- Email: [email protected]
Design of broadband dual-polarized antenna with integrated Marchand balun
The Journal of China Universities of Posts and Telecommunications June 2015, 22(3): 118–124 www.sciencedirect.com/science/journal/10058885
http://jcupt.xsw.bupt.cn
Design of broadband dual-polarized antenna with integrated Marchand balun Hao Honggang1, He Yong1, Ren Yi1 (
), Liu Yanhui2
1. College of Electroning Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, China 2. School of Physics and Mechanical and Electrical Engineering, Xiamen University, Xiamen 361005, China
Abstract A design of broadband dual-polarized antenna with low cross polarization and high isolation was presented. The antenna is composed of a cross dipole, a folded ground, two feeding networks, and a reflector. The impedance bandwidth was enhanced by utilizing the mutual coupling between the two dipoles. A kind of meandering folded Marchand balun was skillfully integrated on the support column of the antenna to excite the dipole differentially, which can deliver both balanced (within 0.5 dB) power splitting and consistent (±5°) phase shifting from 1.71 GHz to 2.17 GHz. The standing wave ratios (SWRs) of each port are less than 1.5. By using this feeding network, the antenna has good performance in isolation (> 45 dB) and cross polarization (> 30 dB) over the entire operating frequency band. Moreover, the gain (~8.6 dB) of the proposed antenna is stable with frequency and the antenna structure is very firm due to the support column. The proposed antenna can be easily formed an array for digital cellular system (DCS), personal communications service (PCS) and 3rd generation (3G) applications. Keywords
Dual-polarized antenna, cross dipole, balun, high isolation, low cross polarization
1 Introduction In recent years, designs of broadband dual-polarized antenna with compact structure become more and more popular in mobile communication systems due to the particular advantages—for instance, dual-polarized antennas are widely used in cellular-phone base stations to realize frequency reuse or polarization diversity schemes. By using these schemes not only can increase the communication capacity but also can improve the rate of data transmission in the communication system. In addition, the dual-polarized antennas with high isolation and low cross polarization are able to reduce the multipath effect and improve the polarization mismatch between the transmitter and receiver. Generally speaking, a good dual-polarized base station antenna should satisfy the requirements such as: sufficient bandwidth, high port isolation, low cross polarization level, high front-to-back Received date: 30-06-2014 Corresponding author: Ren Yi, E-mail: [email protected] DOI: 10.1016/S1005-8885(15)60660-4
ratio, low cost and compact size. However, it is still difficult to achieve these targets simultaneously. Therefore, an overall consideration of the relation among these requirements above is necessary when designing dual-polarized base station antennas. In view of above requirements, various kinds of dual-polarized antennas have been developed [1–10]. A method for realizing dual polarization can be found in Ref. [1], which utilizes a pair of coupled microstrip lines to feed a square microstrip patch via two orthogonal rectangular apertures in a common ground plane. This structure is very simple. However, the bandwidth of the antenna is narrow and the isolation between two input ports is not high either. There are many ways to improve the isolation of dual-polarized antenna and increase its bandwidth [2–5]. A modified H-shaped coupling slots was applied whose two upper side arms are bent inward with a proper angle to improve the isolation (> 34 dB) between the input ports in Ref. [2]. In Ref. [3], the isolation was improved to more than 30 dB by optimizing the position of the meandering probes and employing a vertical metallic
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Hao Honggang, et al. / Design of broadband dual-polarized antenna with integrated Marchand balun
wall surrounding the antenna. Another choice to restrain mutual coupling was using the orthogonal modes of current distribution on the ground which could achieve a good isolation better than 36 dB [4]. Wide bandwidth and high isolation was achieved by adopting two pairs of twin-L probes and a wide meandering M-shaped probe in Ref. [5]. Nevertheless, most of the work has high cross-polarization levels due to the higher-order modes of currents. In Refs. [6–7] differential feeding methods were introduced to achieve purely polarization. Especially in Ref. [7], It showed that the higher-order modes could be suppressed effectively by a dual-feed system with a 180o phase differences, but the − 10 dB impedance bandwidth is only 14%. This is not sufficient for broadband base station application. In this article, a new broadband dual-polarized base station antenna with high isolation and low cross polarization was presented. The proposed antenna is composed of a cross dipole, a folded ground, two feeding networks, and a reflector. The cross dipole fed by two meandering folded Marchand baluns is able to realize ±45o polarized waves with the front-to-back ratio better than 18 dB. The proposed balun delivers both balanced (within 0.5 dB) power splitting and consistent 180o (±5o) phase shifting from 1.71 GHz to 2.17 GHz. A broad impedance bandwidth was achieved with less than 1.5 SWRs in this frequency band which can cover DCS, PCS and 3G. This feeding scheme allows for improving input port isolation (> 45 dB) and reducing H-plane cross-polarization levels (> 30 dB), while maintaining stable radiation patterns over the whole operating frequency band. These performance indexes fulfill the industry standards of modern communications for dual-polarized requirement [11]. Both the balun and the dual-polarized antenna were analyzed and optimized by using High Frequency Structure Simulation software.
