| 1. |
- Karlsson, Anders, et al.
(författare)
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On the physical limit of radar absorbers
- 2010
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Ingår i: 2010 URSI International Symposium on Electromagnetic Theory (EMTS). - 9781424451555 ; , s. 25-28
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A previous investigation has shown that at normal angle of incidence, the integral of the reflectance over wavelength is bounded for a flat metal backed absorber. The bound is applicable to any absorber made of linear, time-invariant, causal and passive materials. We generalize the physical bound to arbitrary angle of incidence and polarization. Different design examples and numerical calculations are provided to investigate the inequalities. It is shown that the theoretical limit for TE polarization results in fair approximations of the integral of the reflectance over wavelength but the TM polarization overestimates the integral value. A simple relation for estimating the optimal thickness of a nonmagnetic absorber is suggested for arbitrary angle of incidence.
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| 2. |
- Kazemzadeh, Alireza, et al.
(författare)
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Capacitive circuit method for fast and efficient design of wideband radar absorber
- 2009
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Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X. ; 57:8, s. 2307-2314
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Tidskriftsartikel (refereegranskat)abstract
- A simple, fast and efficient method for designing wideband radar absorbers is proposed. The idea is to modify the circuit analog absorber method without perturbing the bandwidth. This is done by utilizing the asymptotic behavior of such an absorber at low frequency and replacing the band-stop resonating frequency selective surfaces with low-pass capacitive ones, which can be synthesized by square patches. It is shown that higher frequencies are not influenced by these modifications. A thin wideband capacitive circuit absorber (CCA) is presented with 28% reduction of thickness and 57% increase of bandwidth in comparison to the Salisbury screen. It is also explained why some optimized metamaterial designs fail to compete with the CCA method. For high permittivity layers, it is shown that the CCA is a better solution than the Jaumann absorber and improvements both in thickness and bandwidth are possible. A three layered ultrawideband CCA is presented with total thickness of 15.1 mm.
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| 3. |
- Kazemzadeh, Alireza, et al.
(författare)
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Capacitive Circuit Method for Fast and Efficient Design of Wideband Radar Absorbers
- 2008
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A simple, fast and efficient method for designing wideband radar absorbers is proposed. The idea is to modify the circuit analog absorber method without perturbing the bandwidth. This is done by utilizing the asymptotic behavior of such an absorber at low frequency and replacing the band-stop resonating frequency selective surfaces with low-pass capacitive ones, which can be synthesized by square patches. It is shown that higher frequencies are not influenced by these modifications. A thin wideband capacitive circuit absorber (CCA) is presented with 28% reduction of thickness and 57% increase of bandwidth in comparison to the Salisbury screen. It is also explained why some optimized metamaterial designs fail to compete with the CCA method. For high permittivity layers, it is shown that the CCA is a better solution than the Jaumann absorber and improvements both in thickness and bandwidth are possible. A three layered ultra wideband (4-24 GHz) CCA is presented with total thickness of 15.1 mm. Finally, a design capable of handling oblique angles of incidence for both polarizations and fulfilling different mechanical, thermal and fabrication constraints is given. The absorption band covers the entire C, X and Ku radar bands (4-18 GHz), showing significant improvement compared to the published circuit analog absorbers.
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| 4. |
- Kazemzadeh, Alireza
(författare)
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Design and Analysis of Nonmagnetic Specular Radar Absorbing Materials
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The design and analysis of specular radar absorbing materials are investigated throughout the dissertation. Although the topic has been investigated for more than 60 years by many researchers and numerous designs have been proposed, still many technical challenges are remained unanswered. The basic questions are, e.g., the optimal thickness of a design, the possibility of utilization of unconventional materials in an absorber to meet more design requirements, e.g. thermal or mechanical, than solely the electrical properties, the maximum possible bandwidth achievable with a low profile nonmagnetic coating and the feasibility of designing wideband multilayered absorbers for large scan angles. The thesis proposes systematic solutions for these challenging problems. The dissertation is composed of six peer-reviewed papers. A new versatile design method is proposed in the first paper with outstanding capabilities and remarkable applications. The method is named ''capacitive circuit absorber'' (CCA) and is demonstrated with different design examples to verify its superiority in comparison to the other design approaches. For example, the possibility of utilizing high permittivity dielectric spacers in conjunction with frequency selective surfaces (FSS) in wideband designs is illustrated. The second paper deals with ultra thin absorbers. Recently different proposals based on meta-materials or electromagnetic band-gap structures were suggested for low profile absorbers. The thesis shows that the absorption mechanisms in ultra thin structures are due to excitation of longitude electric field component and no meta-material effect is involved in the absorption process. It is demonstrated that TM cavity modes of patch antennas can approximate fairly accurately the absorption frequency in both periodic and finite extent absorbers. The design of multilayered Jaumann and FSS based absorbers for large scan angles are presented in the third paper. The possibility of extension of the scan and frequency compensation techniques, formerly formulated for single resistive layer designs, to multilayered absorbers is illustrated. It is shown that in contrast to single resistive layer designs, there are some degrees of freedom in the selection of the dielectric layers. Design of ultra wideband absorbers, bandwidth ratios in order of 10:1, with optimal thickness is studied in the fourth paper. It is shown that for achieving an ultra wideband design with optimal thickness, utilization of different spatial periodicities for the periodic layers is essential. By the aid of the physical bound for absorbers it is verified that our design approach leads to optimal total thicknesses. Design of thin wideband absorbers is the topic of the fifth paper, where the challenging problem of reasonable tradeoff between bandwidth and thickness is addressed. The effect of mutual coupling between periodic layers and the ground plane on the frequency response of a thin design is investigated and practical methods for minimizing the couplings are introduced. A thin design is proposed for the X-band which has a total thickness very close to the theoretical limit. Finally in the last paper the physical bound on the absorbers, originally published for normal angle of incidence, is extended to arbitrary angle of incidence for different polarizations. Applicability of the new bounds is examined with different design examples.
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| 5. |
- Kazemzadeh, Alireza, et al.
(författare)
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Multilayered wideband absorbers for oblique angle of incidence
- 2010
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Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X. ; 58:11, s. 3637-3646
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Design procedures of Jaumann and circuit analog absorbers are mostly formulated for normal angle of incidence. Only a few design methods considering oblique angle of incidence are published. The published methods are restricted to single resistive layer circuit analog absorbers or multilayered Jaumann absorbers with low permittivity spacers. General design procedures are developed in this paper for multilayered Jaumann and capacitive circuit absorbers. By expanding the scan and frequency compensation techniques to multilayered structures, Jaumann absorbers with outstanding performances are designed. A capacitive circuit absorber is presented with a stable frequency response up to 45 Degrees for both polarizations, having an ultra wide bandwidth of 26 GHz.
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| 6. |
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| 7. |
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| 8. |
- Kazemzadeh, Alireza, et al.
(författare)
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Nonmagnetic ultra wideband absorber with optimal thickness
- 2010
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Design of ultra wideband absorbers with bandwidth ratios larger than 10:1 is investigated. It is explained that if there is no constraint on the total thickness of the absorber, achieving large bandwidths is straightforward. The problem becomes challenging when the minimization of the total thickness is considered. It is shown that for a given frequency response, the total thickness of a nonmagnetic absorber cannot be less than a theoretical limit. If a design method can reduce the total thickness to the theoretical limit level, its superiority over other design methods is doubtless. It is demonstrated that the capacitive circuit absorber approach has this unique feature. In order to clarify the design ideas and techniques, the optimal absorber is developed in different stages. It is shown that unequal periods for the low-pass frequency selective surfaces are essential for attaining the optimal performance.
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