小型手机天线设计

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 10, 2011 159 Compact Broadband Printed Slot-Monopole-Hybrid Diversity Antenna for Mobile Terminals Gaojian Kang, Zhengwei Du, and Ke Gong, Member, IEEE Abstract—A compact broadband printed diversity antenna is proposed and studied. The antenna, which consists of two symmetric slot-monopole-hybrid (SMH) elements, is printed on a printed circuit board (PCB). A prototype of the proposed antenna is fabricated and measured. It has a 10-dB impedance bandwidth of 460 MHz (1.85–2.31 GHz), covering the PCS (1850–1990 MHz), PHS (1880–1930 MHz), DECT (1880–1900 MHz), and UMTS (1920–2170 MHz) bands, and across the whole band, the isolation of the diversity antenna is higher than 14.8 dB. The measured radiation patterns can cover complementary space regions. The diversity performance is also evaluated by calculating the envelope correlation coefficient, the mean effective gains of the elements, and the diversity gain using the measured results. Index Terms—Broadband antenna, diversity antenna, diversity gain, isolation, mean effective gain. I. INTRODUCTION M ULTIANTENNA technology with diversity schemes iswidely adopted to enhance the performance of wireless communication systems by mitigating multipath fading and cochannel interference. It is more difficult to adopt the antenna diversity technology at the mobile terminals than at the base stations because of the limited space for placing antennas [1]. Some diversity antenna designs have been reported in [2]–[6]. The diversity antennas proposed in [2] and [3] applied in the PCMCIA card of a laptop work at the 2.4/5.2-GHz WLAN bands. A protruding T-shaped ground branch is used to in- crease the isolation between the two elements. Reference [4] presented a dual-slot diversity antenna for mobile handsets. It adopts parasitic elements to enhance the isolation and only covers the UMTS band. In order to meet the ever-growing de- mand for multiple functions at mobile terminals, broadband or multiband antenna design is required. In [5] and [6], dual-band diversity antennas were presented. These antennas, which adopt a T-shaped and dual inverted-L ground structure to enhance the isolation, cover both of the UMTS and WLAN bands. Manuscript received November 21, 2010; revised January 19, 2011; accepted February 16, 2011. Date of publication February 28, 2011; date of current version March 14, 2011. This work was supported in part by the National Basic Research Program of China under Grant 2009CB320205, the National Natural Science Foundation of China under Grant 60971005, the National High Technology Research and Development Plan of China under Grants 863-2007AA01Z284 and 863-2006AA01Z265, and the Tsinghua-QUAL- COMM Associated Research Plan. The authors are with the State Key Laboratory on Microwave and Digital Communications, Tsinghua National Laboratory for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China (e-mail: kgj08@mails.tsinghua.edu.cn; zwdu@tsinghua.edu.cn). Digital Object Identifier 10.1109/LAWP.2011.2119458 However, due to space constraints, it is more difficult to design a diversity antenna working at a lower frequency band, such as the PCS band. In order to cover the PCS (1850–1990 MHz), PHS (1880–1930 MHz), DECT (1880–1900 MHz), and UMTS (1920–2170 MHz) bands, a compact broadband diversity antenna is necessary. In this letter, a compact broadband printed diversity antenna for mobile terminals, covering the PCS, PHS, DECT, and UMTS bands, is proposed. The diversity antenna consists of two symmetric back-to-back slot-monopole-hybrid (SMH) elements. Each SMH element combines an L-shaped slot di- rectly fed by a 50- microstrip feeding line and an inverted-L parasitic monopole. By adopting the inverted-L parasitic monopoles, a wide 10-dB impedance bandwidth of 460 MHz (1.85–2.31 GHz) can be achieved, and the isolation between the two elements is higher than 14.8 dB across the whole band. This letter is organized as follows. In Section II, the antenna geometry with detailed dimensions is illustrated. In Section III, parameter analyses are introduced to obtain the broad band- width of the diversity antenna and good isolation between the two SMH elements. In Section IV, the scattering parameters and the far-field radiation patterns of the prototype antenna are measured. Based on the measured results, the envelope corre- lation coefficient, the mean effective gains (MEGs), and