Quick Revision for Antenna & Wave Propagation

Antenna & Wave Propagation
ANTENNAS AND WAVE PROPAGATION
TWO MARK QUESTIONS
1. Define an antenna.
Antenna is a transition device or a
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?
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?
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 ( WA) consists of the
main beam area ( WM ) plus the minor lobe area ( Wm) .
Thus WA = WM+ Wm .
The ratio of the main beam area to the total
beam area is called beam efficiency.
Beam efficiency = SM = WM / WA.
5.Define Directivity?
The directivity of an antenna is equal to the
ratio of the maximum power density P(q,f)max to its
average value over a sphere as observed in the far field
of an antenna.
D = P(q,f)max / P(q,f)av. Directivity from
Pattern.
D = 4p / WA. . Directivity from beam
area(WA ).
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.
8. Define Aperture efficiency?
The ratio of the effective aperture to
the physical aperture is the aperture efficiency. i.e
Aperture efficiency = hap = 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.?
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 ?
The antenna is a radiating device in which power
is radiated into space in the form of electromagnetic
wave.
W’ = I2 R
Rr = W’/ I2
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?
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
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 WA of an
antenna is given by the normalized power pattern over a
sphere.
WA = ò ò4p Pn ( q,f ) dW
Where dW = Sin q d q .df
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.
It states that the vector product of electric field
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.
26.What is point source?
It is the waves originate at a fictitious volumeless
emitter source at the center ‘O’ of the observation circle.
27.What is meant by array.?
An antenna is a system of similar antennas oriented
similarly to get greater directivity in a desired direction.
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.?
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
31.Define End fire array.?
End fire array is defined as an arrangement in
which the principal direction of radiation is coincides with
the array axis. )RU HQG )LUH DUUD\ - G :KHUH $QG G GLVWDQFH EHWZHHQ WKH HOHPHQWV
32. What is collinear array.?
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.?
:KHQ - G SURGXFHV PD[LPXP ILHOG LQ WKH GLUHFWLRQ
= 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.
The normalized value of the total field is given by, ( Q VLQ Q VLQ
The field is given by the expression E will be
referred to as array factor.
36. Define beam width of major lobe?
It is defined the angle between the first nulls (or) it
is defined as twice the angle between the first null and the
major lobe maximum direction.
37. List out the expression of beam width for broad side
array and end fire array.
For broad side array the expression for beam width
between the first nulls is given by,
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
End fire
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
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/
2
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.
ii) For certain applications, it is highly desirable
that secondary lobes should be eliminated completely or
reduced to minimum desirable level compared to main
lobes.
40. Define power pattern.?
Graphical representation of the radial
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.?
Whenever the variation of the amplitude and the
phase of the field with respect to the absolute angle for any
two sources are same then they are called similar point
sources.
The maximum amplitudes of the individual
sources may be unequal.
42. What is meant by identical Point sources.?
Similar point sources with equal maximum
amplitudes are called identical point sources.
43. What is the principle of the pattern multiplication?
The total field pattern of an array of non
isotropic but similar sources is the product of the
i) individual source pattern and
ii) The array pattern of isotropic point sources each
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? ™ Useful tool in designing antenna ™ It approximates the pattern of a complicated array
without making lengthy computations
45.What is tapering of arrays?
Tapering of array is a technique used for reduction
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.
46.What is a binomial array?
It is an array in which the amplitudes of the
antenna elements in the array are arranged according to the
coefficients of the binomial series.
47.What are the advantages of binomial array?
Advantage: ™ No minor lobes
Disadvantages: ™ Increased beam width ™ Maintaining the large ratio of current
amplitude in large arrays is difficult
48.What is the difference between isotropic and nonisotropic
source ™ Isotropic source radiates energy in all
directions but non-isotropic source radiates
energy only in some desired directions. ™ Isotropic 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 ™ Binomial Array:Tapering follows the
coefficient of binomial series
™ Dolph 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)
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
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
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
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
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?
A loop antenna is a radiating coil of any convenient cross-section of one or more
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
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:
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:
™ It is used as receiving antenna in portable radio and pagers
™ It is used as probes for field measurements and as directional antennas for radio
wave navigation
™ 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
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:
™ It became thw workhouse of space communications for telephone,television and
data,being employed both on satellites and at ground stations
™ Many satellites including weather satellites,data relay satellites all have helical
antennas
™ 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.
Waves that arrive at the receiver after reflection in
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.
