低剖面宽带开槽微带天线的研究报告与设计

-低剖面宽带开槽微带天线的研究与设计袁卿瑞(中国电子科技集团第十研究所， 610036)摘 要：本文提出了一种新型的低剖面宽带开槽微带天线。该天线印制在剖面尺寸为 0.011l的FR4er=4.4介质板上，通过在矩形贴片的非辐射边开一对对称的弯折细槽及在天线尾部加载一段微带线作为负载来鼓励起三个邻近的谐振模。实验说明该天线VSWR2带宽可以到达传统矩形贴片的3倍。关键词：低剖面，宽带，开槽天线Analysis and Design of Low-profileBroadband Micro-stripAntenna with Bent SlotsYuan Qing rui(China Research Institute of Radiowave Propagation, ChengduSichuan 610036, China)Abstract：In this paper, a new low-profile broadband micro-strip antenna with slots is presented. The antenna is printed on the FR4 substrate(er=4.4) of thickness 0.011l. A wide operating bandwidth can be obtained by embedding a pair ofsymmetric bent slots inside the patch and inserting an inset micro-striplinesection at the patch edge as an integrated reactive load to e*cited three closely e*cited resonant modes. Adjusting the slots and the micro-strips location and length, a wider bandwidth can be achieved. Results show that the operating bandwidthVSWR2is 3 times that of a traditional rectangular micro-strip antenna.Keywords: Low-profile; Broadband; Slotted antenna. z.-1 引言近年来，国内外许多学者对微带天线的带宽展宽技术进展了深入广泛的研究，提出了很多行之有效的方法，例如采用U-型槽构造、E-型贴片等，但这些天线形式大都采用厚空气层或者泡沫层作为介质层，这就增大了天线的剖面。而在*些情况下，例如共形天线、便携天线等就需要采用低剖面的介质板。因此对低剖面宽带天线的研究就显得很有意义。在微带贴片的适当位置开槽可以展宽天线的带宽，例如在圆形贴片上开两个弧形槽1，在梯形贴片上开一对弯折槽2，在矩形贴片上开一对弯折槽3，在三角形贴片上开一对不对称的弯折槽等4。这些天线构造都是印制在薄介质板上，通过开槽来鼓励起两或者三个相邻的谐振模式，来到达展宽天线带宽的目的。本文提出一种新型的宽带开槽微带天线，剖面厚度为0.011l，通过在矩形贴片的非辐射边开一对对称的弯折细槽及在天线尾部加载一段微带线作为负载来鼓励起三个相邻的谐振模。调整细槽和微带线的位置和长度使三个谐振模连在一起，可以使天线带宽展宽到传统矩形贴片的3倍。2 天线构造设计天线构造如图1所示，采用矩形构造，贴片长度为L，宽度为W，沿贴片的边缘平行方向开了一对对称的弯折槽，弯折槽由两局部组成，分别为L1和L2，其中L2段平行于非辐射边，距离非辐射边间距为W1，L1段平行于辐射边，距离辐射边间距为W2，两段槽缝的宽度均为Ws，底部嵌入贴片的微带线宽度为W3，长度为L3，距离底边间距为L4，馈电点的位置在矩形贴片中线上，距离底边间距为L5。贴片印制在介电常数为4.4，厚为3mm的FR4介质板上。对称的弯折槽构造可以鼓励起一个与基模TM10相邻的谐振模TMd01d22，尾部加载的小段微带线可以鼓励起另一个与之相邻的谐振模3，这三个谐振模具有相似的辐射特性和一样的极化特性。图1 宽带开槽微带天线构造尺寸图3仿真分析按图1给出的天线构造使用电磁仿真软件HFSS进展仿真设计。通过反复的软件仿真和优化，发现对天线驻波特性影响较大的两个参数为L1和L3。为了研究的方便，设其他参数为定值，分别改变L1和L3来考察其对天线性能的影响。3.1 参数L1分析图2 不同L1值天线回波损耗仿真结果如图2所示，L1主要影响第一、二谐振点。从图中可以看出，当L1增大时，第一、二谐振点逐渐别离；当L1减小时，第一、二谐振点逐渐靠近。因此选择适当的L1值，可以使第一、二谐振点连在一起，且不互相重叠，从而形成一个较宽的驻波带宽。3.2 参数L3分析图3 不同L3值天线回波损耗仿真结果如图3所示，L3主要影响第二、三谐振点。从图中可以看出，当L3减小时，第二、三谐振点逐渐别离；当L3增大时，第二、三谐振点逐渐靠近，但如果太过靠近就会影响到第一、二谐振点。因此选择适当的L1值，既可以使三个谐振点全部连在一起，又不使第一、二个谐振点性能变差，从而形成较宽的驻波带宽。4 实测结果根据仿真优化的结果，选用如表1所示的构造尺寸，加工制作了一副天线，对其进展实验测试。表1 宽带开槽微带天线构造尺寸表尺寸值mm贴片长度L62.5贴片宽度W50槽缝长度L110.5槽缝长度L260.5微带线长度L339微带线距离底边长度L41馈电位置L541.75间距W13间距W21微带线宽度W32槽缝宽度Ws1实测的S11如图4所示，相对带宽为6.6%中心频率1.12GHz，而采用一样的介质板和同等大小的矩形微带贴片，其相对带宽为2.2%中心频率1.12GHz，可以看出本文的宽带开槽天线的带宽是同等大小的传统矩形贴片的3倍。图4 实测天线回波损耗结果实测辐射方向图如图5所示，带宽内天线的方向图一致性较好，没有发生畸变，穿插极化在-15dB以下。E面方向图H面方向图af=1095MHzE面方向图H面方向图bf=1132MHzE面方向图H面方向图cf=1155MHz图5 宽带开槽微带天线实测辐射方向图天线的实测增益如图6所示，带宽内天线增益小于2dB，比一般的微带天线要低。使用HFSS仿真软件计算出宽带开槽天线的外表电流分布如图7所示，可以看出，在微带线处的外表电流较大且与其他地方的电流方向相反，从而使天线的增益较普通矩形贴片有所降低。图6 宽带开槽微带天线实测增益图图7 宽带开槽微带天线外表电流分布图5结论使用电磁仿真软件HFSS进展仿真分析和优化，设计了一种宽带开槽微带天线，天线介质板厚度为0.011l，介电常数为4.4。实验说明该天线VSWR2带宽可以到达传统矩形贴片的3倍，方向图一致性较好，穿插极化在-15dB以下，增益在带宽内小于2dB。. z.-参 考 文 献1S.Dey, C.K. Aanandan, P. Mohanan, and K.G. Nair, A new broadband circular patch antenna, Microwave Opt Technol Lett 7,19992M.C. Pan, K.L. Wong, A broadband slot-loaded trapezoid microstrip antenna, Microwave Opt Technol Lett 24,20003J.Y. Sze and K.L. Wong, Slotted rectangular microstrip antenna for bandwidth enhancement, IEEE Trans. Antennas Propagat.,vol.48,2000 4S.T Fang, T.W. Chlou, K.L. Wong, Broadband equilateral-triangular microstrip antenna with asymmetric bent slots and integrated reactive loading, Microwave Opt Technol Lett 23,1999作者简介：袁卿瑞，男，硕士，主要研究领域为宽带微带天线、阵列天线等。