一篇关于magnetoelectric dipole antenna 的很好的综述性文章.pdf

发布时间：2022-06-03 发布人：admin 分类：说明书资料大小：1.53M 资料格式：pdf 举报版权申诉

kingdl2001-7966103-16359647427936642594.pdf-第1页.png

第1页 / 共11页

kingdl2001-7966103-16359647427936642594.pdf-第2页.png

第2页 / 共11页

kingdl2001-7966103-16359647427936642594.pdf-第3页.png

第3页 / 共11页

kingdl2001-7966103-16359647427936642594.pdf-第4页.png

第4页 / 共11页

kingdl2001-7966103-16359647427936642594.pdf-第5页.png

第5页 / 共11页

kingdl2001-7966103-16359647427936642594.pdf-第6页.png

第6页 / 共11页

kingdl2001-7966103-16359647427936642594.pdf-第7页.png

第7页 / 共11页

kingdl2001-7966103-16359647427936642594.pdf-第8页.png

第8页 / 共11页

文本预览

I N V I T E D P A P E R The Magnetoelectric DipoleVA Wideband Antenna for Base Stations in Mobile Communications Requirements imposed on the design of base station antennas for mobile communications are reviewed in this paper, and a dipole antenna structure for future base stations is presented. By Kwai-Man Luk, Fellow IEEE, and Biqun Wu ABSTRACT | In this paper, stringent requirements imposed on the design of base station antennas for mobile communications are summarized. Conventional techniques for implementing base station antennas are reviewed. The complementary an- tenna concept of combining an electric dipole with a magnetic dipole is reconsidered. Recently, this kind of antenna has been commonly called a BHuygen’s source.[ The purpose is to de- velop wideband unidirectional antennas with stable frequency characteristics and low back radiation. Based on this concept, the magnetoelectric dipole was invented by integrating an electric dipole with an L-probe fed shorted quarter-wave patch antenna. A number of magnetoelectric dipoles with different radiation patterns and different polarizations have been devel- oped in recent years. An overview of the characteristics of this new class of complementary antennas is presented. Major de- sign challenges are explained. Finally, a new magnetoelectric dipole that is low in profile and robust in structure is presented. The magnetic dipole part of this antenna is realized by a triangular-shaped loop antenna. The antenna is inherently direct current (dc) grounded, which satisfies the requirement for outdoor applications. Manuscript received October 10, 2011; revised January 17, 2012; accepted January 21, 2012. Date of publication April 3, 2012; date of current version June 14, 2012. K.-M. Luk is with the State Key Laboratory of Millimeter Waves and the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong (e-mail: eekmluk@cityu.edu.hk). B. Wu was with the State Key Laboratory of Millimeter Waves and the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong. He is now with Broadradio Technologies Ltd., Guangzhou, China (e-mail: biqun.wu@broadradio.cn). Digital Object Identifier: 10.1109/JPROC.2012.2187039 KEYWORDS | Base station antennas; complementary antennas; dipole antennas; L-probe patch antennas; magnetoelectric dipoles; patch antennas; wideband antennas I . I N T R O D U C T I O N In second-generation (2G), third-generation (3G), and long-term evolution (LTE) mobile communications, hand- sets are connected to base stations through wireless micro- waves. For achieving excellent system performance in a complex environment with multiple transmission paths, sophisticated antennas of regular size are required. The operating frequencies of these systems fall into the 698– 2650-MHz frequency band, in which antennas of regular size are too large for handsets. Only base stations have sufficient space to accommodate antennas with specific radiation patterns and diversity capability. Commercially, different requirements are imposed on the performance of base station antennas for indoor and outdoor coverage. In an indoor environment, such as a large shopping mall, an integrated network serving several mobile phone operators is usually installed. The network consists of many broadband or multiband antennas distributed over the areas. These antennas are low in gain. For the wall- mounted case, a multiband antenna is commonly em- ployed. The radiation pattern of the antenna is required to be unidirectional and to have a large beamwidth in the horizontal plane to achieve wide angle coverage. This can be implemented by combining several separate directed dipole arrays operated at different frequency bands. The arrays are connected through a specially designed 0018-9219/$31.00 Ó2012 IEEE Vol. 100, No. 7, July 2012 | Proceedings of the IEEE 2297

