The spot-probe apparatus, which includes the probe antenna and a vector network analyzer, is either handheld or robot-mounted. It is held a known distance above the surface under test to measure the S11 of the test specimen. This method is similar to that of other free-space measurement methods. Calibration measurements are also made and after the data are calibrated a time-domain gate is applied.
When electrical thickness is desired, a specified data-reduction algorithm is used to calculate layer thickness. The algorithm may solve for multiple layer thicknesses simultaneously or may solve for one unknown layer within a known stack. Depending on the application, the surface under the test may be metal-backed or air-backed.
Quality assurance for microwave and millimeter-wave components requires knowledge of material performance or thickness. This test method is useful for evaluating materials used in or applied to components. It may be used to map electromagnetic functionality or physical parameters of structural components, radomes, or absorbers.
