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Printed multi-resonant antenna embedding two inductors in radiating strips for internal mobile phone LTE/ WWAN operation a
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Y.-L. Ban , J.-H. Chen , J.L.-W. Li & W. Hu
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Institute of Electromagnetics, University of Electronic Science and Technology of China, 2006 Xi-Yuan Avenue, Western High-Tech District, Chengdu, Sichuan, 611731, P.R. China b
System Planning Division, Potevio Institute of Technology Co. Ltd, Beijing, 100080, P.R. China Version of record first published: 02 Oct 2012.
To cite this article: Y.-L. Ban, J.-H. Chen, J.L.-W. Li & W. Hu (2012): Printed multi-resonant antenna embedding two inductors in radiating strips for internal mobile phone LTE/WWAN operation, Journal of Electromagnetic Waves and Applications, DOI:10.1080/09205071.2012.729558 To link to this article: http://dx.doi.org/10.1080/09205071.2012.729558
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Journal of Electromagnetic Waves and Applications 2012, 1–11, iFirst Article
Printed multi-resonant antenna embedding two inductors in radiating strips for internal mobile phone LTE/WWAN operation Y.-L. Bana*, J.-H. Chena, J.L.-W. Lia and W. Hub a Institute of Electromagnetics, University of Electronic Science and Technology of China, 2006 Xi-Yuan Avenue, Western High-Tech District, Chengdu, Sichuan 611731, P.R. China; bSystem Planning Division, Potevio Institute of Technology Co. Ltd, Beijing 100080, P.R. China
(Received 27 July 2012; accepted 22 August 2012) In this article, a planar printed antenna for eight-band LTE/WWAN operation applied in the internal mobile phone has been proposed and studied. The presented antenna consists of a feeding strip and a coupling strip, where two chip inductors with different inductances are loaded in place. The desired lower bands of LTE700/GSM850/900 operation can be achieved by the coupling strip loading an inductor of 12 nH and a longer branch loading another inductor of 24 nH in the feeding strip, which provide a quarter-wavelength dualresonance mode at 720 and 900 MHz. While a quarter-wavelength resonant mode at around 2.95 GHz can be generated by the shorter branch of the feeding strip, and other resonant modes at 1.92 and 2.23 GHz are contributed by the higher-order resonant modes of the long feeding strip and coupling strip, covering the desired upper bands of DCS1800/ PCS1900/UMTS2100/LTE2300/2500 operation. In fact, the desired resonant lengths in the lower frequencies can be decreased, and impedance matching can be properly adjusted with the help of the two embedded inductors. Besides, a chip inductor of 2.2 nH is helpful for improving all the bands’ impedance matching. Detailed considerations of the antenna design and main parameters are studied and measured.
1. Introduction Dual and multiband operation of antennas applied in the internal mobile phone has received much attention owing to the tremendous growth in wireless communication technology. Providing more diversified services for mobile users by only one antenna is promising, which in turn leads to the increasing investigation in designing a single antenna that can operate in different bands of interest [1–14]. Generally speaking, these desired operating bands include the 2G (GSM850/900/DCS1800/PCS1900), 3G (UMTS2100), and 4G (LTE700/2300/2500) for practical mobile phone terminals in the near future. The internal antennas that can cover the eight-band LTS/GSM/UMTS WWAN operation are, therefore, preferred for the mobile phone application. Recently, some promising designs have been proposed and studied in the open literature [3–9,14–20]. However, these antenna designs [3,5,6,9,18] are unable to cover the emerging long-term evolution (LTE700/2300/2500 MHz) operating bands, which are attractive for obtaining higher data transmission and more abundant multi-media services than the GSM and UMTS mobile networks. Moreover, with 3D folded/bent metal-plate structures [4,9,14,19], the reported antennas’ height hinders their abilities to integrate with RF (radio *Corresponding author. Email:
[email protected] ISSN 0920-5071 print/ISSN 1569-3937 online Ó 2012 Taylor & Francis http://dx.doi.org/10.1080/09205071.2012.729558 http://www.tandfonline.com
