ANTENNA THEORY ANALYSIS AND DESIGN Chapter.1 Sungjoon YOON 2015.07.06

Contents 1. Antennas 1.1 Introduction 1.2 Types of Antennas 1.3 Radiation Mechanism 1.4 Current Distribution on a Thin Wire Antenna Antennas & RF Devices Lab.

1.1 Introduction antenna transmission line Definition : “a usually metallic device for radiating or receiving radio waves.” by Webster’s Dictionary “a means for radiating or receiving radio waves.” by The IEEE Standard Definitions of Terms for Antennas Transitional structure between free-space and a guiding device antenna transmission line Figure 1.1 Antenna as a transition device. In other words the antenna is the transitional structure between free-space and a guiding device The guiding device or transmission line may take the form of a coaxial line or a hollow pipe (waveguide), and it is used to transport electromagnetic energy from the transmitting source to the antenna, or from the antenna to the receiver. Figure 1.2 Transmission-line Thevenin equivalent of antenna in transmitting mode.

Maximum power is delivered to the antenna under conjugate matching. 1.1 Introduction radiation conduction,dielectric losses Figure 1.2 Transmission-line Thevenin equivalent of antenna in transmitting mode. Maximum power is delivered to the antenna under conjugate matching.

1.1 Introduction standing wave The reflected waves from the interface create, along with the traveling waves from the source toward the antenna pockets of energy concentrations and storage Figure 1.2 Transmission-line Thevenin equivalent of antenna in transmitting mode. If the maximum field intensities of the standing wave are sufficiently large, they can cause arching inside the transmission lines.

1.2 Types Of Antenna 1.2.1 Wire Antennas 1.2.2 Aperture Antennas

1.2 Types Of Antenna 1.2.3 Microstrip Antennas 1.2.4 Array Antennas An aggregate of radiating elements in an electrical and geometrical arrangement (an array) results in the desired radiation characteristics.

1.2 Types Of Antenna 1.2.5 Reflector Antennas 1.2.6 Lens Antennas to communicate over great distance 1.2.6 Lens Antennas Primarily used to collimate incident divergent energy to prevent it from spreading in undesired directions

1.3 Radiation Mechanism total charge Q 1.3.1 Single Wire Basic sources of radiation: “motion of electric charge -> Current” Let us assume that an electric volume charge density, represented by , is distributed uniformly in a circular wire of cross-sectional area A and volume V, total charge Q Figure 1.9 Charge uniformly distributed in a circular cross section cylinder wire. If the current is time varying,

1.3 Radiation Mechanism 1.3.1 Single Wire To create radiation, there must be a time-varying current or an acceleration (or deceleration) of charge. 1. If a charge is not moving, current is not created and there is no radiation. 2. If charge is moving with a uniform velocity: a. There is no radiation if the wire is straight, and infinite in extent. b. There is radiation if the wire is curved, bent, discontinuous, terminated, or truncated, as shown in Figure 1.10. 3. If charge is oscillating in a time-motion, it radiates even if the wire is straight. The internal forces receive energy from the charge buildup as its velocity is reduced to zero at the ends of the wire. Charge acceleration is due to an exciting electric field and deceleration is due to impedance discontinuities or smooth curves of the wire.

1.3 Radiation Mechanism 1.3.2 Two-Wire Applying a voltage source connected to a two-conductor transmission line which is connected to an antenna. Figure 1.11 Source, transmission line, antenna, and detachment of electric field lines. The electric field has associated with it electric lines of force which are tangent to the electric field at each point and their strength is proportional to the electric field intensity. The electric lines of force have a tendency to act on the free electrons (easily detachable from the atoms) associated with each conductor and force them to be displaced. The movement of the charges creates a current that in turn creates a magnetic field intensity. Associated with the magnetic field intensity are magnetic lines of force which are tangent to the magnetic field. Electric charges are required to excite the fields but are not needed to sustain them and may exist in their absence. This is in direct analogy with water waves.

1.3 Radiation Mechanism 1.3.3 Dipole The electric lines of force are detached from the antenna to form the free-space waves. (assuming a sinusoidal time variation) If the disturbance persists, new waves are continuously created which lag in their travel behind the others. There is no net charge on the antenna, then the lines of force must have been forced to detach themselves from the conductors and to unite together to form closed loops.

1.4 Current Distribution On A Thin Wire Antenna The current in a half cycle of one wire is of the same magnitude but 180◦ out-of-phase from that in the corresponding half-cycle of the other wire. the current of each wire are essentially cancelled by those of the other. the two wires of the flared section are not necessarily close to each other, the fields radiated by one do not necessarily cancel those of the other. the fields radiated by the two arms of the dipole will primarily reinforce each other toward most directions of observation. Figure 1.15 Current distribution on a lossless two-wire transmission line, flared transmission line, and linear dipole.

1.4 Current Distribution On A Thin Wire Antenna The fields radiated by some parts of the dipole will not reinforce those of the others. As a result, significant interference and cancelling effects will be noted in the formation of the total radiation pattern. Figure 1.16 Current distribution on linear dipoles.

1.4 Current Distribution On A Thin Wire Antenna Figure 1.17 Current distribution on a λ/2 wire antenna for different times.

Thank you for your attention Antennas & RF Devices Lab.