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# Introduction to Antennas and Radiating Systems

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### Introduction to Antennas and Radiating Systems

1. 1. Introduction to Antennas and Radiating Systems Caleb Wherry∗ Austin Peay State University Department of Computer Science William Cooke† Austin Peay State University Department of Physics and Astronomy (Dated: May 1, 2011) Abstract A basic overview of antennas and radiating systems is presented. The concept of radiation fromaccelerating charges and basic antenna concepts and design are explained. An example half-wavedipole is analyzed. 1
2. 2. INTRODUCTION Antennas are everywhere. From the depths of space to the depths of Earth’s oceans.With the recent emphasis on wireless devices antennas have taken on an ever-increasingimportance. In this paper we will brieﬂy overview what an antenna is, how they work andsome performance criteria, and analyze a simple example.WHAT IS AN ANTENNA To understand how an antenna works, it is important to ﬁrst understand its purpose. Anelectromagnetic wave can radiate through free space and an electric current can ﬂow througha conductive path. The antenna’s purpose is to serve as an interface, or transducer, betweenthe current and the free space wave. It transforms the wave to and from the current[1]. The current ﬂows in a transmission line or other circuit. A transmission line may becoaxial cable or parallel wires or some other medium with a well deﬁned, complex impedance.It is an important part of the antenna system. A free space wave consists of ﬁelds instead of currents. The energy is carried in the ﬁelds.Just as the transmission line has a characteristic impedance, so does free space. As shownby Kraus[1], it is a pure resistance of µ0 / 0 = 377Ω. (1) To convert the current to a wave or vice-versa requires an antenna. A wave imposed onan antenna causes a current to ﬂow within the conductor. The current is proportional tothe ﬁeld strength at a given time and location. Conversely, when a current oscillates in theconductor the accelerating charges emit radiation.HOW AN ANTENNA WORKS Consider an electric dipole separated by a distance, d[2]. By driving the charges to andfrom opposite ends in a sinusoidal motion with angular frequency ω, E and B ﬁelds willbe created that follow the charges. The E ﬁeld is parallel to the movement, the B ﬁeld isperpendicular, and they are in phase. After each complete cycle, the ends of the ﬁeld lines 2