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frequency) or MMIC (Monolithic Microwave Integrated Circuit) modules in the thin mobile handset application. Besides, to obtain wider operating bands, especially for the GSM850/900 (824–960 MHz) and even the LTE700 (698–787 MHz), many techniques have been developed. An effective method among them is the use of the lumped chip elements loaded in radiating strips [3,4,9,14,18]. In the feeding structure, the conventional direct-fed antennas [1–3] are difficult to achieve dual broad bandwidths. A more effective feeding method is capacitively coupled feed [4,5,9,14,19], which can result in an improved impedance matching over the desired operating band [4,14] mainly in the 900 MHz to cover the desired lower band of 698–960 MHz. In [4], a small-size (15 45 3 mm3) wideband internal mobile phone antenna formed by a planar strip monopole with a chip capacitor loaded for achieving eight-band LTE/WWAN operation has been proposed. From the results (see Figure 4 in [4]), the use of the chip capacitor can improve the lower impedance matching to cover the desired lower bands of LTE700/GSM850/ 900. The authors in [5] propose another coupled-fed seven-band WWAN antenna, which consists of a feeding strip and a long coupling strip and occupies a printed size of 15 45 mm2. On the other hand, to reduce the occupied sizes and decrease the resonant lengths for the lower band, loading lumped chip inductors is a good candidate [3,9]. In article [9], the shorted strip has a length of 52 mm (0.16λ at 900 MHz) only, so loading a chip inductor of 12 nH in the shorted strip can effectively compensate for the increased capacitance due to the decreasing resonant length of the antenna in the desired lower operating bands of GSM850/900. In this article, a new planar printed antenna is proposed for the mobile phone application in the 4G LTE/GSM/UMTS mobile communication system. Occupying a no-ground region of 15 50 mm2 the proposed antenna can be easily on the top of an inexpensive FR4 substrate. The presented antenna uses the coupled-fed scheme to excite the radiating strips to obtain two fundamental resonant modes at the lower band. With the presence of two chip inductors loaded in the radiating strips, the desired lower band of 698–960 MHz can be realized due to the decrease of resonant lengths and the improved impedance matching. Several λ/4 and higher-order λ/2 resonant modes can also be generated to obtain the upper band of the 1710– 2690 MHz, i.e. the proposed printed antenna can cover five WWAN bands and three LTE bands. Detailed configurable illustrations and radiation characteristics of the proposed antenna are given in the next several sections. 2. Proposed antenna configuration Figure 1(a) shows the geometry of the planar coupled-fed antenna for eight-band LTE/WWAN mobile phone operation, and detailed dimensions of the antenna are plotted in Figure 1(b). The system circuit board of the mobile phone in this study is simulated by a 1.2-mm thick FR4 substrate of length 120 mm and width 50 mm, which has relative permittivity 4.4 and loss tangent 0.02. The system ground plane having a size of 50 105 mm2 is printed on the back side of the used FR4 substrate, leaving a no-ground region of 15 50 mm2 to design the proposed antenna. The dimensions of the system circuit board and ground plane are reasonable for practical mobile phones, especially for smart phones at present. A 50-Ω mini coaxial feed line is employed to excite the antenna. To simulate the practical case, a 1-mm thick plastic housing (with height 10-mm, relative permittivity 3.3, and loss tangent 0.02) is used in this study. There is a gap of 1 mm between the plastic housing and the edge of the used PCB. The proposed antenna is disposed on the top of the FR4 substrate. Formed by the feeding strip and coupling strip mainly, the antenna shows a simple configuration. To generate a dual-resonance mode at the lower band, two chip inductors with different inductances
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Figure 1. Proposed antenna configuration. (a) Geometry of the wideband antenna for eight-band LTE/ WWAN operation in the internal mobile phone. (b) Detailed dimensions of the metal pattern in the antenna area (unit: mm).
