Broadband low cross-polarization patch antenna

RADIO SCIENCE, VOL. 42,, doi:10.1029/2006rs003595, 2007 Broadband low cross-polarization patch antenna Yong-Xin Guo, 1 Kah-Wee Khoo, 1 Ling Chuen Ong, 1 and Kwai-Man Luk 2 Received 27 November 2006; revised 24 July 2007; accepted 21 August 2007; published 23 October 2007. [1] A broadband 180 microstrip balun is employed as a feed network for the L-probe square patch antenna. The broadband balun delivers impedance matching, equal amplitude power division, and consistent 180 (±10 ) phase shifting over a wide band (45%). We demonstrate that the use of the 180 broadband balun affords significant H-plane cross-polarization suppression across the wide impedance bandwidth designed for L-probe patch antennas. The antenna using the 180 broadband balun exhibits a measured impedance bandwidth (SWR < 2) of 37.15%, from 1.6 to 2.33 GHz. Across our bandwidth of interest (1.7 to 2.3 GHz), the measured boresight gain ranges from 6.16 to 8.5 dbi, and the measured H-plane cross-polarization levels are consistently well suppressed (< 20 db). Citation: Guo, Y.-X., K.-W. Khoo, L. C. Ong, and K.-M. Luk (2007), Broadband low cross-polarization patch antenna, Radio Sci., 42,, doi:10.1029/2006rs003595. 1. Introduction [2] Patch antennas are very popular because they possess many advantageous features such as being low profile and light weight, easy to fabricate, conformable to mounting structures, and compatible with integrated circuit technology. However, patch antennas are inherently constrained by their narrow impedance bandwidth, especially when the radiating elements are printed on thin dielectric substrates. An established method for overcoming this limitation is to use thick low permittivity dielectric substrates that allow for loosely bound electromagnetic fields. A probe feed, which can couple with a radiating patch positioned above the antenna substrate, is commonly used in this bandwidth-widening approach [Chang et al., 1986]. However, the probe inductance limits the impedance bandwidth to less than 10% [Chang et al., 1986]. This probe inductance can be compensated in several ways [Hall, 1987; Huynh and Lee, 1995; Luk et al., 1998]. The L-probe proximity-feed technique [Luk et al., 1998] compensates for probe inductance and extends the achievable impedance bandwidth for probefed patch antennas on thick (0.1 l o ) low-permittivity dielectric substrates. Typically, this design yields 30% impedance bandwidth (SWR 2) and an average gain of 7.0 dbi. Extensive investigations have been dedicated to 1 Institute for Infocomm Research, Singapore. 2 Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China. Copyright 2007 by the American Geophysical Union. 0048-6604/07/2006RS003595 this category of wideband patch antennas [Mak et al., 2000; Lo et al., 2000; Guo et al., 2001, 2002; Wong et al., 2004]. Wideband dual or circular polarization patch antennas can be easily achieved using this L-probe feed approach [Guo et al., 2002; Wong et al., 2004]. The L- probe fed patch antenna is simple in structure and has been used in base stations for various mobile phone systems and other wireless communication systems. [3] On the other hand, the probe leakage radiation, which has serious implications on the radiating performances, remains an important issue for L-probe proximityfed patch antennas. The L-probe feed, though effective in widening the impedance bandwidth, emits probe leakage radiation that causes high cross-polarization levels, especially in the H-plane. A second L-probe feed, provided with an equal amplitude and 180 out-of-phase excitation, can be added to reduce any unwanted probe radiation [Mak et al., 2000; Wong et al., 2004]. Crosspolarization is suppressed when the probe leakage radiation from the added L-probe feed cancels out the probe leakage radiation from the original L-probe feed. Much work has been done to address similar problems [Petosa et al., 1999; Levis et al., 2000; Chen and Chia, 2003]. In prior arts [Petosa et al., 1999; Levis et al., 2000; Mak et al., 2000; Wong et al., 2004], balanced feed networks were used to reduce the cross-polarization. However, the conventional balanced feed networks used in the above cases only provide a consistent 180 (±10 ) phase shift over a very narrow band (10%), thus limiting the frequency range across which proper cancellation of probe leakage radiation can take place. For probe-fed patch antennas, the wideband suppression of cross- 1of8

