Compact Antenna for 4G/5G Metal Frame Mobile Phone Applications Using a Tuning Line

electronics

Article Compact Antenna for 4G/5G Metal Frame Mobile Phone Applications Using a Tuning Line

Daiwei Huang 1 , Zhengwei Du 1,* and Yan Wang 1,2 1 Beijing National Research Center for Information Science and Technology (BNRist), Department of Electronic Engineering, Tsinghua University, Beijing 100084, China; [email protected] (D.H.); [email protected] (Y.W.) 2 Shanghai Huawei Technologies, Ltd., Shanghai 201206, China * Correspondence: [email protected]; Tel.: +86-6278-4107

Received: 20 November 2018; Accepted: 13 December 2018; Published: 14 December 2018

Abstract: A compact antenna with a 6 mm ground clearance for 4G and 5G metal frame mobile phones is proposed in this paper. The proposed antenna consists of a coupled line, a ground branch, a monopole branch, and a tuning line. The ground branch and the coupled line are used to obtain the lower band (698–960 MHz), the monopole branch is used to improve the match at the lower band and obtain the higher band (1710–2690, 3400–3800 MHz), and the tuning line is used to improve the match at the higher band. The novelty of the proposed antenna is that more modes are excited and work together to obtain multiple working bands, by using the coupled line and the folded branches with the help of the tuning line, and then nine bands are obtained under the conditions of a 6 mm, only, ground clearance and a metal frame environment. A prototype has been fabricated and measured. The measured −6 dB impedance bandwidths are 345 MHz (0.685–1.03 GHz) and 2.16 GHz (1.67–3.83 GHz) at the lower and higher bands, respectively. The LTE700, GSM850, GSM900, DCS, PCS, UMTS, LTE2300, and LTE2500 bands for 2G, 3G, 4G, and the 3.5 GHz band that is possible for 5G are covered. The measured efﬁciencies and patterns are also presented.

Keywords: nine bands; metal frame; small ground clearance; 4G/5G; mobile phone

1. Introduction Wireless communications are more and more important for our life, and 5G is coming. The 2G, 3G, 4G, and 5G systems will coexist for a long time. Some important bands, such as the LTE700, GSM850, GSM900, DCS, PCS, UMTS, LTE2300, and LTE2500 bands for 2G, 3G, 4G, and the 3.5 GHz band (3400–3800 MHz) that is possible for 5G, should be covered by the mobile antenna. A mobile phone with a small ground clearance and a metal frame is more attractive in the market. However, it is very difﬁcult to design an antenna that can work at multiple bands while keeping a small ground clearance under a metal frame environment. Many multiple-band antennas for mobile phones with small ground clearances have been proposed in the literature [1–18]. There are some useful methods to achieve compact and multiple-band antennas, such as using coupled feeding [1–3,8–10], lumped elements [4–6,11,12], and reconﬁgurable ways [7,15–17]. The antennas in [1–4,6,7] can work at eight bands, but the 3.5 GHz band is not covered. The antenna in [5] can work at nine bands including the 3.5 GHz band, but its ground clearance is 10 mm. In addition, the metal frame environment is not considered by the antennas presented in [1–5]. The antennas in [8–17] can work at multiple bands for a mobile phone with a metal frame, but the antennas in [8,9,11,13–16] cannot work at the LTE700 and 3.5 GHz bands. Although the antennas in [10,12,17] can work at the LTE700 band, they cannot work at the 3.5 GHz band. The metal frame antennas mentioned above can work at eight bands, but their ground clearances are not smaller than

Electronics 2018, 7, 439; doi:10.3390/electronics7120439 www.mdpi.com/journal/electronics Electronics 2018, 7, 439 2 of 9

