Design and characterization of broadband EM wave absorbers for low-microwave frequency applications

Abstract

In the microwave regime, electromagnetic (EM) interference is considered as a crucial problem and EM shielding techniques are widely used to mitigate this issue. Apart from general application in radar cross-section control, EM wave absorbers have become an important element in modern EM devices. As the EM devices are extensively used almost everywhere, the radiation from those devices can interfere with each other and cause a malfunction in results. To avoid this interference, the EM devices need to be well shielded by a metal-backed absorber, where the metal-back layer provides shielding from potential external EM sources, and the absorber helps to reduce the high reflection from metal walls for internal sensors. Modern EM devices are compact and use a wideband of low-microwave frequencies. As the wavelength of EM waves inversely proportional to the operating frequency, the wideband absorber at low-microwave frequencies suffers from the relatively higher thickness and narrow bandwidth. This thesis aims to design, development and characterization of ultra-thin and broadband absorbers operated over low-microwave frequencies for shielding modern compact EM devices and, in doing so, makes five main contributions to the field of EM devices and absorbers.As the designing resonant absorber requires dielectric and magnetic materials with known electric and magnetic properties, the emphasis is given in developing a broadband and accurate characterization technique to measure the complex permittivity and permeability of solid and semi-solid materials, which is the first contribution of this thesis. The proposed technique is based on classical transmission-line analysis, implemented on a strip-line structure. A new calibration technique, namely Object-Reflect-Line (ORL), is proposed to meet the challenge of an extremely low-reflection line for accurate measurement. In the proposed calibration, the Thru standard of conventional Thru-Reflect-Line (TRL) calibration technique is replaced by a standard with a known object. Since the ORL calibration uses only a single device, it reduces the fabrication cost, the measurement complexity, and the potential error in the Thru measurement needed in the TRL method, which uses an additional device. The simulated and measured results with a variety of non-magnetic and magnetic materials demonstrate the effectiveness of the proposed method over broadband at low-microwave frequencies.The second contribution is the development of a low-cost, broadband, and reliable absorptivity measurement system operated over low-microwave frequencies. This measurement system is used to validate the proposed absorbers' performance. The proposed technique is based on parallel-plate waveguide (PPW) and well-suited for frequency selective surface (FSS)-based resonant absorbers. By employing the PPW structure, only a small sample size, i.e. a 1D array of FSS, is required for the measurement. The TRL calibration is implemented to remove the time-gating ambiguity in the conventional PPW-based measurement technique. A measurement system working up to 6 GHz is designed such that its length is adjustable to ensure all three calibration standards (Thru, Reflect, and Line) can be carried out in a single device. Moreover, the height of the PPW is also adjustable to allow the measurement of absorbers with a different periodicity of FSSs.The third contribution is an accurate equivalent-circuit (EC) model-based approach for designing wideband non-magnetic absorbers operating at low-microwave frequencies. The proposed EC model is based on simulated data and synthetic asymptotes for single- and double-layer FSS-based non-magnetic absorbers. Two simple and commonly used resistive FSS, i.e., square patch and single square loop, are considered in this study. Compared to full-wave simulation, the proposed EC model shows more than 95% accuracy. By employing the proposed model and genetic algorithm (GA)-based optimization, several designs of broadband absorbers are demonstrated. The presented non-magnetic absorbers with single- and double-layer FSS show 126% and 161% fractional bandwidth, respectively, with the total thickness very close to the Rozanov limit.The fourth contribution is the design of a thin and broadband FSS-based magnetic absorber for low-microwave frequencies. The design consists of double-layers of FSS with the combination of two different magnetic substrates, which provides a much broader bandwidth compared to existing single-layered designs. The magnetic materials are in-house made by mixing different ratios of carbonyl-iron (CA) powder with a silicone elastomer called polydimethylsiloxane (PDMS). The simple single-square loop structure is used for both FSSs, which are printed on a standard FR4 material using a lossy resistive sheet. This configuration allows the use of a closed-form EC model for fast and efficient bandwidth optimization. Similar to previous non-magnetic absorbers, GA is utilized to obtain the design parameters for broadband absorption with minimum overall thickness. The measured results confirm that the proposed absorber operates from 0.78 to 4.8 GHz with -10 dB reflectivity. The design achieves 144% fractional-bandwidth with a 10.12 bandwidth-to-thickness ratio, which is very close to the optimum value.The last but not least contribution is the design of high performance FSS-based magnetic absorber for EM portable head imaging system [1], which can play a significant role in saving lives by detecting and monitoring stoke. The proposed absorber design is an FSS-based magnetic absorber like the previous design with the same magnetic materials. Instead of using double-layer FSS, only single-layer lossy FSS with simple single square loop geometry is used. As we have fixed operation bandwidth for a specific application (EM Head Scanner), the cost function mainly focuses on having high absorption, light-weight, and ultra-thin design. The proposed absorber provides -15 dB reflectivity (97% absorption) over a wide bandwidth from 0.6 to 2.2 GHz (114.3% fractional bandwidth). The ratio between the theoretical minimum thickness (Rozanov limit) to the actual one is 0.92, which indicates that the proposed design is very close to the optimum one.The developed measurement techniques, absorbers, and other advances reported in this thesis positively contribute to solve the EM interference problem for EM medical imaging systems and other modern compact EM devices.