利用有限元法分析EMC暗室的NSA_圖文

1、利用有限元法分析EMC 暗室的NSA孟東林,沙斐(北京交通大學(xué)電子與信息工程學(xué)院電磁兼容實(shí)驗(yàn)室,北京100044摘要:EMC 暗室是最常用的電磁兼容測(cè)試場(chǎng)地,常受限于低頻性能。為了在建造EMC 暗室之前,對(duì)其低頻性能作準(zhǔn)確的預(yù)測(cè)分析,利用平面分層等效模型來(lái)模擬暗室中的角錐吸波體,研究了暗室的不同墻面鋪設(shè)不同吸波材料時(shí)其性能的變化情況。仿真結(jié)果表明,適當(dāng)增加鈍口吸波體的高度能夠顯著改善暗室性能;在暗室4個(gè)墻位置中,處于第一菲涅爾區(qū)的側(cè)墻(與電波傳播方向平行位置的吸波材料更加關(guān)鍵。實(shí)測(cè)數(shù)據(jù)的驗(yàn)證,表明上述方法分析暗室低頻性能時(shí)準(zhǔn)確可靠。關(guān)鍵詞:電波暗室;射線追蹤法;歸一化場(chǎng)地衰減;吸波體;有限元法

2、;電磁兼容中圖分類號(hào):TM935;U285文獻(xiàn)標(biāo)志碼:A 文章編號(hào):100326520(20070920041205基金資助項(xiàng)目:北京交通大學(xué)“十五”重大科技專項(xiàng)基金項(xiàng)目資助(DX J 04002。Project Supported by Key Science and Technology Foundation of Beijing Jiaotong University During t he 10t hFive 2yearPlan(DX J04002.Analysis of NSA of Anechoic Chamber with FEMM EN G Dong 2lin ,SHA Fei

3、(EMC Lab ,School of Electronic and Information Engineering ,Beijing Jiaotong U niversity ,Beijing 100044,China Abstract :An accurate method was introduced to simulate and analyze normalized site attenuation (NSA of anechoic chambers (AC in the low f requency range (30100M Hz with the finite element

4、method (FEM .The practical pyramid absorbers were modeled by way of effective permittivity and planar layers ,in order to reduce the mesh size of simulation.Since FEM was good at analyzing high loss ,f requency 2dependent and anisotropic media ,the FEM was adopted to simulate NSA of ACs ,because NSA

5、 is a very important criterion for measuring the performance of a semi 2AC.The principle ,details of simulation ,as well as programming flow chart was provided.The reflectivity of two heights of pyramid absorbers was also provided for convenience.Next the effects of different heights of absorb 2ers

6、lined in different side walls of an AC were studied with this method in order to find the right configuration of ab 2sorbers before constructing an AC.The simulation results show that NSA of ACs can be improved by lengthening pyramid absorbers.The absorbers in the first Fresnel zone are key to ACs ;

7、thus its performance can be improved just by optimize the absorbers in this zone.Finally ,the simulation results are verified by practical measurement da 2ta.In short ,the performance of an AC at low f requency can be simulated with this method in a personal computer.K ey w ords :anechoic chamber ;o

8、pen area test site (OA TS ;NSA ;absorbers ;finite element method ;electromagnetic compatibility0引言電磁兼容(EMC 暗室是最重要的電磁兼容測(cè)試場(chǎng)地之一,其重要性體現(xiàn)為:EMC 暗室不僅是國(guó)際標(biāo)準(zhǔn)和中國(guó)國(guó)家標(biāo)準(zhǔn)規(guī)定的電磁兼容測(cè)試的基本場(chǎng)地之一,而且建造EMC 暗室的費(fèi)用比較昂貴123。事實(shí)上,EMC 暗室集中體現(xiàn)了多種電磁兼容技術(shù)。EMC 暗室屬于修正型半電波暗室,即電磁屏蔽殼體的四周墻壁和天花板均安裝吸波材料,地面鋪設(shè)良導(dǎo)體時(shí),構(gòu)成半電波暗室,本質(zhì)上相當(dāng)于半自由空間;目的是模擬開(kāi)闊試驗(yàn)場(chǎng),用于30

