Band-Notched Dual-Polarized Antennas for 2G/3G/4G/5G Base Station Using the GCPW DGS-DMS Hybrid Filter

Two broadband dual-polarized band-notched crossed dipole antennas based on the grounded coplanar waveguide (GCPW) filter for 2G/3G/4G/5G base station are proposed in this paper. Two antennas with two and three radiation nulls are displayed to demonstrate the design innovation. Firstly, a wideband crossed dipole antenna with GCPW feed structure is designed, and its coverage frequency is 1.7-3.6 GHz. Then, a compact GCPW second-order hybrid bandstop filter is designed using defected ground structure (DGS) and defected microstrip structure (DMS). The DGS-DMS filter are embedded inside the antenna radiator to achieve the stopband covering 2.9-3.1 GHz with only one dielectric substrate. At last, a miniaturized GCPW third-order DGS-DMS highly selective bandstop filter is integrated in wideband antenna to realize the stopband of 2.87-3.17 GHz. The proposed band-notched antenna uses only one substrate. Compared with the antenna using second-order filter, the antenna using third-order filter has one more radiation null, wider stopband of 2.87-3.17 GHz and higher selectivity.


I. INTRODUCTION
For the current application of 2G/3G/4G communication system, broadband dual-polarized antennas with 1.71-2.69GHz working band are widely used in the wireless base station communication.With the maturity of 5G technology, 3.4-3.6GHz band has become the first band for operators to deploy 5G network.Therefore, building the 5G network with 3.4-3.6GHz by reusing 2G/3G/4G sites is more cost-effective and has greater user coverage than building micro-sites on a large scale [1].However, according to the regulations of the International Telecommunication Union, the frequency band between 2G/3G/4G (1.71-2.69GHz) and 5G (3.4-3.6 GHz) is 2.9-3.1 GHz, which is suitable for radio navigation and The associate editor coordinating the review of this manuscript and approving it for publication was Chinmoy Saha .
positioning [2].As a result, for 2G/3G/4G/5G base station, it is of great economic value to research and design a dual-polarized antenna with a notched band of 2.9-3.1 GHz.In order to suppress the narrow-band interference signal, it is an effective method to introduce interference characteristic in wideband antenna without affecting other technical parameters.
Several wideband dual-polarized antennas, such as patch antenna [3], [4], Vivaldi antenna [5], [6], [7], magnetoelectric dipole antenna [8], [9] and crossed dipole antenna [1], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], can be used in base station applications.The crossed dipole antennas are widely used in wireless communication systems due to the advantages of dual polarization, stable radiation pattern, small size and easy fabrication.In paper [20], a stopband of 2.27-2.53GHz is realized by setting a C-shaped stub near the feeding line.Works [21], [22] combines the stub near the feeding line and the slitting split-ring resonator (SRR) to realize the second-order notch characteristic.The difference between works [23] and [22] is that work [23] achieves a first-order notch band and a second-order notch band by adding two additional SRRs to the main radiator.In the paper [2], a parasitic element is introduced on the radiator to generate a notch band of 2.9-3.1 GHz.In work [24], mouseear shaped arm are introduced to the edge of each radiator to realize a notch band of 2.9-3.2GHz, and a radiation director is added for impedance matching.In works [2], [20], [21], [22], [23], and [24], additional feeding structures under the cross dipole antenna or parasitic elements are added, which will increase the installation difficulty and the processing cost.
In paper [25], the notch band is realized by introducing a C-shaped SRR on the arms of the dipole antenna without adding additional filtering circuit.The minimum gain in the notch band is suppressed from 8 dBi to −6 dBi, compared to the wideband dipole antenna.Work [26] proposes a band-notched antenna suitable for 2G/3G/4G base station application using defected ground structure (DGS).Although these two antennas are easy to install and provide good suppression, the bandwidth of working band and the selectivity of notch band is not good enough, which are not suitable for the application of 2G/3G/4G/5G base station.
In this paper, two dual-polarized band-notched crossed dipole antennas based on the grounded coplanar waveguide (GCPW) filter are proposed for 2G/3G/4G/5G base station.Two antennas with two and three radiation nulls are presented to demonstrate the design principle.Firstly, a wideband cross dipole antenna with GCPW feeding structure is designed, and its coverage frequency is 1.7-3.6GHz.Secondly, the second-order DGS-DMS hybrid bandstop filter is designed for VSWR < 1.5 using DGS and defected microstrip structure (DMS).The miniaturized band-notched filter and antenna are printed on the same dielectric substrate, and the stopband of 2.9-3.1 GHz is realized.Compared with the original broadband antenna covering 1.7-3.6GHz band, the proposed second-order notch antenna works in two working bands of 1.7-2.7 GHz and 3.4-3.6GHz, while introducing the notch band of 2.9-3.1 GHz.The proposed notch antenna has an average gain of 8.3 dBi in the low frequency band and 7.1 dBi in the high frequency band.In the stopband, the antenna gain is all lower than 0 dBi, and the minimum gain is −14.5 dBi.Finally, the third-order DGS-DMS hybrid bandstop filter is designed, and the miniaturized and highly selective filter and antenna are integrated on the same substrate, thus realizing the stopband covering 2.9-3.1 GHz.Compared with the original wideband antenna covering 1.7-3.6GHz, the proposed third-order notch antenna works in 1.7-2.7 GHz and 3.4-3.6GHz, and introduces the notch band covering 2.9-3.1 GHz at the same time.The average gain of the proposed third-order notch antenna is about 8.3 dBi in the low frequency band and 6.9 dBi in the high frequency band.In the stopband range of 2.87-3.17GHz, the antenna gain is lower than 0 dBi, and the minimum gain is -14.5 dBi.Compared with the notch antenna using second-order DGS-DMS hybrid filter, the notch antenna using third-order hybrid filter has three radiation nulls, and the stopband of gain less than 0 dBi is 2.87-3.17GHz, which has wider stopband and higher selectivity.

