electronics

Article A Miniaturized Butler Matrix Based Switched Antenna System in a Two-Layer Hybrid Stackup Substrate for 5G Applications

Soyeon Kim, Seongjo Yoon, Yongho Lee and Hyunchol Shin *

Radio Research Center, Kwangwoon University, Seoul 01897, Korea; [email protected] (S.K.); [email protected] (S.Y.); [email protected] (Y.L.) * Correspondence: [email protected]; Tel.: +82-(0)2-940-5553

 Received: 26 September 2019; Accepted: 25 October 2019; Published: 28 October 2019 

Abstract: This work presents a Butler matrix based four-directional switched beamforming antenna system realized in a two-layer hybrid stackup substrate for 28-GHz mm-Wave 5G wireless applications. The hybrid stackup substrate is composed of two layers with different electrical and thermal properties. It is formed by attaching two layers by using prepreg, in which the circuit components are placed in both outer planes and the ground layers are placed in the middle. The upper layer that is used as antenna substrate has εr = 2.17, tanδ = 0.0009 and h = 0.254 mm. The lower layer that is used as a Butler matrix substrate has εr = 6.15, tanδ = 0.0028 and h = 0.254 mm. By realizing the antenna array on the lower-εr layer while the Butler matrix on the higher-εr layer, the Butler matrix dimension is significantly reduced without sacrificing the array antenna performance, leading to significant overall antenna system size reduction. The two-layer substrate approach also significantly suppresses parasitic radiation leaking from the Butler matrix toward the antenna side, allowing overall radiation pattern improvement. The fabricated beamforming antenna is composed of 1 4 patch antenna × array and a 4 4 Butler matrix. The measured return loss is lower than 8 dB at all ports in 28-GHz. × − It demonstrates the switched beam steering toward four distinct angles of—16 , +36 , 39 , and +7 , ◦ ◦ − ◦ ◦ with the sidelobe levels of 12, 11.7, 6, and 13.8 dB, respectively. Antenna gain is found to be − − − − about 10 dBi. Due to the two-layer hybrid stackup substrate, the total antenna system is realized only in 1.7λ 2.1λ, which shows the smallest form factor compared to similar other works. × Keywords: beamforming; Butler matrix; hybrid stackup substrate; microstrip antenna; millimeter wave; 5G

1. Introduction The fifth-generation (5G) millimeter-wave (mm-Wave) frequency range, also referred to as the frequency range 2 (FR2), includes the band between 24.25 and 52.6 GHz. For the 5G mm-Wave wireless applications, beamforming antenna system is essentially needed to cope with high free-space path loss, severe multi-path fading, and possible interference management for improving signal-to-noise ratio [1,2]. Two approaches are possible for the beamforming system realization. The first approach is phased-array beamforming system [2]. It can provide continuously any-directional beam-steering capability, but requires sophisticated beamforming circuitry comprising precise phase shifters, variable gain blocks, and switches, thus it inevitably elevates the system implementation cost and complexity. On the other hand, switched beamforming antenna system can provide discontinuous discrete-directional beam-steering capability, but with much less hardware complexity and easier producibility. Thus, the switched beamforming antenna system is more preferred when precise and continuous beam steering function is not necessarily required, whereas implementation cost and producibility are of

Electronics 2019, 8, 1232; doi:10.3390/electronics8111232 www.mdpi.com/journal/electronics Electronics 2019, 8, 1232 2 of 11 more importance. Passive beamforming matrices such as Blass matrix [3], Nolen matrix [4], and Butler matrix [5–14] are generally adopted for the switched beamforming antenna system. Among them, the Butler matrix is more widely used due to its wideband characteristic and simple structure. When realizing a Butler matrix based switched beamforming antenna system, the Butler matrix usually occupies a much larger area than the antenna array elements. Thus, minimizing the Butler matrix size is more critical for the overall form factor minimization. Many previous works have been reported focusing on the miniaturization of the Butler matrix. The previous techniques can be categorized into a single-layer substrate based design [5–7] and a multi-layer substrate based design [8–14]. For the single-layer substrate designs, several techniques, such as a higher characteristic impedance transmission line based design [5], a directional coupler employing lumped components [6], and an artificial transmission line employing quasi-lumped elements and discontinuities structure [7], were effective for the miniaturization. Nevertheless, since the Butler matrix and the antenna elements were put on the same plane, the spurious radiation leaking from the Butler matrix toward the antenna was a serious issue because it adversely affected the total radiation pattern. The multi-layer substrate design approach can alleviate the spurious radiation issue more effectively because the Butler matrix and the antenna elements are placed in different layers. In addition, such previous multi-layer designs have employed further miniaturization techniques, such as the suspended stripline [8], dual transmission line structure in a multi-layer structure [9], forward-wave directional coupler [10], modularized multi-folded structure [11], slot-directional hybrid coupler [12], and dual-layer substrate integrated waveguide (SIW) based crossover elimination structure [13,14]. Yet, it should be noted that those miniaturization techniques inevitably lead to higher implementation cost and less producibility. It is also interesting to note, that those previous works [8–14] have utilized only the same-property homogenous substrates for the multiple layers. In this work, we propose to adopt a two-layer hybrid stackup substrate for realizing a 28-GHz band Butler matrix based switched beamforming antenna system. The hybrid two-layer stackup substrate is composed of two dissimilar layers with each layer having optimal electrical properties for the Butler matrix and the antenna elements. Such hybrid stackup substrate based design approach was also found to be effective for performance and reliability improvement in the high-frequency serial-link applications [15,16]. This work is carried out to achieve the overall size miniaturization and the spurious radiation reduction in a Butler matrix based switched beamforming antenna system for 28-GHz mm-Wave 5G applications.

