Metamaterial microwave absorbers are widely used in many fields, such as radars, stealth technology, electromagnetic compatibility, anti-electromagnetic interference, and sensors. To increase their usability, the functional requirements of absorbers have been also improved, including not only the broadband absorption properties but also wide-angle absorption characteristics. Furthermore, polarization insensitivity is now required, while all-dielectric metamaterials absorbers show unique potential in improving impedance matching in wide band. It is interesting to note that water, as an abundant natural resource on earth, possesses a high dielectric constant and large dispersion, which is of great significance for all-dielectric absorber applications. In addition, most of the water-based absorbers proposed in the literature cannot circulate the aqueous solution between the cells, or the cross-sectional area of the water flow channel of the adjacent cells is too small. The fact that most of the microwave energy absorbed by water-based metamaterials is converted into heat, if the water itself cannot circulate and dissipate heat, indicates the accumulated heat will seriously deteriorate the working performance of the absorber. However, if a water-based resonant cell with a larger cross-sectional area of the water flow channel is designed, the aqueous solution can be circulated both inside and outside. Hence, the generated heat can be efficiently dissipated during high-power microwave absorption.

First, a microwave absorber based on a water-dielectric resonant structure is designed. More specifically, the resonant cell mainly comprises a cruciform water cavity and its respective encapsulation structure. The absorption spectrum of the model is simulated under different polarization angles and incident angles using the finite element method. Moreover, the electromagnetic wave absorption mechanism of the absorber is systematically studied through the distributions of the electric and magnetic fields and power loss density distribution. The array structure consisting 25×25 cells is processed by the 3D printing technology. The extracted electromagnetic wave absorption characteristics of the samples under different polarization angles and incident angles are tested using the free space method. Furthermore, the comparison and analysis with the simulated results are performed.

In this study, an all-dielectric water-based microwave absorber is proposed (Fig. 1). The array structure consisting 25×25 cells is processed by the 3D printing technology, and the acquired absorption characteristics of the absorber are experimentally investigated by the free space method (Fig. 2). Both the simulated and experimental test results reveal that the absorptivity of the absorber is greater than 90% in the microwave broadband frequency of 21.8-35.9 GHz (Fig. 3). The simulated relative impedances of the water-based absorber are approximately equal to the free space impedance in the operation frequency range, which provides the wideband absorption with high absorptivity (Fig. 4). The electric field, magnetic field, and power loss density at resonant frequency are simulated, and the water layer plays a key role in wideband absorption (Fig. 5). The effects of both the structural parameters and the channel cross-sectional areas on the electromagnetic wave absorption spectrum are also simulated and analyzed (Figs. 6 and 7), and the results provide an important reference for optimizing the structural model. The absorption performances of the absorber at different temperatures are analyzed, and the results suggest that the absorptivity of the absorber is almost insensitive to water temperature changes (Fig. 7). Next, the absorption of the structure at different polarization angles is investigated, and both the simulated and experimental results show that the absorber is insensitive to polarization angles (Fig. 8). Finally, the absorption spectra of the proposed absorber at different incident angles for transverse-electric and transverse-magnetic polarizations are analyzed. The experimental results validate that the structure can maintain wideband absorption at wide incident angle ranges (Figs. 9 and 10).

A microwave absorber based on a water-dielectric resonator is designed. Simulation results reveal that the absorber can achieve electromagnetic absorptivity of more than 90% in the frequency range of 21.8-35.9 GHz, while the relative absorption bandwidth is 48.9%. Compared with the water-based cells that are not connected to each other in the configuration of the traditional water-based wave-absorbing metamaterials, the cross-sectional area of the water flow channel formed by the cruciform cavity in this structure can reach 3 mm×3 mm. As a result, the water-cooling working condition of the absorber can be addressed, and it can be applied to high-power microwave absorption occasions. In addition, the proposed structure possesses polarization-independent absorption characteristics and also operates well on broadband absorption under wide-angle incidence. The absorber sample is processed by the 3D printing technology, whereas the electromagnetic wave absorption characteristics of the sample under both different polarization and incident angles are explored by the free space method. Interestingly, the test outcomes are in good agreement with the simulation results. Our work therefore provides a practical solution for the enhanced broadband absorption of high-power electromagnetic waves.