7 mm. To the best of our knowledge, there is no metal frame antenna that can work at nine bands with a ground clearance less than 6 mm in the open publications. In this paper, a metal frame antenna with a size of 75 mm × 6 mm × 6 mm is proposed for 4G and 5G mobile phones. The proposed antenna can work at nine bands including the 3.5 GHz band, which is the possible band for 5G. The proposed antenna consists of a coupled line, a ground branch, a monopole branch, and a tuning line. The ground branch and the coupled line are used to obtain the lower band (698–960 MHz), the monopole branch is used to improve the match at the lower band and obtain the higher band (1710–2690, 3400–3800 MHz), and the tuning line is used to improve the match at the higher band. A prototype is fabricated and measured. According to the measured results, the proposed antenna can work at the LTE700, GSM850, GSM900, DCS, PCS, UMTS, LTE2300, and LTE2500 bands and the 3.5 GHz band. It is suitable for 4G and 5G metal frame mobile phones. To compare the performance of the antenna proposed in this paper, Table1 presents some antennas that can cover the eight or nine bands. Some important parameters such as the dimensions, working bands, ground clearance, metal frame environment, and efﬁciency are also presented in Table1. The ground clearance of the antenna in [3] is 6 mm and the eight bands are obtained, but the 3.5 GHz band is not realized and the metal frame is not considered. The differences between the antenna in [3] and the antenna proposed in this paper is that the proposed antenna uses a coupled line to excite more high-order modes to cover more bands, it uses the tuning line to match the higher band, and the size is smaller than that of the antenna in [3]. Furthermore, the proposed antenna is very suitable for the metal frame environment. However, if a metal frame is added to the antenna proposed in [3], its match would become much worse. By using a match circuit and a monopole branch, the antenna proposed in [4] can work at the eight bands with an 8 mm ground clearance. By using the multiple modes of a loop and a match circuit, the antenna proposed in [5] can work at the nine bands with a 10 mm ground clearance. Compared to the antenna in [5], the proposed antenna has a ground clearance of nearly half and is suitable for a metal frame environment. The antenna in [6] can work at the eight bands with a 7 mm ground clearance under the condition of a metal frame, but the 3.5 GHz band is not included. The antenna in [10] can work at the eight bands by using multiple branches, and its ground clearance is 8 mm. The antenna in [12] can work at the eight bands with the help of many lumped elements, and its ground clearance is 10 mm. By using coupled feeding and a reconﬁgurable way, the antenna in [17] can work at the eight bands with an 11 mm ground clearance. Compared to the other antennas mentioned above, the proposed antenna has a smaller ground clearance, more working bands, and is suitable for a metal frame environment. The novelty of the proposed antenna is that more modes are excited and work together to obtain multiple working bands, by using the coupled line and the folded branches with the help of the tuning line, and then nine bands are obtained under the conditions of a 6 mm, only, ground clearance and a metal frame environment.

Table 1. Performance of some mobile antennas.

Dimensions Working Ground Metal Frame Efficiency Reference (mm3) a Bands b Clearance Environment (%) c [3] 80 × 6 × 5.8 Eight bands 6 No >67/>63 [4] 80 × 8 × 5.8 Eight bands 8 No >64/>50 [5] 70 × 10 × 5 Nine bands 10 No >30/>59 [6] 70 × 7 × 6 Eight bands 7 Yes >52/>45 [10] 70 × 8 × 5 Eight bands 8 Yes >51/>50 [12] 70 × 10 × 5 Eight bands 10 Yes >47/>43 [17] 68 × 11 × 7 Eight bands 11 Yes >50/>50 Proposed 75 × 6 × 6 Nine bands 6 Yes >51/>49 a Dimension: The length, width and height of the antenna. b Eight bands: The LTE700, GSM850, GSM900, DCS, PCS, UMTS, LTE2300, and LTE2500 bands. Nine bands: The 3.5 GHz and the above eight bands. c Efficiency: The first and second values are the smallest measured efficiency at the lower and higher bands, respectively. Electronics 2018, 7, 439 3 of 9

2. Antenna Structure As shown in Figure1, the antenna proposed in this paper was mounted on a printed circuit board (PCB). The size of the PCB was 139 mm × 75 mm × 0.8 mm, the loss tangent was 0.02, and the relative permittivity was 4.4. The proposed antenna consisted of a coupled line, a ground branch, a monopole branch, and a tuning line. At the back of the PCB, a metal ground with a size of 133 mm × 75 mm, shown in gray, was printed. As shown in Figure1a, the metal frame with the height of 6 mm had three gaps (the widths of the gaps were all 1 mm). The U-shape metal frames with the sizes of 133 mm × 6 mm, 75 mm × 6 mm, and 133 mm × 6 mm (shown in yellow) were all connected to the ground edges.

