曹良足,胡 健,李广文, 3,殷丽霞
(1. 景德镇陶瓷大学机械与电子工程学院,江西 景德镇 333403;2. 南京理工大学电光学院,江苏 南京 210094;3. 景德镇景光电子有限公司,江西 景德镇 333405)
介质陶瓷谐振器天线的研究进展
曹良足1, 2,胡 健1,李广文1, 3,殷丽霞1
(1. 景德镇陶瓷大学机械与电子工程学院,江西 景德镇 333403;2. 南京理工大学电光学院,江苏 南京 210094;3. 景德镇景光电子有限公司,江西 景德镇 333405)
介质陶瓷谐振器天线广泛用于卫星通信、基站和移动通信系统中,本文综述介质陶瓷谐振器天线的馈电结构、小型化、带宽展宽技术和频率可调方法的最近研究成果,并从理论上解释其相关的工作原理,最后提出了介质陶瓷谐振器天线的发展趋势。
天线;介质陶瓷谐振器;馈电结构;带宽展宽技术;频率可调
一直以来,介质陶瓷谐振器(DR)主要用于微波电路,如振荡器和滤波器,它是由高介电常数(εr大于20)的材料制成的,其无载品质因子(Q值)通常在50至500之间,但也可能高达10000[1],例如,(Zr0.8Sn0.2)TiO4陶瓷[2]、CaTiO3-LaAlO3陶瓷[3]。因为在这些传统的应用中,介质陶瓷谐振器是作为贮能器件而不是辐射单元。尽管多年前人们发现开放型的DR能辐射电磁能,直到第一篇关于圆柱介质陶瓷谐振器天线(DRA)的论文发表于1983年后[4],DR用作天线的想法才广泛被人们接受。那时候,人们发现某些系统的工作频率范围扩展到毫米波和近毫米波(100-300 GHz),在这个频段,金属天线的导体损耗变得严重,天线的辐射效率大幅度下降。相反,DRA的介电损耗非常小。自从圆柱DRA研究后[4],Long[5, 6]一直致力于研究长方体DRA和半球DRA。这些工作奠定了DRA未来研究工作的基础。其它形状,如三棱锥DRA[7]、球冠DRA[8]和圆环DRA[9, 10]也得到了开发。图1是常见DRA的照片。
与微带电天线相比,DRA具有相当宽的阻抗带宽(对εr≈10的DRA大约10%)。这是因为微带天线仅通过两条窄的狭缝产生电磁辐射,而DRA除接地面外所有的表面都可以辐射电磁波。DRA还具有另一个明显的优点,就是介质表面能够避免表面波的产生。
图1 各种形状的介质谐振器天线Fig.1 DRAs of various shapes
下面从馈电方式、低背及小型化、带宽展宽技术和频率可调方法四个方面进行综述。
介质陶瓷谐振器天线(DRA)由介质陶瓷制成,所有表面都没有电极,要将电磁信号通过DRA辐射到周围空间,必需采用合适的馈电方式。常见的馈电方式有以下几种:同轴探针[11],窗口耦合的微带线[12],窗口耦合的同轴线[13],直微带线[14],共面波导[15],焊接在微带线上的探针[10],狭缝线[16],带状线[17],共形带状线[18],介质镜像波导[19]。图2示出两种常见的馈电结构。
馈电的本质就是将信号源的能量耦合给DRA,耦合的方式不外乎电耦合和磁耦合以及电磁混合耦合。图3是微带线耦合DRA的等效电路图和反射系数图。图中L1、C1和R1构成并联谐振为DRA的等效电路,L2、C2和R2为微带线的分布参数,L1和L2之间形成磁耦合(图中用M表示)为微带线馈电机制,微带线与DRA距离近耦合强,反射系数更小,反之,则耦合弱,反射系数大于-10 dB。
图2 介质谐振器天线的两种馈电结构Fig.2 Two excitation methods for DRA
图3 微带线耦合介质谐振器天线的等效电路图和反射系数曲线Fig.3 Equivalent circuit and reflection of DRA coupled to microstrip line
许多现代通信系统要求使用低背介质陶瓷谐振器天线,例如可以连接到无线网络的笔记本电脑,低背介质陶瓷谐振器天线本身还可以直接连接到PCMCIA格式的无线网卡上。为了增大辐射,采用低介电常数(εr)的介质材料制作DRA,但是,DRA的高度与εr成反比,即介电常数越高,DRA的高度越低(通常所说的‘低背’), 因此,采用高εr的介质材料可以使DRA低背,从而实现DRA的小型化。 1994年,Mongia[20]采用εr=100的介质材料制作了长方体DRA,阻抗带宽为3%,随后,也研制出低背的圆柱DRA和三棱锥DRA[21-22]。Essele[23]研究了低介电常数(εr=10)DRA的小型化,当DRA的长与高之比等于6(长、宽和高分别为15.2 mm,7.0 mm和2.6 mm)时,谐振频率为11.6 GHz,反射损耗高达38 dB。除了采用高介电常数外, 在圆柱DRA的轴心插入一根短路金属小圆柱,与常规DRA相比,体积可以减小一半以上[24]。最吸引人的方法是将圆柱或长方体沿轴线对半剖开,将剖面紧靠一块垂直的金属板并接地,根据镜像理论,则DRA的体积减小近一半[25-27],如图4所示。
