Influence of Neutral Axis on Performance Stability of Flexible Antennas for ISM Band Wearable Applications

Wearable textile antennas are commonly used in various applications nowadays, such as watches and clothes; however, they are still subjected to degradation in the reflection coefficient and shift in the resonance frequency due to bending or twisting. This study presents a neutral axis-based encapsulation technique to improve the electromagnetic stability of flexible antennas. A 2.45 GHz CPW patch antenna is designed using a polyester substrate, silver conductive layer, and TPU as encapsulation for the encapsulated antenna. By aligning the conductive layer with the neutral axis, the proposed antenna achieved stable resonance characteristics under both convex and concave bending conditions. In contrast, the non-encapsulated antenna exhibited significant resonance degradation under deformation. Comparative analysis further showed that displacing the conductive layer away from the neutral axis increased the maximum resonance frequency shift to 8.38%, whereas the neutral axis aligned configuration maintained a substantially lower shift of only 3.55%. These results validate the effectiveness of neutral axis engineering for enhancing the deformation resilience of flexible wearable antennas.