2. Design of Switched Beamforming Antenna System The switched beamforming antenna system comprises a 4 4 Butler matrix and 4-element array × antenna. Figure1 depicts the block diagram of the proposed antenna system. The 4 4 Butler matrix × has four input and four output ports, and is composed of four 3-dB hybrid couplers, two cross-over structures, two 45◦ phase shifters. Depending on which input port among Port 1, 2, 3, and 4 is fed by input signal, the signal phases at the output ports (Port 5, 6, 7, and 8) are to show constant phase offsets of 135 , 45 , +45 , and +135 with respect to the adjacent output ports, respectively. As a result, − ◦ − ◦ ◦ ◦ the radiated beam is steered to the four discrete directions of 45 , 15 , 15 , and 45 , as illustrated in − ◦ − ◦ ◦ ◦ Figure1. The four steered beams are denoted as 1L, 2R, 2L, 1R with respect to the excited input port 1, 2, 3, and 4, respectively. Electronics 2019, 8, 1232 3 of 11 Electronics 2019, 8, x FOR PEER REVIEW 3 of 11

Figure 1. Block diagram of the antenna array with the 4 4 Butler matrix network. Figure 1. Block diagram of the antenna array with the 4×4× Butler matrix network. For the mm-Wave 5G applications, the microstrip patch-type antenna is widely used because of For the mm-Wave 5G applications, the microstrip patch-type antenna is widely used because of the simple structure, easy fabrication, and small size. The back lobe level is also reduced due to the the simple structure, easy fabrication, and small size. The back lobe level is also reduced due to the back-side ground plane. Signal feeding structure from the Butler matrix to the patch elements should be back-side ground plane. Signal feeding structure from the Butler matrix to the patch elements should carefully designed. Since the Butler matrix and the patch elements are placed in different layers in this be carefully designed. Since the Butler matrix and the patch elements are placed in different layers in work, probe feeding and aperture coupled feeding methods are possible. The aperture coupled feeding this work, probe feeding and aperture coupled feeding methods are possible. The aperture coupled method as adopted in [14,17] is known to be more wideband than the probe feeding. However, it usually feeding method as adopted in [14,17] is known to be more wideband than the probe feeding. requires stricter alignment, otherwise the sidelobe level and impedance matching characteristics can However, it usually requires stricter alignment, otherwise the sidelobe level and impedance matching be significantly affected especially in this mm-Wave high frequency region. In contrast, the probe characteristics can be significantly affected especially in this mm-Wave high frequency region. In feeding through a coaxial-structure through-hole-via does not need such strict alignment and thus contrast, the probe feeding through a coaxial-structure through-hole-via does not need such strict more tolerant to the fabrication inaccuracy, and also has advantages of lower sidelobe and back-lobe alignment and thus more tolerant to the fabrication inaccuracy, and also has advantages of lower levels. Thus, in this work, we employ the probe feeding through direct through- hole-via. sidelobe and back-lobe levels. Thus, in this work, we employ the probe feeding through direct We have first carried out the block level designs for the 1 4 patch antenna array and 4 4 through- hole-via. × × Butler matrix and optimized their performances independently. However, even though the block-level We have first carried out the block level designs for the 1 × 4 patch antenna array and 4 × 4 Butler designs are well carried out, in this 28-GHz mm-Wave band, the overall system performances are very matrix and optimized their performances independently. However, even though the block-level likely to be affected and degraded by interfacing and interconnecting the Butler matrix and antenna designs are well carried out, in this 28-GHz mm-Wave band, the overall system performances are elements. From this point of view, some of the previous works were rather incomplete because they very likely to be affected and degraded by interfacing and interconnecting the Butler matrix and only presented Butler matrix miniaturization designs without realizing full integration with antenna antenna elements. From this point of view, some of the previous works were rather incomplete array [8–11], or only connectorized interconnection [12]. In this work, we have carefully carried out because they only presented Butler matrix miniaturization designs without realizing full integration intensive co-optimization of the total system including the Butler matrix, antenna elements, and the with antenna array [8–11], or only connectorized interconnection [12]. In this work, we have carefully interfacing structures in between. carried out intensive co-optimization of the total system including the Butler matrix, antenna Figure2 shows the proposed beamforming antenna system structure. The key dimensional elements, and the interfacing structures in between. parameters of the structure are given in Table1. Figure2a shows the layout top view, and Figure2b Figure 2 shows the proposed beamforming antenna system structure. The key dimensional shows the cross-sectional view of A-A’ in Figure2a. The 1 4 patch antenna elements and 4 4 Butler parameters of the structure are given in Table 1. Figure 2a ×shows the layout top view, and Figure× 2b matrix are placed on opposite outer sides of the two-layer substrate, and connected by through-hole shows the cross-sectional view of A-A’ in Figure 2a. The 1 × 4 patch antenna elements and 4 × 4 Butler via as illustrated in Figure2b. The two inner planes of the two-layer substrate are used as ground matrix are placed on opposite outer sides of the two-layer substrate, and connected by through-hole planes. The two layers are made of polytetrafluoroethylene (PTFE) material, but having different via as illustrated in Figure 2b. The two inner planes of the two-layer substrate are used as ground properties. The upper antenna layer has a dielectric constant εr = 2.17, tanδ = 0.0009, and thickness planes. The two layers are made of polytetrafluoroethylene (PTFE) material, but having different h = 0.254 mm, and the lower Butler matrix layer has a relatively higher dielectric constant εr = 6.15, properties. The upper antenna layer has a dielectric constant εr = 2.17, tanδ = 0.0009, and thickness h and tanδ = 0.0028, and h = 0.254 mm. The two layers are attached by prepreg that has a dielectric = 0.254 mm, and the lower Butler matrix layer has a relatively higher dielectric constant εr = 6.15, and constant εr = 4, and thickness h = 0.1 mm. The metallization thickness is 35 µm. The via diameter is tanδ = 0.0028, and h = 0.254 mm. The two layers are attached by prepreg that has a dielectric constant 0.3 mm, and the anti-pad diameter is 0.5 mm. Thus, the spacing between the though-hole- via and εr = 4, and thickness h = 0.1 mm. The metallization thickness is 35 μm. The via diameter is 0.3 mm, adjacent ground plane is 0.1 mm. All the dimensions are the minimum allowed by the drilling process. and the anti-pad diameter is 0.5 mm. Thus, the spacing between the though-hole- via and adjacent We have verified through electromagnetic simulations that this via holes do not significantly alter the ground plane is 0.1 mm. All the dimensions are the minimum allowed by the drilling process. We antenna radiation characteristics. have verified through electromagnetic simulations that this via holes do not significantly alter the antenna radiation characteristics.

