The Research Of Microwave Low-frequency Active Absorber Based On Metamaterial

Metamaterials are artificial composite structure or composite materials that have extraordinary electromagnetic(EM)properties which are not available in natural conventional materials.They are formed by periodic arrangement of artificial unit cell whose size is much smaller than the operating wavelength.With the rapid development of electromagnetic simulation technology and manufacturing processes,scientists have conducted a lot of research based on metamaterials in the field of manipulation of EM wave propagation,and have achieved a lot of breakthrough results.EM absorbers have a wide application in military stealth technology and civil EM compatibility.Modern information warfare puts forward the research aim of "thin","wide","light" and "strong" on EM absorbing materials,while absorbers based on metamaterials provides a way to achieve this goal.In recent years,scientists have developed various metamaterial absorbing structures for different application frequency bands and different application scenarios.In this paper,three metamaterial absorbers for microwave low-frequency bands are designed based on active devices:1?A Switchable rasorber/absorber structure with reconfigurable passband between two neighboring absorption bands is proposed based on PIN diodes.First,a polarization-insensitive low-frequency broadband absorber is designed by using the equivalent circuit theory,it can achieve an absorbing band from 0.79 to 2.81 GHz with absorptivity greater than 90%.Next,a transmission passband is designed within the absorbing band,form an absorbing structure with a transmission window.Finally,PIN diodes and resistors are loaded in the bottom structure,the reconfigurability of the transmission window can be realized by changing the switching state of the PIN diode.The operating bandwidth with the reflection coefficient lower than-10 d B ranges from 0.8 to 3.4 GHz.When the diodes are in the off state,there is a passband at 1.6 GHz;when the diodes are switched to the on state,the passband is closed,the structure performs as an absorber.The performance of the reconfigurable structure is verified by the TEM-cell test system,and the switching between the communication mode and the stealth mode is realized.2?A frequency-tunable absorber structure is designed based on the varactor diodes.First,a single-polarized absorber is designed with the help of equivalent circuit theory,and varactor diodes are loaded into the structure.When the capacitance value gradually decreases from 1.46 p F to 0.35 p F,the absorption frequency continuously increased from 0.81 GHz to 1.46 GHz.The overall thickness of the structure is only 5.5 mm.Then the structure is expanded to a dual-polarized frequency-tunable absorber,and the sample is fabricated and tested.When the reverse bias of the varactor diode is changed between 4 V and 20 V,the absorption frequency changes from 0.8 GHz to 1.45 GHz,and the absorption rate is greater than 90%.Finally,the low-frequency tunable absorber is combined with a high-frequency broadband absorbing structure to expand the absorbing bandwidth.Simulation and experiments verify that the EM energy in the low frequency band can be absorbed tunably and the EM energy in the high frequency band can be absorbed broadbandly.3?A microwave low-frequency broadband absorber is designed based on the NonFoster circuit.Non-Foster components have the opposite physical characteristics of ordinary passive devices,and have important application value in the field of broadband matching.We introduce a negative inductor of-80 n H into the absorbing structure,and the simulated absorbing bandwidth ranges from 0.22 GHz to 1 GHz based on the thickness of 31 mm.Then a Non-Foster circuit is designed based on the operational amplifier,and the required negative inductance is realized by simulation.Finally,the circuit structure is brought into the absorbing structure for co-simulation calculation,the ultra-wideband absorbing performance is realized in the range of 0.3 GHz to 1.87 GHz,breaking the limitation between the thickness and bandwidth of the absorbing structure in Rozanov theory.