred to as the condition for

increased directivity.

35.Define array factor.

 ( Q VLQ Q VLQ

referred to as array factor.

The normalized value of the total field is given by,

The field is given by the expression E will be

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36. Define beam width of major lobe?

is defined as twice the angle between the first null and the

major lobe maximum direction.

It is defined the angle between the first nulls (or) it

37. List out the expression of beam width for broad side

array and end fire array.

between the first nulls is given by,

For broad side array the expression for beam width

BWFN = ((+/ - Q G

For End fire array the expression for beam width

between the first nulls is given by,

BWFN = ((+/ - Q G

1/ 2

.

38. Differentiate broad side and End fire array.?

Broad side

array

1. Antennas fed in Phase

2. Maximum Radiation is

perpendicular

to the direction of array

axis.

3. Beam width of major

lobe is twice the

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array

Antenna elements are fed of

out of Phase

 - G

Maximum Radiation is

directed along the array axis.

Beam width is greater than

that for a broad side array of

same length.

BW = ((+/ - Q G 1/

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reciprocal of the array

length.

BW = ((+/ - Q G

.

2

39.What is the need for the Binomial array.?

The need for a binomial array is

i). In uniform linear array as the array length is

increased to increase the directivity, the secondary lobes

also occurs.

that secondary lobes should be eliminated completely or

reduced to minimum desirable level compared to main

lobes.

ii) For certain applications, it is highly desirable

40. Define power pattern.?

component of the pointing vector Sr at a constant radius as

a function of angle is called power density pattern or

power pattern.

41. What is meant by similar Point sources.?

phase of the field with respect to the absolute angle for any

two sources are same then they are called similar point

sources.

sources may be unequal.

Graphical representation of the radial

Whenever the variation of the amplitude and the

The maximum amplitudes of the individual

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42. What is meant by identical Point sources.?

amplitudes are called identical point sources.

43. What is the principle of the pattern multiplication?

isotropic but similar sources is the product of the

i) individual source pattern and

ii) The array pattern of isotropic point sources each

Similar point sources with equal maximum

The total field pattern of an array of non

located at the phase center of the individual

source having the same amplitude and phase.

While the total phase pattern is the sum of the phase

patterns of the individual source pattern and array pattern.0

44.What is the advantage of pattern multiplication?

TMUseful tool in designing antenna

TMIt approximates the pattern of a complicated array

without making lengthy computations

45.What is tapering of arrays?

of unwanted side lobes .The amplitude of currents in the

linear array source is non-uniform; hence the central source

radiates more energy than the ends. Tapering is done from

center to end.

Tapering of array is a technique used for reduction

46.What is a binomial array?

antenna elements in the array are arranged according to the

coefficients of the binomial series.

It is an array in which the amplitudes of the

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47.What are the advantages of binomial array?

Advantage:

TMNo minor lobes

Disadvantages:

TMIncreased beam width

TMMaintaining the large ratio of current

amplitude in large arrays is difficult

48.What is the difference between isotropic and non-

isotropic source

TMIsotropic source radiates energy in all

directions but non-isotropic source radiates

energy only in some desired directions.

TMIsotropic source is not physically realizable

but non-isotropic source is

physically realizable.

49.Define Side Lobe Ratio

Side Lobe Ratio is defined as the ratio of

power density in the principal or main lobe to the power

density of the longest minor lobe.

50. List the arrays used for array tapering

TMBinomial Array:Tapering follows the

coefficient of binomial series

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TMDolph Tchebycheff Array: Tapering follows

the coefficient of Tchebycheff polynomial

51.What is a Short Dipole?

Ans: A short dipole is one in which the field is

oscillating because of the oscillating voltage and current.It

is called so, because the length of the dipole is short and

the current is almost constant throughtout the entire length

of the dipole.It is also called as Hertzian Dipole which is a

hypothetical antenna and is defined as a short isolated

conductor carrying uniform alternating current.

52.How radiations are created from a short Dipole?

