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title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线" title="贴片天线">贴片天线的电特性,结果表明在以未加载微带贴片天线的谐振频率2.2GHz为中心可获得50%的有效阻抗带宽。用最简单的传输线模型估算谐振频率,实验结果与预测相当地一致。

　　关键词: 矩形微带贴片天线  变容管调谐  阻抗带宽

　　微带天线由于其重量轻,制作简单、成本低,易于与载体平台共形以及适合组阵等诸多优点,自20世纪70年代以来越来越受重视并得到广泛应用。它特别适用于各种移动地面设备,如移动通信、无线电话、GPS接收机、车载雷达等,以及飞行载体(如卫星、火箭和飞机等)电子设备。但微带贴片天线的致命缺点是阻抗带宽太窄,只有百分之几,大大限制了它的应用范围。近些年来,已有多种技术成功地用于改善带宽,这些方法中包括使用低介电常数的介质基板、使用水平或垂直方向多层寄生贴片、以及采用匹配结构等。

　　本文提出微带贴片天线加载变容管来提高有效带宽,用最简单的传输线模型理论设计微带贴片天线,研究变容管加载的探针馈电矩形微带天线电特性,重点考查了变容管加载微带天线后的谐振频率变化及可调谐范围,实验结果与预测符合得较好。

1 天线的分析与设计

　　微带天线的分析方法主要可分为三类,即传输线模型,腔模理论以及全波分析法。全波分析法是最严格的分析方法,采用矩量法(MOM)、有限元法(FEM)及时域有限差分法(FDTD)等数值方法比较严格地求解,结果比较精确,但计算量都比较大。在通常的工程应用中,采用传输线模型和腔模理论,只要根据经验公式和实际结构作适当的修正,也能得到满意的设计结果,误差可控制在1%以下。

　　探针馈电的加载变容管矩形微带贴片天线结构,如图1所示,微带贴片印刷在介电常数ε_r=2.2的聚四氟乙烯介质基片上,基片厚度h=1.59mm;设计中心频率f₀=3.0GHz(未加载变容管时),设贴片的宽度和长度分别为W和L,馈电点距两边缘为L1、L2。其等效传输线模型如图2所示,变容管的等效电路如图3所示。

　　微带结构的等效介电常数及特性阻抗分别为:

　　其中W_r=W/h,开路端缝隙边缘效应引起的等效延伸长度为:

　　则可得左边终端导纳Ys1=Gs+jBs,其中

　　且L_C=L+Δl。对于右端(即加载变容管端),其终端导纳还需计入变容管影响,即Y_s2=G_s+j_Bs+g₀+j_BV。另外,采用同轴探针馈电,中心导体需穿过介质板,即在贴片与金属接地板间含有一金属小圆柱,对输入阻抗的影响为引入一感抗

　　令Y_inL和Y_inR分别为由馈电点向左向右看的输入导纳,由传输线公式:

　　根据谐振时输入导纳的虚部为零即可求出天线的谐振频率。这样,控制加在变容管上的电压就可以控制天线的工作频率,从而加大天线的工作频率范围。

2 实验结果

　　首先设计一个微带贴片天线,微带贴片印刷在介电常数为2.2的聚四氟乙烯介质基片上。基片厚度h=1.59mm。微带贴片天线的设计频率为3.0GHz,则由计算得到贴片的物理尺寸,长L=32.85mm,宽W=39.50mm。为使阻抗为50Ω的SMA同轴探针与贴片的输入阻抗匹配,馈电探针的位置应放在距贴片中心约L/6处。所采用的变容管的基本参数为:总电容比10,零偏电容6pF,反向击穿电压-22V,寄生电容0.13pF,串联电感0.4nH,串联电阻0.01Ω;与其对应的变容管的等效电路如图3所示。该变容管具有高电容比,高Q值,恒定的γ值;为获得较宽的调谐范围,变容管加载于贴片辐射边的中心,因为该点的电场最强。

　　再单独测量变容管工作特性曲线,即改变变容管的反向偏压时测电容值;然后加载微带贴片天线作在线测量,改变变容管的反向偏压,用标量网络分析仪来测量反射损耗的频率响应。图4给出了微带贴片天线的谐振与变容管电容的关系曲线的测试结果。不难发现在一个相当宽的频率范围上可以调谐。而且,当变容管反向偏置电压大于10V时,谐振频率数值上变化不明显,这是因为变容管的接头电容在这个工作区上趋近于常数。以2.2GHz为中心,测得的调谐范围为50%。考虑变容管等效电路中所有的参数,图中同时给出了用简单的传输线理论计算谐振频率曲线,测试结果与预测吻合得较好。

　　实验结果表明,将变容管放在贴片辐射边的中心时,获得了相当宽的调谐范围:以2.20GHz为中心可达50%。值得注意的是,采用这种办法并不增加天线的瞬时阻抗带宽;即使如此,这种方案的实现对于频率捷变装置或多频工作的收发系统,如雷达、移动通讯,仍然有着非常重要的实际意义。

参考文献

1 张钧.微带天线理论与工程.北京:国防工业出版社，1988

2 钟顺时.微带天线理论与应用.西安:西安电子科技大学出版社,1991

3 H.A.Atwater.Simplified design equation for microstrip line parameters.Microwave Journal.，1989；(1):109～115

4 E.liter.Improved formulas for input impedance of coaxial-fed microstrip patch antenna.IEE.Proc.H，1982:161～164