N. I COLLEGE OF ENGG.,KUMARACOIL.
Dept. of ECE
ANTENNAS AND PROPAGATION(EC 431)
UNIT I
1.Define Directivity.
Directivity D of an antenna is given by the ratio of the maximum
radiation intensity (power per unit solid angle).
4
D=
A
2.State the ‘Friss Transmission formula’.
Aet.Aer
Pr=Pt
.
(W)
2
Pr=Received power (W)
Pt=Power into transmitting antenna(W)
Aet=Effective aperture of having antenna
Aer=Effective aperture of receiving antenna
=Distance between antenna.
=Wavelength between antenna.
3.What is cross-field? When does it occur?
For elliptical and circular polarization, the electric field vector E at a fixed-point
rotates with time in a plane perpendicular to direction of wave propagation, When E
rotates in a plane parallel to the direction of wave propagation. This condition is called
cross-field. This situation can occur if there is an E in the direction of propagation.
4.Define ‘field pattern’ and ‘power pattern’.
A trace of received power at a constant radius is called power pattern. A graph of
spatial variations of electric/magnetic field along a constant radius is called an
amplitude field pattern.
5.What is an ‘isotropic radiator’?
An isotropic (or) isotropic antenna is defined as hypothetical loss less antenna
having equal radiation is all direction.
It is an ideal and not physically realizable. It is often taken as a reference antenna.
6.What is a ‘radiation lobe’?
A radiation lobe is a ‘portion of the radiation pattern bounded by regions of
relatively weak radiation intensity’.
7.List the type of antenna field region.
The three principal antenna full region are,
a. Reactive near field
b. Radiating near field
c. Far field.
8.Define ‘fresnal region of antenna’.
Radiative near field defined as that region of field of an antenna between reactive
near field region and the far field region where in radiation field predominates and the
angular field distribution on distance of antenna.
9.Fill up.
a. Measure of phase angle is Radian
b. Measure of solid angle is Steradian
c. Poynting vector is a power density.
d. Isotropic radiator radiates power in all direction equally.
e. The radiation intensity is also related to far/franhofer zone E field.
10.Define ‘1’radian.
One radian is defined as the solid angle with its vertex at the center is of a sphere
of radius ‘r’ that is subtended by a spherical surface area equal to half of a square with
each side of length’r’.
11.Define radiation intensity.
Radiation intensity in a given direction is defined as the power radiated from an
antenna per unit solid angle. The radiation intensity is a far field parameter and it can
be obtained by simply multiplying the radiation density by the square of distance.
U=r2rad
where,
U=radiation intensity (w/unit solid angle)
rad= radiation density(w/m2)
12.Define beam solid angle.
The beam solid angle is defined as the solid angle through which all the power
of antenna would flow its radiation intensity is constant for all angles with in A.
13.Define ‘Half power beam width’.
A half power beam width is defined as “in a plane containing the direction
of maximum of a beam, the angle between 2 direction in which the radiation
intensity is one half the maximum value of the beam.
14.Define ‘beam efficiency’.
It is a parameter which is used to describe the quality of transmitting and
receiving antenna .It is defined as ratio of power transmitted with in cone angle to
power transmitted by antenna.
Power transmitted within cone angle 1
BE transmitting
=
Power transmitted by antenna
antenna
Power received within cone angle
BEReceiving
=
Power received by antenna
antenna
15.Define ‘band width of an antenna’.
The bandwidth of an antenna is defined as “the range of frequencies with in which
the performance of the antenna with respect to some characteristics conform to a
specific standard.
16.Define polarization efficiency (or) polarization loss factor PLF.
It is defined as the radio of power received by an antenna from a given phase
wave of arbitrary polarization to the power that would be received by an antenna of
same power flux density and direction of propagation. It is denoted by Pe.
le Einc 2
Pe=
Le 2 Einc
2
Le =vector effective length of antenna.
Einc=Incident electric field.
17.Define input impedance of antenna.
Input impedance is defined as the impedance presented by an antenna at its
terminal or the ratio of voltage to current at a pair of terminal of the ratio appropriate
components of electric to magnetic fields at a point.