低背DRA的10 dB反射损耗带宽和轴比带宽大于微带介质天线。虽然DRA的高度比微带介质天线高,但DRA能提供令人满意的带宽与安装面积之比,而且其高度也是可以接受的,表1列出两种天线的性能。
图4 小型化介质谐振器天线Fig. 4 Miniaturized DRA
表1 矩形微带介质天线和圆盘介质陶瓷谐振器天线的性能比较[28]Tab.1 Comparison of rectangular patch and circular disc DRA
天线的带宽展宽技术已成为研究热点。展宽带宽的方法大致可分为三大类[28],包括降低谐振器的固有品质因素(Q值); 采用外部匹配网络和组合多个谐振器。1989年由Kishk[29]首次堆叠两个不同的DRA来展宽天线的带宽,这是因为它们的谐振频率不同,相当于双通带。Sangiovanni[30]堆叠三个DRA进一步增加带宽,如图5(a)所示。Leung[31]在两个堆叠的DRA之间引入空气间隙,并且用高εr制作低背DRA, 带宽展宽效果很好。Simon[32]采用另一种方法,即将两个额外的DRA放在主DRA旁边来展宽天线的阻抗带宽,如图5(b)所示。其原理相当于主DRA与两个副DRA耦合形成较宽的通带,图6示出两个DRA的等效图和反射系数图,图中L1与L2之间存在磁耦合(图中用M表示)[28],两个DRA与单个DRA相比,-10 dB带宽明显变宽。Leung[33]采用双圆盘的方法展宽高εr低背DRA的带宽。
图5 三个介质谐振器组合的天线Fig.5 3-DRA (a) stacked DRA, (b) co-planar
图6 两个DRA的等效电路图与反射系数曲线Fig.6 Equivalent circuit and reflection of 2-DRA
上述方法需要额外的DRA单元。下面介绍的带宽展宽技术基于单个DRA结构,通过引入一些结构上的改变, 例如在介质陶瓷谐振器天线上挖一个凹槽或增加空气间隙,降低谐振器的固有品质因素(Q值)。Wong[34]在半球DRA内部引入空气间隙来增加阻抗带宽。Ittipiboon[35]对长方体DRA开展了类似的工作,当空气间隙高度为1.5 mm, 虽然谐振频率增加17%,但是阻抗带宽增加84%,如图7所示。Shum[36]在DRA和地平面之间引入空气间隙增加阻抗带宽。Leung[37]研究了导体取代空气间隙的带宽展宽技术。 Chen[38-39]在DRA上加盖介质帽来增加阻抗带宽。还可以引入额外的导体块增加DRA的阻抗带宽[40-41]。在介质陶瓷谐振器底部和地面之间引入一层低介电常数介质板,增加有效辐射的同时,也能提高带宽[28]。
图7 引入空气间隙的介质谐振器天线Fig.7 DRA with air gap
对微带线直接馈电的DRA,可以采用微带线支节进行阻抗匹配,从而达到带宽展宽的目的[28]。
介质陶瓷谐振器天线的频率由其尺寸和介电常数决定,可是,特殊频率的DRA很难在市场上购买到,即使能买到,但是存在组装偏差,测量值和计算的谐振频率不一定相一致,因此诞生了许多调谐频率的方法,例如,顶面加载金属圆盘、多根带状线、狭缝、长方体侧壁上的短路支节、集总电容、变容二极管器等。Z. Li[42]在圆柱体和圆环DRA的顶面加载导电金属盘调谐天线频率,调谐频率范围可达300-500 MHz。H. K. Ng[43]用多对带状线调谐半球体DRA的频率,两对带状线使天线的谐振频率范围扩展到3.1-3.8 GHz。K. K. So[44]采用接地平面上的狭缝调谐半球体DRA带宽和谐振频率,当狭缝长度从14 mm增加至22 mm时,谐振频率从4.35 GHz减小至3.75 GHz。M.I .Sulaiman[45]通过改变长方体侧壁上螺旋线馈线的位置改变天线的频率,从中心位置移至右边,频率变化从 4.1 GHz 至3.1 GHz。B. Wu[46]在DRA下面加入表面金属化的介质片改变天线的谐振频率,前面所采用的调谐频率的方法是静态的,一旦结构和尺寸确定后,频率就固定不变,而电调谐和光调谐能实现动态调频。A. Petosa[47]在长方体侧壁的金属带与接地面之间焊上三个PIN二极管,由高低电平(即数字信号0和1)控制PIN的导通和断开,从而调谐DRA的频率,当数字信号从000变化到111时,频率从4.50 GHz降到3.45 GHz。C. X. Hao[48]在长方体的侧壁焊上贴片电容器或变容二极管,电容从0 pF增到10 pF,频率2.87 GHz 降至 2.12 GHz,如图8所示。其原理很容易从介质陶瓷谐振器天线的等效电路图(图3)中加以理解,加载的电容并联在电路中,满足下列公式,
