A Novel Kind of Wideband Low-Profile Dielectric Resonator Antenna Suitable for Beam-scanning Applications

Wenjian Sun,Yang Yu,Yingdong Hu,Wenwen Yang,2,*,Jianxin Chen

1 School of Information Science and Technology,Nantong University,Seyuan Road,Nantong 226019,China

2 Nantong Research Institute for Advanced Communication Technologies,Chongchuan District,Nantong 226019,China

Abstract:In this paper,a low-profile wideband dielectric resonator antenna(DRA)with a very compact planar size is investigated.The antenna consists of a high permittivity dielectric sheet on the top,a low permittivity substrate in the middle,and a probe feeding structure at the bottom.By digging an annular slot in the designated area of the square dielectric sheet,the resonant frequency of fundamental TE111 mode can be effectively increased to be close to the high-order TE131 mode.The two modes can be finally merged together,yielding a wide impedance bandwidth of 16.6%.Most importantly,the combination of the two modes is done on the premise of a fixed antenna planar size,which can be very compact and suitable for beam-scanning applications.A probe feeding structure is used to excite the DRA,making the antenna simple and practical to be integrated with other RF circuits.For verification,antenna prototypes with singlefeed linear polarization and differential-feed dual polarization were fabricated and measured.Reasonable agreement between the measured and simulated results is observed.

Keywords:dielectric resonator antenna;compact;wideband;beam-scanning;dual-polarization

I.INTRODUCTION

During recent years,beam-scanning antennas are becoming increasingly important for applications in wireless communications as they can offer a significant increase in channel capacity and coverage range[1,2].Generally,the modern beam-scanning antenna technology imposes some requirements on the antenna element design such as: 1)High efficiency.The large number of antennas and radio frequency(RF)channels lead to a sharp increase in energy consumption.To improve the power efficiency,the antenna should have high efficiency.2)Wide bandwidth.The antenna should have a wide bandwidth(e.g.,more than 15%)to support the high data throughput.3)Compact size.In order to achieve good beam-scanning performance and avoid unwanted grating lobes,the antenna array spacing should be kept as about 0.5λ0(λ0refers to the space wavelength at center frequency).The planar size of the array element is thus required to be compact(＜0.4λ0×0.4λ0)enough to fit the spacing demand[3,4].It is thus of great importance for antenna engineers to create novel antenna elements which can satisfy the above requirements and thereby suitable for beam-scanning applications.

Dielectric resonator antennas(DRAs)can achieve high radiation efficiency due to the absence of metallic losses.Additionally,they are lightweight,cost effective,and easy for excitation[5–7].Traditional DRA with high permittivity usually has the advantage of reduced overall size but the impedance bandwidth is limited.In order to reach the demand for broadband communications,many techniques have been proposed.One method is to decrease the relative permittivity(εr).In[8],a conventional DRA obtained a wide bandwidth of about 30% by reducing theεrto about 12.However,the antenna has a high profile(～0.3λ0).Besides,the impedance bandwidth could be improved by using stacked structures[9]or irregular geometric[10,11].Whereas,all these designs bring enlarged DR volumes.

Recently,some interesting low profile DRA designs,known as dense dielectric patch antenna(DDPA)[12–14]and planar dielectric antenna[15],are proposed and investigated owing to their lower profile and higher gain than the traditional ones.However,because of the introduced high permittivity material,the impedance bandwidths are also very limited(typically less than 5%)[14,15].Hence,some new ways have been presented to expand the bandwidths of this kind of DRAs.In[16],the DRA mode and the feeding slot mode are skillfully combined to obtain a wide bandwidth of 10.5%.In[17,18],the higher-order mode can be shifted down to merge with the fundamental mode for bandwidth enhancement by magnifying the length-to-height ratio of the DRA.To acquire a wideband response,the parasitic elements are used to enhance the DRA bandwidths[19,20].For instance,the bandwidth of a DRA is increased to 12% by using two parasitic elements in[20].Nevertheless,all the above methods usually lead to large planar sizes(＞0.5λ0×0.5λ0),making the antennas unsuitable for beam-scanning applications.

