Fundamental Properties of X-rays

Abstract

The beginning of wisdom is the definition of terms. —Socrates An incredible diversity of radiating structures can be found in modern communication and radar systems. Ignoring superficial geometrical or mechanical differences, what distinguishes these structures from each other? In later theoretical work we will find it convenient to classify antennas according to the analytical methods used, and/or the physical basis for the radiation. But in a practical sense, the most important distinguishing characteristics are those that describe the performance of the antenna in a real system. Our goal in this chapter will be to define several important parameters representing the radiation characteristics in transmission and reception, as well as the circuit properties of antennas, and show how these can be computed from a field analysis. This " black-box " description of the antenna will provide well-defined goals for later theoretical work, as well as providing the necessary tools for using antennas in practical systems. 3.1 TRANSMITTING PROPERTIES In a communications system, the antenna is a transducer that couples energy from one electronic system to another at some distant location. Two important aspects of the system are immediately apparent: 1) the antenna-field interaction, and 2) the antenna-circuit interaction. In the first case we are concerned with the directive properties of the antenna and the polarization; that is, we must determine in which direction and how effectively the antenna interacts with electromagnetic waves. In the second case we require the circuit properties of the antenna when used as both a transmitter or receiver; for example, we must know the equivalent impedance of an antenna as perceived by the generator, or the Thevenin equivalent circuit when used as a receiver. We will begin by considering the directive properties of the antenna.