Ground Radiation Antenna for Mobile Devices Using Controlled Endless Metal Rim Mode

ETRI Journal, Volume 4, Number 2, April 218 18 Ground Radiation Antenna for Mobile Devices Using Controlled Endless Metal Rim Mode Jihwan Jeon, Longyue Qu, Hongkoo Lee, and Hyeongdong Kim In this paper, we introduce a ground radiation antenna that uses controlled endless metal rim modes. In the proposed technique, the metal rim mode is tuned and excited as a one-wavelength radiator by a small ground radiation antenna. The proposed antenna occupies a clearance of 1 mm 3 4mmina 3 mm 3 2 mm ground plane. A metal rim with dimensions of 34 mm 3 24 mm surrounds the ground plane, and the metal rim is separated from the ground plane by a gap of 2 mm. In addition, a lumped capacitor is inserted between the metal rim and the ground plane to control the characteristic mode of the metal rim such that the of the metal rim is tuned to be equal to the operating frequency. By performing simulations and measurements, we compare the performance of the proposed antenna with that of a reference antenna that does not have an inserted capacitor between the metal rim and the ground plane. The results show that a significant improvement of the radiation performance is obtained by employing the proposed technique. Keywords: Bluetooth, Characteristic mode, Metal rim, Mobile antenna, Wideband antenna, Wi-Fi, Wimax. Manuscript received Apr. 18, 217; revised Dec. 26, 217; accepted Jan. 18, 218. Jihwan Jeon (ironzeon@hanmail.net), Longyue Qu (rionkorea@gmail.com), Hongkoo Lee (hkl417@naver.com), and Hyeongdong Kim (corresponding author, hdkim@hanyang.ac.kr) are with the Department of Electronics and Computer Engineering, Hanyang University, Seoul, Rep. of Korea. This is an Open Access article distributed under the term of Korea Open Government License (KOGL) Type 4: Source Indication + Commercial Use Prohibition + Change Prohibition (http://www.kogl.or.kr/info/licensetypeen.do). I. Introduction With the rapid development of mobile devices, printed circuit boards (PCBs) are being miniaturized, thereby reducing the area allocated for antennas and increasing the demand for high-performance antennas that can operate in compact PCBs. The design of such an antenna is a difficult task because the quality factor increases as the volume decreases [1] [4], and various studies have sought to overcome these problems [5] [9]. Research has shown that characteristic mode theory is an important method for antenna analysis [1] [13], and that strong coupling to the characteristic modes of the ground plane can effectively enhance antenna performance [14] [2]. Specifically, research has demonstrated that strong coupling between the antenna and the ground plane can be achieved when their frequencies are equal [21], [22]. In commercial PCBs, the size of the ground plane is fixed, so it is difficult to achieve strong coupling because the characteristic mode of the ground plane is very different from the operating frequency, which degrades the radiation performance of the antenna. Therefore, a suitable technique is required to control the characteristic mode of the ground plane in order to achieve good radiation performance without increasing its physical size. In this paper, we propose a simple and efficient method to enhance the antenna performance by controlling the characteristic mode of the metal rim. There have been extensive studies into the development of antennas for smartphones using metal rims as the radiator. These studies showed that good radiation performance can be achieved by forming a loop that connects the metal rim and ground plane [23], [24]. The main aim of our paper is to control the characteristic mode such that the metal rim surrounding the ground plane can be effectively excited by a small, loop-type ground radiation antenna on a ground https://doi.org/1.4218/etrij.217-91 218 pissn: 1225-6463, eissn: 2233-7326

