Transcript Chap2 P2 Basic Antenna Parameters

BASIC ANTENNA
PARAMETERS
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Presentation Outline
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Introduction
Reflection Coefficient
Impedance bandwidth
Radiation patterns
Field Regions
Beamwidth
Directivity
Antenna Efficiency
Antenna Gain
Polarization
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Introduction
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Antenna parameters: describe the performance of
an antenna
Definitions of the various parameters are necessary,
and some are interrelated.
Most parameters are derived from:
 Complex radiation pattern
 Gain (or efficiency)
 Impedance (or scattering parameters)
IEEE Standard Definitions of Terms for Antennas
(IEEE Std 145 – 1983)
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Reflection Coefficient
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Ratio of the reflected voltage amplitude vs the forward
voltage amplitude

V
V


Z L  Z0

Z L  Z0
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Impedance Bandwidth
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A range of frequencies, within which the antenna
characteristics (input impedance) conforms to
certain specifications (e.g. |Γ| = -10 dB):
f max
FBW 
f min
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For narrowband antennas, the FBW is expressed
as a percentage of the frequency difference over
the center frequency:
f max  f min
FBW 
100%
f0
f 0   f max  f min  2
f0 
f max f min
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Radiation Pattern
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Defined as “a mathematical function or a graphical
representation of the radiation properties of the
antenna as a function of space coordinates.
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Is determined in the far-field region.
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Represented as a function of the directional
coordinates.
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Radiation properties include power flux density,
radiation intensity, field strength, directivity, phase or
polarization.”
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Field Regions
Figure 1.11: Field regions of an antenna.
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Field Regions
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Reactive near-field region is defined as “that portion of
the near-field region immediately surrounding the
antenna wherein the reactive field predominates”.
R  0.62
D3

[1.10]
where λ is the wavelength
D is the largest dimension of the antenna
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For a very short dipole (or equivalent radiator) the
outer boundary is commonly taken to exist at a
distance λ/2π from the antenna surface.
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Field Regions
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Radiating near-field region (Fresnel) is defined as “that
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”.
0.62
D3

R
2D 2
[1.11]

where λ is the wavelength
D is the largest dimension of the antenna
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If antenna has a dimension that is not large compared
to wavelength, this region may not exist.
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Field Regions
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Far-field region (Fraunhofer) is defined as “that region
of the field of an antenna where the angular field
distribution is essentially independent of the distance
from the antenna”.
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R
2D

[1.12]
where λ is the wavelength
D is the largest dimension of the antenna
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The far-field patterns of certain antennas, such as
multibeam reflector antennas are sensitive to
variations in phase over their apertures.
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Field Regions
Typical changes of antenna amplitude pattern shape from reactive near field
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toward the far field.
Radiation Pattern: Types
Types of Radiation Patterns:

