【原创总结】二维电磁材料/周期结构的实验测试方法

2017-04-22 by:CAE仿真在线来源:互联网

对于平面型二维电磁屏蔽材料、透波材料或频率选择表面等电磁周期结构的研究或应用,经常需要通过实验测试得到其电磁传输特性。总体来说,实验测试包含基于传输线加载和自由空间加载两类方法,下面分别予以介绍。

(1)基于传输线加载的测试方法

同轴法兰法

基于同轴夹具进行平面材料传输特性测试,美国材料试验学会(ASTM)先后颁布了ES7和D4935两个标准 [1,2],其中ES7标准于1983年颁布,标准规定同轴线内导体连续,给出的有效测试频率范围为0-1.5 GHz,后于1988年被撤回,被D4935代替。现行的D4395标准为2010年修订版本,该标准规定测试夹具内导体断开,使用法兰配合塑料螺钉夹持待测平板状材料,利用测试端面的容性耦合保持交流电连接,测试频率下线为30 MHz,上限依然为1.5 GHz。一种典型的测试场景如图1 所示 [3]。

图1 基于ASTM D4935标准的同轴法兰屏蔽效能装置

由于ASTM D4935的实测频率范围有限,很难满足现代射频测试的需求,2008年,罗马大学的M.S. Sarto教授通过缩减同轴测量罐尺寸的方法,成功将测试频率范围扩大到30 MHz-8.0 GHz [4],在此基础上,此后其学生A. Tamburrano又于2014年将测试频率上限进一步提升到18 GHz [5],如图2 所示,可以明显看出,随着频率上限的不断增加,同轴测量夹具的尺寸越来越小。

图2 不同测试频率上限的同轴法兰测试装置设计

矩形波导法

针对平面形周期电磁结构,例如频率选择表面,一种普遍使用的非标准测试方法为矩形波导法[6-12],图3(a) 和图3(b) 分别展示了S波段和X波段的频率选择表面测试配置以及待测样品外形。

图3 基于矩形波导加载的传输/反射测试系统

受单模工作模式限制,不同工作频段的波导在尺寸上有很大区别,并且无法直接与射频测试仪器直接相连,因此需要进行波导同轴转换。实际测试时还需要特别注意两个问题,一是测试参考面距离波导同轴转换必须足够远,否则由于测试断面以及同轴转换处的不连续性会激励起凋落波,影响传输测量;二是待测样品边缘必须进行导电处理,否则在进行法兰盘装配时,样品加载会影响直接两端波导的电连接。

其它方法

如图4(a) 或图4(b) 所示,当需要扩展加载样品在水平方向上的尺寸时,可以采用平行双板波导或微带波导加工测试夹具[13-19]。

图4 基于特殊波导加载的传输/反射测试系统

其中平行双板波导采用矩形波导进行馈电,波导上下内壁分别与平行双板上相连,上下导电板等宽,双板两侧加载有吸波材料,测试时样品直接放置于平行板和吸波材料之间;与平行双板波导不同,微带波导为开放式传输结构,采用同轴馈电,同轴内芯与微带线相连,同轴外壁与接地板相连,接地板宽度大于微带线宽度,测试时样品直接放置于微带线正下方。由于传输线连续,无需法兰配合固定,测试样品无需特殊导电处理。

(2)基于自由空间加载的测试方法

介质透镜聚束法

1989年宾夕法尼亚州立大学D.K. Ghodgaonkar教授提出利用介质透镜聚束法进行微波频段样品复介电常数测量的方法[20,21]。

图5 基于透镜天线聚焦的自由空间法测试装置

如图5所示,发射天线和接收天线均为聚焦透镜加载天线,天线之间水平距离为透镜焦距的两倍,待测样品通过样品支撑架固定,样品架位于两天线的共焦面,天线和样品架固定于精密导轨,中心位于同一高度水平线上;根据测试频率的高低以及发射电磁波的极化特性,选取合适焦距的透镜和天线,使样品测试满足远场条件[22]。该测试方法也被广泛应用于频率选择表面等周期结构[23,24]。

腔体/挡板开窗法

使空间电磁波透过待测样品的另一种方法是开窗法。如图6(a) 所示[25,26],发射天线和接收天线分别位于金属挡板两侧,金属板中间开窗用于加载测试样品,根据测试频率范围以及天线类型的不同,调整天线与挡板之间的距离,使得样品测试满足远场条件;同时金属板尺寸足够大,使得边缘绕射能量可以忽略不计。为进一步降低边缘散射的影响,可在金属板外围加载吸波材料[27]。实际测试中还可将金属板采用吸波墙代替,吸收窗口以外的入射波能量[28,29]。

图6 基于开窗法的自由空间测试系统

图6(b) 还给出了一种无边缘绕射和免边缘处理的测试方法[30],通过块状吸波材料中心掏空的方式,加工出一个具有吸波边界的传输腔,发射天线和接收天线分别位于腔体两侧,待测样品通过夹接的方式固定于吸波材料中间,该方法由英国约克大学A. Marvin教授于2009年提出,测量可重复性好,已在屏蔽材料和频率选择表面测试中得到应用[31]。与开窗法有关的还有IEEE制定的电磁屏蔽体屏蔽效能的测量方法标准,现行修订版本为IEEE std 299-2006 [32],2014年新增屏蔽室尺寸在0.1 m到2.0 m的测试规范[33]。

值得指出的是,为提高测量精度,上述基于传输线或自由空间加载的测试方法都需要配合相关测量技术使用,如采用直通校准可以较好的去除线缆和测试夹具的附加影响,也可采用更为高级的直通-反射-延时(TRL)校准技术(具体可参考本公众号文章,2017-04-02TRL微波器件测量去嵌入校准--原理详解,2017-02-21TRL微波器件测量去嵌入校准--夹具设计,TRL微波器件测量去嵌入校准--实验测试),将测试参考面直接校准到待测样品加载处,不经能够测出样品的传输特性,还可以准确地得到其反射特性。此外,对于有样品加载造成的多次反射以及挡板开窗法边缘处的散射,可通过时域门技术有效去除[20]。

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