Friday, September 16, 2011

ANTENNA & WAVE PROPAGATION (2 marks question and answer)

1) What is magnetic vector potential?
The source for magnetic vector potential is current element. The
current having both magnitude and direction. Hence the potential in the case
of magnetic field is magnetic vector potential.
The expressions for magnetic vector potential (A)
A = μ/401 J/r dV (volts /metre)
2) What is a monopole antenna?
A monopole antenna consists of one half of a dipole (usually a short
vertical) antenna mounted above the earth or group (reflecting plane).
3) What is a quarter wave monopole?
One-quarter wavelength long .it is also called as Marconi antenna.
4) What is a hertz antenna?
Effective length or height of the antenna becomes half of the actual
length.
5) Define electric scalar potential.
The source for producing a electric potential is charge. The charge is
having magnitude only. Hence the potential in the case of electric charge is
electric scalar potential.
E= -;V
Unit is weber/m²
6) Write down the expressions for magnetic vector potential using three
standard current distributions.
i) for volume current
A = μ/40 1 J/r dV (volts /metre)
ii) for line current
A = μ/40 1 I/r dl (volts /metre)
iii) for surface
A = μ/40 1 K/r dS (volts /metre)
7) What is retarded current?
Since current flowing through a short dipole is assumed to be constant
throughout its length, the effect of this current is not felt instantaneously at a
Distant point P, but only after an interval equal to the time required for the
wave to propagate over a distance r. This current is called as retarded
current.
Expressions for retarded current
[I] = Im e jw(t- r/c) : r/c – retardation current.
8) Define hertzian dipole?
Hertzian dipole is defined as a short linear conductor or short electric
dipole , whose length is very short compared to wavelength(l < current is assumed to be constant throughout its length.
9) Write down the expressions for radiated fields of a half wave dipole
antenna(G/2) .
E I =60 Im /r [( cos(0/2 cosI)/sinI]
H , = Im /20r [( cos(0/2 cosI)/sinI]
10. State reciprocity principle.
If a current I1 at the terminals of antenna no.1 induces an emf E21 at the
open terminals of antenna no.2 and a current I2 at the terminals of antenna
no.2 induces an emf E12 at the open terminals of antenna no.1 then E12= E21
provided I1= I2
11. Define antenna .
An antenna is a structure that has been designed to radiate or receive
electromagnetic waves.
12. Define Radiation pattern
An antenna radiation pattern is defined as a mathematical function or a
graphical representation of the radiation properties of the antennas a
function of space coordinates.
The radiation properties include power flux density, radiation intensity, field
strength,directivity, phase or polarization.
13). What are the two types of radiation pattern.
Field pattern
If the radiation from the antenna is expressed in terms of the field
strength(either E or H ) then the radiation pattern is called as field pattern.
Power pattern
If the radiation from the antenna is expressed in terms of power per unit
area, then the resultant pattern is power pattern.
14) Define Induction field(near field)
The field which predominates at the points closer to the current element
where r is small is known as induction field. The near field is inversely
proportional to square of the distance(1/ r²).
15) Write down expression for retarded vector potential.
Retarded vector potential [A] = μ/40 1v ( Jm e jw(t- r/c) ) / r.
16) Write down the expression for radiation resistance of a hertzian dipole
Radiation resistance Rr = 80 0² ( L / G)² ohms
17) Define retardation time
It is the time required for the wave to propagate over the distance r. It is
given by r/c where c is 3*108 m/s.
18) What is the radiation resistance of a current element whose overall
length is G/50.
Radiation resistance Rr = 80 0² ( L / G)² ohms
Given L = G /50.
Ans = 0.316 ohms
19) Calculate the physical height of a halfwave dipole (G/2) having
antenna Q of 30 and bandwidth of 10 MHz.
f = Q. BW
G= c / f ; ans: 0.5 m
20) Calculate the bandwidth of a 50 cm long half wave diploe having a Q of
15.
Bandwidth = f / Q;
Height of a half wave dipole (G/2) = 50 cm.
f = C/ G
ans : 20 MHz.
21) If the electric field strength(E) of a plane wave is 5
volts/metre,calculate the strength of a magnetic field(H)in free space.
E I = 5 v
R = 120 0 = E I / H S
22). What is the Special feature of folded dipole antennas?
Folded dipole has got a versatile quality of providing any desired input
impedance by increasing the number of elements and selecting suitable
lengths of dipole.
23). What are the Advantages of folded dipole? AN.U.2003
The radiation pattern of a folded dipole and a conventional half wave
is same but the input impedance of the folded dipole is higher. It has low
directivity and bandwidth than simple dipole.
24). What is Application of folded dipole?
Folded dipole is used as a driven element in yagi-uda antenna. It has
built in impedance transforming properties, which makes it easier to match a
transmission line that feeds the antenna .
Unit.ii
1.What is a Loop antenna? AN.U.2003
The 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 likes rectangular, square, triangular, hexagonal and circular loop, most
popular out of these are square and circle.
2.What are the Applications of loop antenna?
1.They are seldom employed for transmission in radio communicating
they are used in such application; it is usually in receiving mode where
antenna efficiency is not important
2.They also use it for finding the direction.
3. What is meant by Broadband antenna?
Antenna which maintain certain required characteristics like gain,
front to back ratio, SWR, polarization, input impedance and radiation pattern
over wide range of frequencies are called wide band or broad band antenna.
EG: Log-periodic
5. What is the Special feature of folded dipole antennas?
Folded dipole has got a versatile quality of providing any desired
input impedance by increasing the number of elements and selecting suitable
lengths of dipole.
6. What are the Advantages of folded dipole? AN.U.2003
The radiation pattern of a folded dipole and a conventional half wave
is same but the input impedance of the folded dipole is higher. It has low
directivity and bandwidth than simple dipole.
7. What is Application of folded dipole?
Folded dipole is used as a driven element in yagi-uda antenna. It has
built in impedance transforming properties, which makes it easier to match a
transmission line that feeds the antenna.
8. What are the disadvantages of loop antenna?