ZA=RA+ jXA
where,
ZA=Antenna impedance at terminal a-b
XA=Antenna reactance at terminal a-b
RA=Antenna resistance at terminal a-b
18.What is antenna temperature?
Every object with a physical temperature above absolute zero radiates energy. The
amount of energy radiated is usually represented by an equation temperature TB called
brightness temperature. The temperature appears at the terminal of antenna is
antenna temperature.
TB(,)
=(1-  2)Tm
19.Define ‘system temperature of antenna’. Give its significance.
It is temperature of the antenna terminal, when antenna terminal, transmission
line, and receiver are connected together.
UNIT II
1.what is point source?
It is the wave originate at a fictitious volume less emitter source at the center 0 of the
observation circle.
2.what is a power pattern?
A graph of (sr)(i.e) radial component ,at a contact radius as a function of angle is a
pointing vector, or power density pattern. this is called as power pattern.
3.compare isotropic and an isotropic point sources?
Isotropic source
1.an isotropic source is a source that radiates energy uniformly in all directions.
2.this is not physically realizable.
An isotropic point source
1.the property of the antenna to radiate more energy insome directions only ran in other
is called an isotropic source.
2.these are physically realizable.
4.what is the total power radiated by isotropic source?
If the pointing vector is known at all ports on a sphere of radius ‘r’ from a point source in
a loss less medium,the total power radiated by the source is integrated over the surface of
the sphere of the radius components (sr) of the average poynting vector.
P= ∫∫ s.ds
Or
P=∫∫ sr .ds
Where p=power radiated.
Sr=radial components of average pointing vector.
Ds= infinitismiley element of area of sphere.
5.define the directivity of the hemispheric source?
Um/uo=directivity.
The ratio of (um)to (uo).it is ratio of maximum radiation intensity to its average radiation
Intensity.
6.compare the directivity of a source with a) hemispheric power pattern.
b) source with unidirectional cosine power
pattern.
c) source with doughnut power pattern.
a) total power radiated by hemispheric source
2пUm=4пUo (or)
Um/uo=2;
b) total power radiated by unidirectional cosine power pattern
пUm=4пUo
Um/uo=4
7.list all possible cases of using 2 isotropic point sources
They are five classes
a) two isotropic point sources of some amplitude a phase.
b) two isotropic point sources of same amplitude but opposite phase.
c) two isotropic point sources of same amplitude and in phase quadrature.
d) two isotropic point sources of equal amplitude and in any phase difference.
e) two isotropic point sources of unequal amplitude and any phase difference.
8.what is pattern multiplication concept?
The array of antenna elements with isotropic point sources contribute to increase in
directivity in the desired direction. The total field of antenna array is the vector sum of
those individual radiating sources. The total field pattern of non isotropic but similar
sources in multiplication of the individual source patterns and the pattern of array of
isotropic point sources each located at the phase center of individual sources. The total
phase pattern is the addition of the phase pattern of individual sources and that of array of
isotropic point sources. This is called pattern multiplication.
9.list some of the advantages of pattern multiplication?
The pattern multiplication technique offers the following advantage.
a) Gives the speedy method of sketching the pattern of complicated array just by
inspection.
b) .Proves to be useful tool in designing antenna.
c). Accurate method since the multiplication of pattern is carried out point by point.
10) What is EFA?
EFA-End Fire Array
To make to field a maximum in direction of array substitute ψ =0 ø =0 is below, we get
the condition for EFA
Ψ=dr cosφ+ό
We get ό=-dr
Hence resulting in an array called EFA. For an end fire array the phase between sources
is retarded progressively by same amount as spacing between sources.
11) Write down the expression for determining the1) equivalent effective aperture 2)Defa
a)
Ae=D(λ*λ)/4п
Where D is the directivity of dipole
λ is the wavelength.
b) Defa the directivity of ordinary EFA
Defa= п L λ √(8 L λe )
Where
L λ is length
Beam width length L λ e
12) How the secondary lobes are eliminated?
