With the rapid growth of wireless communication systems, there is a rising demand for multi-input multi-output (MIMO) antenna systems capable of adapting to various frequency bands and operating conditions. This paper presents an integrated design for MIMO antennas based on a varactor diode as a promising component for achieving frequency agility in the proposed system. A dual-polarized system is achieved by employing a combination of two antennas. One antenna is situated on the exterior surface of the side-edge frame, while the other is positioned on the substrate surface. The spatial configuration enables the creation of orthogonal polarization orientations, specifically vertical and horizontal polarizations. In each element, varactor diodes are positioned to provide reactive loading. By incorporating varactor diodes with a variable bias voltage (0.5-10 V) into the antenna design, the resonant frequency can be dynamically adjusted, allowing the antenna to operate across a wide range of frequencies (4.3 to 6.5 GHz) with more than 18 dB of mutual coupling in the working band. The presented reconfigurable antennas are printed on compact dimensions of 15 x 25 x 0.8 mm3 using a Rogers RT5880 material with a relative dielectric constant 2.2. Because of its flexible frequency range, extensive tuning range, small size, and planar structure, it is well-suited for various current and future wireless communication applications, including cognitive radio, software-defined radio, and next-generation wireless networks.
2. Tawk, Y., M. Bkassiny, G. El-Howayek, S. K. Jayaweera, K. Avery, and C. G. Christodoulou, "Reconfigurable front-end antennas for cognitive radio applications," IET Microwaves, Antennas and Propagation, Vol. 5, No. 8, 985-992, 2011.
3. Valenta, V., R. Marsalek, G. Baudoin, M. Villegas, M. Suarez, and F. Robert, "Survey on spectrum utilization in Europe: Measurements, analyses and observations," 2010 Proceedings of the 5th International Conference on Cognitive Radio Oriented Wireless Networks and Communications, CROWNCom, 2010.
4. Liu, L., S. W. Cheung, and T. I. Yuk, "Compact MIMO antenna for portable devices in UWB applications," IEEE Transactions on Antennas and Propagation, Vol. 61, No. 8, 4257-4264, 2013.
5. Zhang, S. and G. F. Pedersen, "Mutual coupling reduction for UWB MIMO antennas with a wideband neutralization line," IEEE Antennas and Wireless Propagation Letters, Vol. 15, 166-169, 2016.
6. Yang, H. H. and T. Q. S. Quek, Massive MIMO Meets Small Cell?: Backhaul and Cooperation, Springer Cham, 2017.
7. Li, H., J. Liu, Z. Wang, and Y.-Z. Yin, "Compact 1 x 2 and 2 x 2 MIMO antennas with enhanced isolation for ultrawideband application," Progress In Electromagnetics Research C, Vol. 71, 41-49, 2017.
8. Biswas, A. K. and U. Chakraborty, "Compact wearable MIMO antenna with improved port isolation for ultra-wideband applications," IET Microwaves, Antennas and Propagation, Vol. 13, No. 4, 498-504, 2019.
9. Majid, H. A., M. K. A. Rahim, M. R. Hamid, N. A. Murad, and M. F. Ismail, "Frequency-reconfigurable microstrip patch-slot antenna," IEEE Antennas and Wireless Propagation Letters, Vol. 12, 218-220, 2013.
10. Majid, H. A., M. K. Abd Rahim, M. R. Hamid, and M. F. Ismail, "Frequency reconfigurable microstrip patch-slot antenna with directional radiation pattern," Progress In Electromagnetics Research, Vol. 144, 319-328, 2014.
11. Yang, X., Y. Chen, L. Ye, M. Wang, M. Yu, and Q. H. Liu, "Frequency reconfigurable circular patch antenna using PIN diodes," 9th International Conference on Microwave and Millimeter Wave Technology, ICMMT 2016 --- Proceedings, Vol. 2, 606-608, 2016.
12. Mahlaoui, Z., E. Antonino-Daviu, A. Latif, M. Ferrando-Bataller, and C. R. Penafiel-Ojeda, "Frequency reconfigurable patch antenna using pin diodes with directive and fixed radiation pattern," 2018 International Conference on Selected Topics in Mobile and Wireless Networking, MoWNeT, 2018.
13. Madhav, B. T. P., M. Monika, B. M. S. Kumar, and B. Prudhvinadh, "Dual band reconfigurable compact circular slot antenna for WiMAX and X-band applications," Radioelectronics and Communications Systems, Vol. 62, No. 9, 474-485, 2019.
14. Zhou, J., M. Yang, and J. Yu, "Pattern reconfigurable patch antenna with dual band characteristic for WLAN & 5G applications," Progress In Electromagnetics Research M, Vol. 98, 147-158, 2020.
15. Diallo, K., A. Diallo, I. Dioum, S. Ouya, and J. M. Ribero, "Design of a dual-band antenna system for lTE-M and LTE-MIMO by exploiting the characteristic mode theory," Progress In Electromagnetics Research M, Vol. 93, 11-21, 2020.
16. Sailaja, B. V. S. and K. K. Naik, "Design of elliptical-shaped reconfigurable patch antenna with shunt capacitive RF-MEMS switch for satellite applications," International Journal of Microwave and Wireless Technology, Vol. 13, No. 9, 969-978, 2021.
17. Xu, Z. Q., Y. T. Sun, Q. Q. Zhou, Y. L. Ban, Y. X. Li, and S. S. Ang, "Reconfigurable MIMO antenna for integrated-metal-rimmed smartphone applications," IEEE Access, Vol. 5, 21223-21228, 2017.
18. Choi, J., W. Hwang, C. You, B. Jung, and W. Hong, "Four-element reconfigurable coupled loop MIMO antenna featuring LTE full-band operation for metallic-rimmed smartphone," IEEE Transactions on Antennas and Propagation, Vol. 67, No. 1, 99-107, 2019.
19. Hassan, M. M., Z. Zahid, A. A. Khan, I. Rashid, A. Rauf, and F. A. Bhatti, "Two element MIMO antenna with frequency reconfigurable characteristics utilizing RF MEMS for 5G applications," Journal of Electromagnetic Waves and Applications, Vol. 34, No. 9, 1210-1224, 2020.
20. Shruthi, G., C. Y. Kumar, and , "Dual-band frequency-reconfigurable MIMO PIFA for LTE applications in mobile hand-held devices," IET Microwaves, Antennas and Propagation, Vol. 14, No. 5, 419-427, 2020.
21. Sharma, K. and G. P. Pandey, "Two port compact MIMO antenna for ISM band applications," Progress In Electromagnetics Research C, Vol. 100, 173-185, 2020.
22. Dou, Y., Z. Chen, J. Bai, Q. Cai, and G. Liu, "Two-port CPW-fed dual-band MIMO antenna for IEEE 802.11 a/b/g applications," International Journal of Antennas and Propagation, Vol. 2021, 2021.
23. Kareem, Q. H. and M. J. Farhan, "Compact dual-polarized eight-element antenna with high isolation for 5G mobile terminal applications," International Journal of Intelligent Engineering and Systems, Vol. 14, No. 6, 187-197, 2021.
24. Balanis, C. A., Antenna Theory: Analysis and Design, John Wiley & Sons, 2016.