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

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

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

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

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]

1.4 Current distribution on a thin wire antenna

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.

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.

2.2 Radiation Pattern

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.

2.2 Radiation Pattern

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 2.3 (b) Linear 2-D plot of power pattern and its associated lobes and beamwidths.

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.

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.

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

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

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

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

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.

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.

2.2.4 Field Regions

Thank you for attention