A Wide-Band Dual-Polarized Reconfigurable SlotRing Antenna/Array with A Compact CPW Feeding Structure Junyi Huang, Mahmoud Shirazi, and Xun Gong Department of Electrical & Computer Engineering University of Central Florida, Orlando, FL 32816, USA [email protected], [email protected] Abstract—A wide-band dual-polarized reconfigurable slotring antenna/array using a compact CPW feeding structure is presented. By using a CPW feeding mechanism, the mutual coupling between the two orthogonal feeding lines is reduced significantly compared to the traditional microstrip feeding method while maintaining the antenna bandwidth. Furthermore, the CPW feeding network is more compact than the microstrip feeding network which is an important factor in large antenna arrays. Port 2 Port 1 Keywords—CPW feeding structure; dual polarized antenna; reconfigurable antenna; slot-ring antenna; wide-band antenna; I. INTRODUCTION With significant advancements in modern wireless communication systems, antennas with polarization diversity and multi-functional capabilities are in increasing demand. A multi-polarized antenna has the ability to increase communication capacity and is suitable for mitigating fading in a multipath propagation environment. On the other hand, dualband antennas with wide bandwidth in each mode of operation can realize the functions of several independent antennas. Therefore, a wide-band antenna with both dual-polarization and dual-band capabilities will significantly advance the state of the art. Slot antennas are an ideal choice for a dual-band, dualpolarized antenna because they have a planar feeding structure, a wide bandwidth, high isolation, and can be extended to an antenna array . A compact dual-band, dual-polarized loopslot planar antenna was presented in ; a coplanar waveguide (CPW) was used to excite the vertical polarization modes and a microstrip line was used to excite the horizontal polarization modes to obtain dual-polarized performance. In , a wide-band singly-polarized reconfigurable slot-ring antenna/array is presented. This antenna has several advantages: a wide bandwidth in each operating mode, low power consumption, and ease of fabrication. A dual-polarized slot antenna was demonstrated in  that used a compact CPW feeding method to use even and odd modes to excite two orthogonal polarizations in a slot antenna, demonstrating improved isolation between the two feeding lines. In this paper, the same antenna aperture as described in  is used to implement a dual-band, dual-polarized antenna with reduced mutual coupling between the orthogonal feeding lines. Other feed lines are used for the C-band operating mode Two orthogonal feed lines for the S-band operating mode Fig. 1. Schematic view of the wideband dual-polarized, dual-band slot-ring antenna with microstrip feedlines. S Parameter 0 -10 S11 S12 -20 4 5 6 7 8 Frequency(GHz) Fig. 2. S-parameters of the wideband dual-polarized, dual-band slot-ring antenna with microstrip feedlines A compact CPW line is used to feed the wideband slot-ring antenna array to obtain dual-polarization. By exploiting the even and odd modes of the CPW line, the mutual coupling between the two orthogonal feeding lines is decreased while the antenna bandwidth is maintained. The project is supported by the Defense Advanced Research Projects Agency (DARPA) ACT Program under Grant HR0011-14-1-0003. 978-1-5386-3284-0/17/$31.00 ©2017 IEEE 1329 AP-S 2017 II. WIDE-BAND DUAL-POLARIZED RECONFIGURABLE SLOT-RING ANTENNA/ARRAY A. Traditional Microstrip Feeding Structure A dual-polarized version of the same antenna aperture described in  is illustrated in Fig. 1. Two operating modes are defined for this antenna: the S-band and C-band operating modes. The S-parameters of the antenna in the C-band operating mode are shown in Fig. 2. It is seen that the return loss of port 1 is better than 10 dB from 5.3 GHz to above 8 GHz; however, isolation between ports 1 and 2 is as low as 3.5 dB over the frequency range. The horizontal polarization is not well isolated from the vertical polarization, particularly in the C-band operating mode, because the microstrip feed lines which feed the antenna from the horizontal and vertical ports are physically close to each other. Therefore, this antenna suffers from high mutual coupling between two orthogonal feed lines and the worst in-band mutual coupling is approximately 3.5 dB. In the next section, a new feeding structure is applied to