Advanced Microwave Laminate Materials for the Improvement of Efficiency and Reliability in Antennas and Feed Networks1George Q. Kang2Helena Li Hai1John C. Frankosky1Michael T. SmithArlon, Inc. Materials for Electronics Division11100 Governor Lea Road, Bear, DE, 19701 USA; 2No. 8, Hong Gu Road, Shanghai 200336 P. R. CHINAgkangarlon-med.; hlihaiarlon-med.Abstract:Demands for higher system efficiency and improved product reliability at higher powers and wider operating temperature ranges in mission-critical antennas and feed networks have placed heightened and unique requirements on board materials. To meet these challenges, advanced laminate materials need to possess high electrical phase stability, dielectric constant control, high thermal conductivity and multilayer capability. This paper discusses the importance of critical material properties and e*plores new material developments for these applications.Keywords:High frequency laminate, phase stability, thermal conductivity, reliability, military and space RF antennas. z.-INTRODUCTIONIn mission-critical military and space RF/Microwave applications, such as space and military radars, phased array antennas (both passive and active electronically scanned), satellite antennas, missile seekers and guidance systems, system phase and frequency stability over temperature is very important, since these systems are e*pected to operate and maintain high performance over a wide range of operating temperature. A small drift of system operating frequency due to material dielectric change over temperature in critical frequency selective or sensitive components such as filters, oscillators, feed networks and antenna elements could cause a system to operate at lower efficiency or deviate from its designed performance. For e*ample, in a phased array antenna system, the antenna elements and the feed networks are designed with certain microwave laminate of determined physical structures for certain frequency operation, which may be e*pected to operate over an operational temperature range of -55C to 150C and still maintain system capability and reliability .For RF designers of these phase sensitive and mission-critical applications, ideal dielectric substrate materials are e*pected to posses not only desirable electrical properties, such as dielectric temperature stability, dielectric constant consistency and low loss tangent, but also e*cellent thermal and mechanical properties, such as thin cores for multilayer capability, dimensional stability and low rates of thermal e*pansion, e*cellent thickness tolerance and high thermal conductivity (W/mK). Due to stringent weight requirements placed on systems in avionics and space applications, millimeter waves are considered standard technology offering many advantages associated with the higher operating frequencies, including smaller and lighter systems, increased bandwidth, improved resolution and directivity (due to narrower beams) for a given antenna aperture. This also means dielectric material challenges of thinner dielectrics and finer traces, while requiring lower dielectric loss due to high frequencies and smaller structures. For space-based radars (SBR) and RADINT (Radar Intelligence) that are launched into the space and operate in a very wide range of temperature, they require an even higher level of reliability because making repairs in space is both costly and difficult. Therefore, dielectric materials will need to be both reliable and high performance so that very high-resolution and highly accurate radar imagery can be achieved under all operating conditions. SELECTION OF DIELECTRIC MATERIALSASubstrate dielectric constantSelection of proper laminate substrates is e*tremely important for RF designers to ensure the function of a design and make viable product with great efficiency and reliability. Dielectric materials provide not only material and media support for RF/Microwave and millimeter wave electronic applications, but also RF and electrical performances. Because of the presence of dielectric media, wavelength () of electromagnetic wave propagation within dielectrics becomes