Figure 1. The structure and dimensions of the proposed antenna in millimeters. (a) Prospective view.

(b) Unfolded structure of the antenna. (c) Detailed dimensions of some structures.

As shown in Figure1b, the ground branch was the longer branch connected to the ground at the upper left of the PCB, and the monopole branch was the shorter branch at the upper right of the PCB. As shown in Figure1c, the ground branch consisted of a lumped inductance of 8.2 nH, a rectangle strip, and an L-shape folded strip, which was folded ﬁrst at segment C–C2 and then at segment C2–A1, both along the z-axis direction. The monopole branch consisted of a rectangle strip printed on the front of the PCB and an L-shape folded strip, which was folded ﬁrst at segment D–D1 and then at segment D1–C1, both along the z-axis direction. The above mentioned two L-shape folded strips were parts of the metal frame and located at the top edge of the PCB. The tuning line was a T-shape branch connected to the monopole branch at D2 and D3. Last, a 50-Ω feeding line (shown in black) was connected to the coupled line, the monopole branch, and the tuning line at the front of the PCB.

3. Antenna Analysis To study the working mechanism of the proposed antenna, the functions of the coupled line, the ground branch, the monopole branch, and the tuning line were analyzed. The current distributions at the resonant frequencies are presented to better understand the working modes. The following

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rectangle strip, and an L‐shape folded strip, which was folded first at segment C–C2 and then at segment C2–A1, both along the z‐axis direction. The monopole branch consisted of a rectangle strip printed on the front of the PCB and an L‐shape folded strip, which was folded first at segment D–D1 and then at segment D1–C1, both along the z‐axis direction. The above mentioned two L‐shape folded strips were parts of the metal frame and located at the top edge of the PCB. The tuning line was a T‐shape branch connected to the monopole branch at D2 and D3. Last, a 50‐Ω feeding line (shown in black) was connected to the coupled line, the monopole branch, and the tuning line at the front of the PCB.

3. Antenna Analysis To study the working mechanism of the proposed antenna, the functions of the coupled line, Electronics 2018, 7, 439 4 of 9 the ground branch, the monopole branch, and the tuning line were analyzed. The current distributions at the resonant frequencies are presented to better understand the working modes. simulatedThe following results simulated were obtained results were using obtained the software using the of software the High of Frequency the High Frequency Structure SimulatorStructure (HFSS)Simulator 15.0. (HFSS) 15.0.

3.1.3.1. Analysis Analysis ofof thethe MainMain StructuresStructures ToTo study study the the functions functions of of the the main main structures, structures, the the proposed proposed antenna antenna was was formed formed as follows: as follows: First, onlyFirst, the only coupled the coupled line is includedline is included (Ant 1); (Ant then, 1); Ant then, 2 is Ant formed 2 is formed by adding by theadding ground the branchground to branch Ant 1; next,to Ant the 1; monopole next, the monopole branch is added branch to is Ant added 2 to to form Ant Ant 2 to 3; form ﬁnally, Ant the 3; proposed finally, the antenna proposed is formed antenna by addingis formed the by tuning adding line the to Ant tuning 3. The line simulated to Ant 3. S-parameters The simulated of S Ant‐parameters 1, Ant 2, Antof Ant 3, and 1, Ant the proposed2, Ant 3, antennaand the areproposed presented antenna in Figure are presented2. in Figure 2.