In this paper,a novel kind of wideband low profile DRA with a compact planar size of 0.35λ0×0.35λ0is proposed.A simple direct-fed probe is used to provide a good impedance matching,also making the antenna more suited for integration with other RF circuits.For demonstration,a series of designs,including linear-,differential linear-,and dual-polarization antennas,have been presented and investigated.To verify the design,antenna prototypes with single-feed linear polarization and differential-feed dual polarization were fabricated and measured.The measured results of S-parameters and radiation pattern agree well with the simulated results.Table 1 lists the comparison of the proposed antennas with some state-of-theart wideband low-profile DRAs.It can be found the proposed antennas have a wider bandwidth and a much smaller DR planar size than that of[16,20–22].Although the antenna in[17]has a higher gain and a wider bandwidth,its planar size is quite large,making it unsuitable for beam-scanning applications.

Table 1.Performance comparison.

This paper is organized as follows.Section II depicts and analyzes the basic single-feed linear polarized antenna element,followed by measurement results and simulated beam-scanning performance by applying the element into two 1×5 linear arrays alongH-andE-plane respectively;Section III further extended the design concept to differential-fed linearpolarized and dual-polarized antenna designs,with measured results of dual-polarized antenna exhibited.Lastly,the conclusion of the study are summarized in Section IV.

II.SINGLE-FEED LINEAR-POLARIZED ANTENNA DESIGN

2.1 Antenna Configuration

The antenna element is designed at 12.5GHz,which is within Ku-band and can be used for satellite communications.As shown in Figure 1,the proposed design has a simple structure.A square dielectric sheet(l1×l1×h3)has been placed on the top ofSubstrate 1.An annular slot with different widths(dandd1)was cut from that dielectric sheet,leaving a co-central rectangular structure(l3×l2×h3)in the middle.A metal pad,printed on the side surface of the remained dielec-tric sheet,connects to the bottom microstrip feed line through a bonding pad and a metalized via.A ground plane is proposed betweenSubstrate 1and2by using multi-layer printed circuit board(PCB)technology.

Figure 1.The geometric configuration of the proposed compact DRA.(Design parameters: h1 = 0.508,h2 =0.813,h3 =1,l1 =8.4,l2 =4.1,l3 =3.1,d=0.85,d1 =1.35,wp = 0.55,wf = 1.5.Units: mm.).(a)3-D view.(b)Top view of top layer.(c)side view

Figure 2.Simulated E-field distributions of the two modes on xoy-plane of intact dielectric slab(a)TE111 at 8GHz.(b)TE131 at 11.5GHz.

In this design,dielectric sheets are made of ceramic material(εr=45 and tanδ=1.9×10-4).And the substrates are made of Rogers 4003C laminates(εr3=3.55,tanδ= 0.0027).The dielectric sheets and the multilayer PCB substrate are cemented together using glue.

2.2 Operating Principle

To explain the reason of digging slot in the particular place of dielectric sheet,theE-field distributions onxoy-plane of an intact dielectric sheet should be analyzed firstly.Figure 2 presents the overlookEfield distributions of the whole DR(Black dotted box)at two different frequencies,which are considered as fundamentalTE111mode at 8 GHz and higher-orderTE131modes at 11.5 GHz,respectively.

Figure 3.Calculated resonant frequencies of the two modes by CMA with different slot width of(a)Region A.(b)Region B.

Comparing the two modes in Region A,it is clearly seen that the strength of theE-field distribution ofTE131mode is much weaker than that of theTE111mode.Therefore,we can predict that digging out Region A will have a significant effect onTE111mode while there is only a small impact onTE131mode.Conversely,the strength of theE-field distribution in Region B is similar betweenTE111andTE131modes,so that if we dig out Region B,it will have comparable influence on the two resonant modes.

Some theories,such as dielectric waveguide model(DWM)[23]and characteristic mode analysis(CMA)[24–26],can be used to calculate the resonant frequencies of DR.To prove the above hypothesis,CMA method has been utilized in this part.As shown in Figure 3(a),the DRA with an intact dielectric sheet can provide anTE111mode at 8 GHz and anTE131mode at 11.5 GHz.Note that the planar size of the dielectric sheet is set as 0.35λ0×0.35λ0,whereλ0corresponds to the wavelength in the vacuum at 11.5 GHz.We first dig out the material in Region A and it can be seen that as the width of digging area(d)increases from 0 mm to 1.8 mm,the resonant frequency ofTE111mode rises quickly from 8 GHz to 11.7 GHz,while that ofTE131mode moves slowly from 11.5 GHz to 12.6 GHz.The results indicate that by tuning the digging area in Region A(d),theTE111mode can be shifted up much faster than theTE131mode,making the two modes close to each other and easy for combination.