The minor lobes are secondary lobes can be eliminated if
a). The spacing between the two consecutive elements does not exceed half the
wavelength and
b).The current amplitude in the radiating sources are proportional to the coefficient of
successive terms in binomial series.
13).What is Dolph tchebysheff array?
Tchebysheff polynomial is the design of linear in phase antenna array. For a linear in
phase broad side array it is possible to minimize the beam width of the mainlobe
(BWFN).Using tchebysheff array the one can produce narrowest beam width for a given
side lobe or vice versa.
14)Define ‘R’ or ‘Lobe ratio’ ?
R is defined as the ratio of main lobe maximum to the minor lobe level.
R=main lobe level/side lobe level
UNIT III
1.List factors involved in choosing an antenna
There are many factors in choosing an antenna. Some of them are
a. Electrical Characteristics
1.Radiation efficiency
2 Maximum antenna gain
3.Polar diagram
4.Antenna impedance
5. Frequency
b. Structural Characteristics
1.Supporting structure
2.Economical design
c. Installation space
1.Fixed stationary area(Large/small)
2.Mobile
2.What is corona discharge? Give its significance.
If the antenna voltage is high (say order of 30 kv /cm) it will ionize the near by air resulting
in a faint bluish discharge due to near by air ionization . This is called corona discharge.
Presence of corona discharge is undesirable. It increases Q, which causes antenna current to
decay slowly.
3. Fill up:
a. The Fundamental radio antenna is a Metal rod /Tubing or wire
b. The relation between frequency and wave length ‘s c=  or =c/
where c= velocity of light
 = Frequency
 = Operating wavelength
c. Half wave dipole is also called Hertz antenna
4. Mention two simple fundamental antennas
a. /2 (or) Hertz antenna
b. /2 (or) Marconi antenna
5.What is ‘Hertz antenna’?
Hertz antenna is most common, popular ,high frequency antenna .Any antenna complete
is itself capable of self oscillation. Such as half (or) full wavelength (/2) is called Hertz
antenna.
6. What is a Marconi antenna?
When an antenna utilizes the ground (earth) as a part of its resonant circuit it is Marconi
antenna.
Eg: A(/4) quarter wave antenna is Marconi antenna.
7. What is a half wavelength dipole?
A half wavelength dipole (or) half wave dipole or half wave doublet (or) simply dipole
(or) doublet is defined as a symmetrical antenna in which the two ends are at equal
potential w.r.to center point. This is the unit from which many many complex antenna
can be constructed.
8.What is ‘end effect’?
End effect may be regarded as a dielectric effect of the air at the end of the antenna
that effectively lightened it. The result of end effect is to make the half wavelength
antenna wire act as if it were about 5% longer than it actually is. End effect produces
interference between exciting current and oscillating current.
9.How do you get practical  /2 length?
In order to get practical /2 length the free space value must be multiplied by a factor of
k.
/2=(492*k)/f (mHz) (or) 468/f (mHz) feet
Where the value of ‘k’ depends on thickness of conductor and operating frequency.
10.What is ‘two wire folded dipole’?
If two /2 antenna wires are used then the structure is called two wire folded
dipole. As the number of wire increases, it offers a good match with 600.
11.What is a loop antenna? Give its application.
The loop antenna is a radiating coil of any convenient cross section of one/more
turns carrying radio frequency current. It may take any shape, on either on ferrite or air
core. Loops are extensively used in radio receivers, aircrafts, direction finding.
12.Fill Up:
a. A loop of more than one turn is often called frame.
b. The radiation efficiency of closed loop antenna is low for transmission
application.
c. The normal polarization mode of helical antenna is circular polarization.
13.What are direction finders?
A radio direction finder is used to determine the direction of arrival (DOA) of a
radio signal. The receiver and the process of finding the direction of incoming radio
signal from a transmitter and hence its position is a radio direction finder.
14.List the functional blocks of direction finder.
Direction finder has three functional units.
a. A directional antenna.
b.
A receiver.
c. A direction indicator.
