Broadband Dual-Polarized Endfire Array With Compact Magneto-Electric Planar Yagi Antenna for mm-Wave Terminals

Based on a newly proposed planar Yagi antenna with magnetic and electric dipoles, a compact <inline-formula> <tex-math notation="LaTeX">$1\times 4$ </tex-math></inline-formula> broadband dual-polarized (DP) array with endfire radiation is designed in this paper. The magnetic Yagi antenna operates on the vertical polarization while the electric one operates on the horizontal one. The compact configuration of <inline-formula> <tex-math notation="LaTeX">$0.5\,\,\lambda _{0}\times 0.63\,\,\lambda _{0}\times 0.09\,\,\lambda _{0}$ </tex-math></inline-formula> benefits from the novel half-mode magnetic Yagi antenna and the arrangement with the electric Yagi antenna. The broadband characteristic comes from the directors of the magnetic and electric Yagi antennas and the reflector of magnetic Yagi antenna. The simulation and the measurement verify this design. The measured common bandwidth of <inline-formula> <tex-math notation="LaTeX">$\vert S_{11}\vert < -10$ </tex-math></inline-formula> dB for both polarization ports is 23.8–30.1 GHz (23.7%), and the gains vary between 3 dBi and 4 dBi within 24.2–30 GHz band range. The <inline-formula> <tex-math notation="LaTeX">$1\times 4$ </tex-math></inline-formula> DP array with <inline-formula> <tex-math notation="LaTeX">$0.5~\lambda _{0}$ </tex-math></inline-formula> element space has the beam scanning range of ±43 ° at the endfire direction and the gain of higher than 7.1 dBi. The scanning performances are experimentally validated by two phase-shift feed networks for 0 ° and 30 ° main beam directions. The proposed endfire DP array has the good features of compact structure, a low profile and no clearance requirement, which is easy to integrate in 5G terminals.