Luk and Wu: The Magnetoelectric DipoleVA Wideband Antenna for Base Stations in Mobile Communications receiving antennas was commonly used. This technique requires a large installation area for the three antennas. Today, the polarization technique is a favorable choice. This is due to the availability of dual-polarized antenna arrays with high input-port isolation of more than 30 dB. The antenna is an integration of two independent arraysV one for the þ45 polarization and the other for the 45 polarization. In operation, one polarization is used for transmission and two polarizations are used for reception. In doing so, one dual-polarized antenna array is sufficient to achieve diversity performance instead of using three linearly polarized antenna arrays for implementing the space diversity approach. Base station antenna arrays available in the market are mainly developed based on the electric dipole tech- nology. Different brand names can be found, including the vector dipole [1], the butterfly dipole [2], and the directed dipole [3]. These electric dipole arrays produce a direc- tional radiation pattern with the use of a reflector. In general, they can achieve wide impedance bandwidth, low cross polarization, and low back radiation. Some other manufacturers employ patch antennas as basic elements to develop their product lines with different kinds of excitation methods, including the aperture coupling [4], L-probe coupling [5] (Fig. 2), and loop feed [6] (Fig. 3). In general, a patch antenna array can be wide in impe- dance bandwidth and has a low profile structure, which makes it esthetically pleasing. However, the back radiation cannot be too low unless a sophisticated reflector is designed. A common weakness of these two antenna technologies is that they cannot be used to produce wideband base sta- tion antenna arrays with stable performance in gain, beamwidth, and radiation pattern over the operating fre- quencies and with low back radiation. This challenging issue motivated the leading author of this paper to lead his research group to search for innovative wideband antennas with excellent performance in all characteristics. It was found that the complementary antenna concept can be Fig. 2. Base station antenna array based on L-probe patch antennas. Fig. 1. Antenna farm for outdoor base station antennas. multiband feed network. For the ceiling mounted case, a broadband antenna with a conical radiation pattern in the horizontal plane is usually chosen. This antenna can be realized by using a broadband conical monopole. In gene- ral, indoor base station antennas are simple in structure and easy to manufacture. For outdoor coverage, high gain panel antennas for base stations are required. A typical antenna farm for the installation of various base station antennas is shown in Fig. 1. Stringent requirements are imposed on the perfor- mance of these arrays, which is highly challenging to ac- complish. First, a manufacturer is expected to provide a variety of wideband or multiband antenna arrays with dif- ferent values of beamwidth, gain, and downtilt degree for serving different urban and rural areas. Second, the radia- tion patterns of the arrays should be null filled for enhanc- ing the coverage of areas close to the base station antenna. Third, the radiation patterns should have low upper side- lobes for energy saving and low backlobes for reducing interference between neighboring cells in mobile commu- nications. Fourth, the passive intermodulation (PIM) level must be less than 103 dBm when the transmitted power is 43 dBm for minimizing interference among different channels. Diversity techniques are employed in base stations for enhancing channel capacity. In the past, the space diver- sity technique with one transmitting antenna and two 2298 Proceedings of the IEEE | Vol. 100, No. 7, July 2012

