Measurement of the dielectric constant in a microwave near field

Abstract

This study describes a method based on the measurement of a reflection coefficient to determine the dielectric constant of a sample by near-field microwave. A mathematical relation between the reflection coefficient and the parameters of a lumped circuit was derived referring to an impedance model of the probe−sample interaction. Theoretical analysis was performed to verify the equivalent model of the interaction between the probe and the sample which is equivalent to the parallel connection of multiple capacitors in a lumped circuit. By considering an equivalent model between the probe and the sample in the near field, the [inline-formula] parameter measured by the instrument was converted into the impedance parameter between the needle tip and the sample in the near field, and the impedance parameter was used to obtain the equivalent capacitance of the sample, and the dielectric constant of the sample under testing can be deduced. Different samples under near-field probes were modelled in a high-frequency structure simulator. Using a known dielectric constant, the reflection coefficient can be calculated from the coaxial probe model, and compared with the simulation results. The feasibility of the near-field probe in determining dielectric properties is verified.

Keywords

• permittivity
• capacitance
• coaxial probe model
• near-field probe
• dielectric properties
• reflection coefficient
• dielectric constant
• near-field microwave
• mathematical relation
• lumped circuit
• impedance model
• equivalent model
• multiple capacitors
• impedance parameter
• equivalent capacitance
• probe-sample interaction
• high-frequency structure simulator