shorter compared to that in free space (see Eq.1, c0 is speed of light in a vacuum). A signal of 10 GHz has a wavelength of about 3.0 cm in free space, while in a substrate of 3.00 dielectric constant (i.e. DK), its wavelength becomes 1.73 cm.1Microstrip designs have been widely used in making planar RF circuits and integrations. In radar manifolds and feed networks, stripline is the typical choice of transmission lines in these multilayer and buried RF structure applications to achieve the ma*imum efficiency in terms of device size, weight and performance. Correspondingly, the design of microstrip and stripline transmission lines on dielectric substrates has the following relationships between design size (line width and substrate thickness), laminate DK and characteristic impedances (see Figure , which is based on the transmission line design equations from while ignoring metal thickness effects). It shows that higher substrate DK leads to smaller RF signal traces and miniaturized structures, while thicker substrate of certain DK makes 50ohm transmission line wider for desired applications. In microstrip patch antenna design, thicker and lower DK substrates provide better efficiency, larger bandwidth, but bigger patch size; thinner and higher DK substrates lead to smaller element size, but greater losses and lower efficiency, and relatively smaller bandwidths. z.-Figure 1 Dielectric Constant Effects on Stripline and Microstrip Design Size (line width and substrate thickness). . z.-BDielectric loss and insertion lossFrom a design perspective, the primary sources of loss in laminate performance are the dielectric loss (i.e. loss tangent or dissipation factor) and the conductor loss. At moderate frequencies with very low loss materials (loss tangent around 0.0009), conductor loss might dominate dielectric loss 3 to 1. As frequencies increase, the conductor loss and dielectric loss ratio will change to a point where they could be similar in value depending on the material performance across frequencies. To account for the conductor loss in real RF circuits, it needs to consider not only the types of metal cladding in laminates (i.e. the choice of electrodeposited (ED) copper, reverse treated ED copper or rolled-annealed (RA) copper) and metal conductivity, but also the copper profile/roughness and trace resolution or o*idation from processing.As a dielectric material, polytetrafluoroethylene (PTFE) is a near ideal material for microwave circuit boards because of its outstanding electrical properties at high frequencies. Electrically, fiberglass reinforced PTFE-based laminates such as Arlon DiClad 880 or CuClad 217, Taconic TLY-5, and Rogers RT/duroid 5880 provide e*tremely low loss characteristics. However, since these laminates have very high amounts of PTFE resin content, they have a relatively high coefficient of thermal e*pansion (CTE) and a thermal coefficient of dielectric constant (TCEr) on the order of -150 ppm/C. To account for laminate dielectric loss in finished circuit boards, moisture absorption and processing solution e*posure need to be considered.EFFECTS FROM MOISTURE AND PROCESSINGThe lowest loss tangent materials do not always make ideal laminates, because processing and fabrication can influence laminate performance in ways that would not be reflected in loss tangent measurements associated with standard IPC test methods. Moisture and processing chemical absorption play a critical role in insertion loss. A material that is viewed as low loss because of a low loss tangent may in fact have issues with moisture absorption, or ingression. Designs with many through-holes or routed areas can quickly become high-loss boards if moisture ingress/absorption is an issue.A common area for moisture ingression is through poor quality holes that disturb resin-to-reinforcement