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Figure 2.Figure Structure 2. Structure of proposed of proposed antenna antenna system. system. (a) top (a) topview view and and (b) ( bcross) cross section section of of A-A’. A-A’. Table 1. Dimensional parameters. Table 1. Dimensional parameters. Parameters Dimension(mm) Parameters Dimension(mm) W 36.2 W 36.2 L L 32.232.2 a a 3.33.3 b 3.3 b c 5 3.3 d 18.3 c 5 e 22.5 d 18.3

The single antenna elemente has a dimension of 3.3 3.322.5 mm 2, which correspond to 0.3λ where λ × is the free-space wavelength of the operation frequency. The inter-element pitch is 5 mm. The patch The single antenna element has a dimension of 3.3 × 3.3 mm2, which correspond to 0.3λ where λ dimension, inter-element spacing, and via position are carefully optimized for impedance matching is the free-space wavelength of the operation frequency. The inter-element pitch is 5 mm. The patch and antenna gain. Since the antenna dimensions are governed by the substrate dielectric constant dimension, inter-element spacing, and via position are carefully optimized for impedance matching εr, the lower εr (= 2.17) layer is used as the antenna substrate because it is advantageous for higher and antenna gain. Since the antenna dimensions are governed by the substrate dielectric constant εr, radiation efficiency, reduced inter-element coupling, and higher fabrication accuracy, as a result, the lower εr (= 2.17) layer is used as the antenna substrate because it is advantageous for higher leading to much better and reliable radiation pattern with higher gain and lower sidelobe level. On the radiation efficiency, reduced inter-element coupling, and higher fabrication accuracy, as a result, other hand, the Butler matrix is implemented in the higher εr layer because the Butler matrix size leading to much better and reliable radiation pattern with higher gain and lower sidelobe level. On reduction dominates the overall size reduction. We have found that by simply using this higher the other hand, the Butler matrix is implemented in the higher εr layer because the Butler matrix size εr (= 6.15) layer for the Butler matrix, the overall size reduction is significant even without adopting the reduction dominates the overall size reduction. We have found that by simply using this higher εr (= previous sophisticated techniques reported in [8–14]. Also, the proposed two-layer substrate design 6.15) layer for the Butler matrix, the overall size reduction is significant even without adopting the can effectively suppress the spurious parasitic radiation leaking from the Butler matrix side toward the previous sophisticated techniques reported in [8–14]. Also, the proposed two-layer substrate design antenna side, resulting in the overall radiation pattern improvement. can effectively suppress the spurious parasitic radiation leaking from the Butler matrix side toward Figure3 shows the details of the Butler matrix layout and its key dimensional parameters. the antenna side, resulting in the overall radiation pattern improvement. It consists of four 3-dB hybrid couplers, two 45◦ phase shifters, two cross-over structures, and final tuningFigure feed 3 shows lines forthe additional details of finethe adjustmentButler matrix of phase.layout The and final its key tuning dimensional feed lines isparameters. included to It cover ° consistsabout of a four 10◦ variations 3-dB hybrid at the couplers, final output two ports. 45 phase All elements shifters, are two designed cross-over in 50-ohm structures, microstrip and final line type. tuningIn orderfeed lines to estimate for additional the size fine reduction adjustment effect byof phase. using theThe higher final tuningεr (= 6.15) feed layer lines comparedis included to to using ° coverthe about lower a 10εr layer, variations a similar at the previous final output work ports. in [18 Al], whichl elements described are designed a 28-GHz in 50-ohm 4-directional microstrip switched ε line beamformingtype. In order antennato estimate system the size in a reductionεr = 2.2 substrate, effect byis using compared. the higher Compared r (= 6.15) to [layer18], we compared have found ε to usingthat 50theΩ lowermicrostrip r layer, line a similar width is previous reduced work by 54%, in [18], and thewhich 45◦ describedphase-shift a 28-GHz line length 4-directional is reduced by 2 switched37%. Moreover,beamforming the Butlerantenna matrix’s system total in dimensiona εr = 2.2 substrate, of this work is compared. is 13 19 mm Compared, whereas to its [18], dimension we × havein found [18] is that 37 5045 Ω mm microstrip2. Thus, line the overallwidth is size reduced is reduced by 54%, by 85%.and the Meanwhile, 45° phase-shift as shown line length in Figure is 2a, × reducedthe total by 37%. dimension Moreover, including the Butler the antenna matrix’s and total Butler dimension matrix of is 18.3this work22.5 is mm 13 2×, which19 mm corresponds2, whereas to × its dimension1.7λ 2.1λ in. [18] is 37 × 45 mm2. Thus, the overall size is reduced by 85%. Meanwhile, as shown × in Figure 2a, the total dimension including the antenna and Butler matrix is 18.3 × 22.5 mm2, which corresponds to 1.7λ × 2.1λ.

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FigureFigure 3. Butler 3. Butler matrix matrix design. design. (a (a)) layout layout and ((bb)) dimensional dimensional parameters. parameters

