Research On Layered Structural Absorbing Materials With Ultra-thin Thickness And High Efficiency

Although shaping is the first line of RCS reduction and can provide significant reductions in signature over limited aspect angles, many situations require absorption of the incident electromagnetic energy, such as for preventing the detection of multi-static radar system. In this case, the design and application of radar absorbing materials is vital to minimize the total radar signature of targets. There are many reports on the design of radar absorbing materials by means of EM simulation or experimental methods. However, how to design a radar absorbing materials effectively remains a problem.In this dissertation, firstly, an equivalent circuit of types of Salisbury screen absorber has been proposed and developed. The calculations of values of equivalent elements in the circuit have been given. Based on the equivalent circuit, an ultra-thin narrowband single-layered absorbing material is presented. It consists of periodic arrays of single loop printed on a dielectric substrate, with four lumped resistors loaded on each side of the loop. Simulated and measured results coincide well. Additionally, by utilizing the concept of effective aperture similar to antenna array, another ultra-thin narrowband single-layered absorbing structure has been designed.Secondly, a modified multi-layered equivalent circuit is developed for ultra-thin broadband absorbers. In the modified equivalent circuit, serial RC branch circuit is employed to reduce the thickness as well as to avoid the effect of harmonics. For verification, a glass based broadband absorber is presented with a thickness of 13.6mm and a-10dB RCS reduction from 2GHz and 18GHz. To reduce the weight and RCS further, foams have been used as substrates in another design of absorbing material, which can obtain a RCS reduction of-15dB over a broadband frequency band from 2 GHz to 18 GHz with a thickness of 22mm. Basically, the equivalent circuit of an absorbing material is constructed with the consideration of domain mode only, the effects of higher modes exist in a real absorbing material and always deteriorate its performance. To avoid the effect of higher modes, periodic structure with small dimensional square-loop, whose resonating frequency is higher than the highest working frequency of absorber, is chosen to construct absorbing material. A broadband absorber has been fabricated and measured to verify the design.Thirdly, by investigating the equivalent circuit of absorber and antenna, a novel integrated structure with selectable absorbing and radiating capabilities is proposed, which consists of periodic arrays of metallic three-loop nested structure printed on a thin grounded dielectric slab. Lumped resistors with different values are fixed on each side of all three loops while PIN switches are optionally fixed parallel to the lumped resistors on the specific loop. With the switches off or on, the parallel lumped resistors are still working or shorted, the present integrated structure performs absorption function or antenna function. Meanwhile, equivalent circuit models of the present structure of absorber and radiator are developed. A 3X3 periodic array of metallic three-loop nested structure printed on a F4B dielectric slab has been finally designed, fabricated and measured. Simulated and measured results both show that the proposed design can obtain a high absorptivity over a broad frequency band. Meanwhile, by changing the PIN switches’state, the present structure can achieve good radiations at specific frequencies.Finally, negative resistor and negative inductor are firstly introduced in the design of broadband, ultra-thin and highly effective absorbers are firstly investigated in this dissertation. The working principle of negative resistor and negative inductor in the absorbers are given in detail. To verify the idea, negative resistor based and negative inductor based broadband ultra-thin absorbers are presented respectively. Meanwhile, the method of realization of negative resistor and negative inductor are both introduced. Simulated results demonstrate that a great enhancement in performance of absorber can be achieved by the employment of negative resistor and negative inductor.