the antenna array to mitigate the isolation problem. B. Compact CPW Feeding Structure Fig. 3 shows the geometry and dimensions of the compact CPW feeding structure on the wide-band dual-polarized slotring antenna/array. A 50-Ω CPW feed line is used to excite the 2.66 1 6.65 4.3 Port 1 Port 2 Microstrip Feeding CPW Feeding (a) III. RESULT AND DISCUSSION Fig. 4(a) depicts the return loss of ports 1 (microstrip feed line) and 2 (CPW feed line) for the antenna in Fig. 3. When the antenna is excited from port 1, ripples near 6.0 and 7.5 GHz are observed in the return loss. Without considering these ripples, the bandwidth of the antenna with the CPW feeding structure is 55% and 35% when excited from ports 1 and 2, respectively. Compared to the bandwidth of the antenna with microstrip feeding, the bandwidth is enhanced by 15% for the horizontal polarization (the antenna is excited from port 1) and decreased by 4% when the antenna is vertically polarized (the antenna is excited from port 2). Since the same antenna aperture as shown in Fig. 1 is now fed with the compact CPW feeding structure, minor errors will arise because the antenna geometry has been optimized to use the microstrip feeding mechanism. If the antenna geometry is optimized to use the new CPW feeding structure, better results can be achieved. Fig. 4(b) demonstrates the mutual coupling between port 1 and port 2 for the two feeding methods. The red line shows the isolation between the two orthogonal ports of the antenna with microstrip feed lines while the black line illustrates the isolation between two orthogonal feed lines for the antenna with the compact CPW feeding structure; it is seen that the mutual coupling is improved by more than 7 dB from 4 to 6.5 GHz. The lowest isolation between ports 1 and 2 in the microstrip design is 3.5 dB, while the worst isolation between ports 1 and 2 using the CPW feeding structure is 8.5 dB, demonstrating an improvement of 5 dB. IV. CONCLUSION (b) A compact CPW feeding structure has been proposed to feed a wide-band, dual-polarized slot-ring antenna/array to improve isolation between two orthogonal ports and reduce the size of the feeding network. A previously described microstrip feeding method was compared with the proposed CPW feeding structure on the same antenna aperture and it was shown that the compact CPW feeding structure reduced the mutual coupling between the two orthogonal feed lines by more than 5 dB without degrading the bandwidth performance of the antenna. Fig. 3. (a) The close-up view of the CPW compact feeding structure (b) Schematic view of wideband dual-polarized antenna array with compact CPW feeding structure (All dimensions are in mm). 0 0 S12 of New Feed S12 of Former Feed S Parameter S11 S22 S Parameter independent polarizations. When port 1 is excited, the CPW feed line operates in the odd mode and the antenna is horizontally polarized. Conversely, when port 2 is excited, the CPW feed line is in its even mode and vertical polarization is observed. The length and vertical position of the microstrip feed line and CPW line are optimized to obtain a wide bandwidth. -10 -10 REFERENCES  -20 4 5 6 Frequency(GHz) (a) 7 8 -20 4 5 6 7 8 Frequency(GHz)  (b) Fig. 4. (a) Return loss of port 1 and port 2 of the wideband dual-polarized antenna array with CPW compact feeding structure (b) Mutual coupling between two orthogonal feeding lines of new and former feeding structures.  vertical polarization while the orthogonal microstrip feed lines on the back side of the antenna excite the horizontal polarization. As with the previously described microstrip feeding mechanism, the compact CPW feeding structure also supports two  1330 C. R. White and G. M. Rebeiz, "Single- and Dual-Polarized Tunable SlotRing Antennas," in IEEE Transactions on Antennas and Propagation, vol. 57, no. 1, pp. 19-26, Jan. 2009. M. T. Tan and B. Z. Wang, "A Compact Dual-Band Dual-Polarized LoopSlot Planar Antenna," in IEEE Antennas and Wireless Propagation Letters, vol. 14, no. , pp. 1742-1745, 2015. M. Shirazi and X. Gong, "Singly-polarized reconfigurable slot-ring antenna/array with fractal shapes," 2016 IEEE International Symposium on Antennas and Propagation (APSURSI), Fajardo, 2016, pp. 843-844. Yue Li, Zhijun Zhang, Wenhua Chen, Zhenghe Feng, Magdy F.Iskander, “A Dual-Polarization Slot Antenna Using a Compact CPW Feeding Structure,” IEEE Antennas and Wireless Propagation Letters , vol. 9, pp. 191–194, March 2010.