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Hanyang University ANTENNA THEORY by Constantine A. Balanis Chapter 1 Harim KIM 2017.01.20 1/40 Antennas & RF Devices Lab. Hanyang University Contents 1. Antennas 1.1 Introduction 1.2 Types of Antennas 1.3 Radiation Mechanism 1.4 Current Distribution on a Thin Wire Antenna 1.5 Historical Advancement 2/40 Antennas & RF Devices Lab. Hanyang University - Definition : “A usually metallic device for radiating or receiving radio waves.” “A means for radiating or receiving radio waves” Figure 1.1 Antenna as a transition device. Figure 1.2 Transmission-line Thevenin equivalent of antenna in transmitting mode. 3/40 Antennas & RF Devices Lab. Hanyang University Conduction, dielectric losses Radiation resistance Figure 1.2 Transmission-line Thevenin equivalent of antenna in transmitting mode. Imaginary part of impedance from radiation by the antenna - Maximum power is delivered to the antenna under conjugate matching. - Losses from line, antenna and the standing waves are undesirable. when 4/40 Antennas & RF Devices Lab. Hanyang University Figure 1.2 Transmission-line Thevenin equivalent of antenna in transmitting mode. - Maximum field intensities of the standing wave are sufficiently large 5/40 Antennas & RF Devices Lab. Hanyang University 6/40 Antennas & RF Devices Lab. Hanyang University 7/40 Antennas & RF Devices Lab. Hanyang University 8/40 Antennas & RF Devices Lab. Hanyang University 9/40 Antennas & RF Devices Lab. Hanyang University 10/40 Antennas & RF Devices Lab. Hanyang University How are the electromagnetic fields generated by the source. Conducting wires are - Current density 11/40 Antennas & RF Devices Lab. Hanyang University - 1. Charge is not moving No current, no radiation - 2. Charge is moving uniform velocity a. Wire is straight, and infinite, no radiation b. Wire is curved, bent, discontinuous, terminated or truncated it makes radiation. - 3. Charge is oscillating It radiates even if wire is straight. 12/40 Antennas & RF Devices Lab. Hanyang University - Apply voltage, it creates an electric field between the conductors. - Electric lines of force ∝ Electric Field Intensity - Electric charges are required to excite the fields but are not needed to sustain them and may exist in their absence. 13/40 Antennas & RF Devices Lab. Hanyang University - Dipole’s source - First, charge reached its maximum value - After next quarter, charge begins to diminish - Next quarter, charge have neutralized 14/40 Antennas & RF Devices Lab. Hanyang University - Traveling wave current flow to transmission line. At the end complete reflection occurs. - Reflected wave and incident wave combined and forms standing wave pattern. - (a) Net result is non-radiation - (b) Two line’s are not necessarily close to each other, the fields radiated. - (c) It’s called standing wave antenna 15/40 Antennas & RF Devices Lab. Hanyang University - (a) 𝑘𝑙/2 is very small, can approximate 𝑘𝑙 sin 2 ≅ 𝑘𝑙/2 - (b) 𝑙 = 𝜆/2 at feed point, max 𝐼 𝜆 2 - (c) < 𝑙 < 𝜆 - (d) λ < 𝑙 < 16/40 3𝜆 2 Antennas & RF Devices Lab. Hanyang University Thank you for your attention 17/40 Antennas & RF Devices Lab.