Yuyan CAO,Zijun GUO,Zhangcheng HAO?,2,3

1State Key Lab of Millimeter Waves,School of Information Science and Engineering,Southeast University,Nanjing 210096,China

2Frontiers Science Center for Mobile Information Communication and Security,Southeast University,Nanjing 210096,China

3Purple Mountain Laboratories,Nanjing 211111,China

A planar millimeter-wave shared-aperture array antenna is proposed and designed in this paper.By composing the substrate integrated waveguide(SIW)and the stripline,the K-band antenna is embedded inside the Ka-band antenna to achieve a smaller size and a low profile by sharing an aperture.The Ka-band antenna radiates through the parallel slot pairs on the surface of the SIW cavities with horizontal polar‐ization,while the K-band antenna radiates through the butterfly-shaped slots with vertical polarization,which are also designed on the surface.Then the two array antennas can radiate by sharing a common ap‐erture with high isolation.To verify this idea,a proto‐type of an 8×8 shared-aperture array antenna has been designed with center frequencies of 19 and 30 GHz and fabricated using multilayer printed circuit board(PCB)technology.The measurement results show that the-10 dB impedance bandwidths in the K-and Ka-bands are 7.73% and＞20%,and the corresponding isolations are higher than 60 and 44 dB,respectively.The proposed shared-aperture antenna has a small footprint,a low profile,and high isolation,and is a promising candidate to design compact millimeterwave wireless systems.

1 Introduction

With the urgent need for system miniaturization and high integration,planar shared-aperture antennas have received much attention due to their compact sizes and flexible functions,such as multi-band oper‐ation and multi-polarization capability(Smolders et al.,2013;Zhang JD et al.,2016;Tao et al.,2021;Zhou et al.,2021).More importantly,by adopting a sharedaperture array antenna,the aperture utilization can be greatly improved,and the number of antennas in the wireless system can be reduced.Recently,a few tech‐niques have been proposed to develop the sharedaperture antennas with good performance,such as the nested technique(Zhao et al.,2015;Mao et al.,2017a,2017b),the staggered technique(Naishadham et al.,2013;Mao et al.,2017c),and the stacked tech‐nique(Ho and Rebeiz,2014;Wang et al.,2018;Ferrando-Rocher et al.,2019).As millimeter-wave wireless systems have become popular,millimeterwave shared-aperture antennas have drawn great at‐tention of researchers(Ho and Rebeiz,2014;Dingand Cheng,2019;Ferrando-Rocher et al.,2019;Zhang JF et al.,2020).A planar shared-aperture array antenna is highly desired for wireless systems,and can dra‐matically reduce the system size.However,due to the challenges of both the element and array designs in the millimeter-wave band,only a few planar millimeterwave shared-aperture array antennas have been re‐ported with good performance in recent years(Cheng et al.,2017;Xu LM et al.,2019;Zhang JF et al.,2019;Guo et al.,2021;Hong et al.,2021).

2 Antenna and array design

2.1 Operating principle of the proposed antenna element

Fig.1 presents the configuration of the pro‐posed K-/Ka-band orthogonal linear polarization shared-aperture antenna element.It is composed of four Rogers RO4350 substrates(εr=3.66 and tanδ=0.004)and two Rogers RO4450F bondply layers(εr=3.52 and tanδ=0.004).The thickness of the copper is 0.035 mm,and the corresponding planar layouts are illustrated in Fig.2 with their detailed geometries.

Fig.1 Configuration of the proposed K-/Ka-band shared-aperture antenna element:(a)three-dimensional view;(b)cross-sectional view

To achieve a compact profile,the K-and Kaband antennas are integrated and share the same radi‐ation aperture on the top surface of the SIW cavity.Two types of radiation slots are designed for the two bands,i.e.,two rectangular slots and one butterflyshaped slot.On the top surface of the SIW cavity,two rectangular slots are symmetrically designed with re‐spect to the center with an offset for the Ka-band op‐eration,and they are excited by the SIW cavity from port 1.Besides,the butterfly-shaped slot is designed at the center for K-band operation,excited by a strip‐line from port 2.Because the butterfly-shaped slotdoes not cut any surface current of the SIW cavity when the Ka-band element is operated,it has no radi‐ation at the Ka-band.Conversely,when the K-band element is operated,the two rectangular slots func‐tion as two parasitic radiation slots with opposite ra‐diation phases.Their radiation fields are canceled,and the main radiation is from only the butterflyshaped slot.Consequently,the K-and Ka-band ele‐ment antennas can be simultaneously operated with high isolation and share the same radiation aperture.

