The impedance of any transmission medium can be stated in a number of ways depending upon the requirement. For standards purposes it is most useful to give the scattering parameters or voltage reflection coefficient that would apply if an item was connected to the ideal of the medium of interest.

The impedance of a simple transmission line can, in principle, be calculated in terms of its dimensions and the materials of which it has been constructed. However, the uncertainties involved in such a calculation increase rapidly with the number of different materials involved in the construction and the complexity of the electromagnetic fields.

For simple structures, such as coaxial lines or rectangular waveguides, impedance standards can be constructed whose behaviour can be calculated in terms of the fundamental SI quantities of mass, length, time and current. Examples of these standards are air lines, waveguide spacers and plain short-circuits. There is also a second class of standards whose behaviour is predictable but not strictly calculable, e.g. open-circuits with collet depressors and offset short-circuits.

These items can be used to determine the magnitudes and phases of the errors in a reflectometer so that the errors can be numerically eliminated. The degree to which a perfect reflectometer can be created by this method depends upon the accuracy of the mathematical model of the standards, the precision of the standards and the resolution and repeatability of the measuring equipment. The success in approximating a perfect reflectometer can be determined by analysis of the residual errors.

The Impedance section is currently involved in:

• research and development of primary and working standards for coaxial lines from 7/16 to 1.85 mm, rectangular waveguides from WG 16 (R100) to WG27 (R900), and coplanar waveguides;
• assessment of the performance of impedance measurement systems such as vector reflectometers and network analysers;
• error modelling, uncertainty assessment and residual error determination;
• establishing traceability to national standards for impedance measurement systems;
• participating in international impedance measurement comparison exercises.

Relevant clubs:

• Network Analyser Metrology interest group – ANAMET