Introduction
In modern communication systems, Multiple-Input and Multiple-Output (MIMO) antennas are used for receiving and sending many data signal instantaneously over the same radio channel [Reference Malik, Patnaik and Kartikeyan1–6]. The pattern diversity antennas can discriminate a large portion of the angular space, while the polarization diversity is useful for immunizing the system from polarization mismatch, which could be a source of fading in the signal. Isolation between the antennas, working at the same frequency, is a critical factor to consider when designing the MIMO system. Several methods have been suggested to improve the isolation, albeit at the cost of enlarged size and added design complexity of the structure [Reference Malik, Patnaik and Kartikeyan1, Reference Sharma, Sarkar, Saurav and Srivastava2, Reference Yang, Liu, Zhou and Cui7–Reference Alibakhshikenar, Khalily, Virdee, See, Abd-Alhameed and Limiti9]. Although most of the MIMO antennas are designed in the WLAN band (2.4/5.8 GHz) [Reference Malik, Patnaik and Kartikeyan1–Reference Malik, Nagpal and Kartikeyan5], other designs have also been reported in the UWB band [Reference Zhu, Li, Feng and Yin6, Reference Mao and Chu10–Reference Liu, Zhao, See and Chen13]. In some scenarios, one antenna works with linear polarization (LP), while the other operates with circular polarization (CP) [Reference Malik, Patnaik and Kartikeyan1, Reference Sharma, Sarkar, Saurav and Srivastava2]. Furthermore, antennas could be LP [Reference Malik, Nagpal and Kartikeyan5, Reference Mao and Chu10–Reference See and Chen12, Reference Saurav, Mallat and Antar14–Reference Ghosh, Tran and Tho17]. However, both antennas are often chosen to be circularly polarized for a robust system [Reference Kharche, Reddy, Gupta and Mukherjee3, Reference Das, Sharma and Gangwar4, Reference Jalali, Naser-Moghadasi and Sadeghzadeh18]. These antennas are suffered from a narrow CP or axial ratio band. To discriminate large portion of the signal and make use of angular space pattern diversity is exploiting [Reference Malik, Patnaik and Kartikeyan1, Reference Sharma, Sarkar, Saurav and Srivastava2, Reference Malik, Nagpal and Kartikeyan5, Reference Zhu, Li, Feng and Yin6, Reference Chen, See and Qing11, Reference See and Chen12, Reference Saurav, Mallat and Antar14, Reference Ghosh, Tran and Tho17]. Some of the antennas are designed for UWB band with pattern diversity and isolation is improved by using stub in the ground or simple metallic line between the radiating elements [Reference Chen, See and Qing11, Reference See and Chen12, Reference Ghosh and Parui19], while these antennas are linearly polarized. Various types of EBG and metamaterial structures have been employed to reduce the mutual coupling between the radiating elements; this would result in large size and complex structure [Reference Yang, Liu, Zhou and Cui7–Reference Alibakhshikenar, Khalily, Virdee, See, Abd-Alhameed and Limiti9].
This work describes the design of a wideband LHCP/RHCP MIMO antenna in the X-band, which exhibits both the pattern and polarization diversity. Wideband CP with both pattern and polarization diversity is realized by inserting single stub in the ground between the two radiating elements, and this stub also provides high isolation between them. The 3 dB axial-ratio bandwidth (ARBW) of the antenna is 2.45 GHz (8.11–10.56), its impedance matching bandwidth (IMBW) is 3.52 GHz (8.07–11.59), and its isolation is better than 20 dB. All the simulations have been carried out by using the 3D EM simulator ANSYS HFSS.
Antenna design
The proposed MIMO antenna has been designed on low-cost substrate FR-4 having ε r = 4.4, loss tangent = 0.02, and thickness h = 0.8 mm. The detailed dimension with the layout and fabricated antenna structure is shown in Fig. 1. Antenna design starts from the rectangular patch which is step-1 (antenna-1), and then by truncating diagonally from the upper side of the antenna-1 which is step-2 (antenna-2). Next, we truncate the lower portion of the patch to obtain the antenna-3 (step-3), which provides impedance matching from 8.6 to 11.5 GHz, and narrow band CP at center frequency 10.92 GHz. Stub in the ground plane further improves the axial ratio band as well as impedance matching band, which is step-4 (antenna-4). For MIMO application, another similar antenna is placed on the other side of the stub, which is step-5 (final antenna-5). The main objective of the stub in the ground plane is to provide wideband CP, pattern diversity, and high isolation between the two antennas. Proposed MIMO antenna has two monopole antennas, which are facing each other with a small spacing of 5 mm (≈ 0.16λ 0 at f c = 9.83 GHz). Both the antennas have common ground, and both are fed by microstrip line of width (W f) and length (L f). The dimensions of the proposed antenna structure are as follows: W = 29 mm, L = 23 mm, W 1 = 12 mm, L 1 = 12.5 mm, W s = 3.5 mm, L s = 11 mm, L g = 9 mm, W f = 1.4 mm, L f = 9.5 mm, and L t = 10.5 mm. These diagonal truncation and ground stub change the radiation pattern to directional pattern. Since these antennas are facing each other, hence their radiation patterns are in opposite direction thus pattern diversity is observed. The final antenna is antenna-5, which is designed in five systematic steps, illustrated in detail in Fig. 2.