2.2 Design of the antenna element

Ka-band antenna is operated with a slotted SIW cavity,which is fed by the stripline from port 1 at lower substrate layers and then coupled through the coupling slot etched on copper 2.Two types of radia‐tion slots,i.e.,the Ka-band radiation slot pair and the K-band radiation slot in Fig.1a,are orthogonally etched on copper 1.The slots in thexdirection radi‐ate they-polarized waves in the Ka-band,while the slot in theydirection radiates thex-polarized waves in the K-band.The coupling slot is designed at the center of the bottom of the SIW cavity,while the two radiating slots are symmetrically distributed at the center of the SIW cavity.The simulated electrical field distribution of the Ka-band element is shown in Fig.3.Theoretically,the Ka-band SIW cavity can be excited using a straight stripline through the coupling slot.However,because the straight stripline needs to be placed underneath the K-band radiation slot,unwantedradiations from the feeding structure occur.More‐over,a relatively narrow bandwidth would occur if the Ka-band SIW cavity is fed by the straight stripline(Luo et al.,2008).To overcome these drawbacks,a forked stripline is adopted in the design for the Kaband SIW cavity excitation.The forked stripline is equivalent to a two-way power divider.Thus,there is a displacement in thexdirection between the K-band radiation slot and the forked stripline,as illustrated in Fig.1a,and the unwanted radiation from the feed‐ing network can be reduced for the Ka-band applica‐tion.In addition,the forked stripline transforms the single-point excitation of a straight stripline into a double-point excitation with narrow lines.Then,ac‐cording to the coupling theory,the single resonating peak of the SIW cavity can be slightly split into two resonating peaks.This approach is helpful in extend‐ing the operating bandwidth.Ideally,the impedance of the input stripline is expected to be 50 Ω,and the forked striplines have an impedance of 100 Ω.How‐ever,because a thin substrate is adopted to achieve a compact size in the design,the 100 Ω impedance stripline has to be designed with an extremely narrow width of around 0.05 mm,which cannot be realized using the PCB process.Consequently,a relatively low impedance stripline,with an impedance of around 30 Ω,has been used as the input/output for the ele‐ment antenna,and a stripline-to-coplanar waveguide(CPW)transformer is designed for achieving a 50 Ω impedance for the input/output of the array.Accord‐ingly,the forked stripline has an impedance of around 60 Ω.Hence,the initial widths of 0.52 mm and 0.26 mm are adopted for the input and the forked striplines,respectively.The initial sizeLp1of the forked line is chosen as a quarter-wavelength of the Ka-band.Lp3of the forked line is chosen as large as possible to obtain a wide operating bandwidth.In the element design,a space between the end of the cou‐pling slot and the edge of the forked stripline,which is equivalent to the width of the forked line,is initially chosen for theLp3.The initial geometries of the SIW cavity can be determined by classical empirical for‐mulae for the SIW(Zhang Y et al.,2011;Qi et al,2021),and the initial length of the coupling slot and Kaband radiation slot pair are chosen as half-wavelength at 30 GHz.Then,the full-wave high-frequency structure simulator(HFSS)is adopted to tune the geometries of the forked line to obtain a wide bandwidth and good in-band impedance matching.The final geom‐etries are shown in Fig.2.

Fig.2 Geometry of the shared-aperture antenna element:(a)upper substrate layers;(b)lower substrate layers

Fig.3 Simulation of electric field distributions of the Ka-band antenna unit at 30 GHz:(a)lower substrate layers;(b)upper substrate layers

The K-band antenna is operated with the hori‐zontally placed butterfly-shaped slot etched on copper 1 along theydirection,and excited by the K-band stripline at port 2.In this case,the K-band antenna radiatesx-polarized waves and is operated with the transverse electric magnetic(TEM)mode wave,which is coupled from port 2 to the horizontal butterflyshaped slot through the stripline inside the upper sub‐strate layers.To obtain strong coupling and good inband impedance matching,four metallic vias are de‐signed around the butterfly-shaped slot.Because the Ka-band slots are etched on copper 1 and are parallel to the K-band feeding stripline,there is almost no coupling between port 2 and the Ka-band radiation slot pair.Initially,the length of the butterfly-shaped slot is designed as half-wavelength at 19 GHz,and then the butterfly-shaped slot is adjusted throughLs2andWs2to achieve a broadband-10 dB return loss.

