低剖面寬帶開槽微帶天線的研究與設(shè)計(jì)

1、真誠(chéng)為您提供優(yōu)質(zhì)參考資料，若有不當(dāng)之處，請(qǐng)指正。低剖面寬帶開槽微帶天線的研究與設(shè)計(jì)袁卿瑞(中國(guó)電子科技集團(tuán)第十研究所，成都 610036)摘 要：本文提出了一種新型的低剖面寬帶開槽微帶天線。該天線印制在剖面尺寸為 0.011l的FR4（er=4.4）介質(zhì)板上，通過(guò)在矩形貼片的非輻射邊開一對(duì)對(duì)稱的彎折細(xì)槽及在天線尾部加載一段微帶線作為負(fù)載來(lái)激勵(lì)起三個(gè)鄰近的諧振模。實(shí)驗(yàn)表明該天線VSWR2帶寬可以達(dá)到傳統(tǒng)矩形貼片的3倍。關(guān)鍵詞：低剖面，寬帶，開槽天線Analysis and Design of Low-profile Broadband Micro-strip Antenna with Bent

2、SlotsYuan Qing rui(China Research Institute of Radiowave Propagation, Chengdu Sichuan 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

3、 be obtained by embedding a pair of symmetric bent slots inside the patch and inserting an inset micro-strip line section at the patch edge as an integrated reactive load to excited three closely excited resonant modes. Adjusting the slots and the micro-strips location and length, a wider bandwidth

4、can be achieved. Results show that the operating bandwidth（VSWR2）is 3 times that of a traditional rectangular micro-strip antenna.Keywords: Low-profile; Broadband; Slotted antenna13 / 131 引言近年來(lái)，國(guó)內(nèi)外許多學(xué)者對(duì)微帶天線的帶寬展寬技術(shù)進(jìn)行了深入廣泛的研究，提出了很多行之有效的方法，例如采用U-型槽結(jié)構(gòu)、E-型貼片等，但這些天線形式大都采用厚空氣層或者泡沫層作為介質(zhì)層，這就增大了天線的剖面。而在某些情況下，

5、例如共形天線、便攜天線等就需要采用低剖面的介質(zhì)板。因此對(duì)低剖面寬帶天線的研究就顯得很有意義。在微帶貼片的適當(dāng)位置開槽可以展寬天線的帶寬，例如在圓形貼片上開兩個(gè)弧形槽1，在梯形貼片上開一對(duì)彎折槽2，在矩形貼片上開一對(duì)彎折槽3，在三角形貼片上開一對(duì)不對(duì)稱的彎折槽等4。這些天線結(jié)構(gòu)都是印制在薄介質(zhì)板上，通過(guò)開槽來(lái)激勵(lì)起兩或者三個(gè)相鄰的諧振模式，來(lái)達(dá)到展寬天線帶寬的目的。本文提出一種新型的寬帶開槽微帶天線，剖面厚度為0.011l，通過(guò)在矩形貼片的非輻射邊開一對(duì)對(duì)稱的彎折細(xì)槽及在天線尾部加載一段微帶線作為負(fù)載來(lái)激勵(lì)起三個(gè)相鄰的諧振模。調(diào)整細(xì)槽和微帶線的位置和長(zhǎng)度使三個(gè)諧振模連在一起，可以使天線帶寬展寬到

6、傳統(tǒng)矩形貼片的3倍。2 天線結(jié)構(gòu)設(shè)計(jì)天線結(jié)構(gòu)如圖1所示，采用矩形結(jié)構(gòu)，貼片長(zhǎng)度為L(zhǎng)，寬度為W，沿貼片的邊緣平行方向開了一對(duì)對(duì)稱的彎折槽，彎折槽由兩部分組成，分別為L(zhǎng)1和L2，其中L2段平行于非輻射邊，距離非輻射邊間距為W1，L1段平行于輻射邊，距離輻射邊間距為W2，兩段槽縫的寬度均為Ws，底部嵌入貼片的微帶線寬度為W3，長(zhǎng)度為L(zhǎng)3，距離底邊間距為L(zhǎng)4，饋電點(diǎn)的位置在矩形貼片中線上，距離底邊間距為L(zhǎng)5。貼片印制在介電常數(shù)為4.4，厚為3mm的FR4介質(zhì)板上。對(duì)稱的彎折槽結(jié)構(gòu)可以激勵(lì)起一個(gè)與基模TM10相鄰的諧振模TMd0（1d2）2，尾部加載的小段微帶線可以激勵(lì)起另一個(gè)與之相鄰的諧振模3，這三個(gè)