2 Design of balun and antenna configuration As mentioned previously, the proposed antenna adopts the constant amplitude and out-of-phase feeding technology to improve the isolation between two input ports and suppressing cross polarization. Therefore, the design of feeding structure is very important in this work. There are two parts in this section: one is the design of feeding balun; the other is the configuration of antenna.
2.1
119
A kind of meandering folded Marchand balun
A planer view of the proposed balun is shown in Fig. 1. The proposed balun consists of two sections of quarter-wave coupled lines which are shorted to the ground and one section of half-wave coupled line. The balun is designed on the FR-4 epoxy (FR4) substrate with thickness 0.8 mm and dielectric constant ε r =4.4. All of the coupled-line sections are bent to reduce the size of feeding structure. The balun is fed from port 1, then the energy is coupled to the two quarter-wave coupled lines. Finally, two signals of constant amplitude and out of phase can be obtained from port 2 and port 3. The details can be found in Ref. [12].
Fig. 1 Detailed dimensions in the planar view of the proposed balun; w1 =1.4, w2 =2.2, w21 =2.3, s1 =0.05, m=5, n=2.5, d=0.5, wk =5.5, l1 =23.6, l2 =1.72 (Units: mm)
There are three key parameters which are relevant to the performance of the balun. These parameters are the width w2 and w21 of the coupled lines and the line spacing s1 between the coupled lines. Because the value of s1 is relatively small and for convenience of processing, two small round holes were designed between the quarter-wave coupled line and the half-wave coupled line in the folded place, but they hardly affect the performance of the proposed balun. In addition, by using two microstrip stubs from the two output ports of the balun to the feeding points of the dipole, a good impedance matching can be obtained between them. The electromagnetic (EM) simulation results of the designed balun were plotted in Fig. 2. It is observed that
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the return loss of the proposed balun is below − 10 dB and the amplitude and phase imbalance between the two balanced output ports are within 0.5 dB and 5° from 1.5 GHz to 2.2 GHz, respectively. The simulated impedance frequency band cover the operating frequency band of the proposed antenna (1.71 GHz~2.17 GHz), hence we can utilize it to realize the constant amplitude and out-of-phase feeding for each dipole.
2015
folded ground is mounted in the middle of the double-layer FR4 substrate. The two feeding networks are etched on the outer and inner side of the support column, separately. The dipole 1 is fed by the outer balun with six small probes via the holes on the substrate and the dipole 2 is fed by the inner balun directly. To shape the radiation pattern of the antenna, a rectangular reflector of 185 × 185 mm2 is placed under the antenna and the distance H between the patch and the reflector is about a quarter of a wavelength in free space. It was mentioned that the base station antenna should be an array composed of several identical units to gain better performances in the practical application. Another optimization was required in the structure and parameters of the antenna due to the interaction among the units. As a result, the gain and front-to-back ratio can be further controlled in an array.