(L1 ¼ 24 nH, L2 ¼ 12 nH) are loaded in the feeding strip and coupling strip, respectively. Hence, two fundamental resonant modes at around 720 and 900 MHz are successfully achieved by the coupling strip (Section DEGH ¼ 100 mm, 0.24λ at 720 MHz) and the long branch of the feeding strip (Section PMN ¼ 30 mm, 0.1λ at 900 MHz) to cover the desired lower band of LTE700/GSM850/900 (698–960 MHz) operation. Moreover, the Section PQR has a length of 21.5 mm (0.21λ at 2950 MHz), which can obtain a quarter-wavelength resonant mode at 2950 MHz. Another resonant mode at about 2230 MHz can be provided by the Section PQV (length of 27.5 mm, 0.21λ at 2230 MHz). Besides, the Section DEFG having a length of 84 mm (about 0.5λ at 1920 MHz) can obtain a higher-order resonant mode at 1920 MHz. Notice that the third chip inductor of 2.2 nH is designed at the feeding point A to improve all the lower and upper bands of 698–960 and 1710–2690 MHz. Of course, since the presented antenna is an entire structure, any part of the antenna can contribute to the desired bandwidths achievement. In the design, there is a long meandered strip in the coupling strip, which has a uniform narrow width of 0.5 mm. In fact, the long meandered strip acting as a distributed inductor can further compensate additional capacitance, in order to improve the impedance matching over the lower band. The presented antenna can be excited by a 50-Ω mini coaxial feed line at the feeding point A, and the external conductor of the used coaxial feed line and the end portion of the coupling strip are grounded at the grounding points B and C, respectively. 3. 3.1.
Design evolution and parametric study Design evolution
To depict the detailed design evolution and analyze the operating principle of the proposed antenna, two referenced antennas are selected to compare with the proposed antenna. Figure 2 shows compared results of the proposed antenna, the case with monopole only (Case 1) and the case without chip inductors L1 and L2 (Case 2). The corresponding dimensions of the three antennas in the figure are the same. From the results shown in Figure 2(a), it can be seen that there are only two resonant modes at around 2300 and 3200 MHz for Case 1, which
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Figure 2. (a) Simulated reflection coefficient for the proposed antenna, the corresponding antenna monopole strip only (Case1) and the corresponding antenna without chip inductors L1 and L2 (Case 2), (b) comparison of the simulated input impedance for the proposed antenna and Case 1, and (c) comparison of the simulated input impedance for the proposed antenna and Case 2 (other dimensions are the same as given in Figure 1).
cannot cover any of the desired lower band. This behavior can be illustrated by the simulated input impedance in Figure 2(b). This is mainly due to very low input impedance levels of the Case 1, compared with the proposed antenna. By designing the simple feeding strip and the coupling strip without chip inductors loaded (Case 2), an additional resonant mode at around 750 MHz is generated, but the obtained lower band is not enough, especially for the lower edge of the LTE700 band. The obtained upper bandwidth is not enough, although there are two resonant modes at about 1800 and 2400 MHz. In fact, the enhanced input impedance (both the real and imaginary parts) can be obtained for the proposed antenna, as shown in Figure 2(c). Compared with that of Case 2, the variations of the input impedance levels (Re and Im parts) are smaller, leading to obtain a wider lower band of 698–960 MHz and improved impedance matching at the bands of LTE2300/2500. The above results indicate that the monopole cannot obtain the lower band’s resonance, and adding simple feeding strip and coupling strip can provide a resonant mode at 750 MHz. Further, by loading two chip inductors in the radiating strips (the proposed antenna), a wide upper band of 1710–2690 MHz can be achieved, and a dual-resonance mode at 720 and 900 MHz can be generated to cover 698– 960 MHz. 3.2. Parametric study Several important design parameters are studied. Figure 3 shows the effects of the chip inductors on reflection coefficient. As shown in Figure 3(a), results clearly indicate that the
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Figure 3. Simulated reflection coefficient as a function of (a) the chip inductor L1 and (b) the chip inductor L2 (other dimensions are the same as given in Figure 1).