Figure 1. Schematics of the conventional 180 narrowband microstrip balun. polarization has been a very challenging problem critical in base station applications. [4] In this paper, we propose the use of a novel 180 broadband microstrip balun [Zhang et al., 2005] as a feed network for the dual L-probe square patch antenna. The broadband balun delivers impedance matching, equal amplitude power division, and consistent 180 (±10 ) phase shifting over a wide band (45%). We demonstrate that the use of the proposed 180 broadband balun in place of the conventional 180 narrowband balun affords significantly improved H-plane cross-polarization suppression across a wide impedance passband (30%) [Khoo et al., 2005]. As an important application example, we present a wideband patch antenna targeted for emerging broadband mobile base station applications covering three bands, i.e. DCS1800 (1710 1880 MHz), PCS1900 (1850 1990 MHz) and UMTS2000 (1920 2170 MHz). 2. Feed Network Configuration 2.1. Conventional 180 Narrowband Balun [5] The conventional 180 narrowband microstrip balun, as shown in Figure 1, is used commonly in antenna designs as a phase shifting feed network. To provide a 180 phase shift, the lengths of the microstrip branches, d 1 and d 2,mustbesuchthatd 1 d 2 = l g /2, where l g refers to the guided wavelength at a center operation frequency. Here, the characteristic impedances of the two microstrip branches d 1 and d 2 are given by Z 2 =Z 0 =50W. 2.2. Proposed 180 Broadband Balun [6] First introduced in [Zhang et al., 2005], the 180 broadband microstrip balun shown in Figure 2 delivers Figure 2. Schematics of the proposed 180 broadband microstrip balun. 2of8 Figure 3. Simulated and measured return loss comparison between the 180 narrowband and broadband baluns. balanced power splitting and consistent 180 (±10 ) phase shifting across a wide band. This specially designed feed network comprises of a 3-dB Wilkinson power divider for wideband balanced power splitting, cascaded with a broadband 180 phase shifter for wideband 180 phase shifting. 2.3. Simulated Results and Verification [7] All simulations presented in this paper were performed using IE3D 12.0, a commercially available electromagnetic field solver based on the Method of Moment (MoM). The feed networks were modeled on a substrate of thickness 0.8 mm and dielectric constant 3.38. For convenient analysis, the input and output ports of the feed networks were all set to 50 W. [8] Figure 3 shows the simulated and measured return loss comparison between the two baluns. The 180 broadband balun exhibits a wide measured impedance bandwidth (S 11 < 10 db) of 67.3%, from 1.39 to 2.8 GHz, while the 180 narrowband balun exhibits a relatively wider measured impedance bandwidth (S 11 < 10 db) of 150.15%, from 0.41 to 2.88 GHz. Figure 4 shows the simulated and measured output port amplitude response comparison between the two baluns. The 180 broadband balun exhibits balanced measured output ports power distribution (S 21 =S 31 = 3 db (±1.0 db)) over a wide band of 44.73%, from 1.51 to 2.38 GHz, while the 180 narrowband balun exhibits balanced measured output ports power distribution (S 21 = S 31 = 3 db (±1.0 db)) over a relatively wider band of 55.29%, from 1.23 to 2.17 GHz. Figure 5 shows the simulated and measured output ports phase difference comparison between the two baluns. The 180 broadband balun exhibits consistent measured 180 (±10 ) output ports phase difference over a considerably wide band of