the di- versity gain are also calculated. Results show that the proposed antenna can be applied in diversity systems. Finally, a conclu- sion is drawn in Section V. II. GEOMETRY OF THE PROPOSED DIVERSITY ANTENNA Fig. 1(a) shows the geometry of the proposed antenna with detailed dimensions. The proposed antenna consists of two symmetric back-to-back SMH elements. In general, the perfor- mance of each SMH element except for the radiation patterns is consistent with each other because of the symmetric structure. Each SMH element incorporates an L-shaped slot directly fed by a 50- microstrip feeding line and an inverted-L parasitic monopole. The inverted-L parasitic monopoles are used to obtain a broad bandwidth and enhance the isolation between the two SMH elements. As illustrated in Fig. 1, the inverted-L parasitic monopoles and the 50- microstrip feeding lines are printed on the front side of an FR4 substrate with dimensions mm and a relative permittivity of 4.4. On the backside of the substrate, the L-shaped slots are notched on the top of the main rectangular metal ground plane that is treated as the whole circuit part of a mobile terminal. In order to facilitate measurements of the antenna characteristics, the feeding ports, 1 and 2, are placed at the bottom edge of the ground plane. 1536-1225/$26.00 © 2011 IEEE 160 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 10, 2011 Fig. 1. Geometry of the proposed diversity antenna with dimensions in mil- limeters (the front side in black color and the backside in gray color). (a) Top view. (b) Slot-monopole-hybrid antenna element. III. PARAMETER ANALYSES BASED ON SIMULATION The performance of the proposed diversity antenna is related to the antenna geometry. According to numerous simulated re- sults, the performance is affected mainly by the inverted-L par- asitic monopole and the geometric parameters, , , and . Specific discussions are presented. All of the simulated results are carried out with the Ansoft software High Frequency Struc- ture Simulator (HFSS) [7]. A. Effects of Adopting Inverted-L Parasitic Monopole on and As shown in Fig. 1, the two inverted-L parasitic monopoles are not directly connected to the microstrip feeding line, while there are two gaps between them. The inverted-L parasitic monopoles of the SMH diversity antenna have two functions. One function of the parasitic monopoles has been studied in our lab’s previous work [5], in which it is just used to enhance the isolation between the two elements and the diversity antenna has a 6-dB impedance bandwidth only covering the UMTS band. The other function is explored in this letter. That is, as a part of the SMH antenna element, it cooperates with the L-shaped slot antenna to generate two resonant frequencies. The length of the L-shaped slot implies a quarter-wavelength mode at the lower resonant frequency of 1.9 GHz, and the inverted-L parasitic monopole mainly generates the higher resonant frequency of 2.2 GHz. Thereby, the bandwidth of the diversity antenna can be effectively broadened. The characteristics of two designs, namely with and without inverted-L parasitic monopoles, are compared to understand the functions of the inverted-L parasitic monopoles in the design and the simulated results, are illustrated in Fig. 2. One can find that in the case of “with inverted-L parasitic monopoles,” two resonant frequencies are obtained and the 10-dB impedance bandwidth of the diversity antenna is dramatically broadened compared to that in the case of “without inverted-L parasitic monopoles.” The two resonant frequencies in the SMH diversity Fig. 2. Effect of the L-shaped parasitic monopoles on scattering parameters. Fig. 3. �-parameters with different ��. (a) � . (b) � . antenna are generated due to two surface current paths with dif- ferent lengths along the inverted-L parasitic monopole and the L-shaped slot. It is also noticed that the isolation between the elements gets better across the whole band when the inverted-L parasitic monopoles are introduced into the design. B. Effects of the Antenna Parameters on and The performance of the diversity antenna is greatly affected by the parameters , , and , which are illustrated in Fig. 1(a). is related to the geometry of the inverted-L para- sitic monopole. controls the feeding position of the L-shaped slot, while is related to the length of the main ground plane. and are analyzed by varying different parameters