9、M Hz 18GHz 的輻射發(fā)射測(cè)試;相應(yīng)的指標(biāo)為歸一化場(chǎng)地衰減(Normalized Site Attenuatio n 2NSA ,標(biāo)準(zhǔn)有對(duì)于暗室的設(shè)計(jì)和建造527,已經(jīng)由早期的先試驗(yàn)再整改的方法逐步過(guò)渡到采用射線追蹤法在建造暗室之前對(duì)其性能作預(yù)測(cè)分析8。射線追蹤法是一種高頻近似方法,在低頻(100M Hz 時(shí)分析暗室的性能誤差較大9,因此人們采用各種全波分析方法,如矩量法(Met hod of Moment s 2MoM 10,11,有14第33卷第9期2007年9月高電壓技術(shù)High Voltage EngineeringSep.2007限元法(Finite Element Met h

10、od2FEM12,時(shí)域有限差分法(Finite2Difference Time2Domain2 FD TD13215等。其中文10,11推導(dǎo)出均值法計(jì)算公式;文12僅對(duì)鋪設(shè)鐵氧體瓦的情況作了仿真; Ignacio根據(jù)模分析提出了一種安裝了鐵氧體瓦時(shí)的快速算法16,但是忽略了鐵氧體瓦上層的聚氨酯角錐吸波材料的效果,對(duì)于沒(méi)有鐵氧體瓦僅安裝角錐的暗室無(wú)法仿真。本文介紹了采用有限元法(FEM來(lái)分析半電波暗室的歸一化場(chǎng)地衰減(NSA,并用該方法研究暗室不同墻面的吸波體對(duì)NSA的影響情況。1暗室全波分析方法分層等效模型v=(1-g0+gah=0(1+g2(a-0(1+g0+(1-gav=(1-g0+gah

11、=0(1+g2(a-0(1+g0+(1-ga,(1其中0是真空中的介電常數(shù),而a=ar-jal是吸波材料的介電常數(shù)6(ar、al分別為a實(shí)部、虛部;0是真空中的磁導(dǎo)率,a= ar-jal是磁性吸波材料的磁導(dǎo)率(ar、al分別為a實(shí)部 、虛部;g代表吸波材料的占空比,v、h分別表示法向和切向的磁導(dǎo)率,v、h分別表示法向和切向的介電常數(shù)。法向?yàn)榻清F方向,切向?yàn)榕c吸波材料角錐方向相垂直的方向。式(1中法向公式是精確的,切向公式是近似的,誤差為5%。公式(1的本質(zhì)是當(dāng)頻率較低時(shí),角錐結(jié)構(gòu)可以采用各向異性的多層平面來(lái)近似,即每一平面分層的材質(zhì)都是各向異性的,且各層材質(zhì)介電常數(shù)都不相同。原因是,沿角錐方向

12、是慢變化,而沿與角錐相切的方向,是周期性的快變化。根據(jù)該公式,利用有限元法算得的實(shí)際吸波材料和等效分層以后的反射率比較見(jiàn)文18,關(guān)于場(chǎng)均勻性的有限元法仿真情況見(jiàn)文19。分層等效模型見(jiàn)圖1。顯然,上述等效分層模型是高色散、高損耗和各圖1暗室仿真的等效分層方法示意圖Fig.1Schem atic of effective layers for modelinganechoic chamber圖2電波暗室有限元法仿真流程Fig.2F low ch art of simulating anechoic ch amb er by FE M向異性的媒質(zhì)。劃分層數(shù)較多時(shí)比較接近實(shí)際吸波材料,但是計(jì)算時(shí)所需內(nèi)

13、存也越大。通常需要根據(jù)材料高度分成1020層,誤差分析見(jiàn)文18。仿真流程見(jiàn)圖2。2仿真實(shí)例和NSA影響因素研究半電波暗室內(nèi)部共有5個(gè)面安裝吸波體;因此需要對(duì)比研究,哪一個(gè)墻面上的吸波最重要。眾所周知,吸波材料的作用是吸收電磁波使得反射出去的電磁波能量密度盡量小;吸波材料越長(zhǎng),其反射率越低;且正入射反射率優(yōu)于斜入射反射率20。然而當(dāng)吸波材料變長(zhǎng)以后(即圖3中的L+ D值變大,入射角將變大(因?yàn)榘l(fā)射天線T x和接收天線R x的間距必須恒定不變,見(jiàn)圖3。入射角變大將導(dǎo)致吸波材料的反射率R變差,f為頻率,見(jiàn)圖4。其中P TM為平行極化波,指入射波的電場(chǎng)極化方向平行于入射面(平面波的波矢量方向與分界面法