II. GCPW WIDEBAND ANTENNA
The crossed dipole antenna can achieve broadband characteristic by realizing three resonant modes.The antenna achieves the second and third resonant modes by introducing coupling between two cross dipoles and the antenna internal arm, thus greatly broadening the working bandwidth [1], [2], [16], [20], [21], [22], [23].In order to have better matching, the arm spacing of proposed broadband crossed dipoles are designed into an exponential curve [1], [16], [25].The function of the exponential curve can be expressed as Y(x) = Ce kx + B, and the curvature of the curve can be changed by changing the value of the coefficient k.The proposed antennas are very symmetrical, and the characteristics of dual-polarized ports are similar, so the characteristics of only one port are shown in most analyses of the following content.
The wideband reference antenna using modified direct feeding (MDF) in work [1] and the proposed wideband antenna with GCPW feeding structure are shown in FIGURE 1(a) and (b), respectively.Compared with the reference wideband antenna, the inner arm of the antenna is changed from a ring structure to a plane, and the feeding structure is modified to GCPW feeding structure.FIGURE 2 shows the simulation results of two antennas.The proposed wideband antenna can also achieve wide bandwidth (1.7-3.6 GHz) and high isolation (28 dB).Although these two antennas have the same characteristics, the proposed antenna can integrate filters into the GCPW feeding transmission line.Therefore, the proposed wideband antenna is the basis of integrating the filter inside the antenna to realize the filtering characteristic.