(a) Amplitude responses
Fig. 2
(b) Phase responses Simulation results of the proposed balun
2.2 Configuration of the proposed antenna with integrated balun The proposed dual-polarized antenna is designed for wideband operation with high input port isolation and low cross-polarization. We fabricated this antenna with the integrated feeding networks. The antenna and feeding network parameters were optimized for wide impedance bandwidth centering 1.94 GHz. As shown in Fig. 3, two orthogonally situated dipoles are constructed by printing it on a 0.8 mm-thick FR4 substrate with relative dielectric constant 4.4. A center-hollow cuboid made of double-layer FR4 substrate ( ε r =4.4) with the thickness of hs =0.8 mm of each layer is used as a support column for the proposed antenna. The
(a) Top view of the antenna
(b) Side view Fig. 3
Configuration of the proposed antenna: t1 =19.8,
t2 =19.5, ht =4, s=1.13, g=10, wm =6.8, hs =0.8, H=32, W f =185 (Units: mm)
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The presented antenna radiates ±45 ° linearly polarized waves. The E-planes for dipole 1 and dipole 2 are ϕ = 135° and ϕ = 45° respectively according to the coordinate system shown in Fig. 3(a), and correspondingly the H-planes are ϕ = 45° and ϕ = 135° respectively. The radiation patterns of each dipole are calculated when the other dipole is terminated to 50 Ω load. The operating frequency band of the antenna is mainly decided by the parameter t1 . According to general antenna theories, the structure details around feed sections have significant influences on antenna impedance characteristics. However, in the presented design, by properly choosing the values of ht , s, wk , wm , l2 and l21 , good impedance matching and sufficient operating bandwidth for each dipole can be obtained.
Fig. 5
Simulated gain and isolation of the proposed antenna
The radiation patterns of the antenna, at the operating frequencies 1.71 GHz, 1.94 GHz, and 2.17 GHz, are plotted in Fig. 6. It illustrates that the radiation patterns of both polarized-port are very similar and symmetrical to each other, and it shows a broadside radiation.
3 Results The simulated SWRs of the dual-polarized antenna are shown in Fig. 4. A satisfied impedance bandwidth is found for both ports, which fulfils the required operating frequency band from 1.71 GHz to 2.17 GHz. The antenna exhibits more than 23% bandwidth for SWR<1.5 of both ports and is enough to cover the DCS, PCS and 3G frequency bands.
(a) Port 1 1.71 GHz
Fig. 4
Simulated SWRs of the proposed antenna
As shown in Fig. 5, the simulated isolation between the two input ports of the dual-polarized antenna is better than 45 dB over the bandwidth. It also depicts the gain for port 1 and port 2 of the proposed antenna. Within the operating frequency from 1.71 GHz to 2.17 GHz, the average gains are about 8.6 dB for both ports. The efficiency of the antenna is about 85%.
(b) Port 2 1.71 GHz
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(c) Port 1 1.94 GHz
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(f) Port 2 2.17 GHz Fig. 6 Simulated radiation patterns at frequency of 1.71 GHz, 1.94 GHz and 2.17 GHz for the two ports
Moreover, the radiation patterns with low cross polarizations are stable across the operating frequency band in the two principal planes. For port 1, the cross-polarization level is greater than 30 dB in both the E-phane and H-plane. As for port 2, the cross-polarization level is greater than 40 dB in both the E-plane and H-plane. The front-to-back ratio is greater than 18 dB, and the half-power beamwidth is about 60°in E-plane and 70°in H-plane.
4 Discussion
(d) Port 2 1.94 GHz
In this section, the impedance bandwidth, port isolation, and cross polarization of the proposed antenna will be discussed. We only discuss the situation when the dipole 2 is fed and the dipole 1 is terminated with a 50-ohm load due to the structure is symmetrical. 4.1
(e) Port 1 2.17 GHz
Impedance bandwidth
In this design, we neatly utilize the mutual coupling between two orthogonally situated dipoles [13] to achieve a broad bandwidth. Fig. 7 shows the surface current vector distribution on the two dipoles at 1.94 GHz when only the dipole 2 is fed and the dipole 1 is matched by a 50 Ω load. It can be found that there exist currents with large magnitude appear on dipole 1, and this indicates that a strong mutual coupling exists between the two dipoles, the working mechanism can thus be regarded as: when dipole 2 is fed, the dipole 1 works as a parasitic element with anther resonant frequency to effectively broaden the
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Hao Honggang, et al. / Design of broadband dual-polarized antenna with integrated Marchand balun
impedance bandwidth of dipole 2. Similarly, the impedance bandwidth of dipole 1 can also be broadened by dipole 2. From Fig. 4 We can see that the bandwidth of dipole 2 is a little narrower than dipole 1 which is mainly due to the cavity resonance caused by the folded ground, but it still can cover 1.71~2.17 GHz frequency band by careful optimization.