式中f0、Lr、Cr和CL分别为谐振频率,等效电感,等效电容和加载电容。从公式可知,CL越大,f0越低,与图8相一致。
图8 加载电容器/变容管的介质谐振器天线的结构图与反射响应Fig.8 Structure and Reflection of DRA loaded with capacitor/varactor
介质陶瓷谐振器天线的性能参数与其形状、尺寸、介质材料的参数和馈电方式等因素有关。在追求某一项参数高指标时,要适当兼顾其它参数,例如,为了小型化采用高介电常数的介质陶瓷,但天线的带宽变窄,增益减少;再例如,为了扩展天线的调谐范围,采用电容比较大的变容管,但天线在低频端的驻波比小于2,不能满足整机对天线的要求。介质陶瓷谐振器天线的研究课题很多,如圆极化、双极化、多通带和天线阵列等,由于篇幅限制,在此不再综述。介质陶瓷谐振器天线的发展趋势是小型化、高增益、宽频带、易调谐和能够低成本批量生产。
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Progress in Dielectric Ceramic Resonator Antenna
CAO Liangzu1, 2, HU Jian1, LI Guangwen1 3, YIN Lixia1
(1. School of Mechanical and Electric Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, Jiangxi, China; 2. School of Electric and Optical Engineering, Nanjing University of Science & Technology, Nanjing 210094,
Jiangsu, China; 3. Jingdezhen Jingguang Electronic Co. Ltd. Jingdezhen 333405, Jiangxi, China)
Dielectric ceramic resonator antennas (DRAs) have been widely used in satellite communication, base station and mobile communication systems. This paper summarizes the excitation, miniaturization, bandwidth enhancement techniques and frequency tuning in recent years. The relevant principles were given in theory. Finally, the trends for DRAs were proposed.
antenna; dielectric ceramic resonator; excitation; bandwidth enhancement techniques; frequency tuning
date: 2015-10-10. Revised date: 2015-12-21.
TQ174.75
A
1000-2278(2016)02-0127-06
10.13957/j.cnki.tcxb.2016.02.004
2015-10-10。
2015-12-21。
江西省自然科学基金项目(20151BAB207014);景德镇市科技局科研项目(景科字[2013]第55号)。
通信联系人:曹良足(1966-),男,教授。
Correspondent author:CAO Liangzu(1966-), male, Professor.
E-mail:clz4233@liyun.com