III. ANTENNA WITH TWO RADIATION NULLS A. GCPW SECONDE-ORDER DGS-DMS FILTER
In order to integrate the bandstop filter into the proposed GCPW fed wideband antenna, it is necessary to miniaturize the bandstop filter.To achieve 2.9-3.1 GHz notched band, the bandstop filter needs to have a high selectivity.Therefore, a miniaturized GCPW DGS-DMS hybrid filter with two transmission zeros is designed in the following section.In order to clearly illustrate its working principle, the proposed bandstop filter will be simulated using software HFSS, and its equivalent circuit is simulated and analyzed in software ADS.
The philosophy of DGS bandstop filter unit is that the unit is composed of a DGS resonator and an open-stub used as a compensating capacitor [26], [27].According to work [26], the double-turn DGS resonator are adopted to realize smaller size.The traditional DGS resonator model can be equivalent to a parallel RLC circuit [28], [29].The values of resistance, inductance and capacitance of the parallel circuit can be calculated by using the relevant parameters in electromagnetic (EM) simulation and equations ( 1)-( 3), where f 0 is the resonant frequency, f c is the cutoff frequency, and Z 0 is the characteristic impedance of the GCPW transmission line.However, the double-turn DGS resonator suppression performance is not good enough [26].
In order to improve the suppression characteristic of notch filter, capacitance stub can be added above the DGS [26].In this paper, a hexagon GCPW capacitance stub with side length W 2 = 1.62 mm is added to the transmission line above the DGS, as shown in FIGURE 3. The proposed equivalent circuit of DGS filter unit with capacitance stub can be modeled by ADS.Its equivalent circuit is a parallel RLC circuit connected in series with the GCPW transmission line, as shown in FIGURE 4. Since the split of DGS will affect the cascading characteristics, the split is set toward port 1.Looking from port 1 to the inside of the circuit, there is an RLC circuit.Then a GCPW transmission line with length L U = 2.7 mm, width W U = 2.1 mm and gap G = 0.2 mm is cascaded behind the RLC circuit.The EM simulation result is displayed in FIGURE 5.The result shows that resonant frequency f 0 = 2.96 GHz, cutoff frequency f c = 2.889 GHz, |S 11 | = 0.6 dB = 0.93, and characteristic impedance Z 0 is set to 50 .According to formulas (1)-( 3), C = 11.07 nF, L = 0.2611 nH, R = 1392 can be calculated.The circuit simulation result is added to FIGURE 5.The results show that the DGS filter unit has good suppression performance, but the rectangular coefficient is not high enough.As can be seen from the above analysis, the DGS filter has only one transmission zero, and the rectangle coefficient is not big enough.To improve the selectivity performance, the DMS filter is added to the DGS filter unit to form a secondorder DGS-DMS filter, as shown in FIGURE 6.
The DMS filter can realize band-notched characteristic with compact size [30], because the total length of the DMS resonator approaches the half wavelength of the resonant frequency [31].DMS resonator can also be equivalent to a parallel RLC circuit [31], [32] and expressed by formulas ( 1)-( 3).
The corresponding equivalent circuit of the proposed second-order DGS-DMS filter can be understood as a DGS unit and a DMS unit connected in series by a transmission line, as shown in FIGURE 7. The EM and circuit simulation results are depicted in FIGURE 8. Two simulation results are consistent with each other.It can be seen that the proposed DGS-DMS filter can achieve band-notched property in the frequency range of 2.7-3.2GHz and has two transmission zeros and high selectivity.The simulation results of DGS, DMS and the proposed hybrid filter is shown in FIGURE 9 for better comparative analysis.As it can be seen, the DGS and DMS filters, they both have only one transmission zero, the rectangular coefficient is not big enough and the selectivity is poor.With the same length as the DMS unit (F 2 = 12.34 mm), the proposed hybrid filter can realize two transmission zeros, a wider notched band and higher selectivity.In conclusion, the proposed GCPW second-order DGS-DMS hybrid band-notched filter meets the requirements of high selectivity and miniaturization.In order to further study the parameter impacts of DGS and DMS resonator units, and optimize the performance of the filter, two important parameters are analyzed by EM simulation.W 3 and F 2 affect the sizes of DGS and DMS, respectively.FIGURE 10(a) and (b) show the impacts of parameters W 3 and F 2 on the VSWR, respectively.FIGURE 10(a) shows that with the increase of W 3 , the first transmission zero moves to the lower frequency band, and the width of the notch band becomes wider.As shown in FIGURE 10(b), when F 2 increases, the second transmission zero also moves to the low frequency band, and the width of the notch band narrows.In conclusion, it can be seen that two transmission zeros can be adjusted by separately adjusting W 3 and F 2 .Therefore, by selecting the appropriate W 3 and F 2 values, the frequency band and width of stopband can be realized, and the bandstop filter can be designed to meet the requirements and notch suppression performance.Compared with the wideband antenna, the band-notched antenna introduces a second-order hybrid bandstop filter with other designs unchanged.The VSWR of two antennas are shown in FIGURE 12(a).The results show that when VSWR < 1.5, the broadband antenna works in the 1.7-3.6GHz band, while the proposed band-notched antenna works in the 1.7-2.7 GHz and 3.4-3.6GHz bands, and a notch band of 2.9-3.1 GHz is introduced at the same time.In the notch band of 2.9-3.1 GHz, the VSWR of the antenna is greater than 10.FIGURE 12(b) shows a comparison of the realized gain of two antennas in the main radiation direction.The proposed band-notched antenna has an average gain of 8.3 dBi in the low frequency band and 7.1 dBi in the high frequency band.In the stopband, the antenna gain is all less than 0 dBi and the minimum gain is −14.5 dBi.The overall structure of the proposed band-notched antenna is shown in FIGURE 13.It consists of a dielectric substrate, a metal reflector and two coaxial cables.The crossed dipole antenna is printed on the upper and lower layers of the substrate and fed by two coaxial cables.The substrate is the Rogers-4350 with its dielectric constant ε r = 3.66, length L s = 55 mm, thickness H s = 1 mm and copper thickness H m = 0.035 mm.Because the copper thickness H m of the substrate affects the performance of the proposed filter, the value of H m should be considered in the simulation and design of the filter.A square metal reflector is placed 34 mm below the substrate to achieve a directional radiation pattern of about 65 • .FIGURE 14(a) and (b) show the upper and lower metal layers from top view, respectively.The upper and lower metal layers are connected by 21 shorted vias, of which 20 shorted vias are used to connect the upper and lower dipole antennas, and one shorted via is used to connect the GCPW transmission line.Each layer has two pairs of dipole antennas.The edge of the antenna arm between the cross dipoles is designed as an exponential function.The outer conductor of the coaxial cable is connected to an arm of the dipole at the bottom of the substrate, and the inner conductor of the coaxial cable is connected through the substrate to one end of the transmission line.other end of the transmission line to the other arm of the dipole.In order to avoid overlap, one of the transmission lines is modified so that part of the line is printed on the bottom of the substrate, and then the upper and bottom of the transmission lines are connected by a shorted via.