Fig. 7 Surface current vector distribution on the two dipoles at 1.94 GHz when only the dipole 2 is fed
4.2
cross polarization level. Fig. 8 shows the electric field vector distribution between the two dipoles at 1.94 GHz when only dipole 2 is fed and dipole 1 is matched by a 50 Ω load. Fig. 9 is the equivalent schematic of the electric field vector distribution in Fig. 8. We can found that the cross-polarized components (represented by dash lines) of the electric field vector are out-of-phase and annihilate each other, whereas the co-polarization components (represented by dash dot lines) are in-phase and superpose constructively in the far field. Moreover, the two output branches of the feeding networks were supplied equal amplitude and out-of-phase signals, allowing for both the leakage radiation and coupling between them to cancel out. All of this can suppress the cross polarization effectively. From the radiation patterns shown in Fig. 6, it can be found that the cross polarization of port 2 is better than port 1. That is mainly due to the feeding network of port 2 is surrounded by folded ground which is helpful to the cancellation of leakage radiation and coupling between the two output branches of the feeding network.
Port isolation
Generally speaking, poor port isolation will be caused by the strong mutual coupling between the two dipoles. However, high isolation can be achieved in our design though the strong mutual coupling exists between the two dipoles. As shown in Fig. 7, the distribution of the coupled current vectors on the surface of dipole 1 (represented by dash lines) are strictly symmetrical with respect to the center of dipole 1 which indicates that there is no voltage difference between the two feeding points of dipole 1, hence the induced currents will not flow into the feeding port of dipole 1. Similarly, the induced currents will not flow into the feeding port of dipole 2 when the dipole 1 is fed. In addition, as shown in Fig. 3, the two feeding networks are separated by the folded ground and this will reduce the mutual coupling between them. Notably, the feeding network provides two signals which have the same amplitude and out of phase at the two output ports, while the coupling excitation of higher-order mode will be suppressed effectively by this feeding technique. Based on the above analysis, good port isolation is obtained for the proposed antenna. 4.3
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Cross polarization
The symmetric structure is indeed helpful to reduce the
Fig. 8 Electric field vector distribution between the two dipoles at 1.94 GHz when only the dipole 2 is fed
Fig. 9 Schematic of the electric field vector distribution in Fig. 8: electric field vector (solid lines); co-polarization (dash dot lines); cross-polarization (dash lines)
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5 Conclusions In this article, the authors proposed a new broadband dual-polarized antenna suitable for mobile communication base stations. By properly utilizing the strong mutual coupling between the two orthogonally situated dipoles, good impedance bandwidth was achieved. High isolation and low cross polarization were obtained through the out-of-phase feeding mechanism. The proposed antenna yields symmetric radiation patterns and stable gains for both the two polarized ports over a wide operating frequency band from 1.71 GHz to 2.17 GHz. Acknowledgements This work was supported by the National Natural Science Foundation of China (61301032).
References 1. Adrian A, Schaubert D H. Dual aperture-coupled microstrip antenna for dual or circular polarization. Electronics Letters, 1987, 23(23): 1226−1228 2. Wong K L, Tung H C, Chiou T W. Broadband dual-polarized aperture-coupled patch antennas with modified H-shaped coupling slots. IEEE Transactions on Antennas and Propagation, 2002, 50(2): 188−191 3. Lai H W, Luk K M. Dual polarized patch antenna fed by meandering probes. IEEE Transactions on Antennas and Propagation, 2007, 55(9): 2625−2627
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4. Li Y, Zhang Z J, Zheng J F, et al. Design of dual-polarized monopole-slot antenna with small volume and high isolation. IEEE Transactions on Antennas and Propagation, 2012, 60(5): 2511−2514 5. Mak K M, Lai H W, Luk K M. Wideband dual polarized antenna fed by L-and M-probe. Proceedings of the 2012 Asia-Pacific Microwave Conference (APMC’12), Dec 4−7, 2012, Kaohsiung, China. Piscataway, NJ, USA: IEEE, 2012: 1058−1060 6. Huang Y H, Wu Q, Liu Q Z. Broadband dual-polarised antenna with high isolation for wireless communication. Electronics Letters, 2009, 45(14): 714−715 7. Sim C, Chang C C, Row J S. Dual-feed dual-polarized patch antenna with low cross polarization and high