Ans:The dipole has two equal charges of

opposite sign oscillating up and down in a harmonic

motion.The charges will move towards each other and

electric filed lines were created.When the charges meet at

the midpoint, the field lines cut each other and new field

are created.This process is spontaneous and so more fields

are created around the antenna.This is how radiations are

obtained from a short dipole.(See Figure from John. D

.Kraus Book)

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53.Why a short dipole is also called an elemental dipole?

A short dipole that does have a

uniform current will be known as the elemental dipole.Such

a dipole will generally be considerably shorter than the

tenth wave length maximum specified for a short

dipole.Elemental dipole is also called as elementary

dipole,elementary doublet and hertzian dipole.

54.What is a Infinitesimal Dipole?

When the length of the short dipole is

vanishingly small,then such a dipole is called a

infinitesimal dipole.If dl be the infinitesimally small length

and I be the current,then Idl is called as the current element.

55.Why a short dipole is called a oscillating dipole?

A short dipole is initially in neutral condition

and the moment a current starts to flow in one direction,one

half of the dipole require an excess of charge and the other

a deficit because a current is a flow of electrical

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charge.Then ,there will be a voltage between the two halves

of the dipole.When the current changes its direction this

charge unbalance will cause oscillations.Hence an

oscillating current will result in an oscillating

voltage.Since,in such dipole,electric charge oscillates ,it

may be called as Oscilllating electric dipole.

56.What do you understand by retarded current?

Since,the short electric dipole is so

short,the current which is flowing through the dipole is

assumed to be constant throughtout its length.The effect of

this current is not felt instantaneous at a distance point only

after an interval equal to the time required for the wave to

propagate over the distance r is called the retardation time.

The retarded current [I]=Io exp(j W-r/c))

Where U F LV WKH SKDVH UHWDUGDWLRQ

57.Define induction field

The induction field will predominate at points

close to the current element ,where the distance from the

centre of the dipole to the particular point is less.This field

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is more effective in the vicinity of the current element

only.It represents the energy stored in the magnetic field

surrounding the current element or conductor.This field is

also known as near field.

58.Define Radiation field

The radiation field will be produced at a larger

distance from the the current element ,where the distance

from the centre of the dipole to the particular point is very

large.It is also called as distant field or far field.

59.At what distance from the dipole is the induction field

equal to the radiation field?

As the distance from the current element or the

short dipole increases ,both induction and radiation fields

emerge and start decreasing .However,a distance reaches

from the conductor at which both the induction and

radiation field becomes equal and the particular distance

depends upon the wavelength.The two fields will thus have

equal amplitude at that particular distance.This distance is

given by

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 U

60.Define Radiation Resistance

It is defined as the fictitious resistance which when

inserted in series with the antenna will consume the same

amount of power as it is actually radiated.The antenna

appears to the transmission line as a resistive component

and this is known as the radiation resistance.

61.Give the expression for the effective aperture of a short

dipole

The effective aperture of a short dipole is

given by

 $H

2

62.What is a dipole antenna?

A dipole antenna may be defined as a symmetrical antenna in which the two ends are

at equal potential relative to the midpoint.

63.What is a half wave dipole?

A half wave antenna is the fundamental radio antenna of metal rod or tubing or thin

wire which has a physical length of half wavelength in free space at the frequency of

operation

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64.Give the expression for the effective aperture of a Half

wave Dipole

The effective aperture of a half wave dipole is

given by

Ae=0

2

65.What is the radiation resistance of a half wave dipole

The radiation resistance of a half wave dipole is given by

Rr=73 ohm

66.What is a loop antenna?

turns carrying radio frequency current.it may assume any shape (e.g.

rectangular,square,triangular and hexagonal)

67.Give an expression of radiation resistance of a small loop

Radiation resistance of a small loop is given by

A loop antenna is a radiating coil of any convenient cross-section of one or more

Rr=31,200 (A/ 2)2

68.How to increase the radiation resistance of a loop antenna

The radiation resistance of a loop antenna can be increased by:

1. increasing the number of turns

2. inserting a ferrite core of very high permeability with loop antenna‟s

circumference which will rise the magnetic field intensity called ferrite

loop.