7、諧振模具有相似的輻射特性和相同的極化特性。圖1 寬帶開槽微帶天線結(jié)構(gòu)尺寸圖3 仿真分析按圖1給出的天線結(jié)構(gòu)使用電磁仿真軟件HFSS進(jìn)行仿真設(shè)計(jì)。通過(guò)反復(fù)的軟件仿真和優(yōu)化，發(fā)現(xiàn)對(duì)天線駐波特性影響較大的兩個(gè)參數(shù)為L(zhǎng)1和L3。為了研究的方便，設(shè)其他參數(shù)為定值，分別改變L1和L3來(lái)考察其對(duì)天線性能的影響。3.1 參數(shù)L1分析圖2 不同L1值天線回波損耗仿真結(jié)果如圖2所示，L1主要影響第一、二諧振點(diǎn)。從圖中可以看出，當(dāng)L1增大時(shí)，第一、二諧振點(diǎn)逐漸分離；當(dāng)L1減小時(shí)，第一、二諧振點(diǎn)逐漸靠近。因此選擇適當(dāng)?shù)腖1值，可以使第一、二諧振點(diǎn)連在一起，且不互相重疊，從而形成一個(gè)較寬的駐波帶寬。3.2 參數(shù)L3分析

8、圖3 不同L3值天線回波損耗仿真結(jié)果如圖3所示，L3主要影響第二、三諧振點(diǎn)。從圖中可以看出，當(dāng)L3減小時(shí)，第二、三諧振點(diǎn)逐漸分離；當(dāng)L3增大時(shí)，第二、三諧振點(diǎn)逐漸靠近，但如果太過(guò)靠近就會(huì)影響到第一、二諧振點(diǎn)。因此選擇適當(dāng)?shù)腖1值，既可以使三個(gè)諧振點(diǎn)全部連在一起，又不使第一、二個(gè)諧振點(diǎn)性能變差，從而形成較寬的駐波帶寬。4 實(shí)測(cè)結(jié)果根據(jù)仿真優(yōu)化的結(jié)果，選用如表1所示的結(jié)構(gòu)尺寸，加工制作了一副天線，對(duì)其進(jìn)行實(shí)驗(yàn)測(cè)試。表1 寬帶開槽微帶天線結(jié)構(gòu)尺寸表尺寸值（mm）貼片長(zhǎng)度L62.5貼片寬度W50槽縫長(zhǎng)度L110.5槽縫長(zhǎng)度L260.5微帶線長(zhǎng)度L339微帶線距離底邊長(zhǎng)度L41饋電位置L541.75間距

9、W13間距W21微帶線寬度W32槽縫寬度Ws1實(shí)測(cè)的S11如圖4所示，相對(duì)帶寬為6.6%（中心頻率1.12GHz），而采用相同的介質(zhì)板和同等大小的矩形微帶貼片，其相對(duì)帶寬為2.2%（中心頻率1.12GHz），可以看出本文的寬帶開槽天線的帶寬是同等大小的傳統(tǒng)矩形貼片的3倍。圖4 實(shí)測(cè)天線回波損耗結(jié)果實(shí)測(cè)輻射方向圖如圖5所示，帶寬內(nèi)天線的方向圖一致性較好，沒(méi)有發(fā)生畸變，交叉極化在-15dB以下。E面方向圖H面方向圖（a）f=1095MHzE面方向圖H面方向圖（b）f=1132MHzE面方向圖H面方向圖（c）f=1155MHz圖5 寬帶開槽微帶天線實(shí)測(cè)輻射方向圖天線的實(shí)測(cè)增益如圖6所示，帶寬內(nèi)天線增

10、益小于2dB，比一般的微帶天線要低。使用HFSS仿真軟件計(jì)算出寬帶開槽天線的表面電流分布如圖7所示，可以看出，在微帶線處的表面電流較大且與其他地方的電流方向相反，從而使天線的增益較普通矩形貼片有所降低。圖6 寬帶開槽微帶天線實(shí)測(cè)增益圖圖7 寬帶開槽微帶天線表面電流分布圖5 結(jié)論使用電磁仿真軟件HFSS進(jìn)行仿真分析和優(yōu)化，設(shè)計(jì)了一種寬帶開槽微帶天線，天線介質(zhì)板厚度為0.011l，介電常數(shù)為4.4。實(shí)驗(yàn)表明該天線VSWR2帶寬可以達(dá)到傳統(tǒng)矩形貼片的3倍，方向圖一致性較好，交叉極化在-15dB以下，增益在帶寬內(nèi)小于2dB。參 考 文 獻(xiàn)1S.Dey, C.K. Aanandan, P. Mohana