Luk and Wu: The Magnetoelectric DipoleVA Wideband Antenna for Base Stations in Mobile Communications Fig. 4. Complementary antenna concept [16]. Fig. 3. Repeater antenna array based on loop-feed patch antennas. (a) Perspective view. (b) Element design. employed to develop novel wideband base station antennas for future mobile communications. In this paper, the theory of complementary antenna is reviewed. Two different classes of complementary anten- nas suitable for base station antenna application are then described. Both linearly polarized design and dual-polar- ized design are discussed. Some recent new developments of these antennas are presented. I I . T H E OR Y O F C O M P L E M E N T A R Y A NT E N N A Basically, a linearly polarized complementary antenna consists of an electric dipole and a magnetic dipole located orthogonally [7]. The two dipoles can be colocated or se- parated by a small distance. It is common knowledge that the radiation pattern of an electric dipole looks like a figure-8 shape in the E-plane and a figure-O shape in the H-plane whereas the radiation pattern of a magnetic dipole looks like a figure-O shape in the E-plane and a figure-8 shape in the H-plane. Considering the case when the electric dipole and the magnetic dipole are colocated and excited with equal power and equal phase, as shown in Fig. 4, the resulting radiation pattern will have a cardiac shape with identical radiation patterns in the E-plane and the H-plane. More importantly, the back radiation is sup- pressed significantly which is highly desirable in cellular communications. Several complementary antennas consisting of an electric dipole and a slot antenna were developed in the 1970s [8]–[10]. The slot antenna was used to realize the magnetic dipole when the microstrip patch antenna was not yet developed. Excellent results in radiation patterns and bandwidth were achieved but the antennas are not suitable for base stations in mobile communications mainly due to their bulky structures. At this point, we would like to mention that the crossed electric and magnetic dipoles have been implemented for small antenna design with B lower Q-factor and are commonly called Huygen’s sources [11], [12]. [ The complementary antenna concept has been em- ployed again to develop wideband unidirectional antennas. This time, the magnetic dipole is implemented with the use of a shorted quarter-wave patch antenna. From the classical cavity model theory, the radiation of a shorted quarter-wave patch antenna is mainly from the open end which radiates as a folded magnetic dipole. An equivalent circuit of the complementary antenna is shown in Fig. 5. It is well known that the fundamental resonant mode of the patch antenna can be represented by a parallel resonant circuit with resistance Rp, capacitance Fig. 5. Equivalent circuit of magnetoelectric dipole. Vol. 100, No. 7, July 2012 | Proceedings of the IEEE 2299

Luk and Wu: The Magnetoelectric DipoleVA Wideband Antenna for Base Stations in Mobile Communications Cp, and inductance Lp, while the fundamental mode of the electric dipole can be represented by a series resonant circuit with resistance Rd, capacitance Cd, and inductance Ld. If the two resonant circuits are connected in parallel, the input impedance of the equivalent circuit becomes 3 5 2 4 Rdþj !Ld 1 j !Ld 1 !Cd 1 !Cd 1 R2 d j !Ld 1 !Cd þ 1 Rp Yin Ya¼ 1 Rd ¼ 1 Rd þ 1 Rp þ 1 Rp 1 R2 d þ j !Cp 1 !Lp þ j !Cp 1 !Lp !Cp 1 !Lp (1) Here, the feed inductance Lf and feed capacitance Cf are not considered. It can be observed that the imaginary part of the input admittance of the complementary antenna can be canceled out if CdLd ¼ CpLp d ¼ Ld=Cp: R2 (2) (3) Equations (2) and (3) can be satisfied simultaneously if the electric dipole and the patch antenna have the same resonant frequency, and the input resistance of the electric dipole is adjusted to a value related to the reactive compo- nents of the electric dipole and the patch antenna. Al- though this is not a rigorous proof, the result provides us with the insight that the complementary antenna can be very wide in bandwidth if the dimensions of the antenna are selected appropriately. Individually, the two dipoles need not be wide in bandwidth in principle. [ The first attempt to combine an electric dipole and a quarter-wave patch antenna resulted in a wideband an- B tenna designated as a shorted bowtie patch antenna with [13], as depicted in Fig. 6(a). In this antenna, a dipole the magnetic dipole part is realized by two triangular- shape shorted quarter-wave patch antennas, which mirror each other and are arranged like a bowtie. This arrange- ment can ensure a symmetrical radiation pattern of the resulted antenna described as a shorted bowtie patch antenna. Each quarter-wave patch antenna has a wide bandwidth as it has a thickness of about 10% of the ope- rating wavelength. The electric dipole part is implemen- ted by two planar narrow strips, which is not a wideband structure by itself. [ B For proper operation, the separation between the two dipole parts is about 10% of the operating wavelength and they are connected by a two-wire transmission line. The antenna is excited by an air microstrip transmission line 2300 Proceedings of the IEEE | Vol. 100, No. 7, July 2012 Fig. 6. Shorted bowtie patch antenna with an electric dipole. (a) Linearly polarized design [13]. (b) Dual-polarized design [14]. imbedded inside the shorted bowtie patch antenna, which provides the convenience in doing impedance matching. With appropriate selection of dimensions [13], this antenna has a measured impedance bandwidth of 63%, with the standing wave ratio (SWR) 2 from 2.16 to 4.13 GHz. It should be mentioned that without the electric dipole, the shorted bowtie patch antenna can also achieve a measured impedance bandwidth of 62%, with SWR 2 from 2.18 to 4.11 GHz. Therefore, for this case, the addition of an electric dipole can only enhance the gain, suppress the back radiation, and stabilize the frequency response of gain and radiation patterns. G G Dual-polarized outdoor base station antennas for mo- bile communication systems are good diversity antennas for enhancing system performance and reducing installa- tion space. Based on the linearly polarized design, a dual- polarized shorted bowtie patch antenna with electric dipole [14] can also be designed as shown in Fig. 6(b). The antenna is composed of two orthogonal shorted bowtie patch antennas with electric dipoles and can easily be de- signed for the existing 2G and 3G mobile communication systemsVoperated between 1.71 and 2.17 GHzVwith a stable gain of about 6.6 dBi, a stable beamwidth of 80, and a stable front-to-back ratio of about 19 dB. Good