isolation. IEEE Transactions on Antennas and Propagation, 2009, 57(10-2): 3321−3324 8. Khoo K W, Guo Y X, Ong L C. Wideband dual-polarized patch antenna. Proceedings of the 10th IEEE Singapore International Conference on Communication Systems (ICCS’06), Oct 30−Nov 2, 2006, Singapore. Piscataway, NJ, USA: IEEE, 2006: 5p 9. AN W, LI S, HONG W, et al. Design of wideband dual-band dual-polarized dipole for base station antenna. The Journal of China Universities of Posts and Telecommunications, 2012, 19(1): 22−28 10. Li B, Yin Y Z, Hu W, et al. Wideband dual-polarized patch antenna with low cross polarization and high isolation. IEEE Antennas and Wireless Propagation Letters, 2012, 11: 427−430 11. Chen H, Xue F Z. A novel broadband dual-polarized printed dipole for base station. Journal of Chongqing University of Posts and Telecommunications: Natural Science Edition, 2011, 23(1): 58−61(in: Chinese) 12. Zhang Z Y, Guo Y X, Ong L C, et al. A new planar Marchand balun. Proceedings of the 2005 IEEE MTT-S International Microwave Symposium Digest (IMS’05), Jun 12−17, 2005, Long Beach, CA, USA. Piscataway, NJ, USA: IEEE, 2005: 4p 13. Bao Z D, Nie Z P, Zong X Z. A novel broadband dual-polarization antenna utilizing strong mutual coupling. IEEE Transactions on Antennas and Propagation, 2014, 62(1): 450−454
(Editor: Zhang Kexin)
http://jcupt.xsw.bupt.cn
Design of broadband dual-polarized antenna with integrated Marchand balun Hao Honggang1, He Yong1, Ren Yi1 (
), Liu Yanhui2
1. College of Electroning Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, China 2. School of Physics and Mechanical and Electrical Engineering, Xiamen University, Xiamen 361005, China
Abstract A design of broadband dual-polarized antenna with low cross polarization and high isolation was presented. The antenna is composed of a cross dipole, a folded ground, two feeding networks, and a reflector. The impedance bandwidth was enhanced by utilizing the mutual coupling between the two dipoles. A kind of meandering folded Marchand balun was skillfully integrated on the support column of the antenna to excite the dipole differentially, which can deliver both balanced (within 0.5 dB) power splitting and consistent (±5°) phase shifting from 1.71 GHz to 2.17 GHz. The standing wave ratios (SWRs) of each port are less than 1.5. By using this feeding network, the antenna has good performance in isolation (> 45 dB) and cross polarization (> 30 dB) over the entire operating frequency band. Moreover, the gain (~8.6 dB) of the proposed antenna is stable with frequency and the antenna structure is very firm due to the support column. The proposed antenna can be easily formed an array for digital cellular system (DCS), personal communications service (PCS) and 3rd generation (3G) applications. Keywords
Dual-polarized antenna, cross dipole, balun, high isolation, low cross polarization
1 Introduction In recent years, designs of broadband dual-polarized antenna with compact structure become more and more popular in mobile communication systems due to the particular advantages—for instance, dual-polarized antennas are widely used in cellular-phone base stations to realize frequency reuse or polarization diversity schemes. By using these schemes not only can increase the communication capacity but also can improve the rate of data transmission in the communication system. In addition, the dual-polarized antennas with high isolation and low cross polarization are able to reduce the multipath effect and improve the polarization mismatch between the transmitter and receiver. Generally speaking, a good dual-polarized base station antenna should satisfy the requirements such as: sufficient bandwidth, high port isolation, low cross polarization level, high front-to-back Received date: 30-06-2014 Corresponding author: Ren Yi, E-mail: [email protected] DOI: 10.1016/S1005-8885(15)60660-4
ratio, low cost and compact size. However, it is still difficult to achieve these targets simultaneously. Therefore, an overall consideration of the relation among these requirements above is necessary when designing dual-polarized base station antennas. In view of above requirements, various kinds of dual-polarized antennas have been developed [1–10]. A method for realizing dual polarization can be found in Ref. [1], which utilizes a pair of coupled microstrip lines to feed a square microstrip patch via two orthogonal rectangular apertures in a common ground plane. This structure is very simple. However, the bandwidth of the antenna is narrow and the isolation between two input ports is not high either. There are many ways to improve the isolation of dual-polarized antenna and increase its bandwidth [2–5]. A modified H-shaped coupling slots was applied whose two upper side arms are bent inward with a proper angle to improve the isolation (> 34 dB) between the input ports in Ref. [2]. In Ref. [3], the isolation was improved to more than 30 dB by optimizing the position of the meandering probes and employing a vertical metallic