Fig. 1. Proposed antenna structure. (a) Layout with dimensions; (b) top view; (c) bottom view.
Fig. 2. Design steps of the proposed MIMO antenna. (a) Step-1; (b) step-2; (c) step-3; (d) step-4; (e) step-5.
Simulated performances of the proposed antennas (from antenna-1 to antenna-5) are compared in Fig. 3. The IMBW (S 11) starts improving from antenna-3 to antenna-5, while the axial ratio starts improving from antenna-2 to antenna-5.
Fig. 3. Simulated return loss and axial ratio with design steps. (a) Return loss; (b) axial ratio.
Antenna-4 and antenna-5 have IMBW as 3.52 GHz (8.07–11.59 GHz) while ARBW is 2.45 GHz (8.11–10.56 GHz), which is wideband, shown in Figs 3(a) and 3(b).
To realize the CP, we need to excite simultaneously two orthogonal horizontal mode and vertical mode with a 90o phase difference (PD). To understand the generation of CP, the two electric far-field components (E X and E Y) should have the same amplitudes but with 90o PD. The amplitude ratio of the electric far-field components (E X/E Y) and their PD from step-1 to step-5 are illustrated in detail in Figs 4(a) and 4(b). The electric far-field components (E X and E Y) of antenna-4 and antenna-5 have identical amplitude and both have 90o PD from 8.11 to 10.56 GHz, as may be seen in Fig. 4 while antenna-1 to antenna-3 have no identical amplitude and 90o PD at the same time. The relationship between the far-field components demonstrates the generation of CP in the entire axial ratio band.
Fig. 4. Electric field components magnitude (E X/E Y)/phase difference. (a) Magnitude of (E X/E Y); (b) phase difference.
Parametric study
ARBW is mainly controlled by the truncation of lower side length L t of step-4 shown in Figs 1(a) and 2(d). The ARBW starts improving with an increase in length L t of the truncation. There is no significant effect on the IMBW while ARBW is improved with length L t, which is presented in Fig. 5. Truncation in the lower side excites simultaneously two orthogonal horizontal mode and vertical mode with a 90o PD. This can be understood with the help of two electric far-field components (E X and E Y), which have the same amplitudes with 90o PD for L t = 10.5 mm within the entire axial ratio frequency band shown in Fig. 4. Hence these two electric far-field components (E X and E Y) are responsible for the generation of CP.
Fig. 5. Effect of truncation length L t of step-5. (a) Return loss; (b) axial ratio.
Simulated and measured results
Both simulated and measured results are compared in Fig. 6. It can be noted that the isolation is high which can be seen in Fig. 6(a). It is better than −20 dB, obtained without employing extra decoupling structure. The wideband CP, pattern diversity, and isolation are achieved by simply adding one stub in the ground plane which can be seen in Fig. 2(d). There is no electric field coupling between both the antennas, hence it provides a pretty well pattern diversity within the axial ratio band as shown in Fig. 7. Both measured and simulated ARBW is 2.45 GHz (8.11–10.56) while its IMBW is 3.52 GHz (8.07–11.59) as shown in Figs 6(b) and 6(a).
Fig. 6. Simulated and measured return loss, axial ratio and gain. (a) Return loss; (b) gain and axial ratio.
Fig. 7. Radiation pattern at 8.4, 9.2, 10 GHz showing pattern diversity. (a) Excitation at port-1; (b) excitation at port-2.
The simulated and measured gain of the antenna varies from 3 to 4.8 dBi as shown in Fig. 6(b). Figure 8(a) shows the gain measurement in the anechoic chamber. The radiation efficiency is varying from 85 to 93%, which is high within the whole ARBW as shown in Fig. 8(b). This indicates that the proposed antenna is highly efficient.
Fig. 8. Gain measurement and efficiency. (a) Gain measurement; (b) radiation efficiency.
Isolation between two monopoles is better than −20 dB, it further improves with frequency. The 2D simulated radiation pattern shows the pattern diversity phenomena in the proposed antenna structure. Figure 9 shows the simulated and measured radiation pattern for 8.4 GHz which illustrates the pattern diversity. The simulated surface current distribution on the surface of antenna-A and antenna-B is shown in Fig. 10.
Fig. 9. Simulated and measured radiation pattern at 8.4 GHz showing pattern diversity. (a) Excitation at port-1; (b) excitation at port-2.