Figure 4.Design steps of the proposed antenna element.

Apart from investigating the key mechanism for combing the two operating modes,we also should find a proper region for setting the excitation source to effectively stimulate the two modes.Region B with relatively strongE-field distribution for both of the modes is considered as a suitable place.To this end,we can dig out the material within Region B to accommodate the probe feed structure,and the effect of slot digging in Region B is also studied.As shown in Figure 3(b),when the width of digging area in Region B(d1)increases from 0 mm to 1.8 mm,the resonant frequencies of both modes smoothly increase at a similar rate.It means that tuning the digging area in Region B(d1)will cause a slight and synchronous shifting up for both theTE111andTE131modes,and it will almost not change the frequency difference between the two modes.

Based on the above analysis,it can be concluded that we can dig materials in Region B to excite the desiredTE111andTE131modes,while we can dig materials in Region A to asynchronously tuning the two modes,making them approach and merge in the frequency axis.The former operation will not affect the validity of the later operation.A simple design guideline for the proposed DRA is summarized as follows.

1 Determine the initial values of the dielectric sheets:The goal is to design an antenna element suitable for beam-scanning applications at the center frequencyf0,so that the planar size(l1)could be fixed as 0.35λ0×0.35λ0.The initial value ofh3can be calculated by using CMA to make the relatively stableTE131mode be resonating atf0.Besides,choose low permittivity material for substrates and high permittivity material for dielectric sheets,and detailed material selection guidance can be referred to[18].In this case,the center frequencyf0is chosen as 12.5 GHz,thus the planar sizel1is fixed as 8.4 mm.The initial value of the height(h3)of dielectric sheet is calculated as 0.8mm by setting theTE131mode resonating atf0=12.5 GHz as shown in Figure 4(a).

2 Dig the slot and Tune the widths of slot for wideband:Digging slot in the dielectric sheet can combine the two modes for wideband purpose.According to the boundary between the strong and weakE-field distributions in Figure 2(b),the starting point to dig the slot can be chosen asl2≈l1/3.Note that the initial valued1should be proper to fit the bonding pad.As shown in Figure 4(b),tuning the width d of slot makes theTE111mode moving quickly towardsTE131mode.Finally the two modes can be merged together to obtain a wide operating band.

3 Tune h3to adjust the operating band:As shown in Figure 4(b),digging out a part of dielectric sheet will lead to a slight increase of the whole operating band.We can slightly increase the height(h3)of the whole dielectric sheet to bring the operating band back down to the desired center frequency since tuningh3can affect both theTE111andTE131modes at a similar degree.The process is illustrated in Figure 4(c).

4 Optimize the final structure:In this step,the parameters(l2,d,d1,h3)can be further adjusted to obtain an optimized performance.The parameters(wf,wp)of the feed structure can be used for impedance matching.Figure 4(d)shows the influence ofwpon impedance matching.

2.3 Simulated and Measured Results

To verify this design,a prototype has been fabricated and tested.Figure 5(a)shows the photograph of the proposed design.Figure 5(b)shows the simulated and measured results of reflection coefficient and gain.The measured bandwidth for|S11| ＜-10 dB is 16.6%from 11.46 to 13.54 GHz.The measured 1-dB gain bandwidth is 16.7%,which is from 11.4 to 13.6 GHz with a peak value of 7.2 dBi achieved at 13.4 GHz.Figure 6 shows the simulated and measured farfield patterns of the proposed design inE-plane(xozplane)andH-plane(yozplane)at 11.8 GHz and 13.2 GHz,respectively.The measured 3-dB beamwidths are 65◦±5◦atE-plane and 80◦±5◦atH-plane.It has been found that the simulated and measured results are in reasonable agreement.

Figure 5.(a)The photograph of the antenna.(b)Simulated and measured reflection coefficients and gains of the antenna.

Figure 6.Simulated and measured radiation patterns of the antenna.

2.4 Simulated Results of 1x5 Beam-scanning DRA Arrays

Based on the proposed antenna element,two 1×5 linear arrays arranged in theH-plane andE-plane respectively are proposed and analyzed in this part.Figure 7 shows the configuration of the two linear arrays.The spacing among the elements is set to 12mm which corresponds to 0.5λ0at 12.5 GHz,and the size of ground is 75mm×25mm,which is large enough to reduce the influence of the ground size on the beam scanning capability.

