Structural innovations in flexible antennas for bandwidth and resonant frequency enhancement

Abstract

The increasing adoption of wearable, portable, and conformal wireless communication systems has intensified the need for antennas that are lightweight, compact, and mechanically flexible, while maintaining high electromagnetic performance. Conventional rigid and bulky antennas commonly used in wireless systems are often unsuitable for wearable applications due to limited flexibility, poor conformability to the human body, and reduced user comfort. To address these limitations, this work focuses on the design, optimization, and analysis of flexible antennas using advanced flexible substrates that enable improved performance without compromising mechanical adaptability. The primary objective of this research is to enhance key antenna characteristics such as resonant frequency, impedance bandwidth, and return loss, while ensuring suitability for modern wearable and flexible communication platforms. Two flexible antenna designs are proposed and thoroughly investigated. The first design targets operation in the Industrial, Scientific, and Medical (ISM) frequency band from 5.725 to 5.875 GHz. A flexible Rogers RO4003C substrate with a dielectric constant of 3.55 and compact dimensions of 20 × 25 × 0.2032 mm³ is utilized. To improve impedance matching and bandwidth, inset slots are incorporated into both the radiating patch and the ground plane, along with carefully positioned parasitic elements. These design modifications significantly enhance antenna performance, achieving a measured return loss (S11) of −37.05 dB at 5.78 GHz and a bandwidth of 158 MHz, which covers approximately 90.67% of the ISM band, thereby supporting efficient and high-speed wireless data transmission. The second antenna design employs a flexible Thermoplastic Polyurethane (TPU) substrate, selected for its high elasticity, durability, and suitability for wearable electronics. An inset slot feed structure and a partial ground plane are introduced to improve resonant behaviour and achieve wideband operation. This antenna exhibits stable resonance across a broad frequency range from 7.5 to 10.2 GHz, demonstrating a 17.4% increase in bandwidth compared to conventional antenna configurations. Enhanced return loss is observed throughout the operating frequency range, confirming the effectiveness of the proposed structural modifications. Simulation and experimental results show strong agreement for both designs, validating the proposed methodologies. The results demonstrate that combining flexible materials with innovative antenna structures significantly improves electromagnetic performance while maintaining mechanical flexibility. These flexible antennas are therefore well-suited for next-generation wearable and wireless communication systems requiring compact, efficient, and wideband antenna solutions.