A simple multi-band wire inverted-f antenna for cellular application inside handset terminals Tuan Hung Nguyen 1, Takashi Oki 1, Hisashi Morishita 1a), Hiroshi Sato 2, and Yoshio Koyanagi 2 1 Electrical and Electronic Engineering, National Defense Academy, 1 10 20 Hashirimizu, Yokosuka, Kanagawa 239 8686, Japan 2 Panasonic System Networks Limited Company, 600 Saedo-cho, Tsuduki-ku, Yokohama, Kanagawa 224 8539, Japan a) morisita@nda.ac.jp Abstract: In this paper, we present a very simple multi-band technique for a wire inverted-f antenna designed for cellular application inside handset terminals, using multiple branch elements, single feeding port and only one shorting strip to simultaneously obtain good impedance matching at many different frequencies. The results in our investigation show that, the proposed technique is extremely straightforward, and can be simply applied to multiband design of inverted-f antenna, so that it is possible to control easily and independently both the position and the number of all resonant frequencies. Keywords: small antennas, inverted-f antenna, multi-band, cellular application, wire antenna, handset antenna Classification: Antennas and Propagation References [1] A. Boldaji and M. A. Antoniades, Method of isolating and tuning the two dominant modes of a printed inverted-f antenna, IEEE Trans. Antenn. Propag., vol. 61, no. 7, pp. 3420 3426, July 2013. DOI:10.1109/TAP.2013.2256454 [2] S. Sekine, T. Itoh, N. Odachi, Y. Murakami, and H. Shoki, Design method for a broadband inverted-f antenna by parallel resonance mode, IEICE Trans. Commun., vol. J86-B, no. 9, pp. 1806 1815, Sept. 2003. (in Japanese) [3] T. Itoh, Y. Murakami, S. Sekine, and H. Shoki, A single-feed dual-mode antenna consiting of an inverted-f antenna and a meander line antenna with a parasitic element, IEICE Trans. Commun., vol. J87-B, no. 9, pp. 1356 1362, Sept. 2004. (in Japanese) [4] C. W. Chiu and Y. J. Chi, Planar hexa-band inverted-f antenna for portable device applications, IEEE Antennas Wireless Propag. Lett., vol. 8, pp. 1099 1102, 2009. DOI:10.1109/LAWP.2009.2033623 [5] H. Nakano, Y. Sato, H. Mimaki, and J. Yamauchi, An inverted FL antenna for dual-frequency operation, IEEE Trans. Antenn. Propag., vol. 53, no. 8, pp. 2417 2421, Aug. 2005. DOI:10.1109/TAP.2005.852502 [6] T. H. Nguyen, N. Nishiyama, H. Morishita, H. Sato, and Y. Koyanagi, Requisite design volume of small handset antennas, IEICE Technical Report, A P2014-43 281
(2014-6), June 2014. [7] T. H. Nguyen, H. Morishita, H. Sato, and Y. Koyanagi, A simple multi-band linear inverted-f antenna, IEEE Int. Workshop on Electromagnetics (iwem 2014), POS1.9, Aug. 2014. 1 Introduction Possessing a simple but effective structure, the well-known inverted-f antenna (IFA) has attracted much attention from handset antenna designers over the past few decades [1, 2, 3, 4, 5, 6, 7]. There are a large number of studies proposing a variety of handset antenna models with high antenna performance based on the fundamental inverted-f structure of an IFA. In these studies [1, 2, 3, 4, 5], multiband improvement techniques for IFA has been particularly focused on and investigated. However, these proposed multi-band techniques have not utilized exhaustively the possibility of using only one shorting strip to achieve good impedance matching at multiple frequencies. To address this issue, in this study, we further investigate and propose a simple and straightforward multi-band technique based on the most conventional design of a wire IFA, exploiting maximally its simplicity of using the only one shorting strip to improve simultaneously impedance matching at many different frequencies. 2 Proposed multi-band technique Fig. 1(a) shows the conventional single-band design of a fundamental wire IFA designed inside a smartphone terminal. As demonstrated in [6, 7], a large conductive box with the thickness of 10 mm is loaded to a 120 60 mm ground plane due to the reason of taking into account the influence of surrounding components to the antenna [6]. Unlike the content in [6], in this study, we do not focus on the evaluation of requisite design volume of the antenna element inside the terminal, but discuss how to achieve more than one resonant frequency with good impedance matching for js 11 j 6 db (VSWR 3) inside a space of 6 60 10 mm, namely, when the antenna height h is fixed