I. INTRODUCTION
To meet the requirements of the high data rate and large capacity in the fifth and Sixth generation (5G/6G) mobile communication, the operation frequency of wireless communications has been extended to the millimeter wave (mmwave) band. To compensate the losses of the mm-wave in the atmosphere, the high gain with beam scanning antenna arrays are required [1]- [3]. Dual polarized antenna could receive any polarized electromagnetic waves which is preferable for terminals. To ensure high data throughput and cover different communication standards, the antennas should operate on broadband. Thus, beam scanning array antenna with The associate editor coordinating the review of this manuscript and approving it for publication was Tutku Karacolak . high gain, dual-polarization and broadband operations are necessary for the terminals of the next mobile communications. Some dual-polarized millimeter wave arrays with broadside radiation have been suggested for 5G/6G communications [4], [5].
The endfire antennas, having the maximum radiation direction parallel to the antenna array, are more suitable for mobile terminals [6], [7]. Endfire antennas, such as printed dipole [8]- [11], bow tie patch [12]- [14], and Vivaldi antenna [15], [16], operate on the horizontal polarization sense and have wide operation bandwidths. In order to achieve vertical polarization, these antennas need to be placed vertically [17]. As a result, the profiles of those dual-polarized (DP) endfire antennas were high and they are not practical. The monopole, with a length of half a dipole, can reduce the profile of the vertical polarized antenna effectively. The DP endfire antenna had a profile of 0.11 λ 0 (λ 0 is the wavelength of the electromagnetic wave in the air) by using an upright monopole and a printed Yagi dipole in [18]. The overlapped 10-dB impedance bandwidth was from 37.5 to 42 GHz (12.5%). In [19], Hsu loaded a director in front of the driven monopole and the gain was enhanced to 7.3 dBi. However, the profile was increased to 0.24 λ 0 . In [20], Hong designed a quasi-Yagi DP antenna by utilizing the magnetic and electric dipoles. The total height was about 0.07 λ 0 , while the overlapped 10-dB impedance bandwidth was only from 27.1 to 28.1 GHz (3%). Some efforts have been proposed for extending the bandwidth. The designed mm-wave DP arrays in [21], [22] and [23] had the 10-dB impedance bandwidth of 5.3%, 8.3%, and 14%, respectively. To keep the antenna performances from being affected by the metal frame, the aforementioned DP endfire antennas in [18]- [23] need the clearance when they are placed in the terminals.
Microstrip magnetic dipole (MMD) had low profile and could excite V-pol waves in the endfire direction [24]- [28]. The full metal ground at the bottom makes it easy for integration. In [29], Wen designed a MMD antenna with bidirectional radiation at the endfire directions. However, the bandwidth was only about 3%. In [30], the bandwidth of the MMD was broadened to 13.1% by using microstrip patch, while the element size was too large (1.90 λ 0 × 2.08 λ 0 ) to suitable for array design.
Based on the half-mode theory, this paper proposes a novel magnetic Yagi antenna, which has wide operation band and endfire radiation characteristic. Combining this magnetic Yagi antenna with a planar electric Yagi antenna, a compact wideband DP antenna with endfire radiation is suggested. Taking the element space as 0.5 λ 0 , the 1 × 4 antenna array is designed and verified by the measurements. A DP beam coverage of ± 43 • in the endfire direction is obtained with phase control. The overlapped 10-dB impedance band covers the 5G bands of n257 and n258. The bottom of the proposed DP antenna is complete metal ground, so no clearance is required, which make it easy to integrate the DP antenna with chips by the printed circuit board (PCB) technology.
In this paper we presented a DP endfire antenna and Table 1 compares the proposed antenna with those in the published papers. The operation band in the table is the overlapped impedance bandwidths of |S 11 | and |S 22 | less than −10 dB of two polarizations. Compared with other DP antenna, this design has a lower profile and a wider bandwidth of 23.7%. The antenna size is only 0.5 λ 0 × 0.63 λ 0 × 0.09 λ 0 , which is the smallest one among other dual-polarized antennas. Furthermore, the bottom of the antenna is full metal structure, so no clearance will be required. This antenna will be easy to integrate to the chips of 5/6G mobile terminals.
The rest of this paper is organized as follows. Section II illustrates the structure and the operation principle of the newly proposed planar half-mode magnetic Yagi antenna. Section III presents the configuration, performances and experimental verification of the broadband DP endfire Yagi antenna composed of the magnetic and electric dipoles. The simulation and measurement of the 1 × 4 dual polarized endfire antenna array with beam scanning are addressed in Section IV. Finally, conclusions are drawn in Section V.

