Extracted Millimeter Wave Dielectric Variations Impact on Automotive Radar Type Approvals
Substrate dielectric drift at 77 GHz alters antenna phase and EIRP, requiring strict raw material controls to prevent regulatory non-compliance and re-filings.
Quasi-Optical Free Space Measurement
Meaning
Electromagnetic characterization of dielectric substrates at millimeter-wave frequencies requires non-contact methods to avoid the calibration errors introduced by physical coaxial connections or waveguide fixtures. A quasi-optical free space measurement utilizes focused beams of electromagnetic energy to probe a flat material sample in open air. This approach mimics the behavior of light by using lenses or focusing reflectors to direct the signal from the transmitter through the material to the receiver.
The technique is ideal for determining permittivity and loss tangent without requiring complex machining of the test sample.
Propagation Chamber
Measurement setups require an environment free from reflections to ensure that only the signal passing through the sample is measured. The propagation chamber typically contains high-performance absorbing material arranged around the transmitter and receiver horn antennas. These antennas are mounted on a rigid rail system to maintain precise alignment and separation distances.
Precision positioning is necessary because even a sub-millimeter shift in the sample position can cause significant phase errors in the measurements.
Characterization Process
Material parameters are extracted from the transmission and reflection coefficients of the sample. This characterization process involves measuring the amplitude and phase of the signal first without the sample, and then with the sample inserted into the beam path. Software algorithms then resolve the complex equations that relate these coefficients to the dielectric properties of the material.
This procedure provides highly accurate results across a wide frequency range in the millimeter-wave spectrum.
Fixture Limitation
Fixture constraints limit the method due to sample size. This fixture limitation arises because the material must be larger than the beam spot. Energy leaking around the edges of the sample ruins measurement accuracy.