Issue 3
Hao Honggang, et al. / Design of broadband dual-polarized antenna with integrated Marchand balun
wall surrounding the antenna. Another choice to restrain mutual coupling was using the orthogonal modes of current distribution on the ground which could achieve a good isolation better than 36 dB [4]. Wide bandwidth and high isolation was achieved by adopting two pairs of twin-L probes and a wide meandering M-shaped probe in Ref. [5]. Nevertheless, most of the work has high cross-polarization levels due to the higher-order modes of currents. In Refs. [6–7] differential feeding methods were introduced to achieve purely polarization. Especially in Ref. [7], It showed that the higher-order modes could be suppressed effectively by a dual-feed system with a 180o phase differences, but the − 10 dB impedance bandwidth is only 14%. This is not sufficient for broadband base station application. In this article, a new broadband dual-polarized base station antenna with high isolation and low cross polarization was presented. The proposed antenna is composed of a cross dipole, a folded ground, two feeding networks, and a reflector. The cross dipole fed by two meandering folded Marchand baluns is able to realize ±45o polarized waves with the front-to-back ratio better than 18 dB. The proposed balun delivers both balanced (within 0.5 dB) power splitting and consistent 180o (±5o) phase shifting from 1.71 GHz to 2.17 GHz. A broad impedance bandwidth was achieved with less than 1.5 SWRs in this frequency band which can cover DCS, PCS and 3G. This feeding scheme allows for improving input port isolation (> 45 dB) and reducing H-plane cross-polarization levels (> 30 dB), while maintaining stable radiation patterns over the whole operating frequency band. These performance indexes fulfill the industry standards of modern communications for dual-polarized requirement [11]. Both the balun and the dual-polarized antenna were analyzed and optimized by using High Frequency Structure Simulation software.
2 Design of balun and antenna configuration As mentioned previously, the proposed antenna adopts the constant amplitude and out-of-phase feeding technology to improve the isolation between two input ports and suppressing cross polarization. Therefore, the design of feeding structure is very important in this work. There are two parts in this section: one is the design of feeding balun; the other is the configuration of antenna.
2.1
119
A kind of meandering folded Marchand balun
A planer view of the proposed balun is shown in Fig. 1. The proposed balun consists of two sections of quarter-wave coupled lines which are shorted to the ground and one section of half-wave coupled line. The balun is designed on the FR-4 epoxy (FR4) substrate with thickness 0.8 mm and dielectric constant ε r =4.4. All of the coupled-line sections are bent to reduce the size of feeding structure. The balun is fed from port 1, then the energy is coupled to the two quarter-wave coupled lines. Finally, two signals of constant amplitude and out of phase can be obtained from port 2 and port 3. The details can be found in Ref. [12].
Fig. 1 Detailed dimensions in the planar view of the proposed balun; w1 =1.4, w2 =2.2, w21 =2.3, s1 =0.05, m=5, n=2.5, d=0.5, wk =5.5, l1 =23.6, l2 =1.72 (Units: mm)
There are three key parameters which are relevant to the performance of the balun. These parameters are the width w2 and w21 of the coupled lines and the line spacing s1 between the coupled lines. Because the value of s1 is relatively small and for convenience of processing, two small round holes were designed between the quarter-wave coupled line and the half-wave coupled line in the folded place, but they hardly affect the performance of the proposed balun. In addition, by using two microstrip stubs from the two output ports of the balun to the feeding points of the dipole, a good impedance matching can be obtained between them. The electromagnetic (EM) simulation results of the designed balun were plotted in Fig. 2. It is observed that
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the return loss of the proposed balun is below − 10 dB and the amplitude and phase imbalance between the two balanced output ports are within 0.5 dB and 5° from 1.5 GHz to 2.2 GHz, respectively. The simulated impedance frequency band cover the operating frequency band of the proposed antenna (1.71 GHz~2.17 GHz), hence we can utilize it to realize the constant amplitude and out-of-phase feeding for each dipole.