69.What are the types of loop antennas?

Loop antennas are classified into:

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1. (OHFWULFDOO\ 6PDOO &LUFXPIHUHQFH

2. (OHFWULFDOO\/DUJH 'LPHQVLRQ FRPSDUDEOH WR

70.What are Electrically Small loop antennas?

Electrically Small loop antennas is one in which the overall length of the loop is less

than one-tenth of the wavelength. Electrically Small loop antennas have small radiation

resistances that are usually smaller than their loop resistances.They are very poor radiators

and seldom employed for transmission in radio communication.

71.What are Electrically large loop antennas?

Electrically Large loop antennas is one in which the overall length of the loop

approaches the wavelength.

72.List out the uses of loop antenna

Various uses of loop antenna are:

TM It is used as receiving antenna in portable radio and pagers

TM It is used as probes for field measurements and as directional antennas for radio

wave navigation

TM It is used to estimate the direction of radio wave propagation

73.What are the parameters to be considered for the design of an helical antenna?

The parameters to be considered for the design of an helical antenna are:

1. Bandwidth

2. Gain

3. Impedance

4. Axial Ratio

74.What are the types of radiation modes of operation for an helical antenna

The two types of radiation modes of operation possible for an helical antenna are:

1. Normal mode of operation

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2. Axial mode of operation

75.Which antenna will produce circularly polarized waves

Helical antenna radiates circularly polarized wave.

76.List the applications of helical antenna

The applications of helical antenna are:

TM It became thw workhouse of space communications for telephone,television and

data,being employed both on satellites and at ground stations

TM Many satellites including weather satellites,data relay satellites all have helical

antennas

TM It is on many othe probes of planets and comets,including moon and mars,being used

alone,in arrays or as feeds for parabolic reflectors,its circular polarization and high

gain and simplicity making it effective for space application

77.Define Sky wave.

the ionosphere is called sky wave.

78.Define Tropospheric wave.

Waves that arrive at the receiver after reflection from

the troposphere

region is called Tropospheric wave.(ie 10 Km from Earth

surface).

79. Define Ground wave.

Waves propagated over other paths near the earth

surface is called ground wave propagation.

80.What are the type of Ground wave.

i. Space wave

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Waves that arrive at the receiver after reflection in

Ground wave classified into two types.

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ii. Surface wave.

81 What is meant by Space Wave.?

reflected wave. Also includes the portion of energy

received as a result of diffraction around the earth surface

and the reflection from the upper atmosphere.

82. What is meant by Surface Wave.?

an EM wave is guided by a transmission is called surface

wave. Attenuation of this wave is directly affected by the

constant of earth along which it travels.

83. What is meant by fading.?

paths as a result of the atmospheric conditions and it is

called .It can not be predicted properly.

84. What are the type of fading.?

iii. Multi path fading.

85. What is inverse and multi path fading.?

path into an obstructed one.

between the direct and ground reflected waves as well as

interference between two are more paths in the atmosphere.

86.What is meant by diversity reception.?

path interference the technique used are diversity

reception. It is obtained by two ways.

It is made up of direct wave and ground

Wave that is guided along the earth‟s surface like

Variation of signal strength occur on line of sight

Two types. i. Inverse bending.

Inverse bending may transform line of sight

Multi path fading is caused by interference

To minimize the fading and to avoid the multi

i. Space diversity reception.

ii. Frequency diversity reception.

iii. Polarization diversity.

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87. Define Space diversity Reception.

received at different locations do not fade together. It

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and the antenna which high signal strength at the moment

dominates.

88 .Define frequency diversity Reception.

This method exploits the fact that signals

This method takes advantage of the fact that

signals of slightly different frequencies do not fade

synchronously. This fact is utilized to minimize fading in

radio telegraph circuits.

