Meaning
High frequency permittivity measurements rely on the reflection coefficient of a transmission line pressed against a material. An open ended coaxial probe acts as the interface that delivers the electromagnetic wave to the sample under evaluation. This sensor determines the dielectric constant and loss factor of liquids or semi-solids by analyzing the reflected signal phase and magnitude.
It is widely used in radio frequency laboratories to evaluate tissue simulating liquids.
Calibration Routine
Accurate material characterization requires a three-point calibration at the probe tip before taking any measurements. The open ended coaxial probe is measured in air, against a short circuit, and immersed in high purity deionized water. These three reference points allow the network analyzer to calculate and cancel out the systematic cable losses.
This step guarantees that the recorded reflection parameters originate solely from the sample interface.
Sample Preparation
The material under test must have a flat surface to prevent air gaps that introduce measurement error. When using the open ended coaxial probe, the user must ensure that the probe face remains flush with the liquid or gel. Air bubbles trapped at the boundary alter the effective capacitance, leading to false permittivity readings.
The thickness of the sample must exceed the sensing depth of the coaxial field to prevent bottom reflections.
Verification Margin
Quality control protocols require periodic measurement of reference materials to verify probe accuracy. Regular reference checks of the open ended coaxial probe protect against drift caused by temperature fluctuations or mechanical wear.
