Two small size multiple-input-multiple-out (MIMO) antennas with high isolation for ultrawideband (UWB) applications are presented. A two-element MIMO antenna, which is mounted on an FR4 substrate with a compact size of 24 mm × 33 mm, consists of two symmetric circular monopole elements and a modified ground. The protruded ground provides a way to improve isolation and impedance matching. Such a wide band from 2.75 to 11 GHz is achieved by using modified ground technology, and high isolation more than 20 dB is also accomplished. Meanwhile, moderate gain and omnidirectional radiation patterns can be obtained. Based on the circular monopole with modified ground, a four-element antenna array is also constructed and studied. The size of the four-element antenna with orthogonal arrangement is 44 mm × 44 mm. Measured results show that the antenna also exhibits good impedance matching as well as low envelope correlation coefficient over the entire UWB spectrum.
2. Wallace, J. W., M. A. Jensen, A. L. Swindlehurst, and B. D. Jeffs, "Experimental characterization of the MIMO wireless channel: Data acquisition and analysis," IEEE Trans. Wirel. Commun., Vol. 2, 335-343, 2003.
3. Malik, J., A. Patnaik, and M. V. Kartikeyan, "Novel printed MIMO antenna with pattern and polarization diversity," IEEE Antennas Wireless Propag. Lett., Vol. 14, 739-742, 2015.
4. Zhang, S., B. K. Lau, A. Sunesson, and S. L. He, "Closely-packed UWB MIMO/Diversity antenna with different patterns and polarizations for USB dongle applications," IEEE Trans. Antennas Propag., Vol. 60, 4372-4380, 2012.
5. Zhang, S., Z. N. Ying, J. Xiong, and S. L. He, "Ultrawideband MIMO/Diversity antennas with a tree-like structure to enhance wideband isolation," IEEE Antennas Wireless Propag. Lett., Vol. 8, 1279-1282, 2009.
6. Khan, M. S., A. D. Capobianco, A. I. Najam, I. Shoaib, E. Autizi, and M. F. Shafique, "Compact ultra-wideband diversity antenna with a floating parasitic digitated decoupling structure," IET Microw. Antennas Propag., Vol. 8, 747-753, 2014.
7. Khan, M. S., M. F. Capobianco, A. D. Shafique, B. Ljaz, A. Naqvi, and B. D. Braaten, "Isolation enhancement of a wideband MIMO antenna using a floating parasitic elements," Microw. Opt. Technol. Lett., Vol. 57, 1677-1682, 2015.
8. Liu, L., S. W. Cheung, and T. I. Yuk, "Compact MIMO antenna for portable devices in UWB applications," IEEE Trans. Antennas Propag., Vol. 61, 4257-4264, 2013.
9. Chako, B. P., G. Augustin, and T. A. Denidni, "Uniplanar polarisation diversity antenna for ultrawideband systems," IET Microw. Antennas Propag., Vol. 7, 851-857, 2013.
10. Ren, J., W. Hu, Y. Z. Yin, and R. Fan, "Compact printed MIMO antenna for UWB applications," IEEE Antennas Wireless Propag. Lett., Vol. 13, 1517-1520, 2014.
11. Mao, C. X. and Q. X. Chu, "Compact coradiator UWB-MIMO antenna with dual polarization," IEEE Trans. Antennas Propag., Vol. 62, 4474-4480, 2014.
12. Srivastava, G. and B. K. Kanuijia, "Compact dual band-notched UWB MIMO antenna with shared radiator," Microw. Opt. Technol. Lett., Vol. 57, 2886-2891, 2015.
13. Liu, Y. F., P. Wang, and H. Qin, "Compact ACS-fed UWB antenna for diversity applications," Electron. Lett., Vol. 50, 1336-1338, 2014.
14. Vaughan, R. G. and J. B. Andersen, "Antenna diversity in mobile communications," IEEE Trans. Veh. Technol., Vol. 36, 149-172, 1987.
15. Blanch, S., J. Romeu, and I. Corbella, "Exact representation of antenna system diversity performance from input parameter description," Electron. Lett., Vol. 39, 705-707, 2003.
16. Lin, S.-Y. and H.-R. Huang, "Ultra-wideband MIMO antenna with enhanced isolation," Microwave and Optical Technology Lett., Vol. 51, 570-573, 2009.
17. Khan, M. S., A.-D-C. S. Asif, A. Iftikhar, and B. D. Braaten, "A 4 element compact ultra-wideband MIMO antenna array," 2015 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, 2305-2306, 2015.
18. Khan, M. S., A. D. Capobianco, S. Asif, A. Iftikhar, B. Ijaz, and B. D. Braaten, "Compact 4 × 4 UWB-MIMO antenna with WLAN band rejected operation," Electron. Lett., Vol. 51, 1048-1050, 2015.
19. Kiem, N. K., H. N. B. Phuong, and D. N. Chien, "Design of compact 4×4 UWB-MIMO antenna with WLAN band rejection," Int. J. Antennas Propag., 1-11, 2014.