Power pattern: the trace of the angular variation of the
received/radiated power at a constant radius from the
antenna
 Amplitude field pattern: the trace of the spatial variation of
the magnitude of electric (magnetic) field at a constant
radius from the antenna.
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Often the field and power pattern are normalized
with respect to their maximum value, yielding
normalized field and power patterns.
The power pattern is usually plotted on a logarithmic
scale or more commonly in decibels (dB).
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Radiation Pattern: Types
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For an antenna, the
 Field
pattern (in linear scale) typically represents
a plot of the magnitude of the electric or magnetic
field as a function of the angular space.
 Power
pattern (in linear scale) typically
represents a plot of the square of the magnitude
of the electric or magnetic field as a function of
the angular space.
 Power pattern (in dB) represents the magnitude
of the electric or magnetic field, in decibels, as a
function of the angular space.
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Note: the power pattern and the amplitude field pattern
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are the same when computed and plotted in dB.
Radiation Pattern: Example
Figure 1.1: coordinate system for antenna analysis
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Radiation Pattern: Example 3D & 2D
Three and two dimensional power patterns (in linear scale)
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Radiation Pattern: Example 3D & 2D
3-D and 2-D patterns
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Plotting Radiation Patterns
Plotting the pattern
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Example: Monopole
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Example: Yagi antenna
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Example: Dipole antenna
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Radiation Pattern Characteristics
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The plus (+) and the minus (-) signs in the lobes
indicate the relative polarization of the amplitude
between the various lobes, which changes
(alternates) as the nulls are crossed.
To find the points where the pattern achieves its
half-power (-3 dB points), relative to the maximum
value of the pattern:
 Field pattern at 0.707 value of its maximum.
 Power pattern (in linear scale) at its 0.5 value of
its maximum.
 Power pattern (in dB) at -3 dB value of its
maximum.
The angular separation between the two half-power
points is referred to as HPBW.
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Radiation Pattern Characteristics
Two-dimensional normalized field pattern of a 10-element linear
array with a spacing of d = 0.25λ
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Radiation Pattern Characteristics
Two-dimensional normalized field pattern of a 10-element linear
array with a spacing of d = 0.25λ
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Beamwidth
• Defined as the angular separation between two identical
points on opposite side of the pattern maximum.
• The Half-Power Beamwidth (HPBW): in a plane
containing the direction of the maximum of a beam.
• First-Null Beamwidth (FNBW): the angular separation
between the first nulls if the pattern. Often
FNBW≈2⋅HPBW
• Used as a trade-off between it and the side lobe level; as
the beamwidth decreases, the side lobe increases and
vice-versa.
• Used to describe the resolution capabilities of the
antenna; to distinguish between two adjacent radiating
sources and targets.
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Beamwidth
Pattern beamwidth.
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Radiation Pattern Lobes
Radiation lobes and beamwidths of an antenna pattern
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Radiation Pattern Lobes
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A major lobe (also called main beam) is defined as
“the radiation lobe containing the direction of
maximum radiation”.
A minor lobe is any lobe except major lobe.
represent radiation in undesired directions, and they
should be minimized
A side lobe is “a radiation lobe in any direction other
than intended lobe”. Normally adjacent to the main
lobe and occupies the hemisphere in the direction of
the main beam. Largest of the minor lobes.
A back lobe is “a radiation lobe whose axis makes
an angle of approximately 180º with respect to the
beam of the antenna”.
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Radiation Pattern Lobes
Figure 1.6: Linear plot of power pattern and its associated lobes and
beamwidths.
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Radiation Pattern Lobes
Figure 1.7: Normalized three-dimensional amplitude field pattern (in linear
scale) of a 10-element linear array antenna with a uniform spacing of 0.25λ
and progressive phase shift (β = -0.6π) between the elements.
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Isotropic/Directional/Omnidirectional Patterns
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Isotropic pattern
 is the pattern of an antenna having equal radiation in
all directions.
 This is an ideal (not physically achievable) concept.
 Used to define other antenna parameters.
 It is represented simply by a sphere whose center
coincides with the location of the isotropic radiator.
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Directional antenna
 is an antenna, which radiates (receives) much more
efficiently in some directions than in others.
 Applies to antennas whose directivity is much higher
than that of a half wavelength dipole.
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Isotropic/Directional/Omnidirectional Patterns
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Omnidirectional antenna
 is an antenna, which has a non-directional pattern in
a given plane, and a directional pattern in any
orthogonal plane (e.g. single-wire antennas).
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Isotropic/Directional/Omnidirectional Patterns
Figure 1.8: Omnidirectional Antenna Pattern.
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Principal Patterns
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Principal patterns are the 2-D patterns of linearly
polarized antennas, measured in the E-plane (a plane
parallel to the |Ē| vector and containing the direction of
maximum radiation)