Luk and Wu: The Magnetoelectric DipoleVA Wideband Antenna for Base Stations in Mobile Communications isolation of about 28 dB can also be achieved over the operating bandwidth. I I I . T H E M A G N E T O E L EC T R I C D I P O L E A. Basic Configuration For achieving higher gain, narrower beamwidth, sim- pler antenna structure, and wider bandwidth, the magne- toelectric dipole antenna was developed [15]. A prototype of the antenna is shown in Fig. 7. The antenna consists of a wide planar electric dipole placed in front of a ground plane, with the two inner edges shorted to the ground plane through two wide vertical metallic walls with a separation of 10% of the operating wavelength. The feed of the antenna is a simple folded strip which performs as an air microstrip line connected to an L-shaped probe. Geom- etrically, there is not much difference between this an- tenna and the common reflector backed electric dipole excited by a conventional balun. However, the two wide vertical walls together with the folded feed make a sub- stantial difference in the performance of the antenna. The two vertical metallic walls and the portion of the ground plane connecting the two walls together can be interpreted as a vertically oriented thick quarter-wave patch antenna, which has a ground plane size identical to the patch size. Since the quarter-wave patch antenna has a thickness of about 10% of the operating wavelength and is excited by an L-probe, it is equivalent to a wideband folded magnetic dipole in radiation. G The performance of this simple antenna is highly at- tractive. A wide impedance bandwidth of 44% was achieved, with SWR 1.5 and a gain between 7.6 and 8.1 dBi over the frequency range from 1.85 to 2.89 GHz. low cross polarization Symmetrical radiation pattern, G 20 dB), and low back radiation ( G 20 dB) were ( obtained. More importantly, the beamwidth and radiation pattern shapes are very stable over the operating frequency range. Fig. 7. Basic magnetoelectric dipole [15]. Fig. 8. Folded magnetoelectric dipole (version 1). B. Folded Magnetoelectric Dipole The height of the basic magnetoelectric dipole is about 0:25, referring to the center wavelength. For low-profile design, the vertical walls of the antenna can be folded without affecting the antenna performance too much. A prototype is shown in Fig. 8. It was found that the -shaped feed still can excite the antenna with excellent performance even though the vertical walls are folded to a height of 0:15. The bandwidth is slightly reduced to 46%, with SWR 2 from 2.30 to 3.66 GHz. Other character- istics including the gain, beamwidth, and radiation pat- terns do not change noticeably. It should be mentioned that the bandwidth can be restored if two parallel gamma feeds are employed to excite the antenna, which slightly increases the complexity of the structure. G By folding both the horizontal and vertical plates of the magnetoelectric dipole, the height of the antenna can be further reduced to 0:1. The prototype shown in Fig. 9 was found to exhibit good performance. The band- width is about 44%, with SWR 2 from 1.70 to 2.65 GHz. The back radiation and cross polarization are less than 20 and 15 dB, respectively. The only drawback is that it is unstable in gain which drops from 8 to 5.5 dBi when the operating frequency increases from 1.80 to 2.65 GHz. G C. Magnetoelectric Dipole With Horizontally Polarized Conical Radiation Pattern As mentioned before, broadband antennas with ver- tically polarized conical radiation patterns are widely used in indoor mobile phone networks and can be easily realized. In some indoor environments, antennas with horizontally polarized radiation patterns may be helpful to achieve a better system performance. However, li- mited design information is available in the literature. It was shown that a wideband conical beam antenna with horizontal electric field polarization can be imple- mented using four magnetoelectric dipoles that are Vol. 100, No. 7, July 2012 | Proceedings of the IEEE 2301

