Antenna

1. ANTENNA
ANJU IQUBAL
ASSOCIATE PROFESSOR
DEPT OF ECE

4.  Basic antenna parameters
 Antenna Arrays
 Antenna parameter measurements
 Different types of antennas
 Radio wave propagation

5.  Balanis, Antenna Theory and Design, 3/e, Wiley
Publications
 John D. Krauss, Antennas for all Applications, 3/e,
TMH.

6.  Antenna is the
transitional structure
between a guiding
device and free-space
or vice versa

7.  To create radiation , there must be an acceleration
of charge or a time varying current .
 If a charge is not moving , there is no electric current
and thus there will not be any radiation
 If a charge is moving with uniform velocity
 There is no radiation if the wire is straight
 There will be radiation if the wire is curved , discontineous
terminated or truncated .
 If a charge is oscillating , it radiated even if the wire is
straight

8. ANTENNA PARAMETERS
 Radiation pattern
 Beam width
 Radiation power density
 Radiation Intensity
 Directivity
 Gain
 Beam solid angle
 Radiation resistance
 Effective length
 Effective area
 Antenna temperature
 Polarization

9. RADIATION PATTERN
 Mathematical function or a graphical representation of
the radiation properties of the antenna as a function of
space coordinates.
 Radiation pattern is determined in the far field region
and is represented ass a function of the directional
coordinates
 A trace of the received electric (magnetic) field at a
constant radius is called the amplitude field pattern
 A graph of the spatial variation of the power density
along a constant radius is called an amplitude power
pattern.

10.  Often the field and power patterns are normalized with
respect to their maximum value, yielding normalized
field and power patterns.

11. RADIATION PATTERN

14. RADIATION PATTERN LOBES
 A radiation lobe is a “portion of the radiation pattern
bounded by regions of relatively weak radiation
intensity.”

17.  A major lobe ( main beam) is defined as the
radiation lobe containing the direction of maximum
radiation
 A minor lobe is any lobe except a major lobe
 A back lobe is a radiationlobe whose axis makes an
angle of approximately 180◦ with respect to the
main lobe of an antenna

18. TYPES OF RADIATION PATTERN
 An Isotropic radiator is defined as a hypothetical
lossless antenna having equal radiation in all directions.
 A directional antenna : having the property of radiating
or receiving electromagnetic waves more effectively in
some directions than in others
 An Omnidirectional antenna : having an essentially
nondirectional pattern in a given plane (in this case in
azimuth) and a directional pattern in any orthogonal
plane (in this case in elevation). An omnidirectional
pattern is then a special type of a directional pattern

22. FIELD REGION

23.  The space surrounding an antenna is usually
subdivided into three regions:
 (a) reactive near-field,
 (b) radiating near-field (Fresnel)
 (c) far-field (Fraunhofer) regions

24. FIELD REGION
 Reactive near-field region : portion of the near-field region
immediately surrounding the antenna wherein the reactive field
predominates
 Radiating near-field (Fresnel) region : region of the field of an
antenna between the reactive near-field region and the far-field
region wherein radiation fields predominate and wherein the
angular field distribution is dependent upon the distance from the
antenna.
 Far-field (Fraunhofer) region : region of the field of an antenna
where the angular field distribution is essentially independent of
the distance from the antenna

26. RADIAN
 The measure of a plane angle is a radian.
 One radian is defined as the plane angle with its
vertex at the center of a circle of radius r that is
subtended by an arc whose length is r.

27. STERADIAN
 The measure of a solid angle is a steradian.
 One steradian is defined as the solid angle with its
vertex at the center of a sphere of radius r that is
subtended by a spherical surface area equal to that
of a square with each side of length r

29.  The infinitesimal area dA onthe surface of a sphere
of radius r
 The element of solid angle dΩ of a sphere

30. RADIATION POWER DENSITY
 Electromagnetic waves are used to transport
information through a wireless medium or a guiding
structure, from one point to the other.
 The time average Poynting vector (average power
density)
 The 1/2 factor appears because the E and H fields
represent peak values, and it should be omitted for
RMS values

31.  imaginary part represent the reactive (stored)
power density associated with the electromagnetic
fields.

32. AVERAGE POWER RADIATED BY AN ANTENNA

33. PROBLEM 1
The radial component of the radiated power density
of an antenna is given by
where A0 is the peak value of the power density, θ is
the usual spherical coordinate, and ar is the radial unit
vector.
Determine the total radiated power.

36.  An isotropic radiator is an ideal source that radiates
equally in all directions
 The total power radiated
 The Average power density

37. 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
 It can be obtained by multiplying the radiation
density by the square of the distance.