14、向構(gòu)成的平面稱為入射面,P TE為垂直極化波,指入射波的電場(chǎng)極化方向垂直于入射面。EMC164吸波體尺寸為:底座厚度D=20cm、底座24 圖3側(cè)墻吸波材料長(zhǎng)與入射角的關(guān)系Fig.3R elationship betw een absorber sheight and incident angle圖4EMC164和EMC122反射率的比較Fig.4Comparison of reflectivity betw eenEMC 164and EMC122邊長(zhǎng)A =30cm 、角錐頂部鈍口邊長(zhǎng)a =10cm ,角錐高度L =144cm ;EMC122吸波體尺寸為:D =20cm 、A =30cm 、

15、a =10cm ,L =102cm 。增加吸波體高度,一方面其反射率下降,吸波性能變好;另一方面暗室中電磁波的入射角將變大,導(dǎo)致吸波體的性能下降。故需研究屏蔽殼體尺寸不變時(shí),增加側(cè)墻上吸波體的高度是否有用。針對(duì)屏蔽殼體尺寸為1214m 718m 6m 的半電波暗室作仿真,逐一分析影響暗室NSA 的因素。半電波暗室NSA 的實(shí)際測(cè)量方法可參閱文4。本文僅給出半電波暗室兩種情況下長(zhǎng)度方向的中軸線位置上的NSA 值。1暗室地面為理想電導(dǎo)體(PEC ,側(cè)墻安裝吸波體(分別為EMC164或EMC122,其余3個(gè)面均為理想吸波體。此時(shí)的NSA 偏差D 見(jiàn)圖5,D 為實(shí)際的NSA 值與理論的NSA 值之差。

16、其中1m 、TM 指發(fā)射天線距離金屬地面高度為1m ,水平極化方式;1m 、TE 指發(fā)射天線距離地面1m ,垂直極化方式。其余類推。對(duì)比圖5(a 和(b 可見(jiàn),當(dāng)吸波體加長(zhǎng)以后,暗室中的NSA 性能變好,即增加側(cè)墻上吸波體的高度有積極作用。2當(dāng)端墻安裝吸波體,地面為PEC ,側(cè)墻為理想吸波體時(shí)的NSA 偏差D 見(jiàn)圖6。圖6中的(a 和圖5側(cè)墻吸波材料性能對(duì)暗室NSA 的影響Fig.5R ole of side 2w all absorbers on NSA圖6端墻吸波材料性能對(duì)暗室NSA 的影響Fig.6E ffect of end 2w all absorbers on NSA(b 的結(jié)果再

17、次說(shuō)明:當(dāng)吸波體加長(zhǎng)以后,暗室的NSA 性能會(huì)變好,即吸波體高度增加對(duì)NSA 有積極作用。分別對(duì)比圖5和圖6中的(a 以及(b 可見(jiàn),在安裝同樣的吸波體條件下,顯然側(cè)墻的吸波體性能更關(guān)鍵,即改善側(cè)墻吸波體的性能,能更加顯著的改善半電波暗室的NSA 性能。地面為PEC ,其余5個(gè)面均安裝EMC164吸波體以后,得到的NSA 偏差,見(jiàn)圖7。顯然,該性能不達(dá)標(biāo)。本文仿真中相當(dāng)于把吸波體安裝在由理想導(dǎo)體(PEC 構(gòu)成的屏蔽殼體中,但是實(shí)際的吸波體總是342007年9月高電壓技術(shù)第33卷第9期安裝在屏蔽鐵殼上的,屏蔽鐵殼在低頻時(shí)對(duì)磁場(chǎng)具有一定吸收效果,因此鋪設(shè)上述吸波體后,半電波暗室的實(shí)際效果會(huì)比仿真效