Luk and Wu: The Magnetoelectric DipoleVA Wideband Antenna for Base Stations in Mobile Communications Fig. 9. Folded magnetoelectric dipole (version 2). (a) Side view. (b) Perspective view. arranged in a ring configuration, as depicted in Fig. 10 [16]. The planar electric dipoles are designed to have a sectorial shape for achieving a complex structure, which are excited by an in-phase four-way power divider printed on a dielectric substrate located beneath the ground plane. This antenna has a bandwidth of 39%, with SWR 2 from 1.6 to 2.4 GHz. Its gain varies from 3 to 6 dBi over the operating frequency range. It was confirmed that only four elements are enough to achieve an almost omnidirectional radiation pattern in the horizontal plane. G D. Dual-Polarized Magnetoelectric Dipole Dual-polarized antennas are most popular diversity antennas today, owing to their major advantage in reduc- ing the installation space. A challenging issue in designing dual-polarized antennas is to achieve very high input-port isolation. For existing mobile communication systems, the Fig. 10. Conical beam magnetoelectric dipole [16]. 2302 Proceedings of the IEEE | Vol. 100, No. 7, July 2012 Fig. 11. Dual-polarized magnetoelectric dipole [17]. (a) Prototype. (b) Isolation. isolation is required to be more than 30 dB. The basic magnetoelectric dipole is easily modified to have dual polarizations [17]. As shown in Fig. 11(a), the dual- polarized magnetoelectric dipole antenna is composed of a horizontal crossed dipole and a vertically oriented crossed quarter-wave patch antenna. The crossed dipole consists of four planar patches, whereas the crossed quarter-wave patch antenna is realized by four corner walls. The antenna is excited by two -shaped feeds. For achieving high input-port isolation, the vertical portions of each feed are located outside the region between the ver- tical walls, which is different from the linearly polarized case. It was found that this dual-polarized design performs even better than the linearly polarized design. The proto- type has an impedance bandwidth of 66%, with SWR 2 from 1.72 to 3.41 GHz. The gain is higher than 9 dBi from 2.0 to 3.2 GHz. The radiation pattern is very stable over the operating frequency range with only a few degrees variation in beamwidth. More importantly, the isolation is over 36 dB across the operating frequencies, as shown in Fig. 11(b). G E. Magnetoelectric Dipole Antenna With Pattern and Polarization Diversities The dual-polarized antenna is a two-port antenna with polarization diversity. For further enhancing the performance of a wireless system in a fading environment,