Jihwan Jeon et al. 181 plane for a smartwatch. Furthermore, a capacitor C m is inserted between the metal rim and the ground plane to control the frequency of the metal rim mode so that the metal rim is strongly excited by the loop-type ground radiation antenna, and it operates as a good radiator. In Section II, we compare the proposed antenna with a reference antenna that does not use a capacitor C m to tune the frequency of the metal rim mode. We present a theoretical analysis using the coupling equation for the antenna and the metal rim mode to explain the theoretical aspects of the proposed technique. In Section III, we present the control mechanism and parametric studies of the metal rim mode. We measured the performance of the proposed antenna using Agilent 8753ES network analyzers and a 6 m 9 3 m 9 3 m three-dimensional (3D) Cellular Telecommunications Industry Association (CTIA) over-the-air (OTA) chamber, as described in Section IV. The proposed technique realizes greatly improved radiation performance by controlling the metal rim mode. Furthermore, the frequencies of both the ground radiation antenna and the metal rim are easily controlled, making the proposed approach feasible for practical applications. II. Antenna Design and Theoretical Analysis Figure 1 shows the configuration of the proposed antenna. The ground plane has dimensions of 3 mm 9 2 mm, and is printed on a flame-retardant type-4 (FR-4) substrate (e r = 4.4, tan d =.2) with a thickness of 1 mm. As shown in Fig. 1, the loop-type ground radiation antenna occupies a clearance area of 1mm9 4 mm in the ground plane. An endless metal rim with a size of 34 mm 9 24 mm 9 4 mm surrounds the ground plane with a gap of 2 mm, so the metal rim is integrated into the housing of the device. In addition, a capacitor C m is inserted between the metal rim and the ground plane to control the frequency of the metal rim. The reference antenna does not have a capacitor C m. Both the reference and the proposed loop-type ground radiation antennas include a capacitor C f in the feed structure to control the input impedance, and a capacitor C r in the antenna structure to control the frequency of the antenna [16]. The loop-type ground radiation antenna is located in the center of the ground plane in order to generate strong magnetic coupling with the characteristic modes of the metal rim, thus contributing to good radiation performance. Figure 2(a) shows the simulated reflection coefficients of the reference antenna and the proposed antenna. For the reference antenna, the capacitor values of C f and C r are 4 Endless metal rim 24 15 2 4 Unit: mm 1.28 pf and 1.8 pf,respectively,and a 6-dB bandwidth of 3 MHz (from 2.41 GHz to 2.44 GHz) is achieved. For the proposed antenna, the values of C f, C r,andc m are 1.5 pf, 1 pf, and.4 pf, respectively, and a 6-dB bandwidth of 45 MHz (from 2.3 GHz to 2.75 GHz) is achieved. Figure 2 shows that dual and wide bandwidth are achieved because the of the endless metal rim is set such that it remains equal to the operating frequency of the antenna. However, for the reference antenna, the of the metal rim is much higher than the operating frequency, and very poor performance is obtained. To better explain the operation mechanism of the proposed antenna, we present the input impedance in a Smith chart in Fig. 2(b). In the locus of the reference antenna, an additional at 3 GHz is generated owing to coupling between the antenna and the metal rim mode. However, the metal rim frequency is higher than the operating frequency, and the impedance locus of the metal rim is small. To strongly excite the metal rim mode, we set the frequency of the metal rim to be equal to the operating frequency by inserting C m ;this increases the impedance locus of the metal rim. The performance of the proposed technique can be theoretically explained based on the input impedance seen from the antenna port, which is expressed as follows: C r 2 1.5 2 9 3 C f 34 1.5 C m Feed structure Ant. structure Fig. 1. Geometry of the proposed antenna.