Figure 2. Simulated S-parameters of Ant 1, Ant 2, Ant 3, and the proposed antenna.

From Figure2, we found that Ant 1 resonated at about 1.1 GHz, 2.9 GHz, and 4 GHz, but the Figure 2. Simulated S‐parameters of Ant 1, Ant 2, Ant 3, and the proposed antenna. match was not good at about 1.1 GHz. The resonant frequencies at about 1.1 GHz, 2.9 GHz, and 4 GHz were theFrom 0.25, Figure 0.75, 2, and we 1 wavelengthfound that Ant modes 1 resonated of the coupled at about line, 1.1 respectively. GHz, 2.9 GHz, To realize and 4 the GHz, lower but band, the morematch modes was not needed good to at be about introduced, 1.1 GHz. so The the groundresonant branch frequencies was added at about to Ant 1.1 GHz, 1, and 2.9 then GHz, Ant and 2 was 4 formed.GHz were For the Ant 0.25, 2, a new0.75, resonant and 1 wavelength frequency at modes about of 0.725 the GHz, coupled which line, is therespectively. 0.25 wavelength To realize mode the of thelower ground band, branch, more appeared,modes needed and the to be desired introduced, lower band so the was ground nearly branch matched. was The added resonant to Ant frequency 1, and atthen about Ant 1.1 2 was GHz formed. shifted For to about Ant 2, 0.95 a new GHz. resonant At the frequency higher band, at about the resonant 0.725 GHz, frequencies which is the at about 0.25 2.9wavelength GHz and 4mode GHz of shifted the ground to about branch, 2.7 GHz appeared, and 3.7 GHz, and respectively. the desired Therelower were band three was resonant nearly frequenciesmatched. The at aboutresonant 2.2 frequency GHz, 2.7 GHz, at about and 1.1 3.7 GHz GHz. shifted The new to about resonant 0.95 frequency GHz. At the at about higher 2.2 band, GHz was the 0.75 wavelength mode of the ground branch according to the electronic length. To cover the whole higher band, new resonant frequencies needed to be added. Thus, the monopole branch was added to Ant 2, and Ant 3 was formed. For Ant 3, the match at the lower band became much better and the desired lower band was obtained. At the higher band, there was a new resonant frequency at about 1.4 GHz, which is the 0.25 wavelength mode of the monopole branch. The resonant frequency at about 2.7 GHz shifted to about 3 GHz. At last, the tuning line was added, and the proposed antenna was formed. At the lower band, the match and bandwidth had little change. At the higher band, the resonant frequency at about 1.4 GHz shifted to about 1.8 GHz. The resonant frequency at about 3 GHz split into two resonant frequencies at about 2.7 GHz and 3.1 GHz and the desired higher band was matched. As a conclusion, the ground branch and the coupled line were used to obtain the lower band, the monopole branch was used to improve the match at the lower band and obtain the higher band, Electronics 2018, 7, 439 5 of 9 and the tuning line was used to improve the match at the higher band. The two resonant frequencies (0.725 and 0.95 GHz) at the lower band were produced by the 0.25 wavelength mode of the coupled line and the ground branch, respectively. The ﬁve resonant frequencies (1.8, 2.2, 2.7, 3.1, and 3.7 GHz) at the higher band were produced by the 0.25 wavelength of the monopole branch, the 0.75 wavelength mode of the ground branch, and the 0.75 and 1 wavelength modes of the coupled line, respectively. Thus, the proposed antenna was designed by using the folded branches, coupled feeding line, and the tuning line to excite multiple modes to cover multiple bands. The described method is a useful method to design a multi- or wide-band antenna.