18、果稍好一點(diǎn),當(dāng)然,為了提高半電波暗室性能,可采用更長(zhǎng)的吸波體,或采用復(fù)合吸波體。Holloway等采用FD TD方法在時(shí)域作了仿真10。本文的結(jié)果是針對(duì)1214m718m6m尺寸的半電波暗室(顯然,該屏蔽殼體比一般3m法電波暗室的屏蔽殼體大很多,是在32位PC機(jī)的限制下完成的,單個(gè)線程占用最高內(nèi)存達(dá)到2G B。圖8是屏蔽體1260cm780cm600cm的某電波暗室的實(shí)測(cè)數(shù)據(jù)。圖8(a是剛建造完的NSA 偏差數(shù)據(jù),發(fā)現(xiàn)在低頻(37M Hz不達(dá)標(biāo),后側(cè)墻采用反射率降低23dB的新吸波材料,其余條件不變所得實(shí)測(cè)數(shù)據(jù),發(fā)現(xiàn)NSA性能明顯好轉(zhuǎn)(圖8 (b。該實(shí)測(cè)結(jié)果也證明,改善側(cè)墻吸波材料的性能,能夠

19、顯著提高半電波暗室的性能。3計(jì)算方法比較電波暗室通常采用的預(yù)測(cè)分析方法是射線追蹤法,但是在低頻時(shí)射線追蹤法等高頻近似法不再有效,導(dǎo)致采用該法分析暗室低頻性能的誤差很大。針對(duì)該問(wèn)題,開(kāi)始研究采用全波分析方法來(lái)預(yù)測(cè)暗室的低頻性能。然而,對(duì)電波暗室直接做全波分析非常困難,因?yàn)槲w是色散、有耗的,而且電波暗室中的吸波體數(shù)目龐大結(jié)構(gòu)復(fù)雜,無(wú)論對(duì)計(jì)算能力還是內(nèi)存容量的要求都很高。由于FEM非常適合計(jì)算高損耗和高色散媒質(zhì),且FEM采用四面體來(lái)離散劃分網(wǎng)格,使得FEM能夠比較準(zhǔn)確地處理大型、復(fù)雜的結(jié)構(gòu)。因此,本文在采用平面分層等效模型的基礎(chǔ)上,采用FEM來(lái)計(jì)算電波暗室的低頻性能。但需注意在低頻時(shí)采用有限元法

20、來(lái)分析電波暗室時(shí),邊界條件、模型中的網(wǎng)格劃分優(yōu)良情況等,都會(huì)影響計(jì)算精度。如果網(wǎng)格劃分不合理,將產(chǎn)生1 2dB的誤差。此外,對(duì)于暗室的NSA仿真分析,相比場(chǎng)均勻性(FU的仿真要困難一些,因此文14216均研究了此問(wèn)題。本文除了闡釋如何采用有限元法來(lái)預(yù)測(cè)分析電波暗室外,還重點(diǎn)研究了暗室的不同墻面上的吸波體的效果,目的是優(yōu)化暗室吸波體的配置,節(jié)省成本。4結(jié)論a通常電波暗室的低頻性能較差,但是采用射線追蹤法等方法預(yù)測(cè)的誤差較大。 采用本文介紹的圖7高度164cm的吸波材料安裝后所得NSAFig.7NSA of an anechoic chamber linedwith absorber EMC164

21、圖8某電波暗室的實(shí)測(cè)NSAFig.8Measured NSA of an anechoic chamber方法能夠準(zhǔn)確的預(yù)測(cè)半電波暗室的低頻性能。b在半電波暗室的各個(gè)墻面上吸波體的重要程度并不完全相同,側(cè)墻上的吸波體更加重要。c目前出現(xiàn)了許多新型吸波體,如空心吸波體,甚至空殼狀吸波體,而本文主要分析聚氨酯角錐吸波體。如何采用FEM來(lái)仿真分析安裝了這些新型吸波體的電波暗室的性能,有待進(jìn)一步研究。參考文獻(xiàn)1Lin G,Alvarado M J.Construction of a102m semianechoicchamberJ.Compliance Engineering,2001,17(7:62

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