while keeping the remaining dimensions of the antenna geometry the same as shown in Fig. 1(a). (Inverted-L Parasitic Monopole-Related): mainly affects the higher resonant frequency, the isolation at the higher resonant frequency band, and the impedance matching at the whole operating band. Fig. 3(a) and (b) shows the effects of varying on the impedance matching and the isolation between the two SMH elements, respectively. Fig. 3(a) demon- strates that as decreases, the higher resonant frequency moves upward greatly, while the lower resonant frequency changes slightly; the impedance matching across the operating frequency band is affected remarkably as the variance of . As shown in Fig. 3(b), when decreases, the isolation worsens at the whole band, especially at the higher operating frequency band. (L-Shaped Slot-Related): Parameter determines the feeding position of the L-shaped slot. mainly affects the KANG et al.: COMPACT BROADBAND PRINTED SLOT-MONOPOLE-HYBRID DIVERSITY ANTENNA FOR MOBILE TERMINALS 161 Fig. 4. �-parameters with different ��. (a) � . (b) � . Fig. 5. �-parameters with different ��. (a) � . (b) � . lower resonant frequency and the isolation at the lower resonant frequency band. From Fig. 4(a), one can find that as varies from 10 to 12 mm, the lower resonant frequency is lowered and the higher resonant frequency is affected slightly. The 10-dB impedance bandwidth achieves the greatest at mm. As shown in Fig. 4(b), as varies from 10 to 12 mm, the isolation between the two elements is improved remarkably at the lower resonant frequency band. (Ground Plane-Related): Studying the effects of on the impedance matching and isolation is equivalent to studying how the ground length affects the performance of the diversity antenna. affects the impedance matching at the lower and higher resonant frequencies, and it has a remarkable effect on the isolation at the whole working band. From Fig. 5(a), it is no- ticed that the impedance matching at the lower and higher reso- nant frequencies is very sensitive to the variance of , while the 10-dB impedance bandwidth changes slightly with different . Fig. 5(b) illustrates that the isolation between the two ele- ments worsens due to the reduction of the length of the ground plane, but the isolation could still keep above 10 dB when the length of the ground plane shrinks to 39.55 mm. This feature makes the SMH diversity antenna very attractive to miniatur- ization design of the mobile terminals. From the above discussions, we know that the proposed di- versity antenna has a broad bandwidth and high isolation be- cause of adopting the inverted-L parasitic monopoles, and that the parameters , , and have remarkable effects on the performance of the proposed diversity antenna. The optimized dimensions are mm mm , and is chosen as 69.55 mm to accommodate circuit parts of a mobile terminal. IV. EXPERIMENTAL RESULTS AND DISCUSSION A prototype antenna based on the simulated results was fab- ricated and measured. Under a selected combining scheme (that is to say, when one SMH element is excited, the other one is ter- minated to a 50- impedance), the S-parameters are obtained by Fig. 6. Measured and simulated �-parameters of the proposed antenna. Agilent Network Analyzer (E5071B) and the radiation patterns are measured in an anechoic chamber. Based on the measured results, the envelope correlation coefficient, the mean effective gains (MEGs), and the diversity gains are calculated. Specific results are given as follows. A. S-Parameters The measured -parameters of the proposed antenna are shown in Fig. 6, and the simulated results are also given in the same figure for comparison. The measured results reasonably agree with the simulated ones. As illustrated in Fig. 6, the measured 10-dB impedance bandwidth reaches 460 MHz (from 1.85–2.31 GHz), covering the PCS, PHS, DECT, and UMTS bands, and the isolation between two SMH elements is above 14.8 dB across the whole band. B. Radiation Patterns The measured radiation patterns of elements 1 and 2 at 2.05 GHz are given in Fig. 7(a) and (b), respectively. The mea- sured radiation patterns at 2.24 GHz, which are not presented in this letter, show similar characteristics as those at 2.05 GHz. The measured radiation patterns pointing to complementary directions tend to cover the complementary space regions, and it can be observed from Fig. 7(a) and (b), especially the E-phi components in the and planes. This