182 ETRI Journal, Vol. 4, No. 2, April 218 Reflection coefficient (db) 5 1 15 2 25 1.5.2.2.5 Metal rim.5 2. 2.5 3. 3.5 (a) 1. 1. jxða =IÞ 2 Z in ¼ Z ant x 2 x 2 ð1 þ j=qþ : (1) The antenna input impedance can be decomposed into two parts: self-impedance (the first term) and mutual impedance (the second term). The first term Z ant in (1) [25] represents the self-impedance of the antenna structure. Because the antenna structure is too small to be an efficient radiator [26], Z ant is primarily imaginary, indicating that little radiation is generated by the antenna structure. In the second term, Q is the radiation quality factor of the metal rim mode, and a is the mutual coupling between the antenna structure and the metal rim mode. x and x are the frequencies of the antenna structure and the metal rim mode, respectively. Maximal mutual impedance can be achieved as x approaches x. Therefore, significant radiation is contributed by the second term in (1), which provides interesting insight into the proposed technique. In the next section, we explain the controlling mechanism and a parametric study of the proposed technique based on a theoretical analysis of (1). 2. 2. (b) 5. 5. 6 db line Reference ant. Proposed ant. Antenna structure Reference ant. Proposed ant. Fig. 2. Simulated results of the reference and the proposed antennas: (a) reflection coefficient and (b) input impedance shown on a Smith chart. III. Controlling Mechanism and Parametric Studies This section details the verification of the controlling mechanism based on the simulated surface current distributions. We then discuss the parametric studies of C m, C r, and C f. C m is an important factor to control the frequency of the metal rim mode such that the frequency of the metal rim mode is equal to the operating frequency. C r and C f are used in the antenna structure to control the frequency and match the impedance, and the maximum bandwidth is obtained when the frequency of the metal rim is equal to the operating frequency. The characteristic mode of the metal rim is an important component of the proposed antenna, and Fig. 3 shows that the current distributions of the endless metal rim characteristic mode resonates at 3 GHz. Because the frequency of the endless metal rim mode is higher than the operating frequency (2.4 GHz), it is difficult to achieve a large mutual impedance in (1). To control the frequency of the metal rim mode, a lumped capacitor is inserted between the metal rim and the ground plane. The position of the capacitor is determined by the current null position of the metal rim. In Fig. 3, it can be seen that two current nulls exist at the top and the bottom. By inserting the lumped capacitor in this position, the frequency of the metal rim can be easily controlled without modifying the physical size [25]. Figure 4 shows the simulated current distributions of the proposed antenna at 2.45 GHz. It can be clearly seen that the current primarily flow along the length of the metal rim, and two current nulls exist at the top and the bottom. Therefore, the proposed antenna is strongly coupled to the characteristic mode of the metal rim shown in Fig. 3. The simulated surface current distributions Normalized surface current 1..5 = 3 GHz Current null position (suggested C m position) Fig. 3. Current distributions of the characteristic mode in the endless metal rim. https://doi.org/1.4218/etrij.217-91

Jihwan Jeon et al. 183 Jsurf (A/m) 5 1 1 3 1 1 Current null position.2.5 1. 2. 5. Antenna structure 5 1.2 5. Metal rim.5 1. 2. (a) : C r = 1 pf (proposed) : C r = 1.3 pf : C r = 1.6 pf Fig. 4. Simulated current distributions of the proposed antenna at 2.45 GHz. Metal rim.5 Reflection coefficient (db) 2 4 6 8.2.2.5 1. 1. 2. 2. (a) Antenna structure 1 12 C m = (open) C m =.2 pf 14 C m =.3 pf 16 C m =.4 pf (proposed) 18 C m = (short) 2 1.5 2. 2.5 3. 3.5 (b) directly indicate the antenna design and controlling mechanism of the proposed technique. Figure 5 shows the simulated input impedance characteristics and reflection coefficients for different values of C m. As shown in Fig. 5(a), the impedance locus of the metal rim increases and rotates counterclockwise with increasing values of C m, indicating that the frequency of the metal rim decreases and approaches the antenna. Accordingly, Fig. 5(b) shows the reflection coefficients as the value of C m 5. 5. C m = (open) C m =.2 pf C m =.3 pf C m =.4 pf (proposed) C m = (short) Fig. 5. Simulated results of metal rim with variations in C m : (a) input impedance in a Smith chart and (b) reflection coefficient. Reflection coefficient (db) 5 1 15 2 1.5 2. 2.5 3. 