In the previous post, the basics of dielectric material properties were discussed as well as a few possible applications of RF dielectrics. This week’s post will focus on common RF dielectric material characteristic measurement techniques. Namely, the transmission line methods involving transmission and reflection measurements, open-ended coaxial probe method, free-space methods, and resonant methods. For these types of dielectric parameter extraction VNA Test Coaxial Cable Assemblies, High-speed End Launch Connectors, Coaxial GS/GSG Probes and Probe Positioner, and various Waveguide hardware may be needed.
Transmission/Reflection Line Methods
This method can make use of transmission line or waveguide structures to extract the dielectric constant, loss tangent, and even magnetic permeability of known dielectric structures. In the case of coaxial or transverse electromagnetic (TEM) mode planar transmission lines, all of the dielectric material properties can be extracted with the proper techniques over the entire TEM frequency range. In the case of waveguides, the dielectric property extraction is band limited to single mode operation. It is possible to design transmission line and waveguide structures such that dielectric sample can be inserted into the test structures and measured accordingly, though in some cases it is necessary to design and fabricate these types of test structures so that the dielectric is an integrated part of the transmission line or waveguide.
Essentially the transmission/reflection methods involve placing a dielectric within a transmission line or waveguide and using a VNA to measure the transmission and reflection response. Typically, the measurements taken are the complex scattering parameters (S-parameters) S11 and S21. These results must then be run through extraction algorithms that require accurate models of the transmission line or waveguide structures, conductor properties, and precise physical measurements of the structure. The VNA must be calibrated in advance of the measurement, with the calibration plane ideally reaching the ends of the dielectric test structure. Further embedding of the interconnect between the VNA lines and the dielectric test structure can be done by testing structures that only include the interconnect transition area, hence bringing the calibration plane right to the dielectric test zone.
Ultimately, the accuracy of these techniques depend on the transmission line/waveguide models, physical measurements, extraction technique, and VNA performance. Depending on how the transmission line or waveguide structure is fabricated, destructive measurements may be necessary to determine the actual physical measurements of the structure, especially in the case of embedded or layered planer transmission lines or waveguides.
Open-ended Coaxial Probe Method
The open-ended coaxial probe method of dielectric characteristic measurement is a non-destructive and relatively flexible approach. Unlike the transmission/reflection line methods, the coaxial probe method can be used to measure virtually any specimen or dielectric material that is of adequate thickness and the probe can be pressed against or immersed in. This method only uses the reflection coefficient of single-ended VNA measurement to determine the dielectric constant of the sample.
This test method requires that the system is calibrated such that the reflection coefficient measurements are referenced to the probe aperture plan, which typically requires a reference dielectric liquid and calibration standards compatible with open-ended coaxial probes. As this method can only extract dielectric properties from a reflection measurement, it is only possible to measure dielectric constant this way. However, this method also doesn’t require machining or complex sample preparation, and samples can be solid, liquids, or even gasses with the appropriate setup. Contamination of the calibration dielectric standards is a concern with this method, as well as any contamination of the sample.
Stay tuned for Part 3 of “Highlights of Measuring Dielectrics With RF Equipment”