38.  U = radiation intensity (W/unit solid angle)
 Wrad = radiation density (W/m2)
 The total power is obtained by integrating the
radiation intensity, over the entire solid angle of 4π
 dΩ = element of solid angle = sinθ dθ dφ

39.  For an isotropic source U will be independent of the
angles θ and φ, as was the case for Wrad.
 The radiation intensity of an isotropic source as

40. DIRECTIVITY
 The ratio of the radiation intensity in a given
direction from the antenna to the radiation intensity
averaged over all directions
 The average radiation intensity is equal to the total
power radiated by the antenna divided by 4π
 If the direction is not specified, the direction of
maximum radiation intensity is implied

41.  If the direction is not specified, it implies the direction of
maximum radiation intensity
 D = directivity (dimensionless)
 D0 = maximum directivity (dimensionless)
 U = radiation intensity (W/unit solid angle)
 Umax = maximum radiation intensity (W/unit solid angle)
 U0 = radiation intensity of isotropic source (W/unit solid
angle)
 Prad = total radiated power (W)

43.  For an isotropic source, the directivity is unity

44. PROBLEM

48. PROBLEM

50. BEAM SOLID ANGLE
 The beam solid angle ΩA is defined as the solid
angle through which all the power of the antenna
would flow if its radiation intensity is constant (and
equal to the maximum value of U) for all angles
within ΩA
 The total radiated power

52.  For antennas with one narrow major lobe and very
negligible minor lobes, the beam solid angle is
approximately equal to the product of the half-
power beamwidths in two perpendicular planes

55. RADIATION RESISTANCE
 Antenna is a device which interfaces the circuit and
space . From the circuit point of view the antenna
appears to the transmission line as a resistance Rr
called the Radiation resistance
 It is a fictitious resistance which when substituted
with an antenna will consume the same power as
actually radiated by the antenna

56. INPUT IMPEDANCE
 The impedance presented by an antenna at its
terminals or the ratio of the voltage to current at a
pair of terminals or the ratio of the appropriate
components of the electric to magnetic fields at a
point.

59. ANTENNA EFFICIENCY
 Losses associated with an antenna
 Reflection due to mismatch between transmission line
and the antenna
 I2R losses (Conduction and dielectric losses )

60.  Total efficiency

61. ANTENNA RADIATION EFFICIENCY
 The resistance RL is used to represent the
conduction-dielectric losses.
 The conduction-dielectric efficiency ecd is defined as
the ratio of the power delivered to the radiation
resistance Rr to the power delivered to Rr and RL

62. GAIN
 the ratio of the Radiation intensity, in a given
direction, to the radiation intensity that would be
obtained if the power accepted by the antenna were
radiated isotropically
 The gain of the antenna is closely related to the
directivity
 it is a measure that takes into account the efficiency
of the antenna as well as its directional capabilities.

64.  When the direction is not stated, the power gain is
usually taken in the direction of maximum radiation
 Gain is always less than or equal to Directivity

65. EFFECTIVE LENGTH
 Effective length of a linearly polarized antenna
receiving a plane wave in a given direction is
defined as the ratio of the magnitude of the open-
circuit voltage developed at the terminals of the
antenna to the magnitude of the electric-field
strength in the direction of the antenna polarization

67.  Voc = E le
 Voc = open-circuit voltage at antenna terminals
 E = incident electric field
 le = vector effective length

68. EFFECTIVE AREA
 The ratio of the available power at the terminals of
a receiving antenna to the power flux density of a
plane wave incident on the antenna from that
direction, the wave being polarization-matched to
the antenna.

69.  maximum effective aperture

70. POLARIZATION
 Polarization of an antenna ina given direction is
defined as “the polarizationof the wave transmitted
(radiated) by the antenna
 In practice, polarization of the radiated energy
varies with the directionfrom the center of the
antenna, so that different parts of the pattern may
have different polarizations.

71.  Polarization of a radiated wave is defined as that
property of an electromagnetic wave describing the
time-varying direction and relative magnitude of the
electric-field vector
 specifically, the figure traced as a function of time
by the extremity of the vector at a fixed location in
space, and the sense in which it is traced, as
observed along the direction of propagation.

73. DUALITY OF ANTENNAS

74. RECIPROCITY
THEOREM

75.  If an EMF is applied to the terminals of an antenna
A and the current measured at the terminals of
another antenna B , then equal current ( in both
amplitude and phase ) will be obtained at the
terminals of the Antenna A if the same EMF is
applied to the terminals of Antenna B .

76. PROBLEM 1
 The maximum radiation intensity of an antenna with
efficiency 90% is 200mW / unit solid angle .Find
directivity and gain when
 Case 1. Input power = 125.66 mW
 Case 2. Radiated power = 125.66 mW

79. PROBLEM 2
 (a)Estimate the directivity of an antenna with θHP
= 2°, φHP = 1°, and
 (b) find the gain of this antenna if efficiency is 0.5.

81.  D0 = 41253 / ( 1 x 2 ) = 20626.5 = 43.14 dB
 G = 0.5 * 20626.5 = 10313 = 40.13 dB

82. PROBLEM 3
 What is the maximum effective aperture of a
microwave antenna with a directivity of 900 (in
terms of operating wavelength )

83.  Ae = 71.6 λ2

84.  Thank You