Figure 4. Simulated reflection coefficient as a function of (a) the chip inductor L3 and (b) the length m of the long meandered strip (other dimensions are the same as given in Figure 1).
antenna’s second resonant mode at 900 MHz is contributed by the chip inductor L1. For the chip inductor L2, it can be seen that the first resonant mode at 720 MHz will be shifted down by increasing the inductance of the chip inductor L2 in Figure 3(b). In fact, the two chip inductors loaded in the coupling strip and the feeding strip can compensate the additional capacitance owing to the decreased resonant lengths for the lower band. Of course, the higher-order resonant modes over the upper band are also affected by the loaded chip inductors L1 and L2 . However, the effects are small, and the obtained upper bandwidth is enough to cover 1710–2690 MHz. Besides, the chip inductor L3 designed at the feeding point A has a significant role, which is given in Figure 4(a). For the different inductor values (0.5 or 3.9 nH), the impedance matching is poor, leading to insufficient lower bandwidth. When L3 ¼ 2:2 nH, the improved impedance matching can cover the desired operating bands of 698–960 and 1710–2690 MHz. The effects of the length m of the long meandered strip in the coupling strip are plotted in Figure 4(b). When the length m is increased from 36.5 to 48.5 mm, the first resonant mode in the lower band – at about 720 MHz – is shifted to lower frequencies, which results in poor impedance matching over the lower band, and variations in the upper band are also seen clearly. Considering the above observations, the length m is selected as 42.5 mm to combine the mode contributed by the shorter branch of the feeding strip to form the desired lower band for the proposed antenna. Further, the chip inductors loaded can contribute additional inductance to compensate for the increased capacitance of the decreased lengths of the
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feeding strip and the coupling strip in the upper band, which improve the impedance matching over the upper band to cover the DCS1800/PCS1900/ UMTS2100/LTE2300/2500 (1710– 2690 MHz). In addition to those chip inductors, several strips’ dimensions capable of tuning the impedance matching for the antenna are also critical. Figure 5(a) shows the simulated reflection coefficient curves as a function of the end-portion length S of the coupling strip. Big effects on the second resonant mode at 900 MHz are seen by varying the length S from 3 to 13 mm. When S equals 13 mm only, the lower impedance bandwidth is enough. Similar behavior can be found in Figure 5(b), which shows the simulated results of the end-portion length t of the coupling strip. By increasing the length from 10 to 16 mm, the resonant mode at 720 MHz shifts down. However, small variations are observed. From the results of the two parameters, it can be concluded that the two lower resonant modes at 720 and 900 MHz are contributed by the coupling strip and the long branch of the fending strip, respectively. Effects of the length and width of the branch strip in the feeding strip on the impedance matching are shown in Figure 6. Figure 6(a) displays the simulated reflection coefficient curves for different values of w with other fixed parameters. With increasing w, varied from 10.5 to 25.5 mm, the resonant mode at about 2.95 GHz is lowered and the obtained operating band is also enhanced. The width d affects the impedance matching of the proposed antenna. As shown in Figure 6(b), when d is varied, the obtained impedance matching is varied, espe-
Figure 5. Simulated reflection coefficient as a function of (a) the branch’s length S of the feeding strip and (b) the length t of the coupling strip (other dimensions are the same as given in Figure 1).
Figure 6. Simulated reflection coefficient as a function of (a) the length w and (b) the width d of the feeding strip (other dimensions are the same as given in Figure 1).
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Figure 7. Simulated vector surface current distributions on the metal patterns of the mobile phone at (a) 900 MHz, (b) 1920 MHz, and (c) 2560 MHz.
cially for the 720 MHz, because the coupling strength between the feeding strip and the coupling strip can affect the input impedance matching. The simulated vector surface current distributions of the proposed antenna at 900, 1920, and 2560 MHz are plotted in Figure 7. At 900 MHz, it can be seen that the current flows along the long branch of the feeding strip and the coupling strip in different directions. This behavior indicates that the resonant mode at 900 MHz is contributed by the two strips. For the current distributions at 1920 MHz, relatively strong current distributions are seen on the coupling strip, which indicates that the coupling strip generates a higher-order resonant mode at 1920 MHz and there are current distributions on the feeding strip and long meandered strip of the coupling strip at 2560 MHz. Of course, since the proposed printed antenna is an entire structure formed by the feeding strip, coupling strip, and system ground plane of the mobile phone, the whole antenna configuration makes an effective radiating system to cover the two wide operating bands of the 698–960 MHz and 1710–2690 MHz. 4.