Only suitable for LF & MF except for a very small distance.
Transmission efficiency is poor & hence cannot be used for UHF
unless
design is changed.
Spurious induced voltages are produced, if nearby loop, wires &
conductors are present.
It is subjected to antenna effect & night effect, which are minimized
by balancing & using Adcock antenna for direction finding.

9. What is Adcock antenna? & how it eliminates the effects of loop antenna?
AN.U.2003
It consists of 2 spaced vertical antenna.In vertical polarization the
action is same as loop antenna. The horizontally polarized down coming sky
waves have no effect on this antenna due to its circuit arrangement. If the
horizontal wires are placed inside a metal tube shielded from the radio
waves, this antenna system becomes insensitive to horizontally polarize
down coming sky waves & thus it is free from polarization effect.
10. compare half wave dipole & folded dipole? AN.U.2005
s.no Half wave dipole Folded dipole
1. Directivity is unidirectional Directivity is bi-directional, but
because of the distribution of
currents in the parts of folded
dipole, the input impendence is
higher.
Used as impedance matching
devices.
2 Broadness in bandwidth is less
than folded dipole.
Broadness in bandwidth
11.What is a normal mode of helix antenna?
Radiation field is maximum in the direction normal to the helix axis &
wave is nearly or exactly circularly polarized wave . Mode is said to be
normal if the dimensions of helix is small compared with the wavelength.
12. What is a axial mode of helix antenna?
Radiation field is maximum in the end fire direction.(ie) along the
helix axis & polarization is circular or nearly circular.
13. How bandwidth & efficiency is increased? AN.U.2005
Efficiency is increased by increasing the dimension & current feed in
phase with helix axis.
Bandwidth is increased by providing phase shifter at regular intervals.
14.How the axial mode is improved?
When the helix circumference D & spacing S of order of one
wavelegth. Axial mode of radiation is produced by raising helix
circunference (c/G) of order of one wavelength & spacing is approximately
of G/a.
15.Define radiation efficiency
Radiation efficiency=Max power gain/directivity
16.What is the error introduced in direction finding of loop antenna?
Antenna or Vertical effect
Night effect or Aeroplane effect or polarization error.
17. What is the radiation resistance of multi turn loop antenna? & how it
change the o/p power?
Rr = 31,200 (NA/ G 2 ) 2 N= number of turns , G= wavelength, A=
effective area.
18. What is the principle of pattern multiplication?
In case of isotropic antenna arrays the total field of the antenna array
is simply the vector sum of those of individual radiating sources.
In case of non isotropic, but similar sources I s the multiplication of
the individual source patterns and the pattern of an array of isotropic point
sources, each located at the phase centre of individual sources and having
relative amplitude and phase. Whereas the total phase pattern is the
addition of the phase patterns of the individual sources and tat of the array
of isotropic point sources
19. What is the general effect of earth on antennas?
Effect of earth modifies the following:
1.Currents flowing in reflecting surface.
2.Radiation pattern.
20. What is an “array factor”?
It is apparent that the total field of an array is equal to equal field of a
single element positioned at the origin multiplied by a factor which is widely
referred as array factor.
21. What are the disadvantages of binomial arrays?
1. As HPBW increases and hence directivity decreases.
2. For design of a large array, larger amp ratio of sources is required.
22. What is the disadvantage of a binomial array?
A major practical disadvantage of binomial array is the wide
variations between the amplitudes of the different elements of an array,
especially for an array with a large no of elements. This leads to very low
efficiencies, and makes this method not very desirable in practice.
23. What is a binomial array?
The coefficients of binomial expansion represent relative amplitudes
of the elements. Since the coefficients are determined from the binomial
series, expansion the array is known as binomial array.
24. What are the types of antenna arrays?
1.Broad side array