11、n, 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 Tra

12、ns. 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作者簡(jiǎn)介：袁卿瑞，男，碩士，主要研究領(lǐng)域?yàn)閷拵炀€、陣列天線等。Advanced Microwave Laminate Materials for the Impro

13、vement of Efficiency and Reliability in Antennas and Feed Networks1George Q. Kang 2Helena Li Hai 1John C. Frankosky 1Michael T. SmithArlon, Inc. Materials for Electronics Division11100 Governor Lea Road, Bear, DE, 19701 USA; 2No. 8, Hong Gu Road, Shanghai 200336 P. R. CHINAgkangarlon-; hlihaiarlon-A

14、bstract: 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 materia

15、ls 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 explores new material developments for these applications.Keywords: High frequency laminate, p

16、hase stability, thermal conductivity, reliability, military and space RF antennas 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 seeke

17、rs and guidance systems, system phase and frequency stability over temperature is very important, since these systems are expected 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 t

18、emperature 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 example, in a phased array antenna system, the antenna elements and th

19、e feed networks are designed with certain microwave laminate of determined physical structures for certain frequency operation, which may be expected to operate over an operational temperature range of -55C to 150C and still maintain system capability and reliability .For RF designers of these phase

20、 sensitive and mission-critical applications, ideal dielectric substrate materials are expected to posses not only desirable electrical properties, such as dielectric temperature stability, dielectric constant consistency and low loss tangent, but also excellent thermal and mechanical properties, su

21、ch as thin cores for multilayer capability, dimensional stability and low rates of thermal expansion, excellent 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 standar

22、d 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 diel

23、ectrics 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 reliabil

24、ity 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 MATERIALSASubstr

25、ate dielectric constantSelection of proper laminate substrates is extremely 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 wa

26、ve 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

27、 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. （1）Microstrip designs have been widely used in making planar RF circuits and integrations. In radar manifolds and feed networks, stripline is the typical choice of t

28、ransmission lines in these multilayer and buried RF structure applications to achieve the maximum 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 de

29、sign 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 structur

30、es, 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 siz

31、e, but greater losses and lower efficiency, and relatively smaller bandwidths.Figure 1 Dielectric Constant Effects on Stripline and Microstrip Design Size (line width and substrate thickness). BDielectric loss and insertion lossFrom a design perspective, the primary sources of loss in laminate perfo

32、rmance 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 ra

33、tio 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, re

34、verse treated ED copper or rolled-annealed (RA) copper) and metal conductivity, but also the copper profile/roughness and trace resolution or oxidation from processing.As a dielectric material, polytetrafluoroethylene (PTFE) is a near ideal material for microwave circuit boards because of its outsta

35、nding 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 extremely low loss characteristics. However, since these laminates have very high amounts of PTFE resin c

36、ontent, they have a relatively high coefficient of thermal expansion (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 exposure need to be considere

37、d. 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

38、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

39、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 c

40、hemical absorption can also have a role in delamination or blistering if the laminate is exposed to rapid temperatures during post etching processes. Water vapor, when remained at the micro-voids existing at the ceramic-PTFE interface, will have a great effect upon the overall performance of the cir

41、cuit, 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 example, Arlons CLTE-XT laminate, with DK 2.94 and very low loss tangent

42、 (Df of 0.0012), has demonstrated excellent 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 laminates DIELE

43、CTRIC CONSTANT THERMAL STABILITYWithout extra 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 temper

44、ature, 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

45、 addition of phase stable ceramics to reduce TCEr include Rogers RT/duroid 6002, Arlon CLTE-XT 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

46、 constant stability, CLTE-XT 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 aff

47、ects 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 s

48、pecific frequencies, results in lower gain performance. The relationship between frequency or phase stability and dielectric constant drift can be illustrated in the equations (x represents the small change of DK due to varying TCEr and CTE, while l is physical length of circuit elements) as follows

49、. Approximately, 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

50、wave transformers, etc. It also minimizes impedance changes in a transmission line when it is exposed 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 exposed to a heat source of 125C. At the location of the heat source, the impedanc

51、e increased 1.135 ohms.Figure 3 TDR test of 50ohm line impedance change at 125C PRODUCT 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 temper

52、ature 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

53、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

54、 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 h

55、ave 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 fai

56、lure 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

57、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-XT possesses fairly good thermal conductivity, thermal conductivity of TC350 and TC600 has been significantly improved and close to that of

58、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. Figu