II. PLANAR HALF-MODE MAGNETIC YAGI ANTENNA
In order to investigate the operation principle of the novel planar half-mode magnetic Yagi (HMMY) antenna, the evolution process of four antenna prototypes is analyzed. They are the microstrip magnetic dipole (MD), the reflector-loaded MD, the magnetic Yagi (MY) antenna, and the HMMY antenna, as drawn in Fig. 1. It should be noticed that the metallic walls are set as the ideal metal in the HFSS simulation while they are realized by metallic vias of SIW in PCB for fabrication.
The MD antenna is printed on one layer substrate and the structure is shown in Fig. 1 (a). It consists of one patch on the top plane, three shorted metal walls, and a feed probe. Five metal boundaries surround a cavity, while the open aperture radiates the vertical polarized wave (relative to the aperture) to the endfire direction, which is in x direction. This open aperture can be considered as a ''magnetic dipole'' (MD). The length Ld 1 and the width Wd of the top patch determine its operation frequency. This magnetic dipole is fed by a probe from the bottom of the dielectric substrate to the top patch. By shifting the position b of the probe from the open aperture, the input impedance could be matched. The HFSS software is used to simulate and analyze the antenna performances. The simulated |S 11 | versus frequency of the magnetic dipole is plotted as the solid line in Fig. 2. It can be found that |S 11 | < −10 dB is from 26.2 to 29.4 GHz (11.5%).
By loading a magnetic reflector behind this driven MD, as shown in Fig.1 (b), another resonance frequency appears at the low band. This new resonance frequency is determined by length Ld 2 of the magnetic reflector. The reflection coefficients versus frequency are also shown in Fig.2. It can be seen that the low resonance frequency shifts toward lower band when Ld 2 is increased, so the bandwidth is broadened. However, |S 11 | at the low resonance frequency becomes higher and the bandwidth enhancement is not obvious with the increase of Ld 2 . Considering the impedance match characteristic, Ld 2 is 0.85 mm in this design. So and the band of |S 11 | < −10 dB is from 24.5 to 30 GHz (20%).
By loading a magnetic director in front of the driven one, a MY antenna is formed, which is shown in Fig.1(c). In order to reserve enough space for the electric printed dipole, the height h 2 of the magnetic director is slightly lower than the thickness h of the substrate. Fig.3 shows the simulated xoz-and yoz-plane patterns of the antenna prototypes in Fig. 1 (a), (b), and (c) at 25 GHz and 29 GHz. It can be found that the electromagnetic waves are concentrated to the endfire direction (+x axis) by loading the magnetic reflector and director, so the gain could be improved. It can also be seen that the patterns on the xoz plane shift away from 0 • , which is caused by the ground. However, this shift of the maximum direction will not influence the beam scanning characteristic of the phased array. The simulation results have found that the magnetic director has few impacts on the impedance match so the corresponding reflection coefficients were not given in Fig. 2.
The planar HMMY antenna is proposed by cutting half of the MY antenna along the symmetrical plane, as shown in Fig. 1(d). The antenna size is greatly reduced comparing to the full-mode one. To validate the effectiveness of the half-mode theory, the current distributions have been investigated. Fig. 4(a) plots the vector current distributions on the full-mode MY antennas. It can be seen that the vector current is completely symmetrical to the center line, which can be equivalent to a magnetic wall. A reasonable prediction can be drawn that half of the MY antenna would have the same characteristic. Fig.4 (b) draws the simulated vector current distribution on the HMMY antenna, which verifies our prediction. The HMMY antenna has the same current distribution and would have the same radiation characteristics comparing to the full-mode MY antenna. Consequentially, the width Wd of the HMMY antenna could be reduced obviously, and the miniaturization is realized.

III. BROADBAND DUAL-POLARIZED ENDFIRE YAGI ANTENNA
By combing the novel printed HMMY antenna with an electric Yagi antenna, The DP antenna is proposed. Fig. 5 shows the top, side and 3D views of the DP end-fire antenna, which is printed on two dielectric substrate layers.
It should be noticed that the two substrate layers come from the heights of the director dipole and the driven one of the MY antenna and they have the same dielectric constant. The HMMY is feed by a coaxial line at probe 1.
The printed electrical Yagi antenna is on the top plane and is composed of a balun feed network, a printed dipole and an electric director. The driven electric dipole is fed by probe 2 through the balun. The top patch of the magnetic director provides the metal ground for the balun, which makes the DP magnetic-electric Yagi antenna more compact.  Before investigating the DP antenna characteristics, the polarizations should be defined firstly. The direction of x, parallel to the ground plane, is the endfire direction. When the electric field vectors are parallel to the ground plane (xoy plane), it is named as the horizontal polarization (H-pol) wave. When the electric field vectors are perpendicular to the ground plane (xoz plane), it is called the vertical polarization (V-pol) wave. The suggested HMMY and the electrical Yagi antennas excite V-and H-pol waves, respectively.
The bottom of the DP antenna is complete ground plane, which gets the benefit of no requirement of any clearance in the practical mobile terminals. This good feature will be illustrated in following description about the electrical Yagi antenna.
The used substrate is TSM-DS3 with a relative dielectric constant of 2.92 and the loss tangent of 0.002. The total thickness is 1.027 mm (0.09λ 0 ), and the size is 5.5 mm × 7.0 mm (0.5 λ 0 × 0.63 λ 0 ). A. HORIZONTAL-POLARIZED PRINTED ELECTRIC YAGI ANTENNA As shown in Fig. 6(a), the electrical Yagi antenna consists of an electric director, an electric dipole, a balun feed network, and a feed probe, and is printed on the top plane of the substrate. The bottom plane is a full metal ground. The probe feeds the electric dipole through the balun. The balun is located just above the magnetic director. The top patch of the magnetic director not only provides the metal ground for the balun but also can improve the impedance match characteristic, which makes the structure more compact. The electric director dipole can broaden the bandwidth and increase the gain.
In most instances, the covers of mobile phones and other terminals are metal, which will deteriorate the dipole characteristic so a clearance below the dipole is required. The electrical Yagi antenna with and without the clearance are shown in Fig. 6. Generally, the coupling between the electrical dipole and the ground will induce the impedance mismatch. The height between the top electric dipole and its balun is h1. The influence of the substrate thickness h on the impedance match characteristic will be investigated.   By keeping h 1 as 0.127 mm, the influences of height h on the reflection coefficient are plotted in Fig. 7. When h is less than 0.508 mm, the operation band of the electric dipole is very narrow. As h increases, the bandwidth becomes wider and tends to that of the electric dipole without ground, namely with clearance. In this design, h is taken as 1.012 mm for tradeoff between the antenna performance and the profile. The simulated reflection coefficient less than −10 dB is from 24.2 to 29.5 GHz. The director of the V-Pol element offers the ground to the balun of the H-Pol element, which makes the H-Pol and V-Pol element and array have compact size.

