Methods of measuring S-parameters of materials using vector network analyzer
Before proceeding to the measurements of the S-parameters of the sample, it is necessary to make sure that the sample is smooth, homogeneous and has the shape of a cylindrical tube. In this case the data obtained will most accurately correspond to the model used in calculating complex dielectric permittivity and magnetic permeability.
It is also necessary to calibrate the vector circuit analyzer before the start of measurements, which will significantly increase the accuracy. Note: If you selected Unknown adapter (unknown THRU) parameter during calibration, the "effective" length of the Ls measuring cell (in the "Epsilon-Mu Calculator" section) may differ from the physical length.
The vector network analyzer with the coaxial cell connected to it looks as follows:
Figure 1 - Vector network analyzer with coaxial measurement cell
The sample to be tested is placed inside the coaxial cell. It is important not to damage the central contact of coaxial cable, but also to make sure that all elements of the structure fit tightly together and that the axial symmetry of the measuring cell with the sample is not broken anywhere.
During the experiment, it is necessary to fix and save the following parameters, which will be needed in the future for Epsilon-Mu calculations:
- Measuring cell length Lc.
- Sample thickness Ls.
- Distance from the port 1 calibration plane to the sample Lpos. Although this parameter is only used in the NRW algorithm, in any case we recommend that you fix and save the position of the sample in the cell whenever possible.
The S-parameters of the system are then measured and should be saved in .s2p format for further calculations (see the How it works? section).
The general view of a typical coaxial airline measurement cell with the male N-type connectors is presented in Figure 2. After the calibration procedure is performed, the calibration planes are usually located to be in the vicinity of the geometrical boundaries of the cell. In some cases (for example if the "Unknown THRU" option is used during the calibration) the effective length of the cell may be estimated experimentally by measuring group delay of an empty cell.
Figure 2 - Coaxial measurement cell (airline)
Tou achieve consistent repeatable experimental results the studued samples must meet the following requirements in case of mentrioned above standart coaxial line:
- outer diameter 7.00-0.05 mm
- inner diameter 3.04+0.05 mm