89. Define polarization diversity reception.

is found that signal transmitted over the same path in two

polarizations have independent fading patterns.in broad

band dish antenna system, Polarization diversity combined

with frequency diversity reception achieve excellent

results.

90. What is meant by Faraday‟s rotation.?

ionosheric medium becomes anisotropic and the incident

plane wave entering the ionosphere will split into ordinary

and extra ordinary waves/modes.

ionosphere they recombine into a single plane wave again.

have changed, this phenomenon is known as Faraday‟s

rotation.

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It is used in normally in microwave links, and it

Due to the earth‟s magnetic fields, the

When these modes re-emerge from the

Finally the plane of polarization will usually

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91. What are the factors that affect the propagation of radio

waves.?

i. Curvature of earth.

ii. Earth‟s magnetic field.

iii. Frequency of the signal.

iv. Plane earth reflection.

92. Define gyro frequency.

period of an electron in its orbit under the influence of the

earths magnetic flux density B.

93. Define critical frequency.

be reflected back for vertical incidence is

Frequency whose period is equal to the

For any layer , the highest frequency that will

fcr = 9¥1max

94. Define Magneto-Ions Splitting.

two different components (ordinary and extra-ordinary) by

the earths magnetic field is called Magneto-Ions Splitting.

95.Define LUHF.

and transmitter power is defined as the lowest frequency

that will give satisfactory reception for that distance and

power.

It depends on

The phenomenon of splitting the wave into

The lowest useful HF for a given distance

i. The effective radiated power

ii. Absorption character of ionosphere for the

paths between transmitter and receiver.

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iii. The required field strength which in turn

depends upon the radio noise at the

receiving location and type of service

involved .

96. Define Refractive index.

It is defined as n = c / vp

in vacua

n =

the medium

n = ¥ r

97Define maximum Usable Frequency.

reflected back for a given distance of transmission is called

the maximum usable frequency (MUF) for that distance.

98. Define skip distance.

given frequency fails to be reflected back is the skip

distance for that frequency.The higher the frequency the

greater the skip distance.

99. Define Optimum frequency.?

any two points is therefore selected as some frequency

The maximum Frequency that can be

MUF = fcr VHF i

The distance with in which a signal of

Otimum frequency for transmitting between

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lying between about 50 and 85 percent of the predicted

maximum usable frequency between those points.

100. What is wave impedance.?

 0 /¥ - (fc / f)

 LH ¥ - (fc / f)

101. Define wave velocity and Group velocity.?

wave velocity vp = c / ¥ - (fc / f)

Group velocity, vp vg = c

2 / vp

vg = c

16 MARK QUESTIONS

1. Write the potential function in different form.

2. Explain in detail about the aperture Concept

Aperture represents the area of the antenna

confining the effective radiations

The various types of antenna apertures are

i) Effective aperture.

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ii). Scattering aperture.

iii) Loss aperture.

iv) collecting aperture.

v). Physical aperture.

Effective aperture(Ae).

is extrated from the incident

effective aperture.

It is the area over which the power

wave and delivered to the load is called

Scattering aperture(As

.)

It is the ratio of the reradiated

power to the power density of the

incident wave.

Loss aperture. (Ae).

It is the area of the antenna which

dissipates power as heat.

Collecting aperture. (Ae).

It is the addition of above three

apertures.

Physical aperture. (Ap).

This aperture is a measure of the

physical size of the antenna.

The ratio of the effective aperture to the

physical aperture is the aperture efficiency. i.e

Aperture efficiency = ηap = Ae / Ap (dimensionless).

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Antenna matching:

resistance matched to the antenna radiation resistance ,

maximum power is transferred to the load and the power

is also reradiated from the dipole.This is called antenna

matching( Give detailed explanation )

When the antenna is receiving with a load

3.Briefly explain the radiation from a short dipole

Defn: A short dipole is one in which the field is

oscillating because of the oscillating voltage and current.It

is called so, because the length of the dipole is short and

the current is almost constant throughtout the entire length

of the dipole.

