چكيده به لاتين
Metasurfaces, which are considered the two-dimensional counterparts of metamaterials, are two-dimensional arrays of appropriately shaped scatterers embedded in a specific pattern in the cells of a host material boundary and are used for various wave engineering purposes, including radar cross-section reduction. In this thesis, in order to design the optimal shape of the cells for reducing the radar cross-section in a wide bandwidth, in addition to using conventional methods, a new method based on topology optimization is used that provides free-form shapes with a variety of common and unusual designs in the search space. One of the strategies for reducing the radar cross-section is phase cancellation, which is considered here as the main design strategy and after a detailed mathematical discription of this method, its generalization to the multi-phase case is proposed. In practice, two methods can be considered for creating multiple phases, one is to use a multiple AMC designs to create the desired phases in such a way that they cancel each other in the mirror direction, and the other is to use cells with variable heights. In the second method, there are two ways, one using discrete cells and the other considering a continuous height function. Crystallographic-based metasurface radar cross section formulation and RCS formulation of multiple height AMCs in the rectangular and hexagonal layout form the analytical sections of this report. Using the phase cancellation method and a parametric sweep, a metasurface is designed that provides more than 10 dB RCS reduction in the frequency band (9.3-23 GHz). This means RCS reductionin bandwidth is about 85%. The metasurface that is designed by the topology optimization method based on pixellation, produces more than 9 dB RCS reduction, in the frequency range of GHz (5-20) which means RCS reduction bandwidth is about 120%, and the metasurface that is designed by topology optimization method based on continuousization, produces more than 9 dB RCS reduction, in the frequency range of (8.1-38.7) GHz which means RCS reduction bandwidth is about 131%. The first three-phase design proposed is based on parametric sweep, which provides an RCS reduction of 8 dB in the frequency band (7.8-23.3) GHz, equivalent to a bandwidth of about 99.96%. The three-phase design based on parametric scanning has a good performance for reducing RCS by 18 dB in the frequency band (13.3-21.3) GHz, equivalent to a bandwidth of about 46%. The next three-phase design, which is based on continuousization and produced using a cheap FR4 substrate, provides more than 9 dB RCS reduction in the frequency band of (10.2-39.5) GHz, which means RCS reduction bandwidth is about 118%. By help of the formulas obtained, without using conductive layers, a design was produced with only three cells of same height and different permeabilities, which reduced the RCS in the frequency range of (8.2-16.2) GHz, i.e. about 65.6% bandwidth. The same process was used for three cells with the same permeabilities and different heights, which reduced the radar cross section in the frequency range of (5.1-17) GHz, i.e. about 107% bandwidth. Also, three cells with different permeabilities and heights were optimized in such a way that the radar cross section was reduced in the frequency range of (4.4-16.3) GHz, i.e. about 114.9%bandwidth. Finally, a two-phase design with continuously variable height is presented that reduces RCS in the frequency range of (8.3-24.5) GHz, i.e. about 98% bandwidth, and for asymmetry in shape of cells, it is relatively stable for incident angles up to 30 degrees.