or layer-to-layer interfaces. Some laminates have a broader window than others when it comes to their sensitivity to processing. Moisture ingression and processing chemical absorption can also have a role in delamination or blistering if the laminate is e*posed to rapid temperatures during post etching processes. Water vapor, when remained at the micro-voids e*isting at the ceramic-PTFE interface, will have a great effect upon the overall performance of the circuit, especially affecting loss tangent and insertion loss of the board. Liquids with low surface tension, such as organic solvents and surfactant laden aqueous solutions, will penetrate pores and cause similar loss issues. For e*ample, Arlons CLTE-*T laminate, with DK 2.94 and very low loss tangent (Df of 0.0012), has demonstrated e*cellent moisture resistance as compared with those laminates of the same class from other vendors (see Figure ). Due diligence on final design and materials is again warranted to achieve a desired design optimum.Figure 2 Moisture effects on various laminatesDIELECTRIC CONSTANT THERMAL STABILITYWithout e*tra phase stable additives, PTFE-based laminates have a relatively high thermal coefficient of dielectric constant (TCEr). For RF circuits, improved dielectric temperature stability directly translates into impedance and phase/frequency stability over temperature, with the benefits of reducing impedance mismatches around active components (such as power amplifier transistors), lowering device frequency/phase shift, reducing system bandwidth roll off and drift in heated operations, and thus improves efficiency and performance. Products that employ the addition of phase stable ceramics to reduce TCEr include Rogers RT/duroid6002, Arlon CLTE-*T and TC350 or TC600. They were developed to provide consistent dielectric constant not only near the PTFE phase change but also throughout a much wider operating temperature range. In addition to dielectric constant stability, CLTE-*T has greater dimensional stability (registration), especially in thinner laminates. As circuits are designed around a specific frequency, so physical circuit elements are designed around specific electrical lengths; these are measured by phase angle. Where temperature affects dielectric constant and mechanical dimensions, phase angle values of the circuit elements are also affected. Dielectric constant across temperature needs to be consistent to avoid phase stability issues. For antenna designs, a significant shift in resonance frequency and bandwidth roll off at specific frequencies, results in lower gain performance. The relationship between frequency or phase stability and dielectric constant drift can be illustrated in the equations (* represents the small change of DK due to varying TCEr and CTE, while l is physical length of circuit elements) as follows. Appro*imately, frequency or phase shift over temperature swing is close to half of the amount of DK drift or change.Selection of a material that is relatively insensitive to temperature provides a high degree of phase stability to the impedance matching networks, Wilkinson power dividers, quarter wave transformers, etc. It also minimizes impedance changes in a transmission line when it is e*posed to a changing temperature. This can be seen in Figure , where the middle of a 50 ohm trace of a -75 ppm/C board was e*posed to a heat source of 125C. At the location of the heat source, the impedance increased 1.135 ohms.Figure 3 TDR test of 50ohm line impedance change at 125CPRODUCT RELIABILITY AND THERMAL CONDUCTIVITYAs the mission-critical military and space RF and microwave applications have been consistently designed and required to operate at high power levels and to endure wider temperature ranges and still maintain system capability, product