3.2. Current Distributions To further analyze the work principle of the proposed antenna, we simulated the current distributions at 0.725, 0.95, 1.8, 2.2, 2.7, 3.1, and 3.7 GHz. The results are shown in Figure3. At 0.725 GHz, the current mainly distributed at the ground branch and the coupled line, the current had a maximum value at A and a minimum value at A1; thus, the proposed antenna worked at the 0.25 wavelength mode of the ground branch at the section of AA1. At 0.95 GHz, the current had a maximum value at B and a minimum value at B1, thus the 0.25 wavelength mode of the coupled line was excited at the section of BB1. At 1.8 GHz, the current had a maximum value at C and a minimum value at C1 the 0.25 wavelength mode of the monopole branch was excited at the section of CC1. At 2.2 GHz, the current was maximum at A and minimum at A2 and A1, the 0.75 wavelength mode of the monopole branch was excited at the section of AA2A1. At 2.7 GHz, the current was maximum at B and minimum at B1 and B2, thus the 0.75 wavelength mode of the coupled line was excited at the section of BB2B1. At 3.1 GHz, the current was maximum at D and minimum at B1 and B2, thus the 0.75 wavelength mode of the coupled line was excited at the section of DB2B1. At 3.7 GHz, the current was minimum at E, B3, and B1, thus the 1 wavelength mode of the coupled line was excited at the sectionElectronics of EB3B1. 2018, 7, x FOR PEER REVIEW 6 of 10

FigureFigure 3. The3. The simulated simulated current current distributionsdistributions of of the the proposed proposed antenna. antenna.

In conclusion, the proposed antenna worked at the 0.25 wavelength modes of the ground branch, the coupled line, and the monopole branch, and the 0.75 and 1 wavelength modes of the coupled line. They are concurrent with the analysis of Section 3.1.

3.3. Parameter Study To further understand the proposed antenna, the effects of the value of the lumped inductance, the length of the coupled line, and the feeding place were analyzed. The parameters L, T, and Offset (as shown in Figure 1c) are the value of the lumped inductance, the length of the coupled line, and the place of the feeding line, respectively. The influences of these parameters on the S-parameter are shown in Figures 4–6, respectively.

Figure 4. Simulated S11 with different values of L.

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Electronics 2018, 7, 439 6 of 9 Figure 3. The simulated current distributions of the proposed antenna.

In conclusion,conclusion, the the proposed proposed antenna antenna worked worked at the at 0.25the wavelength 0.25 wavelength modes mode of thes groundof the ground branch, thebranch, coupled the line,coupled and line, the monopole and the monopole branch, and branch the 0.75, and and the 1 wavelength0.75 and 1 wavelength modes of the modes coupled of line.the Theycoupled are line. concurrent They are with concurrent the analysis with of the Section analysis 3.1. of Section 3.1.

3.3. Parameter StudyStudy To furtherfurther understand the proposed antenna,antenna, the effectseffects of the value of the lumped inductance, the length of the coupled line, and the feeding place were analyzed. The parameters L, T, and Offset Ethelectronics length 2018 of , the7, x FORcoupled PEER REVIEW line, and the feeding place were analyzed. The parameters L, T, and Offset7 of 10 (as(as shown in Figure1 1cc))are are thethe value valueof of the the lumped lumped inductance, inductance, thethe lengthlength ofof thethe coupledcoupled line,line,and and the place of the feeding line, respectively. The inﬂuences of these parameters on the S-parameter are the placeFrom of Figure the feeding 4, it can line be, respectively.seen that L ha Thed a influences big influence of these on the parameters match at onthe the lower S-parameter band. When are shown in Figures4–6, respectively. theshown value in ofFig L1ures increase 4–6, respectively.d, the match at the lower band became worse. At the higher band, the match became better at about 2.2 GHz, but became worse at about 2.7 GHz. Considering the bandwidth and the match, L = 8.2 nH was chosen. As shown in Figure 5, when T became bigger, at the lower band, the match became worse at about 0.75 GHz, but became better at about 0.95 GHz. At the higher band, the resonant frequencies atElectronics about 20182.1 and, 7, x FOR3.7 PEERGHz REVIEW became smaller whereas the match became better. Trading off the match7 of 10 and the bandwidth, T = 45.5 mm was chosen. From Figure 4, it can be seen that L had a big influence on the match at the lower band. When the value of L1 increased, the match at the lower band became worse. At the higher band, the match became better at about 2.2 GHz, but became worse at about 2.7 GHz. Considering the bandwidth and the match, L = 8.2 nH was chosen. As shown in Figure 5, when T became bigger, at the lower band, the match became worse at about 0.75 GHz, but became better at about 0.95 GHz. At the higher band, the resonant frequencies at about 2.1 and 3.7 GHz became smaller whereas the match became better. Trading off the match and the bandwidth, T = 45.5 mm was chosen. Figure 4.4. Simulated S11 with different valuesvalues of L.

