9.1 Introduction

This chapter gives information concerning some 'impedance' considerations that can be useful when making transmission line measurements at radio frequency (RF) and microwave frequencies. The subject matter is divided into the following three areas:

1. connectors - the mechanisms used to join together two or more transmission lines;

2. air lines - components used to define certain characteristics of transmission lines (such as impedance and phase change); and

3. RF impedance - special considerations needed at lower microwave frequencies (typically, below 1 GHz) when defining the impedance of electrical components and networks.

The treatment of connectors deals only with coaxial connectors used to perform precision transmission line measurements (e.g. of power, attenuation, impedance and noise) ^[1]. Similarly, only air lines used to realise standards of impedance for these connector types are considered. For both connectors and air lines, only the 50 ? variety is dealt with in any detail. Finally, the electromagnetic properties (such as characteristic impedance and propagation constant) of these lines at lower microwave and radio frequencies are considered.

9.2 Historical Perspective

The use of high-frequency electromagnetic signals dates back to the late nineteenth century and the experiments of Hertz [1] validating the theory of electromagnetic radiation proposed by Maxwell [2]. For a majority of these experiments, Hertz chose to use guided...