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```Hanyang University
Antenna Theory
By CONSTANTINE A.BALANIS
Ch1.4, 2.1~2.2.4
O Yeon Jeong
1/22
Antennas & RF Devices Lab.
Hanyang University
Contents
1. Antennas
1.4 Current Distribution on a Thin Wire Antenna
2. Fundamental Parameters of Antennas
2.1 Introduction
2.2.2 Isotropic, Directional, and Omnidirectional Patterns
2.2.3 Principal Patterns
2.2.4 Field Regions
2/22
Antennas & RF Devices Lab.
Hanyang University
1.4 Current distribution on a thin wire antenna
Linear dipole
A transmission lien terminated in an
open circuit.
Current variation along an open
circuited-transmission line.
(a) Current distribution on a lossless two-wire transmission
line

3/22
ZL -Z0
1
ZL +Z0
Antennas & RF Devices Lab.
Hanyang University
1.4 Current distribution on a thin wire antenna
Linear dipole
Current distribution on a flared transmission line, linear dipole.
4/22
Antennas & RF Devices Lab.
Hanyang University
1.4 Current distribution on a thin wire antenna
d
(usually

l 

)
50
10
kl
kl
kl
sin( )
, when
is very small
2
2
2
d : diameter of wire
l : length of dipole arm
 : wave length
k : wave number[1/m]
5/22
Antennas & RF Devices Lab.
Hanyang University
1.4 Current distribution on a thin wire antenna
6/22
Antennas & RF Devices Lab.
Hanyang University
CHAPTER 2
Fundamental Parameters of Antennas
To describe the performance of an antenna,
definitions of various parameters are necessary.
Parameter definitions will be given in this chapter.
7/22
Antennas & RF Devices Lab.
Hanyang University
•
•
•
A mathematical function or a graphical representation of the radiation
properties of the antenna as a function of space coordinates.
Determined in the far-field region
Represented as a function of the directional coordinates.
•
•
•
•
•
•
•
power flux density
Field strength
Directivity
Phase
Polarization
The radiation property of most concern is the two- or three dimensional
spatial distribution of radiated energy as a function of the observer’s
position along a path or surface of constant radius.
8/22
Antennas & RF Devices Lab.
Hanyang University
9/22
Antennas & RF Devices Lab.
Hanyang University
Amplitude field pattern : A trace of the received electric/magnetic
Amplitude power pattern : A graph of the spatial variation of the
power density along a constant radius
a. 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.
b. 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.
c. power pattern( in dB) represents the magnitude of the electric
or magnetic field, in decibels, as a function of the angular space.
10/22
Antennas & RF Devices Lab.
Hanyang University
|E|-pattern
| E|2 -pattern
11/22
| E|2 -pattern in dB scale
Antennas & RF Devices Lab.
Hanyang University
Radiation lobe : portion of the radiation pattern bounded by regions of relatively weak
Figure 2.3 (a) Radiation lobes and
beamwidths of an antenna 3-D
polar pattern
Figure 2.4 Normalized three-dimensional
amplitude field pattern( in linear scale)
Figure 2.3 (b) Linear 2-D plot of
power pattern and its associated
lobes and beamwidths.
12/22
Antennas & RF Devices Lab.
Hanyang University
•
Major lobe (main beam) : The radiation lobe containing the direction of
•
Minor lobe : any lobe except a major lobe.
• Side lobe : The radiation lobe in any direction other than the intended lobe.
(Usually a side lobe is adjacent to the main lobe and occupies the hemisphere
in the direction of the main beam.)
• Back lobe : The radiation lobe whose axis makes an angle of approximately
180◦ with respect to the beam of an antenna.
Usually it refers to a minor lobe that occupies the hemisphere in a direction
opposite to that of the major(main) lobe.
•
Minor lobes usually represent radiation in undesired directions, and they
should be minimized.
•
Side lobes are normally the largest of the minor lobes.
•
The level of minor lobes is usually expressed as a ratio of the power density
in the lobe in question to that of the major lobe. This ratio is often termed
the side lobe ratio or side lobe level.
13/22
Antennas & RF Devices Lab.
Hanyang University
2.2.2 Isotropic, Directional, and Omnidirectional Patterns
Isotropic antenna pattern
-
A hypothetical lossless antenna pattern having
- Ideal, not physically realizable.
- Often taken as a reference for expressing the
directive properties of actual antennas.
Directional antenna pattern
-
Having the property of radiating or receiving
electro-magnetic wave more effectively in some
directions than in others.
This term usually applied to an antenna whose
maximum directivity is significantly greater than
that of a half-wave dipole
Isotropic antenna pattern
Omnidirectional antenna pattern
-
Having an essentially non directional pattern in a given way.
Having a directional pattern in any orthogonal plane.
A Special type of a directional pattern.
14/22
Antennas & RF Devices Lab.
Hanyang University
2.2.2 Isotropic, Directional, and Omnidirectional Patterns
Directional pattern
Orthogonal plane
Nondirectional pattern
15/22
Antennas & RF Devices Lab.
Hanyang University
2.2.2 Principal Patterns
Polarization of an antenna in a given direction is defined as “the polarization of the
wave transmitted (radiated) by the antenna.
Polarization of radiated wave describes the oscillation direction and relative
magnitude of the electric field.
(a)
(b)
(c)
(a) Linear polarization
(b) Circular polarization
(c) Elliptical polarization
• Red and green line : Consisting of (one)two
orthogonal, in-phase components
• Purple line : Polarized along a plane
16/22
Antennas & RF Devices Lab.
Hanyang University
2.2.3 Principal Patterns
Linearly polarized antenna performance – principal E- and H- patterns
•
E-plane : the plane containing the electric-field vector and the direction of maximum
•
H-plane : the plane containing the magnetic-field vector and direction of maximum
Infinite number of principal E-planes
  c
17/22
One principal H-planes
  90
Antennas & RF Devices Lab.
Hanyang University
2.2.4 Field Regions
Reactive near-field region
-
Portion of the near-field region immediately
surrounding the antenna wherein the reactive field
predominates.
-
Region of the field of an antenna between the
reactive near-field and the far-field region.
Angular field distribution is dependent upon the
distance from the antenna.
Far-field(Fraunhofer) region

: Wave length
D : The largest dimension of the antenna
-
Region of the field of an antenna where the
angular field distribution is essentially independent
of the distance from the antenna
18/22
Antennas & RF Devices Lab.
Hanyang University
2.2.4 Field Regions
Hertz Dipole
Dipole placed at the origin of a spherical coordinate system
-
-
19/22
The electrostatic and the
induction fields together are
called the Near Fields.
the Far Fields.
Antennas & RF Devices Lab.
Hanyang University
2.2.4 Field Regions
Hertz Dipole
In near field
In far field
2 I 0l cos   jkr
e
j 4 kr 3
 I l sin 
E   0 3 e  jkr
4 kr
Er  0
Er 
| E | | E r 2|  | E |2 
-
j kI 0l sin   jkr
e
4 r
jkI 0l sin   jkr
H 
e
4 r
E 
 I 0l
1  3cos 2 
3
4 kr
The near field essentially stores the electromagnetic energy around
the dipole. But does not contribute to the power flow from the
antenna.
The far fields are not uniform in all directions. The field strength is
𝜋
maximum along 𝜃 = and zero along 𝜃 = 0, 𝜃 = 𝜋.
2
20/22
Antennas & RF Devices Lab.
Hanyang University
2.2.4 Field Regions
21/22
Antennas & RF Devices Lab.
Hanyang University
Thank you for attention
22/22
Antennas & RF Devices Lab.
```
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