Fig. 10. Surface current distribution at 8.4 GHz. (a) ωt = 0°; (b)ωt = 90°; (c) ωt = 180°; (d) ωt = 270°.
The rotating surface current verifies the CP of both the antennas, when each port of the antennas is excited individually. Antenna-A radiates LHCP while antenna-B radiates RHCP when both of them excited separately. Simulated and measured LHCP/RHCP radiation patterns are at 8.4 and 10 GHz, showing polarization diversity shown in Fig. 11. The proposed antenna radiates LHCP wave when port-1 is excited while it radiates RHCP wave when port-2 is excited.
Fig. 11. Simulated and measured LHCP/RHCP radiation pattern at 8.4 and 10 GHz, showing polarization diversity. (a) Excitation at port-1 (8.4 GHz); (b) excitation at port-2 (8.4 GHz); (c) excitation at port-1 (10 GHz); (d) excitation at port-2 (10 GHz).
To estimate the diversity performance of the proposed MIMO antenna, the envelop correlation coefficient (ECC), Diversity Gain (DG) and total active reflection coefficient (TARC) are calculated. Figure 12 shows simulated ECC using far-field pattern, DG, and TARC. ECC mostly preferred to find the correlation in the received signals while DG provides an enhancement in the system with several antennas compared to a system with single antenna. ECC is calculated from the far-field pattern. The ECC of the proposed antenna is nearly equal to zero while the acceptable ECC is <0.5 and DG is 10 dB, which are shown in Fig. 12(a). TARC plot is shown in Fig. 12(b), which shows stable characteristics as high isolation observed between port-1 and port-2. Hence our antenna has acceptable ECC, DG, and TARC. Our proposed antenna is compared in Table 1 with the earlier published works, which shows its performance is comparable. It has wide band axial ratio and IMBW with polarization and pattern diversities. The efficiency of the proposed antenna varies from 85 to 93%.
Fig. 12. Simulated and measured performances. (a) ECC and DG; (b) TARC.
Table 1. Performance comparison
Conclusion
A wideband circularly polarized MIMO antenna with pattern and polarization diversity is presented in this paper. The ARBW is 2.45 GHz (8.11–10.56) while its IMBW is 3.52 GHz (8.07–11.59). The proposed antenna structure is simple and useful for X-band, and it has both pattern diversity and polarization diversity within the entire axial ratio band. The isolation without using extra decoupling structure between the antennas is better than −20 dB. The performance of the proposed antenna has been compared with those of the recently published works and is shown to be favorable. Compared to legacy circularly polarized MIMO antennas, the proposed antenna has better impedance matching, 3 dB ARBWs, and both pattern and polarization diversity. The simulated and measured results verify the design concept.
Prashant Chaudhary received the B.Sc. (Hon.) degree in Electronics and the M.Sc. degree in Electronics in 2015 and 2017 from the University of Delhi, Delhi, India. He is doing Ph.D. from the Department of Electronic Science, Delhi University. His research interests include planar antennas, MIMO, circularly polarized antennas, and metamaterial. He is a student member of IEEE. He published six research papers in journal and conferences.
Ashwani Kumar received his B.Sc. (Hon.) degree in Electronics, M.Sc. degree in Electronics, M.Tech. degree in Microwave Electronics, and Ph.D. degree in 2000, 2004, 2006, and 2014, respectively, from the University of Delhi, Delhi, India. He was with the Department of Electrical and Computer Engineering, University of Central Florida, Orlando, Florida, USA, for his Post-doctoral research from 2016 to 2017. Currently, he is an Assistant Professor at the Department of Electronics, Sri Aurobindo College, University of Delhi, Delhi, India. His current research interests include design and development of microwave passive components such as microstrip filters, dielectric resonator-based filters, MIMO antenna, UWB antenna, and circularly polarized antennas using metamaterial. He is a Member of IEEE Microwave Theory and Techniques Society. He has published 60 journal and conference technical papers on filters and antennas.
Binod Kumar Kanaujia is working as a Professor in SCIS, Jawaharlal Nehru University, New Delhi since August 2016. Before joining JNU, he had been in the Department of Electronics and Communication Engineering in Ambedkar Institute of Advanced Communication Technologies and Research, as a Professor since February 2011. Dr. Kanaujia had completed his B.Tech. degree in Electronics Engineering from KNIT Sultanpur, in 1994. He did his M.Tech. and Ph.D. degrees in 1998 and 2004, respectively, from the Department of Electronics Engineering, IIT BHU, India. He has a keen research interest in design and modeling of microstrip antenna, DRA, metamaterial antenna, shorted microstrip antenna, UWB antennas, and reconfigurable and circularly polarized antennas. He has published more than 275 research papers in several peer-reviewed journals and conferences. He had supervised 50 M.Tech. and 17 Ph.D. research scholars in the field of microwave engineering. Dr. Kanaujia had successfully executed five research projects of Government of India, i.e. DRDO, DST, AICTE, and ISRO.