2015
folded ground is mounted in the middle of the double-layer FR4 substrate. The two feeding networks are etched on the outer and inner side of the support column, separately. The dipole 1 is fed by the outer balun with six small probes via the holes on the substrate and the dipole 2 is fed by the inner balun directly. To shape the radiation pattern of the antenna, a rectangular reflector of 185 × 185 mm2 is placed under the antenna and the distance H between the patch and the reflector is about a quarter of a wavelength in free space. It was mentioned that the base station antenna should be an array composed of several identical units to gain better performances in the practical application. Another optimization was required in the structure and parameters of the antenna due to the interaction among the units. As a result, the gain and front-to-back ratio can be further controlled in an array.
(a) Amplitude responses
Fig. 2
(b) Phase responses Simulation results of the proposed balun
2.2 Configuration of the proposed antenna with integrated balun The proposed dual-polarized antenna is designed for wideband operation with high input port isolation and low cross-polarization. We fabricated this antenna with the integrated feeding networks. The antenna and feeding network parameters were optimized for wide impedance bandwidth centering 1.94 GHz. As shown in Fig. 3, two orthogonally situated dipoles are constructed by printing it on a 0.8 mm-thick FR4 substrate with relative dielectric constant 4.4. A center-hollow cuboid made of double-layer FR4 substrate ( ε r =4.4) with the thickness of hs =0.8 mm of each layer is used as a support column for the proposed antenna. The
(a) Top view of the antenna
(b) Side view Fig. 3
Configuration of the proposed antenna: t1 =19.8,
t2 =19.5, ht =4, s=1.13, g=10, wm =6.8, hs =0.8, H=32, W f =185 (Units: mm)
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The presented antenna radiates ±45 ° linearly polarized waves. The E-planes for dipole 1 and dipole 2 are ϕ = 135° and ϕ = 45° respectively according to the coordinate system shown in Fig. 3(a), and correspondingly the H-planes are ϕ = 45° and ϕ = 135° respectively. The radiation patterns of each dipole are calculated when the other dipole is terminated to 50 Ω load. The operating frequency band of the antenna is mainly decided by the parameter t1 . According to general antenna theories, the structure details around feed sections have significant influences on antenna impedance characteristics. However, in the presented design, by properly choosing the values of ht , s, wk , wm , l2 and l21 , good impedance matching and sufficient operating bandwidth for each dipole can be obtained.
Fig. 5
Simulated gain and isolation of the proposed antenna
The radiation patterns of the antenna, at the operating frequencies 1.71 GHz, 1.94 GHz, and 2.17 GHz, are plotted in Fig. 6. It illustrates that the radiation patterns of both polarized-port are very similar and symmetrical to each other, and it shows a broadside radiation.
3 Results The simulated SWRs of the dual-polarized antenna are shown in Fig. 4. A satisfied impedance bandwidth is found for both ports, which fulfils the required operating frequency band from 1.71 GHz to 2.17 GHz. The antenna exhibits more than 23% bandwidth for SWR<1.5 of both ports and is enough to cover the DCS, PCS and 3G frequency bands.
(a) Port 1 1.71 GHz
Fig. 4
Simulated SWRs of the proposed antenna
As shown in Fig. 5, the simulated isolation between the two input ports of the dual-polarized antenna is better than 45 dB over the bandwidth. It also depicts the gain for port 1 and port 2 of the proposed antenna. Within the operating frequency from 1.71 GHz to 2.17 GHz, the average gains are about 8.6 dB for both ports. The efficiency of the antenna is about 85%.
(b) Port 2 1.71 GHz
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(c) Port 1 1.94 GHz
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(f) Port 2 2.17 GHz Fig. 6 Simulated radiation patterns at frequency of 1.71 GHz, 1.94 GHz and 2.17 GHz for the two ports
Moreover, the radiation patterns with low cross polarizations are stable across the operating frequency band in the two principal planes. For port 1, the cross-polarization level is greater than 30 dB in both the E-phane and H-plane. As for port 2, the cross-polarization level is greater than 40 dB in both the E-plane and H-plane. The front-to-back ratio is greater than 18 dB, and the half-power beamwidth is about 60°in E-plane and 70°in H-plane.