Luk and Wu: The Magnetoelectric DipoleVA Wideband Antenna for Base Stations in Mobile Communications Fig. 12. A four-port diversity antenna [18]. a four-port antenna with polarization and pattern diver- sities is highly desirable. It was demonstrated that the magnetoelectric dipoles can be employed to develop this kind of diversity antenna with wideband performance. A prototype operated at around 2.4 GHz is depicted in Fig. 12, which consists of four magnetoelectric dipoles ar- ranged in a ring configuration above a ground plane and a vertical electric monopole located at the center of the an- tenna structure [18]. Considering the whole structure as a receiving antenna, the signals received from four mag- netoelectric dipoles are combined by a microstrip feed network located beneath the ground plane to produce three output signals with two from orthogonal broadside modes and one from a conical mode with horizontal po- larization. The electric monopole helps to realize a conical mode with vertical polarization. With this arrangement, the antenna can receive signals from a wide range of directions in the upper hemisphere with any polarization. This diversity antenna was found to exhibit a wide bandwidth of 22% by measurement. The gains of the broadside and conical modes are about 11 and 6 dBi, re- spectively, which is an encouraging result. More impor- tantly, good isolation of more than 26 dB between any two ports was achieved. I V . A N O VE L L O W - P RO F I L E M A G NE T O EL E CT R I C D I PO L E The basic magnetoelectric dipole is thick in profile and not direct current (dc) grounded. Further enhancement on the design is desirable. In this section, the new magnetoelec- tric dipole to be presented is lower in profile and more robust in structure. The antenna itself is dc grounded, which fulfills the requirement for lightning protection in outdoor applications. A. Antenna Description and Design Geometry The geometry of the magnetoelectric dipole antenna is shown in Fig. 13. Dimensions for operation at around 2.3 GHz are shown in Table 1, which were selected after a Fig. 13. Geometry of the proposed antenna. (a) Perspective view. (b) Side view. (c) Perspective view (prototype). (d) Side view (prototype). Vol. 100, No. 7, July 2012 | Proceedings of the IEEE 2303

Luk and Wu: The Magnetoelectric DipoleVA Wideband Antenna for Base Stations in Mobile Communications Table 1 Dimensions of the Proposed Antenna detailed parametric study for good performance. As shown in the figure, the new structure is formed by connecting a rectangular planar electric dipole to a triangular loop an- tenna [19], backed by a horizontal ground plane for back- lobe reduction. Each arm of the dipole has a length L of about 0:23, where refers to the center operating fre- quency of the antenna. The loop antenna was contem- plated by deforming the cross-sectional shape of the quarter-wave patch antenna into a triangular shape. To achieve excellent performance, the cross section should have a shape close to an equilateral triangle with a total length of =2. For this, the angle of the top corner of the triangular loop antenna should be 60. The antenna height H is about 0:16. This structure only needs a small gap width G of about 0:02 that allows feasibility in designing the antenna feed. The width of the dipole and the patch W should be around 0:5 for wideband performance. For low back radiation, the size of the ground plane should be about 1 by 1. This antenna can be excited by the conventional Pawsey stub, which consists of two pieces of coaxial cables. However, it was found that a simplified version of this folded balun is good enough. As shown in Fig. 13, the feed simply has one piece of coaxial cable. One end of the cable is connected to a coaxial launcher mounted on the ground plane. The outer conductor of the other end of the cable is connected to one arm of the electric dipole whereas the inner conductor of the cable is con- nected to the other arm of the electric dipole through a small folded strip. The connection points are located close to the inner edges of the electric dipole. It was found that the input resistance of the antenna is controlled by the total length of the loop antenna, the angle of the top corner of the triangular cross section, and the width of the horizontal planar dipole. Good impe- dance matching can be achieved when the total length of the loop antenna is about =2. B. Electric Current Distribution To obtain more physical understanding on this low- profile magnetoelectric dipole, the current distribution in antenna surfaces at different time within a period of oscil- lation T is studied. As shown in Fig. 14, the electric dipole is strongly excited at time t ¼ 0 and t ¼ T=2, whereas the loop antenna is strongly excited at t ¼ T=4 and 3T=4. The result reveals that two degenerate modes are excited with 2304 Proceedings of the IEEE | Vol. 100, No. 7, July 2012 Fig. 14. Current distribution of proposed antenna at different times: (a) t ¼ 0; (b) t ¼ T=4; (c) t ¼ T=2; and (d) t ¼ 3T=4.

分享到：

赞收藏

资料库

一篇关于magnetoelectric dipole antenna 的很好的综述性文章.pdf

相关推荐

行业

热门标签

最新资料