3.5 (b) : C r = 1 pf (proposed) : C r = 1.3 pf : C r = 1.6 pf Fig. 6. Simulated results of metal rim with variation in C r : (a) input impedance on a Smith chart and (b) reflection coefficient. increases from pf (open) to infinite (short). As is clearly shown, the frequency of the metal rim decreases from 3 GHz (metal rim mode frequency) without affecting the frequency of the antenna structure. When C m is infinite, the frequency of the characteristic mode of the metal rim decreases far below the frequency of the antenna structure, and only the of the antenna structure exists in the 2.4-GHz band. According to (1), a larger mutual impedance, wider bandwidth, and better radiation performance can be achieved when the frequency of the metal rim is set to be closer to that of the antenna structure, as shown in Fig. 5. Figure 6 shows the simulated input impedance characteristics and reflection coefficients for different values of C r. As shown in Fig. 6, the frequency of the antenna structure decreases for frequencies ranging from 2.45 GHz to 2 GHz as the value of C r increases from 1 pf to 1.6 pf. As shown in Fig. 5(a), the impedance locus of the metal rim decreases, which is consistent with (1) because the difference between x and x increases. Figure 6(b) shows the corresponding reflection coefficients for various values of C r, and the frequency of the antenna structure decreases with increasing values of C r. As clearly indicated in (1),

184 ETRI Journal, Vol. 4, No. 2, April 218.2.2 Feed structure.5.5 1. 1. 2. 2. 5. 5. Antenna structure : C f =.5 pf : C f = 1 pf : C f = 1.5 pf (proposed) Fig. 7. Simulated results of input impedance for varying C f on a Smith chart. Cm Reflection coefficient (db) 5 1 15 2 25 6 db line Reference ant. Proposed ant. 1.5 2. 2.5 3. 3.5 Fig. 9. Measured reflection coefficients for the reference antenna and the proposed antenna. 6 Realized efficiency (%) 4 2 Fig. 8. Fabricated proposed antenna. 2.2 Reference ant. Proposed ant. 2.3 2.4 2.5 2.6 2.7 2.8 the mutual impedance becomes smaller as the frequency of the antenna structure is tuned farther from the frequency of the metal rim. Finally, we discuss the effect of C f on impedance matching. Figure 7 shows the variations in the input impedance on a Smith chart for different values of C f. Increasing C f increases the impedance locus of the antenna structure. This mechanism can also be explained using (1). In this case, the second term of (1) can be considered as the mutual impedance between the feed structure and the antenna structure. Using a larger value of C f decreases the frequency of the feed structure; thus, the values of frequencies of the feed structure and the antenna structure are closer to each other. The mutual impedance then increases, as indicated by the larger locus of the antenna structure. Accordingly, optimal impedance matching and a wide bandwidth can be obtained by controlling the value of C f. Note that the mutual impedance between the antenna structure and the feed structure depends on the difference between the frequency of the feed structure and that of the antenna structure. Fig. 1. Measured realized efficiencies for the reference antenna and the proposed antenna. IV. Experimental Results As shown in Fig. 8, the reference and proposed antennas were fabricated and measured using Agilent 8753ES network analyzers and a 6 m 9 3m9 3m3D CTIA OTA chamber. The reflection coefficients are shown in Fig. 9, where the 6-dB impedance bandwidths are 6 MHz (from 2.4 GHz to 2.46 GHz) for the reference antenna and 52 MHz (from 2.28 GHz to 2.7 GHz) for the proposed antenna. The measurement results are in good agreement with the simulation results. Figure 1 shows a comparison of the realized efficiencies of the two antennas. The realized efficiency of the reference antenna averages 34% for frequencies ranging from 2.4 GHz to 2.5 GHz, and the realized efficiency of the proposed antenna averages 56% for frequencies ranging from 2.3 GHz to 2.7 GHz. These results show that greatly enhanced radiation performance is achieved by efficiently using the metal rim radiation. Both the bandwidth performance and the efficiency performance https://doi.org/1.4218/etrij.217-91

Jihwan Jeon et al. 185 3 27 24 3 27 33 21 33-1 -2-3 -4 18-1 -2-3 -4 3 15 (a) 3 6 9 12 6 9 : xy-plane : yz-plane : xz-plane radiation pattern demonstrates that the metal rim mode is excited by the ground radiation antenna, and is operating as a one-wavelength radiator, which is in agreement with the characteristic-mode current distributions shown in Fig. 3. The radiation patterns of the reference and proposed antennas are both affected by the radiation pattern of the rim mode. However, the peak gains of both antennas are different. Figure 12 shows simulated and measured realized peak gains of the reference and proposed antennas at frequencies ranging from 2.2 GHz to 2.8 GHz. For the proposed antenna, the measured realized peak gains range from 2.51 dbi to 2.7 dbi (from 2.2 GHz to 2.8 GHz), and is in good agreement with the simulated peak gains. Experimental results indicate that the difference in the radiation performance depends on the frequency of the metal rim mode relative to the antenna operating frequency, as well as the coupling between the metal rim and the antenna. 