ports power distribution over a relatively wider band. However, its overall performance was inherently limited by its narrowband 180 phase shifting capability; hence we term it a narrowband balun. Figure 4. Output ports amplitude response comparison between the 180 narrowband and broadband baluns. 48.84%, from 1.47 to 2.42 GHz, while the 180 narrowband balun exhibits consistent measured 180 (±10 ) output ports phase difference over a much narrower band of 11.43%, from 1.65 to 1.85 GHz. The simulated and measured results are in generally in good agreement. However, there exist a small frequency shift between simulation and measurement for the narrow band balun, as shown in Figure 5. This may be due to the slightly different reference plane adopted in the actual measurement. [9] Combining the measured results in Figures 3 5, it is observed that the proposed 180 broadband balun delivered low input port return loss (S 11 < 10 db), balanced output ports power distribution (S 21 =S 31 = 3 db (±1.0 db)), and consistent 180 (±10 ) output ports phase difference over a wide band of 44.73%, from 1.51 to 2.38 GHz; hence we term it a broadband balun. The conventional 180 narrowband balun delivered both low input port return loss and balanced output 3. Patch Antenna With a Broadband Balun 3.1. Antenna Structure [10] The single L-probe rectangular patch antenna has been found to deliver a wide impedance bandwidth (SWR < 2) of over 30% [Luk et al., 1998]. However, the L-probe feed emits probe leakage radiation which results in high cross-polarization and pattern distortion, especially in the H-plane. [11] The dual L-probe square patch antenna, as shown in Figure 6, is designed to suppress the H-plane crosspolarization. A second L-probe feed is symmetrically positioned at the opposite radiating edge (W x )ofthe patch element. Probe leakage radiation can be cancelled out by providing the two L-probe feeds with equal amplitude and 180 out-of-phase excitations. Prior to this work, a dual L-probe patch antenna using a conventional 180 narrowband balun has been reported in [Wong et al., 2004]. The use of a feed network with wideband 180 phase shifting capabilities is necessary in order for the probe leakage radiation to cancel out across the wide impedance passband (30%) afforded by the L-probe fed patch antenna. [12] We fabricated the dual L-probe single-element square patch antenna with the proposed 180 broadband balun. The antenna and feed network parameters were optimized for a wide impedance bandwidth centering 2.0 GHz. The square copper patch, of dimensions W x = W y = 53.5 mm (0.357 l 0 ), was positioned at a height above the dielectric substrate to create an air substrate of Figure 5. Simulated and measured output port phase difference comparison between the 180 narrowband and broadband baluns. 3of8

Figure 6. Dual L-probe square patch antenna. 4of8

Figure 7. Simulated and measured SWR comparison for the dual L-probe square patch antenna using either the 180 narrowband or broadband balun. thickness H = 23.5 mm (0.157 l 0 ). The feed network and square copper ground plane of length G = 250 mm (1.5 l 0 ), were respectively printed on the top and bottom of the dielectric substrate. The L-probe feeds, of diameter 2R = 1 mm, with vertical length L h = 12 mm and horizontal length L v = 26.5 mm, were positioned S = 3 mm away from the edge of the patch, and respectively soldered at the output ports of the feed network. A 50 W SMA connector was soldered at the input port of the feed network. The feed substrate used was a Rogers RO4003 laminate of dielectric constant e r = 3.38 and thickness t = 0.8 mm. 3.2. Simulated and Measured Results [13] The Agilent E8364B vector network analyzer and MiDAS far-field measurement software package were used in the impedance and radiation measurements. Figure 7 shows the simulated and measured SWR comparison for the dual L-probe square patch antenna using either baluns. The antenna with the broadband balun exhibits a wide measured impedance bandwidth (SWR < 2) of 37.15%, from 1.6 to 2.33 GHz, while the same antenna with the narrowband balun exhibits a slightly wider measured impedance bandwidth (SWR < 2) of 39.22%, from 1.64 to 2.44 GHz. The simulated and measured SWR