C. EXPERIMENTAL VALIDATIONS
The proposed antenna is fabricated and measured, and the comparison between the measured and the simulated data is completed.FIGURE 15 shows the fabrication of the proposed antenna.
FIGURE 16 shows the simulation and measurement results of VSWR and isolation.The measured bandwidth with VSWR < 1.5 covers 1.7-2.7 GHz and 3.4-3.6GHz, and the isolation of both operating bands is greater than 26 dB.The measured VSWR is larger than 12 in the 3.4-3.6GHz notch band.FIGURE 17 depicts the measured gain in the broadside direction.The average gain is about 8.3 dBi in the lower band and 7.1 dBi in the higher band.In the 2.9-3.1 GHz notch band, the antenna has two radiation nulls, and the antenna gain is both less than 1 dBi and the minimum gain is −15 dBi, which is about 23 dB lower than the gain in the working band.All in all, for VSWR 1.5, the proposed antenna can achieve coverage of two working bands of 1.7-2.7 GHz and 3.4-3.6GHz, and the measured gain is less than 1 dBi in the 3.4-3.6GHz notch band.The measured results agree well with the simulated ones.The deviation between simulation

IV. ANTENNA WITH THREE RADIATION NULLS A. GCPW THIRD-ORDER DGS-DMS FILTER
In order to further improve the selectivity and increase the width of notch band, a GCPW third-order DMS-DGS hybrid notch filter, as shown in FIGURE 19, is proposed and analyzed.Similar to the previous analysis, the corresponding equivalent circuit can be understood as a DMS filter and a second-order DGS filter in series through a GCPW transmission line, as shown in FIGURE 20.The EM simulation data is compared with the circuit simulation data, and the results are shown in FIGURE 21.These two simulation results are consistent.It can be seen from the results that the proposed filter can achieve bandstop property in the frequency range of 2.7-3.3GHz and has three transmission zeros, so it has high selectivity.
FIGURE 22 shows the simulation results of the DMS, second-order DGS and the proposed third-order DGD-DMS filters for better comparative analysis.It can be seen that the DMS filter has only one transmission zeros, while the DGS filter has two transmission zeros.Their rectangular coefficients are not high enough and their selectivity is poor.The proposed hybrid filter can achieve three transmission zeros, a wider notch band and a higher notch selectivity.In conclusion, the proposed hybrid third-order bandstop filter meets the   requirements of impedance bandwidth, high selectivity and miniaturization.