B. DUAL-POLARIZED ENDFIRE MAGNETO-ELECTRIC YAGI ANTENNA
The final configuration and the geometrical sizes of the endfire DP magnetic-electrical Yagi antenna are shown in Fig. 8. In order to make the facilitation easier, the shorted metal walls are all replaced by metallized vias. It should be noticed that there is a little difference between the metalwall-loaded and the metal-vias-loaded DP antenna prototype.  After optimizing, the final geometric parameters are listed in Table 2.
The DP Yagi antenna is manufactured by using the PCB process. The 50 microstrip line has been extended for connecting the end-launch connector for measurement. The radiations have been measured in the chamber and the S-parameters were tested by a VNA. Fig. 9 shows the fabricated sample of the DP antenna including the extended 50 microstrip line and the end-launch connector.
The simulated and measured S-parameters versus frequency are shown in Fig. 10. Port1 and Port2 are the feeding ports for the V-pol and H-pol waves, respectively. The simulated reflection coefficients of |S 11 | and |S 22 | less than −10 dB are from 24.25 to 30.0 GHz (21%), and 24.25 to 29.4 GHz (19.2%), respectively. Meanwhile, those of measured results are from 23.8 to 30.2 GHz (23.7%), and 23 to 30.1 GHz (26.7%), respectively. The measured isolations of |S 12 | are all greater than 21 dB within the whole operation band. It is found that the measured bandwidths of reflection coefficients are slightly wider than the  simulated ones and the measured isolation from 29.5 GHz to 31 GHz is higher than that of the simulated one, which was mainly caused by connecting the end-launch in the measurement processes. However, the simulation and the measurement results have good agreement within a broadband from 23.8 to 30.2 GHz. Fig. 11 shows the gain of the DP Yagi antenna versus the frequency. The measured gains of the two polarized antenna vary between 3 dBi to 4 dBi within 24.2-30 GHz band range. The measured gains are lower than those of simulation. This may be due to the insert losses of the extended microstrip lines, the end-launch connector and solder process.
Figs.12 and 13 show the radiation patterns of the printed electric and the magnetic Yagi antennas at 26 GHz and 28 GHz, respectively. The simulated front-to-back ratios of the V-Pol are 12 dB and 13 dB at 26 GHz and 28 GHz, respectively, while those of the H-Pol are 9 dB and 7 dB, respectively. Although more director can improve the gain of the Yagi antenna, only one director has been applied in this design considering the miniaturization for the mobile terminal applications. It can be found that the simulated and the measured patterns from −90 • to 90 • coincide to each other well and the little difference has mainly been caused by the connectors. Due to the limitation of the chamber measurement system, −90 • ∼ 90 • forward patterns have been measured. It could be found that the measured patterns coincide with the simulated results, and the cross polarization is lower than −15 dB at the main radiation direction.