15. State the emf equation of loop antenna.
eo=[(2AN/)(Em/2)cos ]sin (t+/2)
(Effective height of loop)
Where Erms= Em/2
=Number of wavelength in meters.
A= Area of loop
N=Number of turns
= Angle between place of loop and direction of antenna.
16.Write down the expression for instantaneous electric field at a larger distance ‘r’ from
a loop of any radius ‘a’?
E=(60[I]a/)J1(a .sin)
Where ,
J1(a sin)= Bessel’s function of first order.
r = distance
a=radius.
17.What is helical antenna?
Helical antenna is another basic type of radiator and is the simplest antenna
to provide circularly polarized wave. Helical antenna is broad band VHF and UHF
antenna to provide circular polarization.
18.List any two-prominant modes of radiation in helical antenna?
There are two prominent modes of radiation in helical antenna.
a. Normal/Perpendicular mode of radiation.
b. Axial or end fire/ Beam mode of radiation.
19. Mention some applications of helical antenna.
The following are some applications of helical antenna.
a. Used to receive or transmit VHF signal.
b. Space communication application.
20.List the important characteristic features of helical antenna.
The following are important characteristic features of helical antenna.
a. Wide band width.
b. Simplicity.
c. High Directivity.
d. Circular Polarization.
e. Can be used as array.
f. Efficient.
g. Capable of receiving arbitrary polarization.
21.Fill Up:
a. The intrinsic impedance of free space is pure resistance.
0/H=(0/0)=377
b. Small circular and square loops are identical provided that both have same area.
c. To find radiation resistance of loop antenna , the pointing vector is integrated
over a large sphere.
d. The radiation efficiency of a multi turn loop/ coil antenna can be increased by
introducing ferrite rod.
22.Define Q.
‘Q’ is defined as the ratio of energy stored to energy lost per cycle.
Q=2f0L/(Rr+RL+Rf)
Q=f0/fHP
Where, f0= circular frequency (Hz), L= Inductor, fHP is bandwidth of half power.
23.What is monofilar?
Monofilar /unifilar means a single wire or a single conductor used to distinguish
the one wire helix helices from two or more wires.
24.Distinguish transmission mode and radiation mode.
Transmission mode
1. The transmission mode is used to
describe the manner in which an
Radiation Mode
1.The term radiation mode is used to
describe the general form of the far field
electro magnetic wave is propagated
along an infinite helix.
2.Variety of transmission modes is
possible.
pattern of a finite helical antenna.
2.There are two possible modes of
radiation.
a. Axial/ beam mode(R1)
b. Normal mode of relation(R0)
25.Write note on monofilar axial mode helical antenna.
The radiation from helices with circumference of the order 1(N1) and
number of turns (n>1) is well-defined beam with a maximum in direction of helix axis.
This type of operation is called the “axial” (as beam) mode of radiation. This monofilar
axial mode helical antenna is very non-critical and easiest antenna to build which is
circularly polarized.
26.Write the expression for calculating the total radiation pattern of axial mode monofilar
helical antenna.
E=[sin (900/n)](sin (n/2)/sin (/2)).cos
Where, n= Number of turns.
27. State the important conditions for circular polarization for helical antenna,
Two main conditions for circular polarization are.
a. The radiation in the axial direction from a helical antenna of any pitch
angle and of an integral number of ‘1’ or more turns will be circularly
polarized by k=-1.
b. The radiation in the axial direction from a helical antenna of any pitch
angle and a large number of turns which are not necessarily an integral
number is nearly circularly polarized if k is nearly –1.
28.Write the expression for phase constant and phase difference of an open wire
transmission line or helix.
a. Phase constant ()
=2/0P=360/0P (deg/m)
b. Phase difference ()
=2/0P=360/0P.S (deg)
where, 0=free space wave length
P=relative phase velocity
S=Spacing between sources.
29.Write the general equation relating the electrical distance (free space) and mode
number.
Cos = 1/P + m/(S/0)
Where, S= Spacing between sources
M= Mode number between sources
P=relative phase velocity
0=free space wave length
30. Why with frequency change the beam does not swing the broadside and remains
locked on end fire?