24 21 18 15 (b) : xy-plane : yz-plane : xz-plane Fig. 11. Measured radiation patterns at 2.45 GHz: (a) reference antenna and (b) proposed antenna. Realized peak gain (dbi) 5 5 1 15 2 25 2.2 were significantly improved. Figure 11 shows the radiation patterns of the proposed antenna and the reference antenna at 2.45 GHz in the xy-plane, yz-plane, and xz-plane. Figure 11 shows the measurement results obtained for each antenna, which indicate that there is an omnidirectional radiation pattern in the yz-plane. This 12 Reference ant. (simulated) Proposed ant. (simulated) Reference ant. (measured) Proposed ant. (measured) 2.3 2.4 2.5 2.6 2.7 2.8 Fig. 12. Simulated and measured realized peak gains of the reference antenna and the proposed antenna. V. Conclusions In this paper, we propose a ground radiation antenna that employs a controlled endless metal rim mode for enhanced performance. The proposed technique comprises a ground radiation antenna and an endless metal rim with a capacitor C m inserted at the current null of the metal rim, which provides the frequency control of the metal rim mode. Maximal mutual impedance is achieved between the antenna and the metal rim when the frequency of the metal rim mode is set to be equal to the operating frequency. The proposed technique is able to effectively excite the metal rim as a radiator with a controlled mode, providing wide bandwidth and high efficiency. We theoretically analyzed the proposed technique and then verified it by performing simulations and measurements. For the proposed antenna design, the measurement results show a 6-dB bandwidth of 52 MHz and an average realized efficiency of 56% at frequencies ranging from 2.3 GHz to 2.7 GHz, indicating significant enhancement in both bandwidth and efficiency. The radiation patterns are basically omnidirectional, which is suitable for mobile antennas. Therefore, the proposed antenna can be effectively used in mobile devices, providing high performance and compact size. Acknowledgements This work was supported by the ICT R&D program of the MSIP/IITP, Republic of Korea (213--41, Ground radiation technique for mobile devices).

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Jihwan Jeon et al. 187 Jihwan Jeon received his BS degree in electrical engineering from the Department of Electric Wave Engineering, Kwangwoon University, Seoul, Rep. of Korea, in 213. He is currently working towards his MS and PhD degrees in engineering at the Hanyang University, Seoul, Rep. of Korea. His research interests are mobile antenna theory and design based on ground characteristic mode analysis and antenna performance measurement methods. Longyue Qu received his BS degree in electronic engineering from the Yanbian University, Yanji, China, in 213, and his MS degree in microwave engineering from the Hanyang University, Seoul, Rep. of Korea, in 215. He is currently pursuing the PhD degree with the Hanyang University. From 213 to 215, he was a recipient of the Korean Government Scholarship Program, and since 215, he has been funded by the China Scholarship Council. He serves as a reviewer for several international journals, such as IEEE Access, IEEE Antennas and Wireless Propagation Letters, and IEEE Antennas and Propagation Magazine. His current research interests include antenna theory and design, in particular mobile antennas, circularly-polarized antennas, MIMO technology, millimeter-wave technology, and metamaterial-based antennas. Hongkoo Lee received his BS and MS degrees in electrical engineering from the Department of Electronics and Computer Engineering, Hanyang University, Seoul, Rep. of Korea, in 212 and 214, respectively. He is currently working towards his PhD degree in engineering at the Hanyang University. His research interest is mobile antenna research using characteristic mode analysis. Hyeongdong Kim (S 89-M 91) received his BS and MS degrees from Seoul National University, Rep. of Korea, in 1984 and 1986, respectively, and his PhD degree from the University of Texas, Austin, USA in 1992. From May 1992 to February 1993, he was a Post-Doctoral Fellow with the University of Texas. Since 1993, he has been a Professor with the Department of Electrical and Computer Engineering, Hanyang University, Seoul, Rep. of Korea. His recent research interests include antenna theory and design based on ground characteristic mode analysis, i.e., wideband, highefficiency, circular polarization, MIMO antennas, and highsensitivity antenna.