Figure 5. Simulated S-parameters with different values of T.

From Figure 6, one can see that the feeding place influenced the S11, not only at the lower band but also at the higher band. When the value of Offset became larger, at the lower band, the match became better at about 0.75 GHz, but became worse at about 0.95 GHz. At the higher band, the match became worse at about 2.1, 2.7, and 3.5 GHz. The resonant frequency became smaller at about 3.1 GHz, but became larger at 3.7 GHz. Considering the match and the bandwidth, Offset =

16.5 mm was chosen. Figure 5. Simulated S S-parameters-parameters with different valuesvalues of T.

Figure 6. Simulated S S-parameters-parameters with different lengths of Offset.

4. Experimental Results and Discussion To verify the design of the proposed antenna, we fabricated a prototype and tested it. The front and back views of the prototype are shown in Figure 7. In Figures 8–10, the measured S-parameter, efficiency, and patterns are presented.

Figure 6. Simulated S-parameters with different lengths of Offset.

4. Experimental Results and Discussion To verify the design of the proposed antenna, we fabricated a prototype and tested it. The front

and back views of the prototype are shown in Figure 7. In Figures 8–10, the measured S-parameter, efficiency, and patterns are presented.

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From Figure4, it can be seen that L had a big inﬂuence on the match at the lower band. When the value of L1 increased, the match at the lower band became worse. At the higher band, the match became better at about 2.2 GHz, but became worse at about 2.7 GHz. Considering the bandwidth and the match, L = 8.2 nH was chosen. As shown in Figure5, when T became bigger, at the lower band, the match became worse at about 0.75 GHz, but became better at about 0.95 GHz. At the higher band, the resonant frequencies at about 2.1 and 3.7 GHz became smaller whereas the match became better. Trading off the match and Ethelectronics bandwidth, 2018, 7, x T FOR = 45.5 PEER mmREVIEW was chosen. 8 of 10 From Figure6, one can see that the feeding place inﬂuenced the S11, not only at the lower band but also at the higher band. When the value of Offset became larger, at the lower band, the match became better at about 0.75 GHz, but became worse at about 0.95 GHz. At the higher band, the match became worse at about 2.1, 2.7, and 3.5 GHz. The resonant frequency became smaller at about 3.1 GHz, but became larger at 3.7 GHz. Considering the match and the bandwidth, Offset = 16.5 mm was chosen.

4. Experimental Results and Discussion To verify the design of the proposed antenna, we fabricated a prototype and tested it. The front and back views of the prototype are shown in Figure7. In Figures8–10, the measured S-parameter, efﬁciency,Electronics 2018 and, 7 patterns, x FOR PEER are REVIEW presented. 8 of 10

Figure 7. Photos of the fabricated antenna.

4.1. S-Parameters Figure 8 presents the simulated and measured S-parameters of the proposed antenna. The simulated −6 dB impedance bandwidths were 319 MHz (0.681–1 GHz) at the lower band and 2120 MHz (1.68–3.8 GHz) at the higher band. The measured −6 dB impedance bandwidths were 345 MHz (0.685–1.03 GHz) at the lower band and 2160 MHz (1.67–3.83 GHz) at the higher band. The simulated and measured S-parameters coincided with each other very well. The little difference seen was caused by the manufacture errors. According to the measured results, we can find that, the LTE700, GSM850, GSM900, DCS, PCS, UMTS, LTE2300, and LTE2500 bands, and the 3.5 GHz band are covered. Figure 7.7. PhotosPhotos ofof the fabricated antenna.