With the frequency change, the beam does not swing the broadside and
remains locked on end fire because,
a. Phase velocity changes automatically by just the right amount to not
only compensate for frequency change but also to provide increased
directivity and super gain.
b. The monofilar helical antenna’s input impedance is an almost constant
resistance over an octave bandwidth.
Note: The above are important properties of the monofilar axial mode helical antenna.
Unit: IV
1.What is Yagi-Uda antenna?
Yagi uda antenna /yagi antenna are high gain antenna. It consists of a driven
element , a reflector and one or more directors.Yagi uda antenna is an array of a driven
element and one or more parasitic elements.
2.List some of the non-electrical radiational characteristics of yagi uda antenna.
The following are some of the non-electrical radiational characteristics of yagi
uda antenna.
a. Light weight
b. Simple to build
c. Low cost
d. Simplicity in feed design
e. Array length space can be varied.
f.
3.Why yagi uda antenna is called ‘super directive’ or ‘ super gain’ antenna?
Yagi uda antenna is also known as super directive or super gain antenna , due
to its high gain and beam width per unit area of array. This antenna provides directive
gain appreciably greater than that obtained from uniform distribution. Hence it is super
directive or super gain antenna.
4.What is turnstile antenna?
Turnstile antenna is two half wave dipole place at right angles to each other in
the same plane excited 90 out of phase with each other. The dipoles carry equal current
in magnitude but in phase quadrature.
5.What is the application of turnstile array antenna?
A turnstile antenna is used for transmission of FM signal and television
broadcast signal.
6.Define a frequency independent antenna.
A frequency independent (FI) antenna may be defined as “ the antenna for
which the impedance, pattern and directivity remain constant as a function of the
frequency”. For an antenna to be FI, the antenna should expand or contract in proportion
to wavelength.
7. What is log periodic antenna?
The geometry of log periodic antenna structure is chosen such that the
electrical properties most repeat periodically with the logarithm of frequency. The design
of low periodic antenna involves a basic geometry structure that is repeated but with
change in size of structure, by a constant scale factor.
8.Mention the applications of log periodic antenna.
a. Log periodic antenna lie mainly in field of high frequency communication.
b. Log periodic is also used in television reception.
c. Best suited for all round monitory in which case a single L-P antenna will
cover all the higher frequency bands.
9.Mention some advantages of log periodic antenna.
Advantages of log periodic antenna are:
a. Ease in design.
b. No power is wasted in terminating resistance.
c. All channels even up to UHF band.
10.List the parameters involved in designing the rhombic antenna.
a. The tilt angle ()
b. Length of leg (L)
c. Height above the ground (h)
11. List the advantages of the rhombic antenna.
a. The input impedance of radiation pattern do not change rapidly over a
considerable frequency range when compared to resonant dipole.
b. Gives highly directional broad band characteristics.
c. Very efficient and widely employed.
d. Cheap or easy to erect.
e. Input impedance is twice that of a single side radiation.
f. Vertical angle of radiation is low.
12.List some disadvantages of rhombic antenna.
a. Requires large space for installation.
b. Besides major lobe , a large number of minor lobes are present and reduces
radiation efficiency.
c. Power wastage in terminal resistance.
13.What is horn antenna?
A horn antenna is required as a flared out or opened out wave guide. A wave
guide is capable of radiating into open space provided the same is excited at one end and
opened at other end.
14.List the various types of horn.
a. Sectorial H- plane horn
b. Sectorial E-plane horn
c. Pyramidal horn
d. Path difference
e. Conical horn
15.List some applications of horn
a. Horn antenna are used at microwave frequencies under the condition of
moderate power gain
b. Standard for calibrating other antenna
c. Feed for reflectors and lens
d. Pick up horn for sampling power
e. Transmitting / receiving antenna
16.List the practical antennas in microwave frequency.
There are three practical antennas in microwave frequency.
a. Parabolic reflector (microwave dish)
b. Lens antenna
c. Horn antenna
17.List some special parabolic reflectors.