H
the H-plane (a plane parallel to the
vector,
orthogonal to the E-plane, and containing the direction
of maximum radiation).
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Principal Patterns
Figure 1.9: Principle E- and H-plane patterns for a pyramidal horn antenna.34
Polarization
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The polarization of the wave can be defined in terms
of a wave radiated (transmitted) or received by an
antenna in a given direction.
Polarization may be classified as linear, circular or
elliptical.
If the vector that describes the electric field at a
point in space as a function of time is always
directed along a line, the field is said to be linearly
polarized.
If the figure that the electric field traces is an ellipse,
the field is said to be elliptically polarized.
Linear and circular polarizations are special cases of
elliptical, and they can be obtained when the ellipse
become a straight line or a circle, respectively.
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Polarization
Types of polarization
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Polarization
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Polarization of an antenna in a given direction is
defined as “the polarization of the wave transmitted
(radiated) by the antenna. When the direction is not
stated, the polarization is taken to be the
polarization in the direction of maximum gain”.
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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.
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Polarization is the curve traced by the end point of
the arrow (vector) representing the instantaneous
electric field. The field must be observed along the
direction of propagation.
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Polarization
Rotation of a plane electromagnetic wave and its polarization
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Polarization
Rotation of a plane electromagnetic wave and its polarization
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Polarization
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At each point on the radiation sphere, the
polarization is usually resolved into a pair of
orthogonal polarizations, the co-polarization and
cross-polarization.
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Co-polarization represents the polarization the
antenna is intended to radiate (receive) while crosspolarization represents the polarization orthogonal
to the co-polarization
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Radiation Pattern Measurement
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Radiation Pattern Measurement
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Radiation Pattern Measurement
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Radiation Pattern Measurement
Calculated radiation patterns of a paraboloid antenna for different distances
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from the antenna.
Directivity
• Defined as “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.”
• The directivity of a non-isotropic source is equal to the
ratio of its radiation intensity in a given direction over that
of an isotropic source.
U 4U
D

U0
Prad
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Directivity
• If the direction is not specified, it implies the direction of
maximum radiation intensity (maximum directivity)
expressed as:
Dmax  D0 
U
max
U0
U max 4U max


U0
Prad
[1.28]
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)
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Antenna Efficiency
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Total efficiency of an antenna et is used to estimate the
total loss of energy at the input terminals of the antenna
and within the antenna structure.
Includes
all
mismatch
losses
and
the
dielectric/conduction losses (described by the radiation
efficiency, e, as defined by the IEEE Standards)
The overall efficiency:
e0  er ec ed
e0
= total efficiency (dimensionless)
er
= reflection (missmatch) efficiency (dimensionless)
ec
= conduction efficiency (dimensionless)
ed
= dielectric efficiency (dimensionless)
Γ
= voltage reflection coefficient at the input terminals of the
antenna.
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Antenna Efficiency
Reference terminals and losses of an antenna
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Antenna Gain
• G is the ratio of the radiation intensity U in a given
direction and the radiation intensity that would be
obtained, if the power fed to the antenna were radiated
isotropically.
U  ,  
G  ,    4
Pin
• Normally we deal with relative gain.
• Defined as “the ratio of the power gain in a given
direction to the power gain of a reference antenna in its
referenced direction”.
• The power input must be same for both antennas.
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Antenna Gain
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The reference antenna usually a dipole, horn, or any
other antenna whose gain can be calculated or it is
known.
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When the direction is not stated, the power gain is
usually taken in the direction of maximum radiation.
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Gain Measurement
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Popular: Gain comparison/substitution method
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A standard gain antenna (with known gain) is
used to calculate the gain of the antenna under
test (AUT)
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From the Friis Transmission equation:
Pr
  
 Got Gor 

Pt
 4R 
2
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Standard Gain Antennas
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Gain Measurement
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Step 1: Measure the standard gain antenna
(SGH) as the AUT
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Step 2: Measure the AUT
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Gain Measurements
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Step 3: Gain of AUT can be calculated using:
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Note:
 Tx and AUT needs to be co-polarized
 AUT need to be placed in the far field
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Extra Slides
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Radian & Steradian
• 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.
C  2r
•There are 2π rad (2πr/r) in a full circle.
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Radian & 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 circle of a sphere radius r that
is subtended by a spherical surface area equal to that of
square which each side of a length r.
A  4r
2
•There are 4π sr (4πr2/r2) in a closed sphere.
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Radian & Steradian
The infinitesimal area dA on the surface of a sphere of
radius r, is given by:
dA  r sin dd
2
(m2)
Therefore, the element of solid angle dΩ of a sphere can
be written as:
dA
d  2  sin dd
r
(sr)
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Radian & Steradian
Geometrical arrangements for defining a radian and a steradian
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