Figure 8. Simulated and measured S-parameters of the proposed antenna. Figure 8. Simulated and measured S-parameters of the proposed antenna.

4.2. Radiation Performance The efficiency and 3D patterns of the proposed antenna were measured in an ETS-Lindgren AMS8500 anechoic chamber, and the measured results are presented in Figure 9 and 10, respectively. From Figure 9, the measured efficiencies were 51–66% at the lower band, and 49–75% at the higher band. The measured 3D radiation patterns at 0.725, 0.95, 1.8, and 2.2, 2.65, and 3.5 GHz are shown in Figure 10. The proposed antenna had a dipole-like 3D pattern at the lower frequencies and more nulls at the higher frequencies.

Figure 8. Simulated and measured S-parameters of the proposed antenna.

4.2. Radiation Performance

The efficiency and 3D patterns of the proposed antenna were measured in an ETS-Lindgren AMS8500 anechoic chamber, and the measured results are presented in Figure 9 and 10, respectively. From Figure 9, the measured efficiencies were 51–66% at the lower band, and 49–75% at the higher band. The measured 3D radiation patterns at 0.725, 0.95, 1.8, and 2.2, 2.65, and 3.5 GHz are shown in Figure 10. The proposed antenna had a dipole-like 3D pattern at the lower frequencies and more nulls at the higher frequencies.

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FigureFigureFigure 9. Measured 9 9. .Measured Measured efﬁciencies efficiencies efficiencies of of the the the proposed proposed proposed antenna. antenna. antenna.