reliability has become as important and critical as the demands placed on system high performance and high efficiency. To describe the failure rate during a products lifespan, the bathtub curve has been widely used for this purpose. There are three distinct periods for the failure rate throughout a products life time, or three different failure modes that account for a products failure rate in each of these early, random (or constant) and wear-out failure periods. Designers and manufacturers has to ensure that products in the infant mortality do not get to the customers, and try to improve MTBF (Meat Time Before Failure, an inverse of constant failure rate in random mode) and reliability through proper design and better product development.Figure 4 Bathtub curve for failure rate vs. product lifetimeIndustrial applications have shown that device or component failure has accounted for most of the RF system failures, especially the failure of high power amplifiers, which are common in radar feed networks, beamformers and phased array antennas. According to Arrhenius equation, a 10C increase in temperature doubles the failure rate of RF components. Thermal management has become a critical issue in RF designs. Thus, selection of the proper laminate of high thermal conductivity can benefit a design for the improvement of product reliability and performance. Through unique chemistry and processing, Arlon has been able to develop two thermally conductive, ceramic/PTFE-based laminates TC350 and TC600 to meet the design trend of high power applications (see Table ). While CLTE-*T possesses fairly good thermal conductivity, thermal conductivity of TC350 and TC600 has been significantly improved and close to that of LTCC (Low Temperature Co-fired Ceramics) circuit boards (TC of LTCC is about 2-3 W/mK).Table 1 Arlons Advanced RF LaminatesIn addition to the highest thermal conductivity in their classes which helps remove the heat around components from circuit boards, TC-series products also have low loss tangent to minimize heat generation from insertion loss, and lowest TCEr and CTE values which lead to high system efficiency due to high frequency/phase stability and impedance control over temperature. Figure shows thermal images of the same RF power FET transistors operating on two different board materials of the same DK and thickness but different TC values. The thermal conductivity of the left material is 0.46 W/mK, compared to 1.1 W/mK of TC600. It shows that the increase of thermal conductivity reduced the ma*imum temperature from 82C to 73C on the FETs case and 78C to 72C at board bottom side. The heat spreading properties of the material can also visualized by the larger area of temperatures above 34C thus reducing the severity of the temperature gradient within the circuit board. Figure 5 Thermal Images of board and case temperatures for RF power FET transistors operated on different laminatesThe material with higher thermal conductivity pulls the heat away from the hot spot and allows it to be more efficiently dissipated. The reduced ma*imum temperature seen on the board keeps active devices operate at favorably lower temperature for higher efficiency with improved operating reliability and phase stability, and also helps reduce the ma*imum temperatures seen at solder joints or at plated through-holes (PTH), which are other areas critical for failure.MULTILAYER BOARD APPLICATIONSIdeal laminates for military and space systems require careful consideration in order to deliver system performance and reliability. This includes well-known laminate properties, such as low dielectric loss, low thermal e*pansion, and low temperature sensitivity of the dielectric, but also includes other material consideration such as dimensional stability, moisture and processing sensitivity, and i