International Journal of Reconfigurable and Embedded Systems (IJRES) Vol. 12, No. 2, July 2023, pp. 186~194 ISSN: 2089-4864, DOI: 10.11591/ijres.v12.i2.pp186-194  186 Journal homepage: http://ijres.iaescore.com S11 parameter results comparison in reconfigurable antennas under simulation and measurement V. Reji 1 , C. T. Manimegalai 2 1 Department of Electronics and Communication Engineering, SRM Institute of Science and Technology, Ramapuram, India 2 Department of Electronics and Communication Engineering, SRM Institute of Science and Technology, SRM Nagar, Kattankulathur, India Article Info ABSTRACT Article history: Received Aug 11, 2022 Revised Dec 10, 2022 Accepted Jan 25, 2023 In this paper, a simulation and measurement return loss parameter results comparison in frequency reconfigurable antenna is proposed. More low- profile and compact microstrip antennas have been developed in recent years for 5 GHz, 5G, WLAN, Wi-Fi, and ISM band applications. These antenna frequency bands may be single, dual, or multiband. The small microstrip antenna, without connecting any external devices like switches, resonators, and passive elements, does not show any variations in their simulation and measurement results like return loss (S11 parameter), gain, and efficiency. However, in the S11 parameter most frequency reconfigurable antennas show a mismatch between simulation and measurement results. The reason for this mismatch between the simulation and measurement results are given in the paper. Keywords: Bias Diode Microstrip Substrate Switch This is an open access article under the CC BY-SA license. Corresponding Author: C. T. Manimegalai Department of Electronics and Communication Engineering, SRM Institute of Science and Technology Katankulathur, Chennai, Tamil Nadu, India Email: manimegc@srmist.edu.in 1. INTRODUCTION Reconfigurable antennas are attractive solutions for cognitive radio and multiband applications with different time slots. In the early days, to change the frequency bands and shift the radiation pattern of the antenna, some mechanical parts were used in the reconfigurable antenna. These mechanical parts were controlled manually, they had little influence on the antenna parameters. Later, very small microstrip antennas were developed to connect easily with portable electronic devices for reducing the complexity of big antenna structures. Recently, more microstrip reconfigurable antennas have been developed in small size for changing the frequency, pattern, and polarization of the antenna without connecting the big size mechanical rotary parts. But most of the frequency reconfigurable antennas are designed with different diode switches to shift the characteristics of the antenna. These diode switches can be connected easily with the substrate of the microstrip antennas, they are controlled by an externally applied DC bias voltage because the switches are commercially available in the market. Most of the frequency reconfigurable antennas have shown acceptable results in simulation and measurement of their gain and efficiency values, but they showed more variation in the return loss parameters of the antenna. The return loss parameter of the antenna is also called the reflection co-efficient or S11 parameter. This parameter is more significant in the antenna design because it shows how much input power is accepted by the antenna. Based on the return loss (S11) value, the antenna gives its gain and efficiency. The gain of the antenna is the ratio of radiation intensity of the antenna to the total input power applied to the antenna and the efficiency is the ratio of the radiation intensity of the antenna to the total input power accepted by the