Electromagnetic Modelling of Microwave Absorbers (EMMA)
Final Report Abstract
The objective of the collaborative project which was conducted at the RF and Microwave Research Group was the systematic investigation of the angle-dependent reflection behaviour of microwave absorbers with regard to their modelling in ray-optical simulation methods. The study particularly looked at dielectric foam absorbers, the most common type in the microwave range. The reflectivity value is greatly influenced by the size of the absorber in relation to the wavelength scale. Different pyramid absorbers of the same electrical size have similar reflectivity values. Absorbers with a larger electrical size are characterised by a lower specular reflection, but waves with different propagation directions can occur when the spatial period of the absorber elements exceeds the value of one wavelength. To investigate the systematics behind the angle- and frequency-dependent reflectivity, a total of 18 test absorbers were examined analytically, numerically and experimentally, for which a bistatic measurement method and extended calibration procedure for the angle-dependent reflectivity was developed during the course of the project. Based on the numerical results, reflectivity models were developed for pyramid and wedge absorbers which can be used for ray tracing applications. Another key finding of the investigations was that assuming purely spectral reflection from absorbers in absorber chambers may not be sufficient. The numerical investigations on periodic unit cells of absorbers show that at certain frequencies and angles of incidence not the specular but rather other directions of reflection can dominate the reflection. The analytical and numerical results were confirmed by the experimental measurements, even if larger measurement distances would have been necessary to detect the discrete modes. Nevertheless, the measurement results show that at a constant angle of incidence, the same reflection values occur at different observation angles. The project results provide important insights into the basic mechanisms of microwave absorbers as a key element of anechoic measurement environments and form the basis for further work on the investigation of angle-dependent reflectivity, an issue that extends beyond the field of absorbers.
Publications
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Advanced Calibration Method For Accurate Microwave Absorber Reflectivity Measurements At Oblique Illumination Angles. 2020 14th European Conference on Antennas and Propagation (EuCAP), 1-5. IEEE.
Hofmann, Willi; Schwind, Andreas; Botnkessel, Christian & Hein, Matthias A.
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“Geometry- and angle-dependent monostatic scattering of microwave absorbers,” 2020 Antenna Measurement Techniques Association Symposium (AMTA), Newport, RI, USA, 2020, pp. 1-6
W. Hofmann, A. Schwind, C. Bornkessel & M. A. Hein
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Bi-static reflectivity measurements of microwave absorbers between 2 and 18 GHz. 2021 Antenna Measurement Techniques Association Symposium (AMTA), 1-6. IEEE.
Hofmann, Willi; Schwind, Andreas; Bornkessel, Christian & Hein, Matthias A.
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Analysis of Microwave Absorber Scattering Using Ray-tracing and Advanced Measurement Techniques. 2022 16th European Conference on Antennas and Propagation (EuCAP), 1-5. IEEE.
Hofmann, Willi; Schwind, Andreas; Bornkessel, Christian & Hein, Matthias A.
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Angle-Dependent Reflectivity of Microwave Absorbers at Oblique Wave Incidence. 2021 51st European Microwave Conference (EuMC), 233-236. IEEE.
Hofmann, Willi; Schwind, Andreas; Bornkessel, Christian & Hein, Matthias A.
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Comparison of Angle-Dependent Scattering of Convoluted and Straight Microwave Absorbers. 2022 Antenna Measurement Techniques Association Symposium (AMTA), 1-6. IEEE.
Hofmann, Willi; Schwind, Andreas; Bornkessel, Christian & Hein, Matthias A.
