**Standard designation:** National Standard of Reflection and Transmission Coefficient

**Internal identifier:** ECM 240-2/03-023

**Year of approval:** 2003

**Department:** Primary metrology of RF electrical quantities 1013, Regional branch Prague

**Person in charge:** Ing. Karel Dražil, Ing. František Hejsek (2001 – 2008)

**Number of CMC rows:** 6

Basic metrological parameters of the standard:

Frequency range | 0.1 MHz - 50 GHz |

Reflection coefficient measurement range | 0 - 1 |

Uncertainty (k = 2) | 0.002 - 0.038 |

Transmission coefficient measurement range | 0.0003 - 1 |

Uncertainty (k = 2) | 0.0003 - 0.003 |

Measurement of the reflection and transmission coefficients (scattering parameters, s-parameters) is of crucial importance for measurement of RF power and electromagnetic field strength and it is also often required by laboratories dealing with, e.g., electromagnetic compatibility (EMC).

Precise air lines (coaxial or waveguide) together with a convenient calibration method (usually TRL) comprise the primary standard of impedance (reflection coefficient) at microwave frequencies. For determination of the reflection coefficient phase, short-circuited sections of transmission lines are used. Both the characteristic impedance of the air lines and the phase of the short-circuited transmission line can be calculated from their geometrical dimensions. The measurment accuracy of the dimensions is in the order of micrometers (three-coordinate measurement machine). The linearity of the vector network analyser is also important for provision of the s-parameters measurement accuracy. The linearity measurement is performed using calibrated attenuators (traceable to the RF power standard).

The characteristic impedance *Z _{0}* of a lossless coaxial transmission line is defined by

where:

| ...........…. | inner diameter of the outer conductor, |

| ...........…. | outer diameter of the inner conductor, |

| ...........…. | permeability of the dielectric |

| ...........…. | permittivity of the dielectric |

Losses caused by the skin effect are taken into account as well, as they may change the values of characteristic impedance and propagation constant, respectively (both the characteristic impedance and the propagation constant are complex numbers).

The national standard comprises the HP 8510SX and E48364B vector network analysers and additionally the BM553 vector analyser with a directional coupler and symmetrical power divider. Other parts of the national standard comprise coaxial calibration kits (N, PC7, PC3.5, PC2.4) and waveguide calibration kits (R220, R320, R400). The traceability is achieved through measurement of geometrical dimentsions of the air lines and short-circuited sections of transmission lines for all types of calibration kits (Department of technical length, CMI).

The national standard has successfully participated in a KCDB key comparison. Participation in further comparisons is under preparation.