1. Antenna Theory
By CONSTANTINE A.BALANIS
Ch1.4, 2.1~2.2.4
O Yeon Jeong

2. Contents Antennas 1.4 Current Distribution on a Thin Wire Antenna
2. Fundamental Parameters of Antennas
2.1 Introduction
2.2 Radiation Pattern
2.2.1 Radiation Pattern Lobes
2.2.2 Isotropic, Directional, and Omnidirectional Patterns
2.2.3 Principal Patterns
2.2.4 Field Regions

3. 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

4. 1.4 Current distribution on a thin wire antenna
Linear dipole
Current distribution on a flared transmission line, linear dipole.

5. 1.4 Current distribution on a thin wire antenna
d : diameter of wire
l : length of dipole arm
: wave length
k : wave number[1/m]

6. 1.4 Current distribution on a thin wire antenna

7. Fundamental Parameters of Antennas
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.

8. 2.2 Radiation Pattern Antenna (radiation) pattern Radiation properties
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.
Radiation properties
power flux density
Radiation intensity
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.

9. 2.2 Radiation Pattern

10. 2.2 Radiation Pattern
Amplitude field pattern : A trace of the received electric/magnetic field at a constant radius
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.

11. 2.2 Radiation Pattern

12. 2.2.1 Radiation Pattern Lobes
Radiation lobe : portion of the radiation pattern bounded by regions of relatively weak radiation intensity
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 (b) Linear 2-D plot of power pattern and its associated lobes and beamwidths.

13. 2.2.1 Radiation Pattern Lobes
Major lobe (main beam) : The radiation lobe containing the direction of maximum radiation.
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.

14. 2.2.2 Isotropic, Directional, and Omnidirectional Patterns
Isotropic antenna pattern
A hypothetical lossless antenna pattern having
equal radiation in all directions.
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.

15. 2.2.2 Isotropic, Directional, and Omnidirectional Patterns
Orthogonal plane
Nondirectional pattern

16. 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
Blue line : Electric field of a radiated/received wave
Red and green line : Consisting of (one)two orthogonal, in-phase components
Purple line : Polarized along a plane

17. 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 radiation
H-plane : the plane containing the magnetic-field vector and direction of maximum radiation
Infinite number of principal E-planes
One principal H-planes

18. 2.2.4 Field Regions 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 and the far-field region.
Radiation fields predominate.
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
: Wave length
D : The largest dimension of the antenna

19. 2.2.4 Field Regions Hertz Dipole
Dipole placed at the origin of a spherical coordinate system
The electrostatic and the induction fields together are called the Near Fields.
The radiation fields are called the Far Fields.

20. 2.2.4 Field Regions Hertz Dipole
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 maximum along 𝜃= 𝜋 2 and zero along 𝜃=0,𝜃=𝜋.

21. 2.2.4 Field Regions

22. Thank you for attention