Once the K-and Ka-band antennas are initially designed,they are integrated and tuned using the full-wave HFSS to obtain the final geometries with good performance.As discussed above,because the coupling between the K-and Ka-band excitations is pretty weak,final geometries can be obtained through a fast full-wave optimization process.

Fig.4a shows the electric field distribution of the K-band element at 19 GHz.When feeding from port 2,the butterfly-shaped slot cuts off the current to generate thex-polarization radiation field.In this case,a very weak coupling can be found from port 2 to port 1.Similarly,the electric field distribution of the Ka-band element at 30 GHz is shown in Fig.4b,where the antenna is fed from port 1.In this case,most of the quasi-TE120mode field is radiated by thex-direction slot pair.Consequently,ay-polarized radi‐ation wave of the Ka-band element is obtained.Again,a very weak coupling can be found from port 1 to port 2.This is because the two elements only support different operation modes and orthogonal polariza‐tions,and the mode and polarization couplings are small enough to achieve high port isolation.

Fig.4 Electric field distributions of the shared-aperture antenna element at 19 GHz(a)and 30 GHz(b)

To support the planar array application,the pro‐posed shared-aperture element is designed with a very compact form.Both its width and length are 4.55 mm,i.e.,0.288λKat 19 GHz and 0.455λKaat 30 GHz.The reflection coefficients and the radiation gains of the K-and Ka-band are plotted in Fig.5.In the K-band,the reflection coefficient of port 2 is lower than-10 dB,from 18.82 GHz to 19.25 GHz(relative band‐width is 2.26%),and the maximum realized gain is 6.06 dBi.In the Ka-band,the reflection coefficient of port 1 is below-10 dB,from 28.58 GHz to 31.06 GHz(relative bandwidth is 8.32%),and the maximum realized gain is 6.53 dBi.A filtering radiation gain character can be observed in Fig.5b,which is the result of a two-order filtering coupling of the SIW cavity with a double-excitation strategy using the forked striplines.In addition,because the feeding net‐works of the K-and Ka-band are designed in differ‐ent layers with different operating modes and polar‐izations,the isolation between these ports is relatively high.It can be seen from Fig.6 that the isolations in the K-and Ka-band are higher than 65 dB and 23 dB,respectively.

Fig.5 Simulated reflection coefficients and realized gains of the shared-aperture antenna unit:(a)K-band;(b)Ka-band

Fig.6 Simulated isolation between port 1 and port 2 of the antenna element

2.3 Design of the antenna array

The design of an 8×8 array is described in this subsection to demonstrate the planar array applica‐tion of the proposed shared-aperture element.In the array design,the spacing between the elements is chosen as sp=7.5 mm,i.e.,0.475λKat 19 GHz and 0.75λKaat 30 GHz.This spacing avoids undesired grating lobes of the K-and Ka-band while leaving enough space to design the required feeding net‐works.Two feeding networks are designed for the Kand Ka-band arrays,respectively,and their layouts and geometries are shown in Fig.7.Because a very weak coupling exists between the two networks,theycan be designed independently.Both the K-and Ka-band feeding networks are designed in parallel(Xu et al.,2020).Using multipleλ/4 impedance con‐versions and T-junction power dividers,the equal am‐plitude and in-phase output response are realized for 64 ports of each feeding network.Because the strip‐line feeding networks introduce the parallel plate mode in the operation,metallic vias are added on both sides of the stripline to suppress the parallel plate mode(Mukherjee,2017).To feed the array antenna with an end launch connector in the experiments,two shielded stripline(SSL)-to-grounded coplanar waveguide(GCPW)transitions are designed for the array(Li et al.,2015).The final three-dimensional configuration of the proposed array antenna is shown in Fig.8.Fig.9 shows the simulated insertion loss and reflection coefficients for the designed feeding networks.At the K-band,the reflection coefficient is smaller than-12 dB,and its transmission coeffi‐cient is around-20 dB.At the K-band,the reflection coefficient is smaller than-10 dB,and its transmis‐sion coefficient is around-21 dB.The insertion losses of the K-and Ka-band feeding networks are around-2 dB and-3 dB,respectively,which is a re‐sult of the substrate and metal losses.