B. BAND-NOTCHED ANTENNA WITH THREE RADIATION NULLS
FIGURE 23(a) and (b) show the structures of the reference GCPW wideband antenna and the proposed GCPW third-order band-notched antenna, respectively.Compared with the wideband antenna, the proposed antenna introduces a hybrid third-order bandstop filter while other designs remain unchanged.The VSWR of two antennas are shown in FIGURE 24(a).The results show that when VSWR < 2, the broadband antenna works in 1.7-3.6GHz frequency band, while the proposed bandstop antenna works in 1.7-2.7 GHz and 3.4-3.6GHz frequency band, and introduces the notched band covering 2.9-3.1 GHz.In the notch band of 2.87-3.17GHz, the VSWR of the antenna is greater than 10.FIGURE 24(b) shows a comparison of the radiation gain of two antennas in the main radiation direction.The average gain of the proposed notch antenna is about 8.3 dBi in the low frequency band and 6.9 dBi in the high frequency band.In the stopband, the antenna gain is all less than 0 dBi and the minimum gain is −14.5 dBi.The overall structure of the proposed band-notched antenna with three radiation nulls is shown in FIGURE 25.The substrate is also the Rogers-4350 with the same size as the one in FIGURE 13.FIGURE 26(a) and (b) show the upper and lower metal layers from top view, respectively.Compared with the antenna in with two radiation nulls FIGURE 14, the antenna proposed in this section has one more DGS unit and the antenna parameters are displayed below the picture in FIGURE 26.

C. EXPERIMENTAL VALIDATIONS
The proposed antenna with three radiation nulls is also fabricated and measured.FIGURE 27 shows the fabrication of the proposed antenna.
FIGURE 28 shows the simulated and measured results of VSWR and isolation.The measured bandwidth with VSWR <2 covers 1.7-2.7 GHz and 3.4-3.6GHz, and the isolation    FIGURE 29 depicts measured gain in the main radiation direction.The average gain is about 8.3 dBi in the low band and 6.85 dBi in the high band.In the frequency band of 2.7-3.4GHz, the antenna has three radiation nulls.The measured third null is at 3.2 GHz frequency, which is higher than the simulation result.In the notched band, the antenna gain is lower than 3 dBi, which is 3 dB higher than the simulated one.The minimum gain is −12 dBi, about 20 dB less than the gain in the operating band.All in all, the proposed notch antenna works in the two operating bands of 1.7-2.7 GHz and 3.4-3.6GHz with VSWR < 2, while the notch band of 2.87-3.17GHz is introduced.The proposed notch antenna has three radiation nulls, which solves the problem of filter selectivity.The deviation between simulated and measured data is caused by the tolerance of manufacturing and wideband measurement.Deviations can be reduced through multiple iterations of processing and measurement.Two proposed band-notched antennas are compared with some published antennas, as shown in Table 1.These conference antennas are band-notched antennas with directional radiation pattern suitable for base station applications.The antenna in [20] realizes the notch band by setting a C-shaped stub near the feeding line, but the selectivity performance of this method is poor and the gain suppression is not obvious.Compared with reference [20], the antennas in [21] and [22] achieve second-order filtering characteristics by etching SRR and introducing the stub near the feeding line.The antenna in [2] uses parasitic elements above the dipole to introduce the notch band.Although the antenna has a bandwidth of 1.7-3.6GHz, but the gain suppression in the notch band is only about 4 dB.In reference [24], mouse-ear shaped arms are added to the radiator to introduce the 2.9-3.2GHz notch band, and a radiation director is added for impedance matching.The maximum gain suppression of the antenna reaches 12 dB with only one radiation null.In addition, additional feeding balun structure and parasitic elements increase the processing cost and installation difficulty [2], [20], [21], [22], [23], [24].It is easy to printing a notch antenna using one substrate in [25].However, the notch band of this antenna is too wide to be used in 2G/3G/4G/5G base station applications.It can be seen that two proposed band-notch antennas can achieve broad bandwidth (1.7-3.6 GHz), covering 2G/3G/4G/5G bands at the same time, and band-notched characteristic (2.9-3.1 GHz).The minimum gain of the notch band is only −14.5 dBi.Since the proposed antenna can use only one substrate to realize wideband and band-notched characteristic, there is more freedom to design the feeding structure in mass production with lower processing cost and more convenient installation.Although the integration design of the filter and the crossed dipole on the same PCB slightly affect the current distribution of the antenna radiator, there is only a slight impact on the antenna radiation performance.Moreover, this proposed antenna could provide a good option for 2G/3G/4G/5G base station applications that require band-notched characteristic, miniaturization and integration.