2.End fire array
3.Collinear array
4.Parasitic array
25. Define the length of an array
The length of an array is the length measured along the axis of the
array from the first element to the last element.
26. What is array factor or space factor?
The factor by which the antenna array increases the field strength over
that of a single element radiating the same total power is called array factor
or space factor.
27. Define antenna array. AN.U.2003
Antenna array is an arrangement of several individual antennas so
spaced and phased that their individual electric field contributions combine
in one preferred direction and cancel in all other directions to get greater
directivity.
28. How to convert broad side array radiation pattern into unidirectional?
AN.U.2004
Bidirectional pattern of a broad side array can be converted into
unidirectional by installing an indentical array behind this array at a distance
G/4 and exciting it by current leading in phase by 90*.
29. What are the applications of arrays?
Because of their physical size, their use is generally limited to HF
bands and higher but they are used in LF bands for point to point
communications. They are also used in overseas broadcasting stations in
MF and HF bands for repeated broad casting to some distant area.
30.Give the formula for beam width between first nulls(BWFN) of a broad
side array.
BWFN=115/(L/G)
L=length of array
G=wave length
31.Give the formula for BWFN of an end-fire array.
BWFN=115/X(L/G)
32.Give the formula for half power beam width of a broad side array.
BWFN=51/(L/G)
L=length of array
G=wave length.
33.Give the formula for HPBW of an end-fire array.
HPBW=52/X(L/G)
34.Give the formula for calculating directive gain of broad side array.
Directive gain=2L/G(when the elements along with axis parallel to
array axis)
Directive gain=4L/G(when the elements at right angles to array axis)
35.What is the formula for directive gain when the elements right angels to
array axis?
Directive gain=4L/G
36. What is a collinear array? AN.U.2004
In this array the antennas are arranged co-axially and the elements are
fed with equal inphase currents. The radiation pattern of a collinear array
has circularly symmetry with its main lobe every where perpendicular to the
principle axis.
37. When will be the gain of the collinear array be maximum?
When the spacing between elements is of the order of 0.3G to 0.5G.
38. Define antenna .
An antenna is a structure that has been designed to radiate or receive
electromagnetic waves.
39. Define Radiation pattern
An antenna radiation pattern is defined as a mathematical function or
a graphical representation of the radiation properties of the antennas a
function of space coordinates.
The radiation properties include power flux density, radiation intensity, field
strength,directivity, phase or polarization.
40. What are the two types of radiation pattern.
1) Field pattern
If the radiation from the antenna is expressed in terms of the field
strength(either E or H ) then the radiation pattern is called as field
pattern.
2) Power pattern
If the radiation from the antenna is expressed in terms of power per
unit area, then the resultant pattern is power pattern.
41. What is broadside array?
The simplest array consist of a number of dipoles of equal size,equally
spaced along a straight line with the dipole fed in the same phase from the
same sources. Such an arrangement is called broad side array.
42.What is an end-fire array?
In the end-fire array each element is fed with same magnitude but
different phase and this phase is progressive from left to right.
43.What is the direction of maximum radiation is maximum only in one
direction that is in the direction of array axis.
(180 & 360 (or) 0 degree)
44.What are the characteristics of an end-fire array and broad side array?
1.linear 2.Resonant 3.Narrow bandwidth
45.What is a collinear array? AN.U.2004
In this array the antennas are arranged co-axially and the elements are
fed with equal inphase currents. The radiation pattern of a collinear array
has circularly symmetry with its main lobe every where perpendicular to the
principle axis.
46. When will be the gain of the collinear array be maximum?
When the spacing between elements is of the order of 0.3G to 0.4G

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