Description
This method allows a broad-band measurement in millemetric waves of the complex permittivity of dielectric plates.

It is based on the use of a "Quasi-Optical" free-space test bench together with focusing- and slotted-horn antennas (Top photo).

Principle
A free-space test bench devoted to linear measurement in Ka  and W bands has been developed within our laboratory. While designing specific pieces of equipment, i.e. antennas, we defined a characterization method able to give the phases of the measured S parameters with the best accuracy possible. Then, this test bench can be also easily used to characterize dielectric materials without time filtering.

The dielectric plate to be characterized is placed on the "Quasi-Optical" free-space test bench at right angle to the direction of the wave propagation. The reflection and transmission coefficients of the  device are measured within the Ka and W bands using an AB millimeter-type vector network analyzer. The measured S-parameter data are directly analyzed by the analyzer-driving computer through a built-in procedure; this enables one to quickly and easily assess them together with the experimental parameters, i.e. calibration, adjustments....  

Data analysis
The theory used for S-parameters data analysis is based on the assumption that the propagation of a beam is gaussian. In consequence, the focusing- and slotted-horn antennas have been designed in order to check this hypothesis in Ka- and W-band. The sample under test must be positioned at the place where the wave can be considered as being plane (TEM wave). Because of the beam size in Ka and W bands the sample diameters must be at least 10cm to avoid uncertainty in relation with boundary effects.

The dielectric sample thickness being known, the characterization procedure consists in extracting its complex permittivity from the measured transmission coefficient