Figure 10. Measured 3D patterns of the proposed antenna. FigureFigure 10 10. .Measured Measured 3D 3D patterns patterns of of the the proposed proposed antenna. antenna. 4.1. S-Parameters 5.5. Conclusions Conclusions Figure8This Thispresents paper paper propose propose the simulatedss a a nine nine--bandband and metal metal measured frame frame antenna antenna S-parameters with with a a size size of of 75 75 the mm mm proposed × × 6 6 mm mm × × 6 6 mm antenna. mm for for The simulated4G4G− and and6 dB 5G 5G impedance mobile mobile phones. phones. bandwidths It It consists consists of wereof a a coupled coupled 319 MHz line, line, (0.681–1a a ground ground branch, GHz)branch, at a a monopole themonopole lower branch bandbranch,and ,and and 2120a a MHz tuning line. The novelty of the proposed antenna is that more modes are excited and work together (1.68–3.8tuning GHz) line. at theThe novelty higher of band. the proposed The measured antenna is that−6 more dB impedancemodes are excited bandwidths and work together were 345 MHz toto obtain obtain multiple multiple working working bands bands, ,by by using using the the coupled coupled line line and and the the folded folded branches branches with with the the help help (0.685–1.03 GHz) at the lower band and 2160 MHz (1.67–3.83 GHz) at the higher band. The simulated ofof thethe tuningtuning line, line, and and then then nine nine bands bands are are obtained obtained under under the the conditions conditions of of a a 6 6 mm mm, ,only, only, ground ground and measuredcleaclearancerance S-parameters and and a a metal metal coincided frame frame environment. environment. with each TheThe other measured measured very − well.−66 dB dB Theimpedanceimpedance little difference bandwidths bandwidths seen areare 345 was345 caused by the manufactureMHzMHz (0.685(0.685––1.031.03 errors. GHz)GHz) According atat thethe lowerlower to bandband the andmeasuredand 21602160 MHzMHz results, (1.67(1.67––3.83 we3.83 can GHz)GHz) find atat thethe that, higherhigher the band. LTE700,band. The The GSM850, GSM900,LTE700,LTE700, DCS, PCS,GSM850, GSM850, UMTS, GSM900, GSM900, LTE2300, DCS, DCS, PCS, PCS, and UMTS, UMTS, LTE2500 LTE2300, LTE2300, bands, and and andLTE2500 LTE2500 the 3.5bands bands GHz for for 2G band2G, ,3G 3G are, ,4G 4G, covered. ,and and t hethe 3.53.5 GHz GHz band band that that is is possible possible for for 5G 5G are are covered. covered. The The measured measured efficiencies efficiencies are are 51 51––66%66% at at the the lower lower band, and 49–75% at the higher band. 4.2. Radiationband, Performanceand 49–75% at the higher band. AuthorAuthor Contributions: Contributions: D. D.HH. .conceived conceived the the idea, idea, simulat simulateded and and measure measuredd the the proposed proposed antenna, antenna, and and wrote wrote the the Thepp efficiencyaperaper. .Z Z.D.D. .proposed proposed and 3Dto to do do patterns this this work, work, ofg gaav thevee the the proposed analysis analysis, ,and and antenna prepare preparedd the the were paper paper measured. .Y Y.W.W. .g gaavvee the the in analysis analysis an ETS-Lindgren and and AMS8500somesome anechoic important important chamber, suggestions. suggestions. and the measured results are presented in Figure9 and 10, respectively. From FigureFunding:Funding:9, the ThisThis measured work work was was supported efficiencies supported by by the werethe National National 51–66% Natural Natural at the Science Science lower Foundation Foundation band, of and of China China 49–75% under under at Grant Grant the higher band. The61771277.61771277. measured 3D radiation patterns at 0.725, 0.95, 1.8, and 2.2, 2.65, and 3.5 GHz are shown in Figure 10ConflictsConflicts. The proposed of of I Interest:nterest: antennaThe The authors authors had declare declare a dipole-like no no conflicts conflicts of of 3Dinterest. interest. pattern at the lower frequencies and more nulls at the higher frequencies. ReferencesReferences 5. Conclusions1.1. Tang,Tang, R.; R.; Du, Du, Z. Z. Wideband Wideband monopole monopole without without lumped lumped elements elements for for octa octa-band-band narrow narrow-frame-frame LTE LTE smart smart phone.phone. IEEE IEEE Antennas Antennas Wirel. Wirel. Propag. Propag. Lett. Lett. 2017 2017, ,16 16, ,720 720––723.723. This2.2. paper Chen,Chen, proposes H.R.; H.R.; Wang, Wang, a H.Y.; nine-bandH.Y.; Sim, Sim, C.Y. C.Y. metalD.D. Single Single frame open open-slot- antennaslot antenna antenna with for for LTE/WWAN LTE/WWAN a size of 75smartphone smartphone mm × 6application. application. mm × 6 mm for 4G and 5G mobileIEEEIEEE Trans. Trans. phones. Antennas Antennas It Propag Propag consists. .2017 2017, of,65 65, a,4278 4278 coupled––4282.4282. line, a ground branch, a monopole branch, and a tuning line. The novelty of the proposed antenna is that more modes are excited and work together to obtain multiple working bands, by using the coupled line and the folded branches with the help of the tuning line, and then nine bands are obtained under the conditions of a 6 mm, only, ground clearance and a metal frame environment. The measured −6 dB impedance bandwidths are 345 MHz (0.685–1.03 GHz) at the lower band and 2160 MHz (1.67–3.83 GHz) at the higher band. The LTE700, GSM850, GSM900, DCS, PCS, UMTS, LTE2300, and LTE2500 bands for 2G, 3G, 4G, and the 3.5 GHz band that is possible for 5G are covered. The measured efficiencies are 51–66% at the lower band, and 49–75% at the higher band. Electronics 2018, 7, 439 9 of 9

Author Contributions: D.H. conceived the idea, simulated and measured the proposed antenna, and wrote the paper. Z.D. proposed to do this work, gave the analysis, and prepared the paper. Y.W. gave the analysis and some important suggestions. Funding: This work was supported by the National Natural Science Foundation of China under Grant 61771277. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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