Figure 7.The geometric configuration of 1×5 linear arrays(a)along H-plane.(b)along E-plane.

Figure 8.The simulated beam scanning performance in(a)H-plane at 11.8 GHz.(b)H-plane at 13.2 GHz.(c)Eplane at 11.8 GHz.(d)E-plane at 13.2 GHz.

Figure 9.The simulated S-parameter in(a)H-plane at different scanning angles.(b)E-plane at different scanning angles.

We select low(11.8 GHz)and high(13.2 GHz)frequencies to elaborate the beam-scanning characteristics of the linear arrays.Beam-scanning is done by phasing the array with progressive phase shifts between the element.The scanning patterns of the proposed arrays at the two frequencies are reported in Figure 8.It is worth mentioning that themaximumscan angle here is defined to be attained if the 3 dB scan loss or the-10 dB sidelobe level(SLL)is met,and thelimitingscan angle is defined to be attained if the gain of main lobe is equal to the grating lobe.In general,the maximum scan angles in theH-andE-plane linear array is±45◦and±42◦at low frequency,while this set of data changes to±40◦and±38◦at high frequency.On the other hand,the limiting scan angles in theH-plane linear array at two frequencies are±58◦and±50◦,respectively.However,the limiting scan angles in theE-plane linear array are asymmetric,and they are(-58◦,60◦)at low frequency and(-45◦,50◦)at high frequency.It is mainly due to the asymmetric single-end feeding structure along theE-plane.

The simulated active S-parameters,which include the mutual coupling effect between the elements,of theE-andH-plane linear arrays at different scanning angles are given in Figure 9.We select the first element(E1)and the center element(E3)as the representative ones to characterize its reflection coefficient.For theH-plane linear array,it can be found that the active|S11|s of the two elements are better than-8.7dB from 11.6 GHz to 13.5 GHz when scanning coverage of the array is changing from 0◦to maximum angle.As arriving to the limiting scanning angle,the active|S11|s deteriorate to be better than-6.5dB within the band.In theE-plane linear array,the active|S11|s of the two elements are better than-9.2dB from 11.5 GHz to 13.5 GHz when scanning coverage of the array is changing from 0◦to maximum angle.The active|S11|s deteriorate to be better than-7.2dB within the band when arriving to the limiting scanning angle.In addition,the simulated mutual coupling between the adjacent elements in the two linear arrays is shown in Figure 9,where we can learn that the mutual coupling is lower than-15.5 dB and-16.9 dB in theH-andE-plane arrays within the operating band,respectively.All the results show that the proposed antenna element is suitable for beam-scanning applications.

III.DIFFERENTIAL-FEED ANTENNA DESIGNS

Most of the current antennas are designed to be singleended since it has a simple structure and can easily be integrated with single-ended systems.However,nowadays,differential systems have attracted more and more attention due to their merits of harmonic suppression,noise immunity,and mode current elimination.In order to integrate differential systems with the single-ended antennas,baluns are usually required.But the usage of baluns will cause some side effects such as additional losses,complex structure,and deteriorated impedance bandwidth.Differential-fed antennas overcome the need for baluns,and they can be directly integrated with differential systems.Further,for a differential-fed antenna,the balanced signal is beneficial to cancel the cross-polarization radiation in the far-field and thus enhance the polarization purity,which is very suitable for dual-polarized antenna design for increase of channel capacity.

3.1 Differential-feed Linear-Polarized Design

Various kinds of differential antennas have been investigated,including microstrip patches[27,28],magneto-electric/multiple dipoles[29,30],cavitybacked antennas[31],and others.In this part,the original single feed structure has been expended to differential feed structure as shown in Figure 10.The structure is basically the same as above and all the parameters have been optimized,but the planar size of the design is still kept as 0.35λ0×0.35λ0.

Figure 11 shows the simulated results of reflection coefficient and gains.The impedance bandwidth better than-10 dB is 17%from 11.34 GHz to 13.46 GHz.The simulated 1 dB gain bandwidth is 18.9%,which is from 11.25 to 13.8GHz with a peak gain value of 7.6 dBi obtained at 13.46 GHz.The simulated far-field patterns inE-plane(xozplane)andH-plane(yozplane)at 11.8 GHz and 13.1 GHz are shown in Figure 12.The 3-dB beamwidths are 64◦±5◦atE-plane and 75◦±3◦atH-plane.Compared to the design in Section II,the proposed one can provide much better performance of cross polarization isolation while the other metrics are in the same level.