V. CONCLUSION
In this paper, two dual-polarized band-notched crossed dipole antennas are proposed for 2G/3G/4G/5G base station with two and three radiation nulls, respectively.The proposed antenna with two radiation nulls works in two working bands of 1.7-2.7 GHz and 3.4-3.6GHz, while introducing the notch band of 2.9-3.1 GHz for VSWR < 1.5.The antenna has an average gain of 8.3 dBi in the low frequency band and 7.1dBi in the high frequency band.In the stopband, the antenna gain is all less than 0 dBi and the minimum gain is −14.5 dBi.The proposed antenna with three radiation nulls works in 1.7-2.7 GHz and 3.4-3.6GHz for VSWR <2, and introduces the notch band covering 2.87-3.17GHz.The average gain is about 8.3 dBi in the low frequency band and 6.9 dBi in the high frequency band.In the notched band, the antenna gain is all lower than 0 dBi and the minimum gain is −14.5 dBi.The antenna with two radiation nulls has better impedance matching.Compared with the antenna with two radiation nulls, the antenna with three radiation nulls has a wider stopband range of 2.87-3.17GHz and higher selectivity, which can better suppress the interference signal.

FIGURE 2 .
FIGURE 2. Simulation results of two antennas.

FIGURE 4 .
FIGURE 4. Equivalent circuit of the proposed GCPW filter unit.

FIGURE 5 .
FIGURE 5. EM and circuit results of the proposed GCPW filter unit.

FIGURE 7 .
FIGURE 7. Equivalent circuit of the proposed second-order DGS-DMS filter.

FIGURE 8 .
FIGURE 8. EM and cirtcuit results of the proposed second-order DGS-DMS filter.

FIGURE 15 .
FIGURE 15.Fabrication of the proposed antenna.(a) Overall view.(b) Top view.

FIGURE 16 .
FIGURE 16.Simulated and measured results of VSWR and isolation.

FIGURE 18
FIGURE 18  shows the simulated and measured radiation patterns of the proposed band-notched antenna in the horizontal plane at the three frequency points of 1.7 GHz, 2.7 GHz and 3.6 GHz.It can be seen from the figure that the radiation pattern is stable in the full spectrum of 2G/3G/4G/5G, and the

FIGURE 17 .
FIGURE 17. Simulated and measured results of realized gain.

FIGURE 20 .
FIGURE 20.Equivalent circuit of the proposed third-order DGS-DMS filter.

FIGURE 21 .
FIGURE 21.EM and cirtcuit results of the proposed third-order DGS-DMS filter.

FIGURE 27 .
FIGURE 27.Fabrication of the proposed antenna.(a) Overall view.(b) Top view.(c) Bottom view.

FIGURE 28 .
FIGURE 28.Simulated and measured results of VSWR and isolation.

FIGURE 29 .
FIGURE 29.Simulated and measured results of realized gain.

FIGURE 30
FIGURE 30  shows the simulation and measurement results of the proposed notch antenna radiating direction in the horizontal plane at the three frequency points of 1.7 GHz, 2.7 GHz and 3.6 GHz.It can be seen that the proposed notch antenna has a stable radiation pattern in the full frequency band of 2G/3G/4G/5G, and the cross-polarization component of the main radiation direction is less than −23 dB.The measured radiation pattern is in good agreement with the simulation results.

TABLE 1 .
Comparison of different antennas.