TheThe hybrid hybrid two two layers layers have have different different thermal thermal proper propertiesties as as well well as as different different electrical electrical properties. properties. Thus,Thus, the the thermal thermal issue issue during during fabrication fabrication process process st stepseps needs needs to to be be carefully carefully taken taken into into account account for for reliablereliable mechanical mechanical characteristics. characteristics. Typical Typical fabrication fabrication process process of of the the two-layer two-layer printed printed circuit circuit board board (PCB)(PCB) is is as as follows. follows. First, First, inner inner plane plane of of each each layer layer is is independently independently fabricated fabricated through through patterning, patterning, etching,etching, and drilling processes.processes. Then,Then, aa prepreg prepreg sheet shee ist is placed placed in in the the middle middle of theof the two two layers. layers. They They are arepiled piled up accordingup according to the to the wanted wanted construction, construction, and and bonded bonded together together by using by using heated heated press press plates plates with withhigh high pressure. pressure. Finally, Finally, through-hole through-hole vias vias for electricalfor electrical connections connections between between the the two two outer outer layers layers are areformed formed by by drilling. drilling. The The last last step step is is patterning patterning and and plating plating for for the the outer outer layers.layers. Since the two layers layers havehave different different coefficients coefficients of of temp temperatureerature expansion expansion (CTE), (CTE), registrati registrationon and and warpage warpage issues issues are are likely likely toto occuroccur duringduring the the lay-up lay-up and and heated heated press press steps st [19eps]. The[19]. two The layers two usedlayers in thisused work in this are anisotropicwork are anisotropicmaterials. The materials. CTE’s forThe the CTE’s antenna for the layer antenna are 20 layer and 280 are ppm 20 and/◦C, 280 for ppm/ x-y andoC, zfor directions, x-y and z respectively, directions, respectively,and the CTE’s and for thethe ButlerCTE’s matrix for the layer Butler are 9,matrix 8, and layer 69 ppm are/◦ C,9, for8, and x, y, and69 ppm/ z directions,oC, for x, respectively. y, and z directions,Assuming therespectively. process steps Assuming impose the very process even distribution steps impose of heat very and even pressure, distribution and the of whole heat PCBand 2 size is 1 1 m , the maximum dimensional o2ffset in x-y direction will become 12 µm during the heated pressure,× and the whole PCB size is 1 × 1 m , the maximum dimensional offset in x-y direction will becomeprocess, 12 which μm during is only the 12% heated considering process, the which anti-pad is only clearance 12% considering rule of 100 theµm. anti-pad Thus, weclearance have found rule ofthat 100 this μm. is Thus, not a we critical have issue found for that at least this is the not given a critical PCB issue size. However,for at least this the issuegiven shouldPCB size. become However, more thiscritical issue if theshould fabricated become PCB more size critical is larger if the in massfabricated production. PCB size is larger in mass production.

3.3. Results Results TheThe whole whole antenna antenna system system is is simulated simulated and and optimized optimized by by using using a commercial a commercial full-wave full-wave 3- dimensional3-dimensional electromagnetic electromagnetic field field simulator. simulator. Several Several selected selected key keydimensions dimensions and andjig structure jig structure are tunedare tuned to achieve to achieve optimal optimal antenna antenna performances performances through through intensive intensive electromag electromagneticnetic simulations. simulations. The The antenna system is fabricated in the hybrid two-layer substrate, for which the lower εr and antenna system is fabricated in the hybrid two-layer substrate, for which the lower εr and the higher the higher εr substrates as depicted in Figure2b are chosen to be Taconic TLY-5A and RF-60A εr substrates as depicted in Figure 2b are chosen to be Taconic TLY-5A and RF-60A substrates, respectively.substrates, respectively.The The radiation microwave characteristic radiations are characteristics measured in are an anechoic measured chamber. in an anechoic Careful calibrationschamber. Careful were carried calibrations out to were de-embed carried the out cable to de-embed and connector the cable effects. and connectorFor a test characterization effects. For a test purpose,characterization a single purpose,element patch a single ante elementnna is first patch fabricated antenna in is the first proposed fabricated two-layer in the proposed substrate. two-layer Figure 4substrate. shows a Figurephotograph4 shows of athe photograph fabricated of antenna. the fabricated The low antenna. dielectric The lowconstant dielectric increases constant the increasesradiated powerthe radiated and efficiency. power and efficiency.

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Figure 4. Photographs of single patch antenna: (a) top view and (b) bottom view. Figure 4. Photographs of single patch antenna: (a) top view and (b) bottom view. TheFigure simulated 4. Photographs and measured of single results patch ofantenna: the return (a) top loss view and and radiation (b) bottom pattern view. are plotted in Figure The simulated and measured results of the return loss and radiation pattern are plotted in 5. According to the simulation without considering fabrication inaccuracies and non-idealities, the FigureThe5. According simulated toand the measured simulation results without of the considering return loss fabrication and radiation inaccuracies pattern are and plotted non-idealities, in Figure impedance band with return loss < −10 dB was found to be 28.0–28.9 GHz. However, measured return the5. According impedance bandto the with simulation return loss without< 10 dBconsidering was found fabrication to be 28.0–28.9 inaccuracies GHz. However, and non-idealities, measured return the loss shows that the impedance matching− band is shifted down to 24.2–25.0 GHz, and at the same time lossimpedance shows thatband the with impedance return loss matching < −10 dB bandwas found is shifted to be down28.0–28. to9 24.2–25.0GHz. However, GHz, andmeasured at the return same 27.3–27.6 GHz band also appears as a matched band. We found that such discrepancy be attributed timeloss shows 27.3–27.6 that GHz the impedance band also matching appears asband a matched is shifted band. down to We 24.2–25.0 found thatGHz, such and discrepancyat the same time be to the fabrication inaccuracies and non-idealities. For further investigation of the effects, intensive attributed27.3–27.6 GHz to the band fabrication also appears inaccuracies as a matched and non-idealities. band. We found For that further such investigation discrepancy of be the attributed effects, simulations are performed again with several possible fabrication inaccuracies assumed. Three intensiveto the fabrication simulations inaccuracies are performed and non-idealities. again with several For further possible investigation fabrication of inaccuracies the effects, assumed.intensive fabrication inaccuracies assumed here are too much soldering on the connector pin, which can widen Threesimulations fabrication are inaccuraciesperformed again assumed with here several are too possible much solderingfabrication on inaccuracies the connector assumed. pin, which Three can the feed line, slightly wider patterning during etching process, which can increase the patch widenfabrication the feed inaccuracies line, slightly assumed wider here patterning are too much during soldering etching process,on the connector which can pin, increase which can the patchwiden dimension, and slight misalignment of drilling position, which can move the probe feeding point. dimension,the feed line, and slightly slight misalignment wider patterning of drilling during position, etching which process, can movewhich thecan probe increase feeding the point.patch When such inaccuracy parameters are changed by about 10–20%, as can be seen in Figure 5a, the Whendimension, such inaccuracyand slight misalignment parameters are of drilling changed position, by about which 10–20%, can move as can the be probe seen feeding in Figure point.5a, simulation result shows the excellent agreement with the measured result. This observation indicates theWhen simulation such inaccuracy result shows parameters the excellent are changed agreement by about with 10–20%, the measured as can result.be seen This in Figure observation 5a, the that the fabrication inaccuracies and non-idealities must be minimized and avoided as much as indicatessimulation that result the fabrication shows the inaccuraciesexcellent agreement and non-idealities with the measured must be minimizedresult. This and observation avoided asindicates much possible in this mm-Wave frequency band. Meanwhile, as shown in Figure 5b, the simulated and asthat possible the fabrication in this mm-Wave inaccuracies frequency and non-idealities band. Meanwhile, must as be shown minimized in Figure and5b, avoided the simulated as much and as measured radiation patterns are well matched. The peak antenna gain is 7.4 dBi, and the front-to- measuredpossible in radiation this mm-Wave patterns frequency are well matched. band. Meanwhile, The peak antenna as shown gain in is Figure 7.4 dBi, 5b, and the the simulated front-to-back and back ratio is 25 dB. ratiomeasured is 25 dB. radiation patterns are well matched. The peak antenna gain is 7.4 dBi, and the front-to- back ratio is 25 dB.