at 6 mm of a low-profile design. Antenna simulation is conducted by the Time Domain Solver (FDTD) of CST- Microwave Studio ver. 2013. Naturally, the resonant frequency of the fundamental design in Fig. 1(a) is driven by adjusting the length l, and impedance matching at this resonance is controlled by tuning the distance ds between the feeding and shorting points. When l ¼ 27 mm, ds ¼ 5 mm, the fundamental single-band design has a single resonance around 2 GHz, as the result indicated in Fig. 2(a). For the multi-band design, our target is to construct a simple antenna model inside the fixed space of 6 60 10 mm, to simultaneously achieve multiple resonant frequencies with good impedance matching in 800 MHz, 1.5 GHz, 1.7 GHz, and 2 GHz bands of cellular application. In general, the very standard solution to attain this target is to increase the number of IFA elements corresponding to the number of target frequencies. Certainly, because increasing number of feeding ports is not promising in the design, we propose a simple idea for multi-band design using a single feeding port and branch elements as shown in Figs. 1(b d). The most 282
(a) Fundamental single-band design (b) Proposed dual-band design (c) Proposed triple-band design (d) Proposed quad-band design Fig. 1. Fundamental single-band and proposed multi-band designs. (a) Fundamental single-band design (b) Proposed dual-band design (c) Proposed triple-band design (d) Proposed quad-band design Fig. 2. Reflection coefficients of fundamental single-band and proposed multi-band designs shown in Figs. 1(a d). 283
schematic core of the proposed technique for multi-band design here is the relocation of the only one shorting strip, so that two, three, or four branch elements can share this shorting strip together to operate like independent IFAs to simultaneously achieve good impedance matching at two, three, or four different resonant frequencies. The evidence for this issue can be easily found by the calculated results of reflection coefficients js 11 j illustrated in Figs. 2(b d), and additionally verified by the measured result in Fig. 2(d) of a fabricated quad-band model, which is practically similar to that in simulation. To further clarify the high effectiveness of the proposed multi-band technique, antenna operations of the proposed quadband model in Fig. 1(d) will be representatively cited as below. 3 Antenna operations of the proposed quad-band model Here, we aim to attest that the four resonances obtained in four cellular bands denoted in Fig. 2(d) are independently generated by each IFA element of the proposed quad-band model shown in Fig. 1(d). To address this issue, we investigate the variation of reflection coefficients, current distributions, and radiation characteristics in the cases whereby only one IFA element is remained, and other three IFA elements are completely eliminated from the proposed quad-band model in Fig. 1(d). Due to the limitation on number of figures in this letter, we do not show here the results of reflection coefficients and current distributions but those of radiation patterns at four resonances between before and after eliminating three of four IFA elements of the proposed quad-band model. However, the variations of reflection coefficients in these cases can be easily observed by referring the results shown in Fig. 3 of [7], and the whole results of reflection coefficients and current distributions will be subsequently explained in detail. Realized gain radiation patterns [dbi] of the proposed quad-band model at four resonant frequencies are respectively shown in Fig. 3(a), namely, from Fig. 3(a-1) to Fig. 3(a-4), revealing sufficient radiation characteristics in both calculation and measurement results at all 855 MHz, 1500 MHz, 1710 MHz, and 2045 MHz. Next, when the IFA2, IFA3, and IFA4 elements are completely eliminated, and only the IFA1 element is remained, we confirmed that the second, third and fourth resonances (in 1.5 GHz, 1.7 GHz, and 2 GHz bands) disappear, and only the first resonance (in 800 MHz band) remains (see Fig. 3 of [7]). In this case, although the appearance of the fifth resonance around 2.3 GHz (in Fig. 2(d)) becomes clearer, this resonance is no more than the 3=4 resonant mode of the IFA1 as mentioned in [1, 6]. It is also confirmed that, even though the three IFA elements are completely removed from the proposed quad-band model, impedance matching, bandwidth, and current distributions at the first resonance change very slightly. In addition, radiation patterns before and after removing the IFA2, IFA3, and IFA4 elements are respectively shown in Fig. 3(a-1) and Fig. 3(b-1), in which radiation patterns at the first resonance almost have no change in both E and E polarization components. All these results clearly highlight that the IFA1 element in the proposed quad-band design act like an independent IFA, that is mostly not influenced by the presences of the IFA2, IFA3, and IFA4 elements. 284
