This article studies security aspects of data transmission in on-body Internet of Things (IoT) devices through the use of Directional Modulation (DM) from a smart watch integrated antenna. DM is a beamsteering-based Physical Layer Security (PLS) method used to transmit baseband constellations to known secure locations, while simultaneously scrambling them in other directions. However, in the state-of-the-art DM is implemented using large arrays, which are out-sized for many on-body IoT devices. To overcome this challenge, this study proposes multimodal ultra-thin compact antennas suitable for integration into smart watches. These antennas present advanced beamsteering capabilities that allow the implementation of DM from a low-thickness structure of up to 0.57 mm or 1/100th of wavelength at the center operating frequency. Two different DM implementations
(simultaneous multiport transmission and an energy-efficient single-port transmission) are tested using a multi-layer phantom. Overall, secure steerable transmissions are realized, and experimental verifications are carried out to validate the proposed concept. Ultimately, this article demonstrates the feasibility of applying new energy-efficient and low-cost security techniques to enhance the privacy of resource-constrained IoT devices.Â