Result and discussion
The proposed antenna is fabricated and measured by an Agilent N5247A vector network analyzer in the experiment. Figure 8 shows photos of the fabricated antenna. Figure 9 gives the measured reflection coefficient as a function of the operating frequency range of 500– 3500 MHz. The measured results show that the obtained 3:1 VSWR (or of –6 dB reflection coefficient) bandwidths are 315 MHz (1010–695 MHz) and 1340 MHz (2825–1485 MHz), enough to cover all the three lower bands of LTE700/GSM850/900 and five upper bands of DCS1800/PCS1900/UMTS2100/LTE2300/2500. In the design, 3:1 VSWR is used as the impedance matching bandwidth, which is generally acceptable for practical mobile phone antennas [1–9,11–14,18–20]. As for the slight deviation between the two results, as depicted in this figure, it could be due to the inserted feeding line or the unexpected tolerance during the fabricating process of the proposed antenna. The radiation characteristics of the proposed antenna are also measured and studied in SATIMO anechoic chamber. Figure 10 plots the measured radiation patterns at 900, 1920,
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Figure 8. Photos of the manufactured printed antenna for eight-band LTE/WWAN operation in the internal mobile phone. (a) Front side, (b) back side, and (c) top side.
Figure 9.
Simulated and measured reflection coefficient against frequency for the proposed antenna.
and 2560 MHz, respectively. At 900 MHz, a fundamental resonant mode over the LTE700/ GSM850/900, a similar half-wavelength dipole-like radiation pattern is observed in the azimuthal plane (xy-plane) shown in Figure 10(a), which indicates that stable radiation characteristic is obtained over the antenna’s lower band. The other two frequencies at 1920 and 2560 MHz are also plotted in Figures 10(b) and (c). The measured radiation patterns show several nulls in the azimuthal plane, owing to their higher-order resonant modes. Comparable Eu and Eh components in the three principal planes are seen, and the radiation patterns also
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Figure 10. Measured and simulated 2D radiation patterns at (a) 900 MHz, (b) 1920 MHz, and (c) 2560 MHz for the proposed antenna (dotted line is Eu , solid line is Eh ).
show no special distinction compared to those of the conventional internal mobile phone antennas [1–6,9,11–14]. The measured antenna peak gain and radiation efficiency results of the proposed coupledfed antenna, which are acceptable for practical application, are shown in Figure 11. For the lower band of LTE700/GSM850/900 (698–960 MHz), the antenna gain varies from about – 0.5 to 0.2 dBi, and the radiation efficiency is larger than about 43%. For the upper band including DCS, PCS, UMTS, and LTE, the obtained antenna gain is 1.2–4.3 dBi, and the corresponding radiation efficiency is larger than 60%. As a result, the measured radiation charac-
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Figure 11. Measured and simulated peak antenna gain and simulated radiation efficiency across the operating bands for the proposed antenna. (a) The lower operating bands of LTE700/GSM850/900 and (b) The upper operating bands of DCS1800/PCS1900/UMTS2100/ LTE2300/2500.
teristics of the proposed antenna within the operating band are suitable to meet the requirement for mobile phone systems. 5.