Figure 5. Simulated and measured results of single patch: (a) return loss and (b) radiation pattern. Figure 5. Simulated and measured results of single patch: (a) return loss and (b) radiation pattern. The Butler matrix characteristics are evaluated using the same electromagnetic simulator. Table2 Figure 5. Simulated and measured results of single patch: (a) return loss and (b) radiation pattern. showsThe the Butler simulation matrix results characteristics of the relative are evaluated phases and using insertion the same losses electromagnetic at all ports. The simulator. ideal insertion Table 2loss shows of this the Butlersimulation matrix results must beof 6the dB relative since it phases contains and two insertion 3-dB hybrid losses couplers. at all ports. In simulation,The ideal The Butler matrix characteristics are evaluated using the same electromagnetic simulator. Table insertionthe insertion loss lossof this is found Butler to matrix be 5.3–10.0 must dBbe with6 dB ansince average it contains of 8.1 dB.two The 3-dB relative hybrid output couplers. phases In 2 shows the simulation results of the relative phases and insertion losses at all ports. The ideal simulation,in ideal and the simulated insertion conditionsloss is found are to also be 5.3–10.0 given in dB Table with2 .an The average maximum of 8.1 dB. error The of relative the simulated output insertion loss of this Butler matrix must be 6 dB since it contains two 3-dB hybrid couplers. In phasesphase within ideal respect and to simulated the ideal phase conditions is 16 forare the also signal given paths in Table of Port 2. 1-to-Port The maximum 5 and Port error 3-to-Port of the 8. simulation, the insertion loss is found to be◦ 5.3–10.0 dB with an average of 8.1 dB. The relative output phases in ideal and simulated conditions are also given in Table 2. The maximum error of the

Electronics 2019, 8, x FOR PEER REVIEW 7 of 11 simulatedElectronics 2019 phase, 8, 1232 with respect to the ideal phase is 16o for the signal paths of Port 1-to-Port 5 and7 Port of 11 3-to-Port 8. The overall phase error in root-mean-squared value is calculated to be 7.4°. Applying the simulated phase and amplitude results of the Butler matrix to the antenna array input ports, we have foundThe overall that, compared phase error to in the root-mean-squared ideal excitation condit valueions, is calculated the gains toare be degraded 7.4◦. Applying by 2 and the 2.8 simulated dB and beamphase steering and amplitude angles resultsare tilted of theby − Butler2° and matrix+2°, for to 1L/1R the antenna and 2L/2R array beams, input respectively. ports, we have found that, compared to the ideal excitationTable conditions, 2. Butler the matrix gains aredesign degraded results. by 2 and 2.8 dB and beam steering angles are tilted by 2 and +2 , for 1L/1R and 2L/2R beams, respectively. − ◦ ◦ Input Port 1 (1L) Port 2 (2R) Port 3 (2L) Port 4 (1R)

Output Phase Phase Insertion PhaseTable 2.PhaseButler Insertion matrix designPhase results.Phase Insertion Phase Phase Insertion Ideal Simulated Loss Ideal Simulated Loss Ideal Simulated Loss Ideal Simulated Loss Input Port 1 (1L) Port 2 (2R) Port 3 (2L) Port 4 (1R) Port 5 135Phase o 119Phase o 9.0Insertion dB 50Phase o 50Phase o 8.7Insertion dB 0Phase o Phase1 o 7.7Insertion dB 85(455)Phaseo Phase88 o Insertion5.7 dB Output Ideal Simulated Loss Ideal Simulated Loss Ideal Simulated Loss Ideal Simulated Loss PortPort 6 5 90 o135 ◦ 94 o119 ◦ 8.7 9.0dB dB 185 50o ◦ 17350 o ◦ 10.08.7 dB dB 45 0o ◦ 481 o◦ 5.37.7 dB dB -40(320)85(455)o◦ -4388◦ o 8.85.7 dB dB Port 6 90 94 8.7 dB 185 173 10.0 dB 45 48 5.3 dB 40(320) 43 8.8 dB ◦ ◦ ◦ ◦ ◦ ◦ − ◦ − ◦ Port 7 o 45 o47 8.9 dB o40 o43 5.3 dB 90 o 94 o 9.8 dB 185 o 173 o 8.7 dB Port 7 45 ◦ 47 ◦ 8.9 dB -40− ◦ -43− ◦ 5.3 dB 90 ◦ 94 ◦ 9.8 dB 185 ◦ 173◦ 8.7 dB Port 8 0◦ 1◦ 5.9 dB 85◦ 88◦ 7.8 dB 135◦ 119◦ 8.8 dB 50◦ 50◦ 9.3 dB Port 8 0o 1 o 5.9 dB 85 o 88 o 7.8 dB 135 o 119 o 8.8 dB 50 o 50 o 9.3 dB The fabricated 4-directional switched beamforming antenna system is shown in Figure6. The fabricated 4-directional switched beamforming antenna system is shown in Figure 6. The The antenna array is located at the center of the structure in order to minimize radiation disturbances that antenna array is located at the center of the structure in order to minimize radiation disturbances that may be caused by asymmetric backside ground plane. Also, note that the ground pattern surrounding may be caused by asymmetric backside ground plane. Also, note that the ground pattern each antenna element at the front side as shown in the single patch antenna in Figure4a, is not adopted surrounding each antenna element at the front side as shown in the single patch antenna in Figure in this array implementation for the sake of simplification and miniaturization. Nevertheless, we have 4a, is not adopted in this array implementation for the sake of simplification and miniaturization. verified that such surrounding ground plane does not make any noticeable effects on the overall Nevertheless, we have verified that such surrounding ground plane does not make any noticeable antenna characteristics. For measurements, input signal is fed to one of the four input ports via a 2.92 effects on the overall antenna characteristics. For measurements, input signal is fed to one of the four mm coaxial end-launch connector while three unused input ports are terminated by 50 Ω resistors. input ports via a 2.92 mm coaxial end-launch connector while three unused input ports are The 50 Ω resistors are used as available from [20] as a part number CH0603-50RJNT. They are mm-Wave terminated by 50 Ω resistors. The 50 Ω resistors are used as available from [20] as a part number SMD components with dimension of 1.52 0.75 mm2 and guaranteed performance up to Ka-band. CH0603-50RJNT. They are mm-Wave SMD× components with dimension of 1.52 × 0.75 mm2 and The PCB jig is designed as small as possible just enough to hold the connector, but not to adversely guaranteed performance up to Ka-band. The PCB jig is designed as small as possible just enough to affect the original antenna radiation characteristics. The total dimension of the fabricated switched hold the connector, but not to adversely affect the original antenna radiation characteristics. The total beamforming antenna system is 36.2 44.3 mm2 or equivalently 3.4λ 4.1λ including the jig, while it dimension of the fabricated switched× beamforming antenna system is× 36.2 × 44.3 mm2 or equivalently is 36.2 36.2 mm2 or equivalently 3.4λ 3.4λ excluding the jig. 3.4λ × 4.1× λ including the jig, while it is 36.2× × 36.2 mm2 or equivalently 3.4λ × 3.4λ excluding the jig.