results are in good agreement. From 1.7 to 2.3 GHz (30%), it is observed that the impedance matching is good (SWR < 2) for the antenna using either the narrowband or broadband balun. This common impedance passband, sufficient to cover the DCS 1800, PCS 1900 and UMTS 2000 bands, will be the designated bandwidth of interest when we compare the radiation performance of the antenna utilizing either baluns. [14] Figure 8 shows the simulated and measured boresight gain for the dual L-probe square patch antenna using either baluns. Within the bandwidth of interest (1.7 to 2.3 GHz), the measured boresight gain of the antenna utilizing the broadband balun ranges from 6.16 to 8.5 dbi, while that of the same antenna utilizing the narrowband balun ranges from 6.49 to 8.46 dbi. [15] Figure 9 shows the simulated radiation patterns for the dual L-probe square patch antenna using the 180 narrowband balun at the start frequency (1.7 GHz), center frequency (2.0 GHz), and end frequency (2.3 GHz) of our bandwidth of interest. Across this passband, the antenna using the 180 narrowband balun exhibits symmetrical E- and H-plane co-polarization patterns, and consistently low E-plane cross-polarization levels (< 32 db). The H-plane cross-polarization levels are generally higher than that of the E-plane and can be seen to appreciate considerably at the end frequency point (up to 11 db). [16] Figure 10 shows the simulated radiation patterns for the dual L-probe square patch antenna using the 180 broadband balun at the start frequency (1.7 GHz), center frequency (2.0 GHz), and end frequency (2.3 GHz) of our bandwidth of interest. Across this passband, the antenna using the 180 broadband balun exhibits symmetrical E- and H-plane co-polarization patterns and consistently low E- and H-plane cross-polarization levels (< 28 db). From Figure 5, it is observed that at 1.7, 2.0, and 2.3 GHz, the simulated phase shift for the narrowband balun, are respectively 210, 177, and 215, while the simulated phase shift for the broadband balun are respectively 178, 178, 180. These simulated results suggest that H-Plane cross-polarization levels can be kept sufficiently low when the phase difference between the two L-probe feeds are kept within 180 (±10 ). [17] Figure 11 shows the measured radiation patterns for the dual L-probe square patch antenna using the 180 Figure 8. Simulated and measured boresight gain comparison for the dual L-probe square patch antenna using either the 180 narrowband or broadband balun. 5of8

Figure 9. Simulated radiation pattern for the dual L-probe square patch antenna utilizing the 180 narrowband balun. narrowband balun at the start frequency (1.7 GHz), center frequency (2.0 GHz), and end frequency (2.3 GHz) of our bandwidth of interest. Figure 12 shows the measured radiation patterns for the dual L-probe square patch antenna using the 180 broadband balun at the start frequency (1.7 GHz), center frequency (2.0 GHz), and end frequency (2.3 GHz) of our bandwidth of interest. While still maintaining symmetrical E- and H-plane co-polarization patterns and considerably low E-plane cross-polarization, it is evident that the use of the broadband balun provides the added advantage of H-plane cross-polarization suppression, across the bandwidth of interest. [18] Across this passband, the antenna using the 180 broadband balun exhibits symmetrical E- and H-plane co-polarization patterns and consistently low E- and Figure 10. Simulated radiation pattern for the dual L-probe square patch antenna utilizing the 180 broadband balun. 6of8