Ground wave classified into two types.
i. Space wave
ii. Surface wave.
81 What is meant by Space Wave.?
It is made up of direct wave and ground
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.?
Wave that is guided along the earth’ s surface like
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.?
Variation of signal strength occur on line of sight
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.?
Two types. i. Inverse bending.
iii. Multi path fading.
85. What is inverse and multi path fading.?
Inverse bending may transform line of sight
path into an obstructed one.
Multi path fading is caused by interference
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.?
To minimize the fading and to avoid the multi
path interference the technique used are diversity
reception. It is obtained by two ways.
i. Space diversity reception.
ii. Frequency diversity reception.
iii. Polarization diversity.
87. Define Space diversity Reception.
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.
88 .Define 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.
89. Define polarization diversity reception.
It is used in normally in microwave links, and 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.
90. What is meant by Faraday’ s rotation.?
Due to the earth’ s magnetic fields, the
ionosheric medium becomes anisotropic and the incident
plane wave entering the ionosphere will split into ordinary
and extra ordinary waves/modes.
When these modes re-emerge from the
ionosphere they recombine into a single plane wave again.
Finally the plane of polarization will usually
have changed, this phenomenon is known as Faraday’ s
rotation.
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.
Frequency whose period is equal to the
period of an electron in its orbit under the influence of the
earths magnetic flux density B.
93. Define critical frequency.
For any layer , the highest frequency that will
be reflected back for vertical incidence is
fcr = 9¥1max
94. Define Magneto-Ions Splitting.
The phenomenon of splitting the wave into
two different components (ordinary and extra-ordinary) by
the earths magnetic field is called Magneto-Ions Splitting.
95.Define LUHF.
The lowest useful HF for a given distance
and transmitter power is defined as the lowest frequency
that will give satisfactory reception for that distance and
power.
It depends on
i. The effective radiated power
ii. Absorption character of ionosphere for the
paths between transmitter and receiver.
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
Velocity of light
in vacua
n =
Phase velocity in
the medium
n = ¥ r
97Define maximum Usable Frequency.
The maximum Frequency that can be
reflected back for a given distance of transmission is called
the maximum usable frequency (MUF) for that distance.
MUF = fcr VHF i
98. Define skip distance.
The distance with in which a signal of
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.?
Otimum frequency for transmitting between
any two points is therefore selected as some frequency
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)2
Group velocity, vp vg = c2
vg = c2 / vp
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.
ii). Scattering aperture.
iii) Loss aperture.
iv) collecting aperture.
v). Physical 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.
The ratio of the effective aperture to the
physical aperture is the aperture efficiency. i.e
Aperture efficiency = hap = Ae / Ap (dimensionless).
Antenna matching:
When the antenna is receiving with a load
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 )
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)
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:
Defn:Self impedance of an antenna is defined as its
input impedance with all other antennas are
completely removed i.e away from it.
Write the formula required
Mutual Impedance:
Defn: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.
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
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:
A linearly polarized wave is one in which the
electric field remains in only one direction.For a linearly
polarized wave,the axial ratio is infinity.
Elliptical polarization
The electric field vector rotates and form a
ellipse called polarization ellipse. The ratio of the major to
the minor axes of the polarization ellipse is called the
Axial Ratio. (AR). AR is greater than 1 .
Circular polarization
The electric filed vector rotates and form a
circle and this wave is called circularly polarized
wave.AR is unity.
6.Explain in detail the different cases of the array
containing two isotropic sources
™ Case 1: Arrays of two isotropic sources fed with
currents of equal amplitude and in phase ™ Case 2: Arrays of two isotropic sources fed with
currents of equal amplitude and opposite phase ™ Case 3: Arrays of two isotropic sources fed with
currents of unequal amplitude and any phase ™ Case 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
· 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.
End fire array is defined as an arrangement in
which the principal direction of radiation is coincides with
the array axis. )RU HQG )LUH DUUD\ - G :KHUH $QG G GLVWDQFH EHWZHHQ WKH HOHPHQWV
· Field pattern of a linear array with n isotropic
sources
· Determine the maxima ,minima direction and
half power point direction
· Draw the radiation pattern.
9.Explain the principle of pattern multiplication with some
examples.