Fig.7 Layouts of the feeding networks on different layers:(a)K-band feeding network;(b)Ka-band feeding network

Fig.8 Three-dimensional configuration of the proposed K-/Ka-band shared-aperture antenna array

Fig.9 Simulated insertion losses and reflection coefficients of the designed feeding networks:(a)K-band;(b)Ka-band

3 Experiments and results

The designed 8×8 array prototype was fabricated using the PCB process.Generally,a multilayer process can be adopted to fabricate the full structure with a multiple pressing procedure,but that would be expen‐sive.To reduce the fabrication cost,we individually fabricated the top two substrates and the bottom two substrates using a double-layer PCB process.Then,these two fabricated boards were aligned through an alignment hole with screws.Fig.10 shows photographs of the fabricated array antenna.TheS-parameters were measured using the vector network analyzer,and the radiation gain and radiation pattern were measured in a far-field anechoic chamber.

Fig.10 Photographs of the fabricated shared-aperture array antenna:(a)front view;(b)back view

The measuredS-parameters and realized gains as well as the simulation results are illustrated in Fig.11.The measured-10 dB impedance bandwidth for the K-band is 7.73%(18.27–19.74 GHz),and the band isolation is higher than 60 dB.The measured maximum realized gain is 18.5 dBi,and the 3 dB gain bandwidth is 9.53%(18.18–20 GHz).In the Ka-band,the measured reflection coefficient is below-10 dB from 27 to 33 GHz,and the bandwidth is higher than 20%.In addition,the band isolation is higher than 44 dB.The measured maximum gain is 20.02 dBi,and the 3 dB gain bandwidth is 10.54%(28.4–31.56 GHz).Generally,the measurement and simulation results agree well,and the small discrepan‐cies are from mainly the alignment and fabrication tolerances.

Fig.11 Simulation and measurement results of the shared-aperture array antenna:(a)magnitude responses for the S-parameters;(b)realized gain

The typically normalized co-and cross-polariza‐tion radiation patterns of the proposed K-/Ka-band shared-aperture antenna are shown in Figs.12 and 13.Generally,the measured patterns are in goodagreement with the simulated ones,and a low crosspolarization level below-30 dB is achieved for the proposed array antenna.Note that the proposed an‐tenna can provide only linear polarization.To support millimeter-wave low Earth orbit satellite communica‐tion,circular polarization is required for the antenna.To this end,a cross-radiation slot can be adopted in the design instead of two types of radiation slots in the proposed antenna,and can be used to generate circular polarization using a 90° phase shifting stripline feed‐ing network and the forked stripline feeding SIW cavity for the K-and Ka-band applications,respectively.

Fig.12 Simulated and measured normalized radiation patterns of the shared-aperture array antenna at K-band:E-plane(xoz plane)at 18.4 GHz(a),19.0 GHz(b),and 19.6 GHz(c);H-plane(yoz plane)at 18.4 GHz(d),19.0 GHz(e),and 19.6 GHz(f)

Fig.13 Simulated and measured normalized radiation patterns of the shared-aperture array antenna at Ka-band:E-plane(yoz plane)at 29 GHz(a),30 GHz(b),and 31 GHz(c);H-plane(xoz plane)at 29 GHz(d),30 GHz(e),and 31 GHz(f)

Table 1 shows the comparison between the pro‐posed and relative millimeter-wave shared-aperture array antennas in the literature.Generally,the proposed antenna has good performance in terms of bandwidth,port isolation,and aperture size.Note that the profile of the antenna is only 1.91 mm,which is suitable for planar applications.

Table 1 Comparisons between the proposed and relative millimeter-wave shared-aperture array antennas

4 Conclusions

A compact 8×8 dual-polarized shared-aperture array antenna is presented in this paper.A hybrid slot that combines the butterfly-shaped transverse slots and parallel slot pairs was designed for K-and Kaband radiations,independently.Different radiationmodes were used for the K-and Ka-band radiations,along with orthogonal polarizations.Consequently,high band isolation was achieved.The designed pro‐totype was fabricated using PCB technology,and the measurement results were in good agreement with the simulation ones.The proposed antenna has the advantages of dual-band operation capability,high band isolation,planar form,and small size,and has great potential to reduce the size of wireless systems.

Contributors

Zhangcheng HAO proposed the idea.Yuyan CAO de‐signed and measured the antenna,processed the experimental data,and drafted the paper.Zijun GUO helped design the an‐tenna and organize the paper.All the authors revised and final‐ized the paper.

Compliance with ethics guidelines

Yuyan CAO,Zijun GUO,and Zhangcheng HAO declare that they have no conflict of interest.