Fields from Oscillating Dipole: The dipole has two

equal charges of opposite sign oscillating up and down in

a harmonic motion.The charges will move towards each

other and electric filed lines were created.when the

charges meet at the midpoint, the field lines cut each other

and new field are created.This process is spontaneous and

so more filed are created around the antenna.This is how

radiations are obtained from a short dipole.(See Figure

from John. D .Kraus Book)

Antenna Field Zones:The regions containing the

radiations that are present around the antenna are called

Zones.The fields around an antenna ay be divided into

two principal regions.

a)Near field zone (Fresnel zone)

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b)Far field zone (Fraunhofer zone)

Electric and Magnetic field components of short

Dipole: Write the derivations by referring The

Book,K.D.Prasad.

4.Gives notes on the antenna impedances. Find the

effective aperture and Directivity of a short dipole

antenna.

Self Impedance:

input impedance with all other antennas are

completely removed i.e away from it.

Defn:Self impedance of an antenna is defined as its

Write the formula required

Mutual Impedance:

induces a current in the antenna no.1 indicates that

presence of antenna no.2 changes the impedance of the

antenna no.1.This effect is called mutual coupling and

results in mutual impedance.

Defn:The presence of near by antenna no.2

State Reciprocity theorem

Formula required

Effective aperture and Directivity of a short dipole

antenna.

Consider a plane wave incident on a short

dipole.The wave is assumed to be linearly polarized with

electric field in the y direction. The current in the dipole is

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assumed constant and in the same phase over its entire

length,and the terminating resistance is assumed equal to

the dipole radiation resistance.

The effective aperture of this dipole is given by

 $H

The directivity is found to be

 ' $H VT

5.Define Polarization? Explain the different types of

polarization in detail.

Polarization is defined as the orientation of

electric field as a function of direction. The polarization

of the radio wave can be defined by direction in which the

electric vector E is aligned during the passage of atleast

one full cycle.Also polarization can also be defined the

physical orientation of the radiated electromagnetic waves

in space.The polarization are of three types. They are:

Elliptical polarization

Circular polarization

Linear polarization.

Linear Polarisation:

electric field remains in only one direction.For a linearly

polarized wave,the axial ratio is infinity.

Elliptical polarization

ellipse called polarization ellipse. The ratio of the major to

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A linearly polarized wave is one in which the

The electric field vector rotates and form a

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the minor axes of the polarization ellipse is called the

Axial Ratio. (AR). AR is greater than 1 .

Circular polarization

circle and this wave is called circularly polarized

wave.AR is unity.

The electric filed vector rotates and form a

6.Explain in detail the different cases of the array

containing two isotropic sources

TMCase 1: Arrays of two isotropic sources fed with

currents of equal amplitude and in phase

TMCase 2: Arrays of two isotropic sources fed with

currents of equal amplitude and opposite phase

TMCase 3: Arrays of two isotropic sources fed with

currents of unequal amplitude and any phase

TMCase 1: Arrays of two isotropic sources fed with

currents of equal amplitude and in phase

quadrature.

Write about the following:

• Field pattern of the individual cases

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• Find the maxima ,minima direction and half

power point direction

• Draw the radiation pattern.

7. What is broadside array? Derive the maxima ,null

directions and also the beamwidth of a broadside array.

Broad side array is defined as an arrangement in

which the principal direction of radiation is perpendicular

to the array axis and also the plane containing the array

element. For Broad side array the phase difference

EHWZHHQ DGMDFHQW HOHPHQW LV

• Field pattern of a linear array with n isotropic

sources

• Determine the maxima ,minima direction and

half power point direction

• Draw the radiation pattern.

8.What is End Fire array? Derive the maxima ,null

directions and also the beamwidth of a Endfire array.

which the principal direction of radiation is coincides with

the array axis.

 )RU HQG )LUH DUUD\ - G

:KHUH $QG G GLVWDQFH EHWZHHQ WKH HOHPHQWV

End fire array is defined as an arrangement in

• Field pattern of a linear array with n isotropic

sources

• Determine the maxima ,minima direction and

half power point direction

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• Draw the radiation pattern.

