M. C. Lim, S. K. A. Rahim, M. R. Hamid, P. J. Soh, M. I. Sabran, A. A. Eteng, and M. R. Ramli. “Single-, Dual- and Wide-Band Frequency-Reconfigurable Antenna With Annular Ring Slots”. Applied Computational Electromagnetics Society Journal (ACES), vol. 32, no. 11, July 2021, pp. 987-93, https://journals.riverpublishers.com/index.php/ACES/article/view/9433.