Figure 6. Photographs of the antenna system with jig: (a) top view, (b) bottom view, and (c) Figure 6. Photographs of the antenna system with jig: (a) top view, (b) bottom view, and (c) key key dimensions. dimensions. Figure7 shows the simulated and measured return loss with respect to the four input ports. Figure 7 shows the simulated and measured return loss with respect to the four input ports. The The simulated return loss is found to be less than 10 dB at 28 GHz, but the measured results show simulated return loss is found to be less than −10 dB− at 28 GHz, but the measured results show slight slight degradation, and is found to be about 8 dB at 28 GHz. Such degradation may be accounted degradation, and is found to be about −8 dB −at 28 GHz. Such degradation may be accounted for by for by fabrication inaccuracies and non-idealities such as swollen solder bumps, through-hole via fabrication inaccuracies and non-idealities such as swollen solder bumps, through-hole via misalignment, jig misalignment, and so on. misalignment, jig misalignment, and so on.

Electronics 2019, 8, x FOR PEER REVIEW 8 of 11 Electronics 2019, 8, x FOR PEER REVIEW 8 of 11 Electronics 2019, 8, 1232 8 of 11

FigureFigure 7. 7. Return loss of the antenna system: (aa) simulated results and (bb) measured results. Figure 7.Return Return loss loss of of the the antenna antenna system: system: ( ()a) simulated simulated results results and and ( (b) measured) measured results. results. TheThe radiation radiation patterns patterns are are measured measured in in an an anechoic anechoic chamber chamber by by using using the the fabricated fabricated antenna antenna as as The radiation patterns are measured in an anechoic chamber by using the fabricated antenna as aa receiving receiving one. one. Figure Figure 8 illustrates the four-directional beam patterns by comparing the simulation a receiving one. Figure 8 illustrates the four-directional beam patterns by comparing the simulation andand measurement measurement results. results. Figure Figure 9 also shows the same measured radiation patterns in Cartesian and measurement results. Figure 9 also shows the same measured radiation patterns in Cartesian formatformat for for the the sake sake of of more more clarity. clarity. The The peak gains are found to bebe ++9.9,9.9, +8.8,+8.8, +9.9,+9.9, +8.5+8.5 dBi dBi for for 1L, 1L, 2L, 2L, format for the sake of more clarity. The peak gains are found to be +9.9, +8.8, +9.9, +8.5 dBi for 1L, 2L, 1R,1R, and and 2R directions. The beambeam steeringsteering anglesangles are are measured measured to to be be 16−16,°, −3939°,, ++77°,, and and +36+36° whilewhile the the 1R, and 2R directions. The beam steering angles are measured to− be ◦−16−°, −◦39°, +7◦ °, and +36◦ ° while the simulatedsimulated beam beam steering steering angles angles are are −1313°, −,4343°, +13, +°,13 +43, +°, 43for ,1L, for 2L, 1L, 1R, 2L, and 1R, and2R, respectively. 2R, respectively. They They are simulated beam steering angles are− −13◦°, −−43◦°, +13°,◦ +43°, for◦ 1L, 2L, 1R, and 2R, respectively. They are comparableare comparable to previously to previously reported reported works works in [5,13, in14,17]. [5,13,14 The,17]. simulated The simulated and measured and measured sidelobe sidelobe levels comparable to previously reported works in [5,13,14,17]. The simulated and measured sidelobe levels arelevels observed are observed as −9.5, as−7.2,9.5, −9.8, 7.2,−7.3 and9.8, −12,7.3 −6, and −13.8,12, −11.76, dB13.8, for 1L,11.7 2L, dB 1R, for and 1L, 2R, 2L, respectively. 1R, and 2R, are observed as −9.5, −−7.2, −9.8,− −7.3− and− −12, −6, −−13.8, −−11.7− dB for− 1L, 2L, 1R, and 2R, respectively. Noterespectively. that the Notetypical that sidelobe the typical levels sidelobe are comparab levels arele comparableto the previous to the works, previous except works, for the except worst for Note that the typical sidelobe levels are comparable to the previous works, except for the worst sidelobethe worst level sidelobe of −6 leveldB for of 2L6 beam. dB for The 2L beam.overall Theagreement overall agreementbetween the between simulated the and simulated measured and sidelobe level of −6 dB for− 2L beam. The overall agreement between the simulated and measured resultsmeasured are satisfactory. results are satisfactory. results are satisfactory.

Figure 8. Simulated and measured radiation patterns at (a) 1L (Port 1), (b) 2R (Port 2), (c) 2L (Port 3), andFigure (d) 1R8. Simulated (Port 4). and measured radiation patterns at (a) 1L (Port 1), (b) 2R (Port 2), (c) 2L (Port Figure 8. Simulated and measured radiation patterns at (a) 1L (Port 1), (b) 2R (Port 2), (c) 2L (Port 3), and (d) 1R (Port 4). 3), and (d) 1R (Port 4).