4 Discussion
(d) Port 2 1.94 GHz
In this section, the impedance bandwidth, port isolation, and cross polarization of the proposed antenna will be discussed. We only discuss the situation when the dipole 2 is fed and the dipole 1 is terminated with a 50-ohm load due to the structure is symmetrical. 4.1
(e) Port 1 2.17 GHz
Impedance bandwidth
In this design, we neatly utilize the mutual coupling between two orthogonally situated dipoles [13] to achieve a broad bandwidth. Fig. 7 shows the surface current vector distribution on the two dipoles at 1.94 GHz when only the dipole 2 is fed and the dipole 1 is matched by a 50 Ω load. It can be found that there exist currents with large magnitude appear on dipole 1, and this indicates that a strong mutual coupling exists between the two dipoles, the working mechanism can thus be regarded as: when dipole 2 is fed, the dipole 1 works as a parasitic element with anther resonant frequency to effectively broaden the
Issue 3
Hao Honggang, et al. / Design of broadband dual-polarized antenna with integrated Marchand balun
impedance bandwidth of dipole 2. Similarly, the impedance bandwidth of dipole 1 can also be broadened by dipole 2. From Fig. 4 We can see that the bandwidth of dipole 2 is a little narrower than dipole 1 which is mainly due to the cavity resonance caused by the folded ground, but it still can cover 1.71~2.17 GHz frequency band by careful optimization.
Fig. 7 Surface current vector distribution on the two dipoles at 1.94 GHz when only the dipole 2 is fed
4.2
cross polarization level. Fig. 8 shows the electric field vector distribution between the two dipoles at 1.94 GHz when only dipole 2 is fed and dipole 1 is matched by a 50 Ω load. Fig. 9 is the equivalent schematic of the electric field vector distribution in Fig. 8. We can found that the cross-polarized components (represented by dash lines) of the electric field vector are out-of-phase and annihilate each other, whereas the co-polarization components (represented by dash dot lines) are in-phase and superpose constructively in the far field. Moreover, the two output branches of the feeding networks were supplied equal amplitude and out-of-phase signals, allowing for both the leakage radiation and coupling between them to cancel out. All of this can suppress the cross polarization effectively. From the radiation patterns shown in Fig. 6, it can be found that the cross polarization of port 2 is better than port 1. That is mainly due to the feeding network of port 2 is surrounded by folded ground which is helpful to the cancellation of leakage radiation and coupling between the two output branches of the feeding network.
Port isolation
Generally speaking, poor port isolation will be caused by the strong mutual coupling between the two dipoles. However, high isolation can be achieved in our design though the strong mutual coupling exists between the two dipoles. As shown in Fig. 7, the distribution of the coupled current vectors on the surface of dipole 1 (represented by dash lines) are strictly symmetrical with respect to the center of dipole 1 which indicates that there is no voltage difference between the two feeding points of dipole 1, hence the induced currents will not flow into the feeding port of dipole 1. Similarly, the induced currents will not flow into the feeding port of dipole 2 when the dipole 1 is fed. In addition, as shown in Fig. 3, the two feeding networks are separated by the folded ground and this will reduce the mutual coupling between them. Notably, the feeding network provides two signals which have the same amplitude and out of phase at the two output ports, while the coupling excitation of higher-order mode will be suppressed effectively by this feeding technique. Based on the above analysis, good port isolation is obtained for the proposed antenna. 4.3
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Cross polarization
The symmetric structure is indeed helpful to reduce the
Fig. 8 Electric field vector distribution between the two dipoles at 1.94 GHz when only the dipole 2 is fed
Fig. 9 Schematic of the electric field vector distribution in Fig. 8: electric field vector (solid lines); co-polarization (dash dot lines); cross-polarization (dash lines)
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The Journal of China Universities of Posts and Telecommunications
5 Conclusions In this article, the authors proposed a new broadband dual-polarized antenna suitable for mobile communication base stations. By properly utilizing the strong mutual coupling between the two orthogonally situated dipoles, good impedance bandwidth was achieved. High isolation and low cross polarization were obtained through the out-of-phase feeding mechanism. The proposed antenna yields symmetric radiation patterns and stable gains for both the two polarized ports over a wide operating frequency band from 1.71 GHz to 2.17 GHz. Acknowledgements This work was supported by the National Natural Science Foundation of China (61301032).
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(Editor: Zhang Kexin)