TABLE OF CONTENTS From this point of view, the arms of an antenna form the banks of a channel in which the waves excited by the source are confined before they emerge into unlimited space. In this sense antennas are similar to waveguides.
Sergei Schelkunoff, 1952
The discovery of the relationship between voltage ( V) and current ( I) by Georg Simon Ohm (1789 1854) was a giant leap forward in understanding electrical physics. Ohm placed the concept of electrical resistance ( R) on a mathematical footing by identifying it as the ratio of voltage to current:
| (3.1) |  |
Heaviside generalized Ohm's concept of electrical resistance to the case of alternating current and dubbed it "impedance." Later, Schelkunoff extended Heaviside's transmission-line impedance concept to include the impedance of fields in free space [1], Schelkunoff identified the field impedance ( Z) as the ratio of the electric ( E) to the magnetic ( H) field intensities:
| (3.2) |  |
The concept of impedance bridges the gap between fields guided by transmission lines on the one hand and fields propagating in free space on the other. Both the RF system engineer and the antenna designer need to understand impedance to achieve a good, high-efficiency match between a UWB antenna and an RF front end.
This chapter will begin by introducing the idea of antenna impedance, by discussing some common transmission line geometries and by providing examples of how to couple transmission lines into antenna elements. Then, this chapter will examine...