There are three prominent special parabolic reflectors. They are:
a. Cut parabolic reflector (or) truncated paraboloid
b. Parabolic cylindrical reflector with source of ‘n’ dipole
c. Pill box or cheese antenna
18.Define beam width.
Beam width is the angular width between half power points. Thus half power
beam width corresponds to 3dB down /-3db.
19.List the method used to determine directivity.
The following are used to determine directivity.
a. Orange slice method
b. Conical cut method
20.Calculate the directivity of 20 turn helix having =12, circumference=.
Directivity of a helical antenna is
D= 15NSC2 /3
Where, C=  ,= 12, tan =S/C
S= C tan *0.2126
D=15*20*0.2126*2/3
D=10 log 63.78=18.4 dB
21. Estimate the diameter of a paraboloidal reflector required to produce a beam of 5o
width at 1.2 GHz. How would you make this reflector?
BWFN= 5o
F= 1.2 GHz
= 300/1.2*103 =3/12 =1/4
BWFN=5o=140/D
D= 140*1/(4*5)
=7 meters.
22.Mention some antennas used in medium frequencies.
There are 4 antennas used in medium frequencies.
a. Centre feed / folded dipole
b. BSA
c. Rhombic antenna
d. 3 vertical radiators of length (1/6  to 5/8 )
23.List the antennas used in UHF and UHP range
The antenna used in UHF and UHP range is
a. Half wave antenna
b. HF loops
c. Parabolic reflectors
d. Slot radiators
e. Yagi antenna
f. Turn stile antenna
24. What is slewing?
Slight change in the angle of maximum radiation can be effective to an array
antenna by suitably shifting the phase of feed to adjacent section of array. This is called
slewing
25.What is CATR ?
CATR is Compact Antenna Test Range, is a collimating device which generates nearly
planar wave fronts in a very short distance say 10 to 20 meters, compared to 2D2/
26.Define radiation efficiency?
Radiation efficiency is defined as the ratio of total power radiated by antenna to
total power accepted by antenna.
Radiation Efficiency = Gain/Directivity.
Unit: V
1.List the prominent modes of propagation.
There are three prominent modes of propagation.
a. Ground wave / Surface wave propagation
b. Sky wave / Ionospheric Propagation
c. Space wave Propagation
2. What is ground wave/ Surface wave propagation?
The ground wave is a wave that is guided along the surface of the earth.
Surface wave permits the propagation around the curvature of the earth. This mode of
propagation exist when transmitting and receiving antenna are close to surface of the
earth.
3.What is space wave?
Space wave is a type of ground wave . It is made up of direct wave, the
signal that travels the direct path from the transmitter to the receiver and the ground
reflected wave, which is the signal arriving at the receiver after being reflected from the
surface of the earth.
4.What is critical frequency?Give its significance.
For any given layer the highest frequency that will be reflected back for vertical
incidence fcr =(81 Nmax)1/2.
fcr-critical frequency
Nmax-Maximum ionization density
Critical frequency for any layer represents the height frequency
6.What is skip distance?
The distance within which a signal of given frequency fails to be reflected back is the
skip distance for that frequency. Higher the frequency the greater the skip distance.
7. What is MUF?
The maximum frequency that can be reflected back for a given distance of transmissions
is called maximum usable frequency.
MUF=fcrsecI
fcr- critical frequency
I-angle of incidence
The maximum usable frequency from layer is greater than the critical frequency by factor
of secI.
8. List some of irregularities variations of ionoshere.
Sudden ionospheric disturbance
Gradual disturbance
Ionospheric storms
Polar cap absorption.
9. What is faraday rotation?
Any linearly polarized wave may regarded as the vector sum of two counter –rotating
circularly polarized wave. If such a wave propagation is in the direction of H field in
lossless plasma, the two circularly polarized components will travel at different phase
velocity and thus the plane of polarization will rotate with distance . This phenomenon is
called Faraday rotation.
10.Whaty is Fading
The strength of a sky wave signal varies with time because of fluctuations in the
properties of the the ionosphere and multipath transmission. The relatively given period
variations is called fading.