Figure 10.The geometric configuration of the differentialfeed single polarized DRA.(Design parameters: h1 =0.508,h2 = 0.813,h3 = 1.1,l1 = 8.4,l2 = 3.3,l3 =3.0,d = 1.2,d1 = 1.35,wf = 1.7,wp = 0.39.Units:mm.).(a)3-D view.(b)Top view of top layer.(c)side view.

Figure 11.Simulated reflection coefficients and gains of the differential-feed linear polarized DRA.

Figure 12.Simulated radiation patterns of the linear polarized DRA at(a)11.8 GHz.(b)13.2 GHz.

Figure 13.The geometric configuration of the differentialfeed dual polarized DRA.(Design parameters: h1 =0.508,h2 = 0.813,h3 = 1.15,l1 = 8.4,l2 = 3.2,l3 =3.1,d = 1.35,d1 = 1.4,wf = 1.8,wp = 0.36.Units:mm.).(a)3-D view.(b)Top view of top layer.(c)side view.

Figure 14.(a)The photograph of the dual-polarized antenna.(b)simulated and measured S-parameters and gains of the antenna.

3.2 Differential-feed Dual-Polarized Design

Dual-polarized antennas have inherent advantages of enhanced channel capacity,reduced multipath fading,and polarization diversity,and therefore been widely used in wireless communications.Many studies in this area have been reported for different types of dualpolarized antenna[29,30,32].

In this part,another differential-feed has been added along the y-axis,which improves the design to dualpolarization.Figure 13 shows the structure of the proposed design after optimizing all the parameters.Figure 14 shows the photograph of this design,along with its simulated and measured results of S-parameters and gains.It is observed that the dual-polarized antenna achieves measured impedance bandwidths of 20%(11.2–13.7GHz)and 20.3%(11.25–13.8 GHz)for differential Port 1 and Port 2,respectively.The measured isolation between the two differential ports across the entire operating band is greater than 41 dB.It is pointed out that a wideband balun was used to measure the radiation performance[33].The insertion losses of the balun have been taken into account in the measured gain values.When measuring the radiation performance of one port,two 50Ω resistors have been used to match the other port to reduce the influence from SMA connectors on radiation patterns.For differential Port 1,the measured 1 dB gain bandwidth is 17.6%,which is from 11.4 to 13.6 GHz with a peak gain value of 6.98 dBi found at 13.4 GHz.For differential Port 2,the measured 1 dB gain bandwidth is 17.6%,which is also from 11.4 to 13.6 GHz with a peak gain value of 6.93 dBi achieved at 13.4 GHz.The measured far-field patterns at 12.5 GHz are shown in Figure 15.The 3-dB beamwidths are 80◦atE-plane and 85◦atH-plane for Port 1 and the 3-dB beamwidths of Port 2 are 77◦atE-plane and 88◦atH-plane.Due to the differential feed mechanism,the cross-polarization level is always better than-30dB within the 3-dB beamwidth for both polarizations.

Figure 15.Simulated and measured radiation patterns of the antenna in two planes at 12.5 GHz.

In comparison to the design in Part 3.1,Section III,it is clear that the differential dual-polarized is a useful promotion.

IV.CONCLUSION

In this paper,we investigated a novel kind of compact DRA for beam-scanning application.The structure is found to be able to pull the fundamental mode closer to the high-order mode by digging an annular slot in the dielectric sheet.Based on this mechanism,the planar size of the antenna element can be kept compact,which is the key point to get good beam-scanning ability.On this basis,we further propose two 1×5 linear arrays to prove their excellent potential in beamscanning array applications.Prototype antennas have been constructed and tested for the validity of the proposed ideas.

ACKNOWLEDGEMENT

This work was supported by the National Natural Science Foundation of China under Grant 62071256,National Natural Science Foundation of Jiangsu under Grant BK20201438,and also supported by State Key Laboratory of Millimeter Waves(Nanjing)and Nantong Research Institute for Advanced Communication Technologies(Nantong),and also sponsored by Qing Lan Project of Jiangsu Province.