results in low correlation of the signals. Therefore, the proposed antenna can provide pattern diversity in a wireless communication system to mitigate multipath fading and enhance the system performance. C. Diversity Performance The diversity performance of an antenna diversity system can be evaluated by diversity gain, which depends on the correlation and relative signal strength levels between the received signals. The envelope correlation coefficient , whose computing for- mula is presented in (1) [8], and the MEG [9] are calculated using the measured radiation patterns to characterize the corre- lation and average power from each element (1) 162 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 10, 2011 Fig. 7. Measured radiation patterns at 2.05 GHz. (a) Element 1. (b) Element 2. where in which and are the - and -components of the com- plex electric field radiation pattern, respectively; and are the - and -components of the probability distribution func- tion of the incoming wave, respectively. The asterisk denotes the complex conjugate. The parameter is the average cross-po- larization discrimination (XPD) of the incident field, which is assumed to be 0 or 6 dB for the indoor or urban fading environ- ments, respectively, [10] in this paper. The envelope correlation coefficient, MEGs, and diversity gain at 1% of the cumulative distribution functions are listed in Table I. The calculated results in Table I satisfy the criteria of low correlation and comparable average received power dB [11], and good diversity per- formance can be expected in practical multipath environments. TABLE I DIVERSITY PERFORMANCE OF THE PROPOSED ANTENNA V. CONCLUSION A compact broadband printed diversity antenna for mobile terminals is proposed in this letter. The antenna, consisting of two symmetric slot-monopole-hybrid antenna elements, is printed on a planar printed circuit board. Based on numerous simulations, a prototype of the proposed antenna is fabricated and measured. The measured 10-dB impedance bandwidth is 460 MHz (1.85–2.31 GHz), which covers the PCS, PHS, DECT, and UMTS bands, and the isolation of the diversity antenna is higher than 14.8 dB across the whole band. The measured radiation patterns can cover complementary space re- gions. The envelope correlation coefficients, the mean effective gains of the elements, and diversity gains are calculated using the measured radiation patterns for evaluating the diversity performance. It is proved that the proposed antenna can provide pattern diversity to mitigate the multipath fading. REFERENCES [1] T. S. Chu and L. J. Greenstein, “A semi-empirical representation of antenna diversity gain at cellular and PCS base stations,” IEEE Trans. Commun., vol. 45, no. 6, pp. 644–646, Jun. 1997. [2] G. M. Chi, B. Li, and D. S. Qi, “Dual-band printed diversity antenna for 2.4/5.2 GHz WLAN applications,” Microw. Opt. Technol. Lett., vol. 45, no. 6, pp. 561–563, Jun. 2005. [3] T.-Y. Wu, S.-T. Fang, and K.-L. Wong, “A printed diversity dual-band monopole antenna for WLAN operation in the 2.4- and 5.2-GHz bands,” MIcrow. Opt. Technol. Lett., vol. 36, no. 6, pp. 436–439, Jun. 2003. [4] Z. Y. Li, Z. W. Du, and K. Gong, “A dual-slot diversity antenna with isolation enhancement using parasitic elements for mobile handsets,” in Proc. Asia–Pac. Microw. Conf., Singapore, 2009, pp. 1821–1824. [5] Y. Ding, Z. Du, K. Gong, and Z. Feng, “A novel dual-band printed di- versity antenna for mobile terminals,” IEEE Trans. Antennas Propag., vol. 55, no. 7, pp. 2088–2096, Jul. 2007. [6] X. Wang, Z. W. Du, and K. Gong, “A compact wideband planar di- versity antenna covering UMTS and 2.4 GHz WLAN bands,” IEEE Antennas Wireless Propag. Lett., vol. 7, pp. 588–591, 2008. [7] “HFSS Ansoft Corporation,” ANSYS, Canonsburg, CA [Online]. Available: http://www.ansoft.com [8] R. H. Clarke, “A statistical theory of mobile radio reception,” Bell Labs Syst. Tech. J., vol. 47, pp. 957–1000, 1968. [9] T. Taga, “Analysis for mean effective gain of mobile antennas in land mobile radio environments,” IEEE Trans. Veh. Technol., vol. 39, no. 2, pp. 117–131, May 1990. [10] S. C. K. Ko and R. D. Murch, “Compact integrated diversity antenna for wireless communications,” IEEE Trans. Antennas Propag., vol. 49, no. 6, pp. 954–960, Jun. 2001. [11] R. G. Vaughan and J. B. Andersen, “Antenna diversity in mobile communications,” IEEE Trans. Veh. Technol., vol. VT-36, no. 4, pp. 149–172, Nov. 1987.