Conclusion
In this article, a planar printed coupled-fed antenna consisting of two chip-inductor-loaded feeding strip and coupling strip for achieving eight-band LTE/GSM/UMTS WWAN operation in the mobile phone has been proposed and studied. The proposed antenna shows an allprinted structure occupying a size of 15 50 mm2 making it easy to fabricate at low cost for practical mobile phone applications. In the design, the use of the two chip inductors loaded in the radiating strips can reduce the resonant lengths at the lower band, and the impedance matching can also be improved. So the obtained lower bandwidth can cover the LTE700/ GSM850/900 operation. In addition, two additional higher-order modes at 1920 and 2230 MHz are contributed by the long feeding strip and coupling strip, in order to effectively widen the antenna’s upper band. The obtained measured results, including reflection coefficient and radiation efficiency, indicate that the presented antenna is promising for the thin mobile phone in practical applications. References [1] Sze JY, Wu YF. A compact planar hexa-band internal antenna for mobile phone. Progress In Electromagnetics Research. 2010;107:413–25. [2] Lin DB, Tang IT, Hong MZ. A compact quad-band PIFA by tuning the defected ground structure for mobile phones. Progress In Electromagnetics Research B. 2010;24:173–89. [3] Wong KL, Chen SH. Printed single-strip monopole using a chip inductor for penta-band WWAN operation in the mobile phone. IEEE Transaction on Antennas and Propagation. 2010;58(3):1011–14. [4] Chen SH, Wong KL. Planar strip monopole with a chip-capacitor-loaded loop radiating feed for LTE/WWAN slim mobile phone application. Microwave and Optical Technology Letters. 2011;53 (4):952–8. [5] Chen JH, Ban YL, Yuan HM, Wu YJ. Printed coupled-fed PIFA for seven-band GSM/UMTS/LTE WWAN mobile phone. Journal of Electromagnetic Waves and Applications. 2012;26(2–3):390–401. [6] Chen WS, Lee BY. A meander PDA antenna for GSM/DCS/PCS/UMTS/WLAN applications. Progress In Electromagnetics Research Letters. 2010;14:101–9. [7] Ban YL, Chen JH, Ying LJ, Li JLW, Wu YJ. Ultrawideband antenna for LTE/GSM/UMTS wireless USB dongle applications. IEEE Antennas and Wireless Propagation Letters. 2012;11:403–6.
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[8] Ban YL, Yuan HM, Chen JH, Li LW, Wu YJ. A novel ultra-wideband antenna with distributed inductance for wireless USB dongle attached to laptop computer. Journal of Electromagnetic Waves and Applications. 2012;26(2–3):179–91. [9] Wong KL, Lee CT. Small-size wideband monopole antenna closely coupled with a chip-inductorloaded shorted strip for 11-band WWAN/WLAN/WiMAX operation in the slim mobile phone. Microwave and Optical Technology Letters. 2011;53(2):361–6. [10] Panda JR, Kshetrimayum RS. A printed 2.4 GHz/5.8 GHz dual-band monopole antenna with a protruding stub in the ground plane for WLAN and RFID applications. Progress In Electromagnetics Research. 2011;117:425–34. [11] Chiu CW, Chang CH. Multiband folded loop antenna for smart phones. Progress In Electromagnetics Research. 2010;102:213–26. [12] Kusuma AH, Sheta AF, Elshafiey I, Siddiqui Z, Alkanhal MA, Aldosari S, Alshebeili SA. A new low SAR antenna structure for wireless handset applications. Progress In Electromagnetics Research. 2011;112:23–40. [13] Lin DB, Tang IT, Chang ET. Interdigital capacitor IFA for multiband operation in the mobile phone. Progress In Electromagnetics Research C. 2010;15:1–12. [14] Chen SC, Wong KL. Wideband monopole antenna coupled with a chip-inductor-loaded shorted strip for LTE/WWAN mobile handset. Microwave and Optical Technology Letters. 2011;53 (6):1293–8. [15] Elsharkawy ZF, Saharshar AA, Elhalafawy SM, Elaraby SM. Ultra-wideband A-shaped printed antenna with parasitic elements. Journal of Electromagnetic Waves and Applications. 2010;24(14– 15):1909–19. [16] Secmen M, Hizal A. A dual-polarized wide-band patch antenna for indoor mobile communication applications. Progress In Electromagnetics Research. 2010;100:189–200. [17] Nishamol MS, Sarin VP, Tony D, Anandan CK, Mohanan P, Vasudevan K. A broadband microstrip antenna for IEEE 802.11a/WiMAX/HIPERLAN2 applications. Progress In Electromagnetics Research. 2010;19:155–61. [18] Cho O, Choi H, Kim H. Loop-type ground antenna using capacitor. Electronics Letters. 2011;47:11–2. [19] Ban YL, Lei CQ, Chen JH, Sun SC, Xie ZX, Ye F. Compact coupled-fed PIFA employing T-shaped monopole with two stubs for eight-band LTE/WWAN in internal mobile phone. Journal of Electromagnetic Waves and Applications. 2012;26(7):973–85. [20] Liao WJ, Chang SH, Li LK. A compact planar multiband antenna for integrated mobile devices. Progress In Electromagnetics Research. 2010;109:1–16.