Figure 11. Measured radiation pattern for the dual L-probe square patch antenna utilizing the 180 narrowband balun. H-plane cross-polarization levels (< 21 db). Consistent with our analysis from the simulated results, it is observed that as long as the phase difference afforded by the balun is kept within 180 (±10 ), the measured H-plane cross-polarization levels will not exceed 20 db. Table 1 provides a summary of the simulated and measured H-plane cross-polarization levels for the dual L-probe square patch antenna utilizing either baluns, across our bandwidth of interest. Both simulated and measured results indicate that, unlike the narrowband balun, the broadband balun consistently provides good H-plane cross-polarization suppression throughout the 30% passband. 4. Conclusion [19] A broadband 180 microstrip balun has been employed as a feed network for the L-probe square patch antenna. The broadband balun provides impedance matching, equal amplitude power division and consistent out-of-phase excitations over a wide band (45%). We have shown that for the dual L-probe square patch Figure 12. Measured radiation pattern for the dual L-probe square patch antenna utilizing the 180 broadband balun. 7of8

Table 1. Simulated and Measured H-Plane Cross-Polarization Comparison for the Dual L-Probe Square Patch Antenna Using Either the 180 Narrowband or Broadband Balun With 180 Narrowband Balun With 180 Broadband Balun Frequency, GHz Simulated X-Pol, db Measured X-Pol, db Simulated X-Pol, db Measured X-Pol, db 1.7 27.2 26.7 28.6 24.1 1.8 27.0 24.9 28.3 27.2 1.9 27.9 22.9 28.6 22.8 2.0 27.2 15.6 31.9 26.3 2.1 20.6 12.9 31.0 24.4 2.2 6.4 8.3 27.5 22.1 2.3 10.1 12.1 32.1 21.1 antenna, the use of the proposed 180 broadband balun as a feed network affords good H-plane cross-polarization suppression across a wide passband. In this way, we can now achieve high gain, stable and symmetrical E- and H-plane co-polarization patterns, and consistently low E- and H-plane cross-polarization levels, throughout the wide impedance bandwidth (30%) designed for L-probe patch antennas. References Chang, E., S. A. Long, and W. F. Richards (1986), Experimental investigation of electrically thick rectangular microstrip antennas, IEEE Trans. Antennas Propag., AP-34, 767 772. Chen, Z. N., and M. Y. W. Chia (2003), A novel center-fed suspended plate antenna, IEEE Trans. Antennas Propag., AP-51, 1407 1410. Guo, Y.-X., C. L. Mak, K.-M. Luk, and K. F. Lee (2001), Analysis and design of L-probe proximity fed patch antennas, IEEE Trans. Antennas Propag., AP-49, 145 149. Guo, Y.-X., K.-M. Luk, and K. F. Lee (2002), Broadband dual polarization patch element for celluar-phone base stations, IEEE Trans. Antennas Propag., AP-50, 251 253. Hall, P. S. (1987), Probe compensation in thick microstrip patches, Electron. Lett., 23, 606 607. Huynh, T., and K. F. Lee (1995), Single-layer single-patch wideband microstrip antenna, Electron. Lett., 31, 1310 1312. Khoo, K.-W., Y.-X. Guo, Z. Y. Zhang, L. C. Ong, and K.-M. Luk (2005), Dual L-probe proximity-fed rectangular patch antenna using a broadband balun feeding network, paper presented at International Workshop on Antenna Technology (IWAT05), 7 9 March, Inst. of Electr. and Electron. Eng., Singapore. Levis, K., A. Ittipiboon, and A. Petosa (2000), Probe radiation cancellation in wideband probe-fed microstrip arrays, Electron. Lett., 36, 606 607. Lo, W. K., J.-L. Hu, C. H. Chan, and K.-M. Luk (2000), Circularly polarized patch antenna with an L-shaped probe fed by a microstrip line, Microwave Opt. Technol. Lett., 24(6), 412 414. Luk, K.-M., C. L. Mak, Y. L. Chow, and K. F. Lee (1998), Broadband microstrip patch antenna, Electron. Lett., 34, 1442 1443. Mak, C. L., K.-M. Luk, K. F. Lee, and Y. L. Chow (2000), Experimental study of a microstrip antenna with an L-shaped probe, IEEE Trans. Antennas Propag., AP-48, 777 783. Petosa, A., A. Ittipiboon, and N. Gagnon (1999), Suppression of unwanted probe radiation in wideband probe-fed microstrip patches, Electron. Lett., 35, 355 357. Wong, H., K. L. Lau, and K.-M. Luk (2004), Design of dualpolarized L-probe patch antenna arrays with high isolation, IEEE Trans. Antennas Propag., AP-52, 45 52. Zhang, Z. Y., Y.-X. Guo, L. C. Ong, and M. Y. W. Chia (2005), A new wideband planar balun on a single-layer PCB, IEEE Microwave Wireless Comp. Lett., 15, 416 418. Y.-X. Guo, K.-W. Khoo, and L. C. Ong, Institute for Infocomm Research, 20 Science Park Road, #02-21/25, TeleTech Park, 117674 Singapore. (guoyx@i2r.a-star.edu.sg) K.-M. Luk, Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China. 8of8