IV. 1 × 4 DUAL-POLARIZED ENDFIRE YAGI ARRAY
A dual-polarized 1 × 4 magnetic-electric Yagi array with endfire radiation is formed by using the proposed DP antenna element, which is shown in Fig. 14. Fed with appropriate phase for every element, the array can scan the beam in different directions. Due to the advantages of the miniaturized DP Yagi antenna, the space between the antenna elements can be taken as d = 0.5 λ 0 = 5.5 mm. Therefore, the side lobe level of the array can remain a low level, and a wide scanning angle range can be obtained. Fig.15 shows the simulated reflection coefficient of the DP array versus frequency. Ports 1 to 4 are the vertical polarization ports, and ports 5 to 8 are the horizontal ones. The simulated bandwidth of reflection coefficient less than −10 dB is from 24.2 to 29.5 GHz, which is similar to that of the element. Fig. 16 shows the isolation between ports 1 to 8 versus frequency. The isolations are all higher than 17 dB within the operating band.   With the same amplitude and appropriate phase at each port, the beam-scanning performance at the endfire direction can be achieved. Fig. 17 plots the simulated beam-scanning performance of H-pol and V-pol waves at 26 and 28 GHz. The array yielded a scan angle up to 43 • at both frequencies.

A. SIMULATIONS AND ANALYSIS OF THE DP ARRAY
The gains for the two polarizations at 26 and 28 GHz are 7.1/8.1 dBi and 7.8/8.7 dBi, respectively. Fig. 18 gives the simulated radiation and total efficiencies of the H-pol and V-pol array versus frequency. Within the whole operation band, the radiation and total efficiencies of the H-pol are above 91.7% and 90.7%, respectively, while those of the V-pol are above 97.7% and 89.5%, respectively.

B. BEAM SCANNING PERFORMANCE AND MEASUREMENTS OF THE DP ARRAY
In order to verify the beam scanning performance of the DP array, the T-type phase-shift feed networks are designed to excite the DP waves with the main beams at 0 • and 30 • directions. Fig. 19 shows the layouts and the fabricated prototypes of the DP arrays with V-and H-pol waves at 0 • scanning angle. The T-type power divider with 4 outputs is designed to feed 4 elements in the same amplitude and phase. Fig. 20 gives out the Photograph of the fabricated prototypes of the proposed array, and Fig. 21 shows the simulated and measured reflection coefficients of V-and H-pol ports versus frequency. The    10-dB impedance bandwidth of V-and H-pol ports are 24.0∼30.1 GHz and 24.7∼29.5 GHz, respectively. The measured results are slightly worse than the simulated ones, which might mainly be caused by processing errors, soldering and experiment process. Fig. 22 shows the simulated and measured normalized patterns of H-pol and V-pol at 26 and 28 GHz. It can be seen that the measured patterns are nearly the same as the simulated ones. When the main beam is at the 0 • direction, the measured side lobes are about −10 dB and −9 dB at         Fig. 25 shows the simulated and measured reflection coefficients of V-and H-pol versus frequency. The measured 10-dB impedance bandwidth of V-and H-pol ports are 23.8∼30.5 GHz and 24.2∼30.5 GHz, respectively. Although the measured reflection efficient are higher than the simulated ones, the trends and operation bands coincide with each other well. The higher reflection coefficient have been caused by processing errors, soldering and experimental process. Fig. 26 plots the simulated and measured normalized patterns of H-pol and V-pol at 26 and 28 GHz. It can be seen that the measured main beams are pointed at 30 • direction. The side lobes are −10 and −9 dB, respectively, which is same as those of the 0 • direction. The measured pattern is nearly the same with the simulated ones except that the side lobe is slightly higher than the simulated ones.

V. CONCLUSION
This paper proposed a compact Yagi magneto-electric dipole antenna with a newly suggested half-mode magnetic Yagi antenna. It operates on broadband, dual-polarization and endfire radiation. The 10-dB impedance bandwidth covers the 5G bands of n257, n258. The size of the element is only 0.5 λ 0 × 0.63 λ 0 , and the profile is only 0.09 λ 0 , which is easy to design arrays. A 1 × 4 array for beam scanning is designed by using this compact DP element. This array can achieve DP beam coverage of ± 43 • in the endfire direction with appropriate phase differences among four feed ports. Simulation and measurement results verify the design. This compact 1 × 4 array can be directly integrated with chips with no clearance requirement for 5G terminal applications.