9.Explain the principle of pattern multiplication with some

examples.

Principle of pattern multiplication:

isotropic but similar sources is the product of the

iii) individual source pattern and

iv) The array pattern of isotropic point sources each

The total field pattern of an array of non

located at the phase center of the individual

source having the same amplitude and phase.

While the total phase pattern is the sum of the phase

patterns of the individual source pattern and array pattern.0

TMSituation 1: Array of two point sources fed in

phase with the amplitude of the individual

source to be Eo=Eo

TMSituation 2: Array of two point sources fed in

phase with the amplitude of the individual

source to be Eo=Eo

TMSituation 3: Array of four point sources fed in

phase with the amplitude of the individual

source to be Eo=Eo

1

Sin (theta)

1 Cos (theta)

1

Sin (theta)

10.Explain the different techniques used for tapering of

arrays

Array Tapering:

reduction of unwanted side lobes .The amplitude of

currents in the linear array source is non-uniform; hence the

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Tapering of array is a technique used for

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central source radiates more energy than the ends. Tapering

is done from center to end.

Techniques used for array tapering:

TMBinomial Array:Tapering follows the

coefficient of binomial series

TMDolph Tchebycheff Array: Tapering follows

the coefficient of Tchebycheff polynomial.

1. Binomial Array:

antenna elements in the array are arranged according to the

coefficients of the binomial series.

The need for a binomial array is

increased to increase the directivity, the secondary lobes

also occurs.

that secondary lobes should be eliminated completely or

reduced to minimum desirable level compared to main

lobes.

Advantage:

Disadvantages:

It is an array in which the amplitudes of the

i). In uniform linear array as the array length is

ii) For certain applications, it is highly desirable

TMNo minor lobes

TMIncreased beam width

TMMaintaining the large ratio of current

amplitude in large arrays is difficult

2. Dolph Tchebycheff Array:

TMTapering follows the coefficient of

Tchebycheff polynomial.

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11.Derive the fields radiated from a short electric dipole.List the far field

components.Determine its radiation resistance and directivity.

TM Fields radiated from the short dipole and radiation resistance(Refer

Antennas & propagation By K.D.Prasad,Page No.210 to 227)

TM Directivity is 1.5(Refer Antennas & propagation By K.D.Prasad,Page

No.251 to 252)

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12.Derive the expressions for the fields and power radiated from a half wave dipole

antenna.Find its radiation resistance and directivity.

TM Fields radiated from the short dipole and radiation resistance(Refer

TM Directivity is 1.5(Refer Antennas & propagation By K.D.Prasad,Page

Antennas & propagation By K.D.Prasad,Page No.229 to 234)

No.252 to 253)

13.Derive the field radiated from a small loop antenna

TM Small loop radiated fields(Refer “Antennas” By John.D.Kraus,Page

No.200 to 208)

14.Explain in detail about the helical antenna

TM Definition of helical antenna

TM Helical Geometry

TM Radiated fields of helical antenna

TM Types of helix

Refer “Antennas” By John.D.Kraus

15.Explain the different modes of operation of helical antenna

TM Normal mode of operation

TM Axial mode of operation.

16. Explain Ground wave Propagation.

Sky wave.

the ionosphere is called sky wave.

Tropospheric wave.

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Waves that arrive at the receiver after reflection in

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Waves that arrive at the receiver after reflection from

the troposphere

region is called Tropospheric wave.(ie 10 Km from Earth

surface).

Ground wave.

Waves propagated over other paths near the earth

surface is called ground wave propagation.

Type of Ground wave.

iv. Space wave

v. Surface wave.

Space Wave.

reflected wave. Also includes the portion of energy

received as a result of diffraction around the earth surface

and the reflection from the upper atmosphere.

Surface Wave.

an EM wave is guided by a transmission is called surface

wave. Attenuation of this wave is directly affected by the

constant of earth along which it travels.

