Transactions on Antennas and Propagation
BSE. The results reveal the EM characteristics of inhomogeneous
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| BSE. The results reveal the EM characteristics of inhomogeneous radomes which is potential in realizing high-performance air- borne radomes superior to traditional VTRs. The second part of the paper studies the bandwidth performance, and the effect of radome coating, thickness errors and the layer number in order to acquire an overall understanding of the performance of inhomo- geneous radomes. Index Terms—Electromagnetic performance, Inhomogeneous radomes, Phase distortion, Variable thickness radomes A BBREVIATIONS : BSE Boresight Error CTR Constant Thickness Radome EM Electromagnetic GO Geometrical Optics IPD Insertion Phase Delay IPL Inhomogeneous Planar Layer PSO Particle Swarm Optimization TL Transmission Loss Manuscript received August 30, 2016. This work was supported by National Natural Science Foundation of China (Grant No. 51605362, No. 51490660 and No. 51475348), the Fundamental Research Funds for the Central Universities and the 111 Project B. Wanye Xu, BY. Duan and Peng Li are with the Key Laboratory of Electronic Equipment Structure Design, Ministry of Education, Xidian University, Xian, Shaanxi 710071, China (email xuwanye@163.com; yin- hong0523@163.com). Yuanying Qiu is with the National Key Laboratory of Antenna and Microwave Technology, Xidian University, Xian, Shaanxi 710071, China. VTR Variable Thickness Radome II NTRODUCTION irborne radomes, due to the aerodynamic requirements, have to be streamlined, which bring about large incident angles between electromagnetic (EM) waves and radome wall, and fail to maintain the radome symmetry as seen by the antenna during antenna scanning [1][2]. Typical airborne ra- domes include conformal radomes, such as the nosecone ra- domes of missiles and airplanes, and non-conformal radomes, such as those ellipsoidal or balance-beam ones above the fuselage of early warning aircrafts. From the viewpoint of geometrical optics (GO, large incident angle introduces large amplitude attenuation and large phase change of antenna aperture field. Amplitude attenuation is the main inducement of radome transmission loss (TL, whereas phase change, in combination with the radome asymmetry, causes asymmetric phase distortion which leads to boresight error (BSE), and can also contribute to TL [3]. The thickness of conventional constant thickness radomes (CTRs) can be optimized to realize EM performance improvement, which, however, is usually limited. Through properly designing the radome thickness profile, the variable thickness radome (VTRs) can significantly improve the phase distortion, thus reducing BSE and TL [4]-[9]. Nevertheless, VTR essentially belongs to the category of half-wave wall radome, which achieves reflectionless transmission by constructive cancellation of reflected energy from each of the two air-dielectric interfaces, and is known to be inherently nar- row-banded [2]. Moreover, VTR tends to be sensitive to the radome errors, which is inevitable in manufacturing process [10], [11], as well as the variation of radome coating layer, which, for high-speed applications, can ablate to absorb the aerodynamic heating and protect the radome from excessive temperature increment [12]-[13]. These aspects limit the development of high-performance airborne antenna-radome systems. An inhomogeneous medium is described by a permittivity function of a spatial variable [14]. Due to the advantage in providing less scattering and better coupling effects than homogeneous media, inhomogeneous media are widely used in A X (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information DOI TAP, IEEE Transactions on Antennas and Propagation > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERETO EDIT) < 2 microwave and antenna engineering. Inhomogeneous planar layers (IPLs) are extensively used as optimum shields, filters, absorbers and radomes, etc [15]-[21]. To provide a good im- pedance match to improve radome transmission characteristics, the permittivity variation of IPLs can be described as inverse of distance (1/r), inverse of distance with power two (1/r 2 ), exponential of distance (e r ) or Fourier series expansion concerning distance, etc [22]. Philippe [23] studied the reflection and transmission coefficients of IPLs with the relative permit- tivity decreasing symmetrically according to the 1/r 2 law from the middle plane to the slab walls bounded by air. The superior performance of the IPLs over broadband at both small and large incident angles motivates its application in radome design. With the further consideration of loss tangent variation, RU. Nair et al. [20] introduced this IPLs into radome design, and the transmission coefficient, reflection coefficient and insertion phase delay (IPD) of the IPL radome were reported. The transmission efficiency and power reflection of the IPL radome are far superior to that of the traditional monolithic radome over broadband due to the excellent impedance matching effect, whereas the reduced IPD of the IPL radome imply a potential BSE improvement of inhomogeneous radomes. Such an IPL radome wall can be accomplished by stacked laminations of varying permittivity. F. Chen et al. [24] reported a broadband ceramic IPL radome with graded porous structure, and corresponding experiments validate the theoretical results. The multilayered ceramic radome was prepared by orderly stacking the prepared individual ceramic layer, which employ the same material and possess specified permittivity value by controlling the material porosity. The proposed radome has excellent broadband transmission efficiency, and is more suitable for high temperature environment than traditional sandwich ra- domes, as the constituent layer is made of the same material and therefore, is immune to the thermal mismatch stress which exists between adjacent layers with different coefficient of thermal expansion. M. Khalaj-Amirhosseini [21] optimized the permittivity profile of IPL radome to minimize reflection, where the permittivity profile was described by truncated Fourier series expansion. Y. M. Pei et al. [25] presented a dual-band IPL radome with optimal transmission efficiency. A. Solovey [26] studied the EM performance of IPL radome with structural performance as constraints. These works mainly focused on the planar radome and thus, the concerned EM performance can only be the transmission coefficient, reflection coefficient and, at most, insertion phase delay. It should be pointed out that the results of such planar radome designs do not naturally signify equally good EM performance in the case of streamlined radomes for airborne applications, because the situation may become rather complicated as the incident angle within the area of antenna aperture can vary radically and change during antenna scanning. RU. Nair et al. [27] proposed an optimized inhomogeneous nosecone radome design for airborne applications and reported the radome TL and BSE. The inhomogeneous radome consists of 7 layers with varying permittivity and has conspicuously superior transmission efficiency than variable thickness ra- dome (VTR) designs, whereas the BSE is somewhat inferior to that of the monolithic VTR design. This seems contradictory to the results that inhomogeneous radome has reduced IPD [20], which intuitively imply abetter BSE as radome IPD is the main cause of BSE. Moreover, to motivate the application of inho- mogeneous radome in airborne areas, more results are necessary to obtain an overall understanding of the EM characteristics of inhomogeneous radomes, including comparison with VTR design which is the common strategy for high-performance airborne radomes. In this paper, the typical continuous permittivity variation according to the 1/r 2 law, which has been proven exceedingly effective in IPL radome, is employed in streamlined inhomo- geneous radome, where the permittivity profile can be readily controlled to realize a spiky shape or a blunt shape. The phase distortion of antenna aperture field caused by inhomogeneous radomes is studied, and, through a comparison with that of the monolithic CTR and VTR design, explains the cause of the relatively large BSE of inhomogeneous radome which has small IPD. Moreover, a method is proposed to estimate the IPD distribution and the BSE from the parallel and perpendicular polarization transmission coefficients. This paper is the first part of a two-paper sequence. In Part II [28], considering transmission loss (TL) and boresight error (BSE), the two most important EM characteristics of airborne radomes, the bandwidth performance, and the effect of radome coating, thickness errors and layer number are further analyzed to acquire an overall understanding of the performance of in- homogeneous radomes. II. M ETHODOLOGY Download 1 Mb. Share with your friends:
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