32.1 Introduction

The range of frequencies defined as microwave is 3 to 30 GHz, where a gigahertz (GHz) is 1 billion (1 10 ⁹) cycles per second. The frequency range below this is called radio frequency (rf) and spans 300 kHz to 3 GHz; the range above it is called millimeter wave and spans 30 to 300 GHz.

The definition of a microwave device is not set by the frequency range of use; it is set by the design techniques used to create it and the manner in which it is applied. Microwave devices have been created and used in the rf range as low as 200 MHz and in the millimeter frequency range all the way to 300 GHz.

Microwave passive devices are used inside microwave measurement instruments, and they are used to combine instruments to create more complex measurement systems. In all cases, these devices will split, combine, filter, attenuate, and/or shift the phase of a microwave signal as it propagates through a particular transmission system. This chapter describes the most useful and prevalent transmission systems for microwave signals and the passive devices that are most effectively achieved in each system.

32.2 Coaxial Transmission Lines

A coaxial transmission line, or coaxial cable, is used to transport electromagnetic power from one place to another (one device to another) with minimum loss of power and maximum isolation from other electromagnetic signals that may be present.