7.1 Introduction

The number of different transmission lines has greatly increased in recent years. This range of lines enables the microwave circuit designer to choose particular features which meet the specifications. For instance, it may be that certain values of characteristic impedance, phase constant, dispersion or attenuation are required. Or it may be that the ease with which these lines can be used to couple to various solid-state devices is important. Or, finally, it may be that the line has unique properties in certain configurations, for example, gives good coupling to other circuits or radiates in a special way. As many microwave circuits are now completely integrated so that the largest dimension of the whole circuit could be less than a millimetre, special lines are required to couple signals into them.

Measuring microwave circuits is still as important as ever. Almost without exception, most microwave measurement equipment has remained with the same input and output transmission lines. For most microwave frequencies, a coaxial output is used. Above 20 GHz, a waveguide output is sometimes used. One of the major problems in circuit measurement is designing transitions from the standard coaxial and waveguide ports to numerous transmission lines that now exist in modern circuits. In the case of integrated circuits special surface probes are used that involve a tapered transmission line. As many microwave measurements of impedance, noise, gain, etc. involve using transitions from either coaxial or waveguide to these other transmission lines the properties of transitions are...