(a-1) 855 MHz (a-2) 1500 MHz (a-3) 1710 MHz (a-4) 2045 MHz (a) Proposed quad-band model Only IFA1 Only IFA2 Only IFA3 Only IFA4 (b-1) 843 MHz (b-2) 1419 MHz (b-3) 1689 MHz (b-4) 1995 MHz (b) Models with only one IFA remained Fig. 3. Realized gain radiation patterns in xz plane [dbi] of the proposed quad-band model in Fig. 1(d) and the comparison with those in the cases whereby three IFAs are completely eliminated and only one IFA is remained. Similarly, the same investigations were conducted for the cases whereby only IFA2, IFA3, or IFA4 is remained from the proposed quad-band model. Results of all these investigations, including those shown in Fig. 3(a-2) + Fig. 3(b-2), Fig. 3(a-3) + Fig. 3(b-3), and Fig. 3(a-4) + Fig. 3(b-4) indicate unequivocally that, the IFA2, IFA3, and IFA4 elements of the proposed quad-band model mostly operate like independent IFAs, too, because the variations of reflection coefficients, current distributions, and radiation patterns at the second, third, and fourth resonances are relatively small. Strictly speaking, in all cases, resonance of each IFA element shifts slightly to lower frequency side when the three of four IFA elements are completely eliminated. Moreover, the resonant bandwidth of each IFA widens after eliminating the other ones, and the higher frequency is, the more the resonant bandwidth widens. That is to say, in the strict sense of the word, the operation of each IFA element is not absolutely independent due to the minor mutual coupling raised by the close proximities between them in the quad-band design. However, after verifying carefully all the small variations of input impedance, current distributions, and radiation patterns in all cases, we assure that it is possible to affirm the approximate independency of each resonant mode induced by each IFA element in the proposed quad-band design. On the other hand, we also confirmed that the adjustment of the distance ds between the feeding and shorting points plays a large role in maintaining good impedance matching in all resonant bands of the multi-band design. Because increasing ds makes the capacitive reactance components of input impedance become larger, it needs to be adjusted properly, so that capacitive reactance 285
components in each operation band does not increase excessively, and a good balance of impedance matching between all the four bands is maintained. We found that the optimal range of ds in the quad-band design is about from 4 mm to 9 mm. With any value of ds chosen within this range, the significant effectivity of the proposed technique in the multi-band design is available. This effectivity is, namely, the multi-band design is implemented simply and similarly to single-band design with single feeding port and only one shorting strip, and furthermore, all the resonant frequencies and the number of resonant modes can be easily controlled by adjusting the number of IFA elements and the length l i (i ¼ 1; 2; 3; 4) of each element (such like l 1 and l 2 shown in Fig. 1(b)). 4 Conclusion In this letter, we proposed a simple matching technique for multi-band improvement of a wire IFA designed for cellular application inside a handset terminal. The key point of the proposed technique is the fair sharing of a shorting strip between multiple branch elements to construct approximately independent IFAs that can singly operate at different frequencies. Due to the limitation of this letter, it is impossible for us to further discuss the operation principles of the proposed multiband antenna models in detail, but we hope to continue researching this issue by extending the similar investigation to the case whereby the antenna is placed upon an infinite ground plane to validate the effectivity of the proposed technique. 286