17.Explain diversity reception.?

multi path interference the technique used are

diversity reception. It is obtained by two ways.

Ground wave classified into two types.

It is made up of direct wave and ground

Wave that is guided along the earth‟s surface like

To minimize the fading and to avoid the

1. Space diversity reception

2. Frequency diversity reception.

3. Polarization diversity.

Space diversity Reception.

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This method exploits the fact that signals

received at different locations do not fade together. It

UHTXLUHV DQWHQQDV VSDFHG DW OHDVW DSDUW DUH SUHIHUUHG

and the antenna which high signal strength at the moment

dominates.

Frequency diversity Reception.

This method takes advantage of the fact that

signals of slightly different frequencies do not fade

synchronously. This fact is utilized to minimize fading in

radio telegraph circuits.

Polarization diversity reception.

it is found that signal transmitted over the same path in

two polarizations have independent fading patterns.in

broad band dish antenna system, Polarization diversity

combined with frequency diversity reception achieve

excellent results.

18. Explain in detail ionospheric propagation.

propagation through ionosphere is ionospheric

propagation..

It is used in normally in microwave links, and

Waves that arrive at the receiver after the

The ionosphere is that region of the

earths atmosphere in which the constituent gases are

ionized by radiation from the outer space.

The region is 50 Km to 400 Km.

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• Effective Dielectric and conductivity of an ionized

gas.

• Reflection and refraction waves by the ionosphere.

( Refer Page no 667 to 681. Electro magnetic waves and

radiating Systems .By. C.JORDAN and G.BALMAIN )

19. Explain

a. Effect of the earth magnetic field.

b. Faraday rotation in Sky wave Propagation.

Effect of the earth magnetic field

Electrons and ions in the ionosphere are influenced not

only by the fields of a passing elecro magnetic wave but

also by the earth magnetic field, which causes the charged

particles to move in circular or spiral paths.

( Refer Page no 687. Electro magnetic waves and radiating

Systems .By. C.JORDAN and G.BALMAIN )

Faraday rotation in Sky wave Propagation

ionosheric medium becomes anisotropic and the incident

plane wave entering the ionosphere will split into ordinary

and extra ordinary waves/modes.

ionosphere they recombine into a single plane wave again.

have changed, this phenomenon is known as Faraday‟s

rotation.

( Refer Page no 693. Electro magnetic waves and radiating

Systems .By. C.JORDAN and G.BALMAIN )

Due to the earth‟s magnetic fields, the

When these modes re-emerge from the

Finally the plane of polarization will usually

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20. Derive the expression for Permitivity and conductivity

of ionized gas.?

Plasma: .Assembly of charged particles in which the time

average charge density is

Zero.

Plasma is formed Whenver the atoms ina gas

are ionized to produce equal number of ions and electrons.

Eg earth ionosphere.

 r

 >1H

,

= 1- Ne

2

/ ( P v

2

P

2

Note: Conductivity is maximum means the wave will pass

through that medium.

( Refer Page no 670

fn. Electro magnetic waves and radiating Systems .By.

C.JORDAN and G.BALMAIN )

21.Explain the concept of Reflection and refraction waves

by the ionosphere.

the ionosphere is a function of frequency.

i. Reflection at low frequency.

ii. Reflection at high frequency.

iii. Maximum usable frequency.

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Conductivity is small, waves gets reflected.

The reflection and refraction of radio waves by

Briefly describe the following.

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iv. Optimum frequency.

v. Skip distance.

Important formula for problems:

i Critical frequency,

ii. Relative Dielectric constant

iii. Phase constant,

iv. Wave impedance.

v. Wave velocity.

vi Group velocity,

fcr = 9¥1max

 r = 1- Ne

 P

2

β = (2π/λ) ¥ - (fc / f)

 0 /¥ - (fc / f)

vp = c / ¥ - (fc / f)2

2

vii. Incident angle .

vp vg = c

2 / vp

vg = c

sin i = ¥ - (fc / f)

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