Electronics 2019, 8, x FOR PEER REVIEW 9 of 11 Electronics 2019, 8, 1232 9 of 11

0 2L 1L 1R -5 2R

-10

-15

-20

Normalized Gain(dBi) -25

-30 -90 -60 -30 0 30 60 90 Angle (degree)

Figure 9. Measured radiation pattern. Figure 9. Measured radiation pattern. Table3 compares this work with previously reported Butler matrix based switched beamforming antennaTable systems 3 compares operating atthis the work mm-Wave with frequencypreviously range. reported In literature, Butler verymatrix limited based number switched of worksbeamforming can be found antenna to demonstrate systems operating full integration at the mm-Wave of Butler frequency matrix and range. antenna In literature, array elements very limited as wellnumber as switched of works beam can steering be found performance. to demonstrate Only full [13] integration is found to operateof Butler in matrix the same and 5G antenna mm-Wave array frequencyelements band as well as thisas switched work, whereas beam othersteering works performa operatence. at Only a higher [13] frequency is found to band operate of 38 in GHz the [ 14same], 425G GHz mm-Wave [17], and frequency 60 GHz [5 band]. The as maximum this work, beam whereas steering other angle works and operate sidelobe at a levelhigher of frequency this work areband foundof 38 to GHz be comparable [14], 42 GHz or [17], slightly and worse60 GHz than [5]. others. The maximum Nonetheless, beam they steering are practically angle and su sidelobefficient forlevel 5Gof mm-Wave this work radio are requirements.found to be comparable or slightly worse than others. Nonetheless, they are practicallySince this sufficient work focuses for 5G on mm-Wave the miniaturization radio requirements. technique, form factor comparison is of the most importance.Since this For work fair comparison focuses on ofthe the miniaturization form factor, we tec introducehnique, form a normalized factor comparison size factor, is of which the most is calculatedimportance. by dividing For fair comparison the given physical of the form area byfactor free-space, we introduce wavelength a normalized squared andsizebeam factor, steering which is directioncalculated count. by dividing For example, the given the physical beam steering area by count free-space is 5 for wavelength [17], and 4squared for [5,13 and,14 ].beam From steering this normalizeddirection count. size factor, For example, it can be the seen beam that steering this work co demonstratesunt is 5 for [17], the and smallest 4 for form[5,13,14]. factor. From Also, this notenormalized that this size work factor, is the it onlycan be one seen that that employs this work the demonstrates hybrid stackup the substratesmallest form for thefactor. form Also, factor note miniaturization,that this work whereasis the only all others one that are fabricatedemploys the in ahybrid homogeneous stackup stackupsubstrate substrate for the withform more factor sophisticatedminiaturization, miniaturization whereas all techniques others are employed. fabricated in a homogeneous stackup substrate with more sophisticated miniaturization techniques employed. TableTable 3. Results 3. Results summary summary and comparisons. and comparisons.

Substrate Structure Maximum Substrate Structure Maximum Sidelobe Freq. Butler No. of Beam Sidelobe Area Normalized Freq. Antenna Butler No. of Dielectric Beam Level Area2 Normalized (GHz) Antenna Matrix Dielectric Stackup Steering Level (cm ) Size Factor† ConstantDielectric (dB) 2 † (GHz) Matrix DielectricLayers Stackup (degree)Steering (cm ) Size Factor Constant (dB) This 1 4 4 4 Layers 2.17* (degree) 4.12 28 × × 2 Hybrid ** 39 6 0.9 work Patch Microstrip 6.15 * − (1.7λ 2.1λ) This 1 × 4 4 × 4 2.17 ×4.12 28 1 4 4 4 2 Hybrid 39 - 6 1.28 0.9 [5] 60 × × 1 Homogeneous 2.2** 40 9 1.3 work PatchPatch MicrostripMicrostrip 6.15 − (2(1.7λ λ2.6 ×λ 2.1) λ) × 1 8 4 8 28.71 [13] 28 1× × 4 4 × 4 2 Homogeneous 2.2 45 12 1.28 6.3 [5] 60 Slot SIW§ 1 Homogeneous 2.2 40 − -9 (6.1λ 4.1λ) 1.3 × 8Patch10 Microstrip4 8 (238.64λ × 2.6λ) [14] 38 × × 3 Homogeneous 2.2 36 8 16.4 Patch SIW§ − (8.4λ 7.8λ) 1 × 8 4 × 8 ×28.71 [13] 28 8 8 5 8 2 Homogeneous 2.2 45 -12 24‡ 6.3 [17] 42 × × § 2 Homogeneous 2.94 21 6 9.5 PatchSlot MicrostripSIW − (6.9(6.1λ λ6.9 ×λ 4.1) λ) ‡ × ‡ 8 × 10 4 × 8 § 38.64 [14]* For antenna,38 ** For Butler matrix, Substrate3 IntegratedHomogeneous Waveguide, 2.2† Normalized 36 size factor-8 is calculated by (total 16.4 area)/(λ2 beamPatch steering directionSIW§ count), Only including the Butler matrix without antenna. (8.4λ × 7.8λ) × ‡ 8 × 8 5 × 8 24‡ [17] 42 2 Homogeneous 2.94 21 -6 9.5‡ Patch Microstrip (6.9λ × 6.9λ) ‡ 4. Conclusions *For antenna, **For Butler matrix, § Substrate Integrated Waveguide †NormalizedA four-directional size factor switched is calculated beamforming by (total area) antenna / (λ2 × beam system steering is realized direction for count) 28-GHz mm-Wave 5G wireless‡Only applications.including the Butler For the matrix overall without size antenna reduction and miniaturization, a hybrid two-layer stackup substrate based design is employed, in which the higher-εr layer is used for the 4 4 Butler matrix × ε and4. Conclusions the lower- r layer is used for the 4-element antenna array. Measurement results show that the

Electronics 2019, 8, 1232 10 of 11 fabricated antenna system successfully steers the radiation beam to 16 , 39 , 7 , and 36 with the − ◦ − ◦ ◦ ◦ sidelobe levels of 6 to 13.8 dB. The total area including the antenna and Butler matrix is 1.7λ 2.1λ, − − × which is found to be the smallest normalized size and form factor in comparison with state-of-the-art mm-Wave Butler matrix based switched beamforming antenna systems.

