The measurement of the characteristic impedance of transmission lines using nanoscale resistive films

The value of the characteristic impedance, Z₀, is well known for most transmission lines either from mathematical formulations or computer modelling (Wadell, 1991). So much so, that when an impedance, Z_L, is evaluated, a value of Z₀ is assumed and only a measurement of the reflection coefficient, ρ, is needed. However, there are instances where an experimental value of Z₀ is required, for example in the cases of some lines with attenuation or exotic lines where mathematical equations are not available and where computer models are not rigorous. This paper shows that by reversing the procedure described above, a value Z₀ can be found from a known value of Z_L, and a measurement of ρ. In order to demonstrate the validity of this technique, some well-characterised transmission lines were measured at the National Physical Laboratory (NPL), using nanoscale sheet resistances to give Z_L, and the results are given in this paper. The technique can now be extended to examine those instances mentioned above.