Principle of pattern multiplication:
The total field pattern of an array of non
isotropic but similar sources is the product of the
iii) individual source pattern and
iv) The array pattern of isotropic point sources each
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
™ Situation 1: Array of two point sources fed in
phase with the amplitude of the individual
source to be Eo=Eo1 Sin (theta) ™ Situation 2: Array of two point sources fed in
phase with the amplitude of the individual
source to be Eo=Eo1 Cos (theta) ™ Situation 3: Array of four point sources fed in
phase with the amplitude of the individual
source to be Eo=Eo1 Sin (theta)
10.Explain the different techniques used for tapering of
arrays
Array Tapering:
Tapering of array is a technique used for
reduction 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.
Techniques used for array tapering: ™ Binomial Array:Tapering follows the
coefficient of binomial series ™ Dolph Tchebycheff Array: Tapering follows
the coefficient of Tchebycheff polynomial.
1. Binomial Array:
It is an array in which the amplitudes of the
antenna elements in the array are arranged according to the
coefficients of the binomial series.
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.
ii) For certain applications, it is highly desirable
that secondary lobes should be eliminated completely or
reduced to minimum desirable level compared to main
lobes.
Advantage: ™ No minor lobes
Disadvantages: ™ Increased beam width ™ Maintaining the large ratio of current
amplitude in large arrays is difficult
2. Dolph Tchebycheff Array: ™ Tapering follows the coefficient of
Tchebycheff polynomial.
11.Derive the fields radiated from a short electric dipole.List the far field
components.Determine its radiation resistance and directivity.
™ Fields radiated from the short dipole and radiation resistance(Refer
Antennas & propagation By K.D.Prasad,Page No.210 to 227)
™ Directivity is 1.5(Refer Antennas & propagation By K.D.Prasad,Page
No.251 to 252)
12.Derive the expressions for the fields and power radiated from a half wave dipole
antenna.Find its radiation resistance and directivity.
™ Fields radiated from the short dipole and radiation resistance(Refer
Antennas & propagation By K.D.Prasad,Page No.229 to 234)
™ Directivity is 1.5(Refer Antennas & propagation By K.D.Prasad,Page
No.252 to 253)
13.Derive the field radiated from a small loop antenna
™ Small loop radiated fields(Refer “Antennas” By John.D.Kraus,Page
No.200 to 208)
14.Explain in detail about the helical antenna
™ Definition of helical antenna
™ Helical Geometry
™ Radiated fields of helical antenna
™ Types of helix
Refer “Antennas” By John.D.Kraus
15.Explain the different modes of operation of helical antenna
™ Normal mode of operation
™ Axial mode of operation.
16. Explain Ground wave Propagation.
Sky wave.
Waves that arrive at the receiver after reflection in
the ionosphere is called sky wave.
Tropospheric wave.
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.
Ground wave classified into two types.
iv. Space wave
v. Surface wave.
Space Wave.
It is made up of direct wave and ground
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.
Wave that is guided along the earth’ s surface like
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.?
To minimize the fading and to avoid the
multi path interference the technique used are
diversity reception. It is obtained by two ways.
1. Space diversity reception
2. Frequency diversity reception.
3. Polarization diversity.
Space diversity Reception.
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 used in normally in microwave links, and
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.
Waves that arrive at the receiver after the
propagation through ionosphere is ionospheric
propagation..
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.
· 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
Due to the earth’ s magnetic fields, the
ionosheric medium becomes anisotropic and the incident
plane wave entering the ionosphere will split into ordinary
and extra ordinary waves/modes.
When these modes re-emerge from the
ionosphere they recombine into a single plane wave again.
Finally the plane of polarization will usually
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 )
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 , = 1- Ne2 / ( P v 2 2 )
>1H2 P 2 2 )]
Note: Conductivity is maximum means the wave will pass
through that medium.
Conductivity is small, waves gets reflected.
( 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 reflection and refraction of radio waves by
the ionosphere is a function of frequency.
Briefly describe the following.
i. Reflection at low frequency.
ii. Reflection at high frequency.
iii. Maximum usable frequency.
iv. Optimum frequency.
v. Skip distance.
Important formula for problems:
i Critical frequency,
fcr = 9¥1max
ii. Relative Dielectric constant r = 1- Ne2 P 2 + v2 )
iii. Phase constant,
b = (2p/l) ¥ - (fc / f)
iv. Wave impedance. 0 /¥ - (fc / f)
v. Wave velocity.
vp = c / ¥ - (fc / f)2
vi Group velocity,
vp vg = c2
vg = c2 / vp
vii. Incident angle .
sin i = ¥ - (fc / f)