A Study On Metamaterial Electromagnetic Absorbers Loaded With Lumped Resistors

Metamaterials represent artificial materials or structures comprised by periodically arranged sub-wavelength unit cells by specific rules.With proper design,metamaterials can exhibit characteristics non-existent amongst natural materials,for example,the anomalous refraction or anomalous reflection of electromagnetic?EM?waves.Metasurfaces are the two-dimensional forms of metamaterials.Apart from less difficulty in design and fabrication,they also have sub-wavelength thickness and lighter weights.EM absorbers are able to absorb energy of incident EM wave and convert it to other forms,making it undetectable to EM detectors.Therefore,EM absorbers have important applications on military purposes,such as radar stealth.Compared with EM absorbers made of natural materials,metasurfaces feature flexibility in surface unit-cell design and adjustability in performance.Currently,meta-material absorbers have both sub-wavelength thickness and wide absorption band.These advantages make them quite promising for multiple application scenes.In this thesis,studies are carried out on EM wave absorption utilising lumped resistor-loaded metasurfaces.The surface unit-cell patterns of metasurfaces are designed according to several theories,so that the designed metasurfaces absorb incident electromagnetic wave at a specific broadband or narrow band frequencies as different application scenes may require.Detailed contents of this thesis are as follows:1.Three wideband resistor-loaded metasurface absorbers working across 6 GHz-18 GHz are designed,including two based on magnetic resonance and one based on circuit analogue method.For the absorbers based on magnetic resonance,single-polarisation and dual-polarisation versions are offered,their performances verified by numerical simulations and experiments.The measured relative bandwidth of single-polarisation absorber reaches 95.7%,while the relative bandwidth of dual-polarisation absorber reaches 86.8%,and both of them exhibit stable performance under large-angle transverse magnetic?TM?incidence.The dual-polarisation absorber based on circuit analogue method is also proven to have a relative bandwidth of 100.8%by simulation,with a stable performance under large-angle TM incidence as well.Furthermore,the problems that can be introduced during the fabrication process,such as the real forms of resistors and imperfect soldering,are also discussed.2.The design and working principles of narrowband absorbers based on metasurfaces are introduced from the perspective of the polarisability of unit cells.An absorber based on this principle that aims to absorb WiFi signals near 2₄ GHz is designed.The design process goes first by a proper layout of an anisotropic unit structure,followed by a proper arrangement of these unit structures so that the incident electromagnetic wave has strong transmission with little reflection in a wide frequency range,while experiences absorption near 2.4 GHz.The sample structure is optimised by simulation,and the performance is further verified by experiment.Simulation and experiment also demonstrate the stable performance of the sample under large incident angles of electromagnetic wave.The designed absorber can be fabricated by standard printed circuit board process,suggesting easiness for mass production.