Author Contributions: Conceptualization, S.K., S.Y. and H.S.; Resources, S.K., S.Y., and Y.L.; Data Curation, S.Y. and Y.L.; Investigation, S.K., S.Y. and Y.L; Writing—Original Draft Preparation, S.K.; Writing—Review and Editing, H.S.; Visualization, S.K. and H.S.; Supervision, H.S.; For other cases, all authors have participated. Funding: This work was supported by Institute of Information and Communications Technology Planning and Evaluation (IITP) grant funded by the Korea government (MSIT) (2017-0-00959, University ICT Basic Research Center) and Kwangwoon Research Grant 2019. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Hong, W.; Baek, K.H.; Lee, Y.; Kim, Y.; Ko, S.T. Study and prototyping of practically large-scale mmWave antenna systems for 5G cellular devices. IEEE Commun. Mag. 2014, 52, 63–69. [CrossRef] 2. Parchin, N.O.; Alibakhshikenari, M.; Basherlou, H.J.; Abd-Alhameed, R.A.; Rodriguez, J.; Limiti, E. Mm-Wave quasi-yagi antenna for the upcoming 5G cellular communications. Appl. Sci. 2019, 9, 978. [CrossRef] 3. Chen, P.; Hong, W.; Kuai, Z.; Xu, J. A double layer substrate integrated waveguide Blass matrix for beamforming applications. IEEE Microw. Wirel. Compon. Lett. 2009, 19, 374–376. [CrossRef] 4. Fonseca, N.J.G. Printed S-band 4 4 Nolen matrix for multiple beam antenna applications. IEEE Trans. × Antennas Propag. 2009, 57, 1673–1678. [CrossRef] 5. Tseng, C.H.; Chen, C.J.; Chu, T.H. A low-cost 60-GHz switched-beam patch antenna array with Butler matrix network. IEEE Antennas Wirel. Propag. Lett. 2008, 7, 432–435. [CrossRef] 6. Jeong, Y.S.; Kim, T.W. Design and analysis of swapped port coupler and its application in a miniaturized Butler matrix. IEEE Trans. Microw. Theory Tech. 2010, 58, 764–770. [CrossRef] 7. Wang, C.W.; Ma, T.G.; Yang, C.F. A new planar artificial transmission line and its applications to a miniaturized Butler matrix. IEEE Trans. Microw. Theory Tech. 2007, 55, 2792–2801. [CrossRef] 8. Bona, M.; Manholm, L.; Starski, J.P.; Svensson, B. Low-loss compact Butler matrix for a microstrip antenna. IEEE Trans. Microw. Theory Tech. 2002, 50, 2069–2075. [CrossRef] 9. Ausordin, S.F.; Rahim, S.K.A.; Seman, N.; Dewan, R.; Sa’ad, B. A compact 4 4 Butler matrix on double-layer × substrate. Microw. Opt. Technol. Lett. 2013, 56, 223–229. [CrossRef] 10. Lin, T.H.; Hsu, S.K.; Wu, T.L. Bandwidth enhancement of 4 4 Butler matrix using broadband forward-wave × directional coupler and phase difference compensation. IEEE Trans. Microw. Theory Tech. 2013, 61, 4099–4109. [CrossRef] 11. Tudosie, G.; Vahldieck, R.; Lu, A. A novel modularized folded highly compact LTCC Butler matrix. In Proceedings of the 2008 IEEE MTT-S International Microwave Symposium Digest, Atlanta, GA, USA, 15–20 June 2008; pp. 691–694. 12. Kim, S.W.; Choi, D.Y. Analysis of beamforming antenna for practical indoor location-tracking application. Sensors 2019, 19, 3040. [CrossRef][PubMed] 13. Lian, J.W.; Ban, Y.L.; Xiao, C.; Yu, Z.F. Compact substrate-integrated 4 8 Butler matrix with sidelobe × suppression for millimeter-wave multibeam application. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 928–932. [CrossRef] 14. Cao, Y.; Chin, K.S.; Che, W.; Yang, W.; Li, E.S. A compact 38 GHz multibeam antenna array with multifolded Butler matrix for 5G applications. IEEE Antennas Wirel. Propag. Lett. 2017, 16, 2996–2999. [CrossRef] 15. Ruiz, C.M.; Ye, C.; Ye, X.; Lopez, E.; Yin, M.; Hsu, J.; Su, T. Improve signal integrity performance by using hybrid PCB stackup. In Proceedings of the 2013 IEEE International Symposium on Electromagnetic Compatibility, Denver, CO, USA, 5–9 August 2013; pp. 317–321. 16. Ren, Y.; Yin, M.; Ye, C.; Ye, X. Application of hybrid PCB stackup. In Proceedings of the 2017 IEEE Electrical Design of Advanced Packaging and System Symposium (EDAPS), Haining, China, 14–16 December 2017; pp. 1–3. Electronics 2019, 8, 1232 11 of 11

17. Koubeissi, M.; Freytag, L.; Decroze, C.; Monediere, T. Design of a cosecant-squared pattern antenna fed by a new Butler matrix topology for base station at 42 GHz. IEEE Antennas Wirel. Propag. Lett. 2008, 7, 354–357. [CrossRef] 18. Louati, S.; Talbi, L.; Elhassen, M.O. Design of 28 GHz switched beamforming antenna system based on 4 4 × Butler matrix for 5G applications. In Proceedings of the 2018 5th International Conference on Internet of Things: Systems, Management and Security (IoTSMS), Valencia, Spain, 15–18 October 2018; pp. 189–194. 19. Xu, P.; Deng, D.; Liu, D.; Gao, T.; Wu, F.; Zhou, L.; Peng, W. Deformation analysis of multilayer board in the lamination process. In Proceedings of the 2010 11th International Conference on Electronic Packaging Technology & High Density Packaging, Xi’an, China, 16–19 August 2010; pp. 612–615. 20. Vishay Intertechnology, Co. Available online: http://www.vishay.com (accessed on 28 October 2019).

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