An antenna is defined as "electrically small" when its physical dimensions are considerably smaller than the wavelength of the electromagnetic waves it is designed to transmit or receive. Such antennas are advantageous for compact applications, enabling integration into space-constrained devices like smartphones and portable electronics [1]. Other studies have explored various methods such as direct ink printing [1], selective laser sintering 3D printing [2], laser cutting [3], and photolithography [4] for fabricating 3D antennas. Although these methods vary, they all rely on a support structure, either as solid or thin-film polymer substrates.
In this work, we suggest self-supporting thin film antennas without having any polymer substrate to hold the structural integrity. The antennas are fabricated with Cu-NiTi bimetallic system using photolithography, magnetron sputtering, and etching. Magnetron sputtered NiTi thin films are amorphous. Thus, a crystallization step is necessary to achieve the superelastic effect in the NiTi. This step is also used to shape set the antenna from a 2D flat state to 3D complex shape, in this study a hemisphere with a 11 mm diameter. NiTi thickness (1 µm) kept as thin as possible to just be able to allow us to shape set a regular metal Cu (25 µm) (Figure 1a). This fabrication method also offers flexibility in material choice, allowing for the use of metals like gold, silver or magnetic materials to tailor antenna performance for specific applications. Unlike traditional designs that require substrates for mechanical support, our robust self-supporting structure overcomes substrate-induced limitations, such as extra weight, bulkiness, dielectric detuning, and poor heat dissipation. The antenna simulations are performed using CST Microwave Studio (Dassault Systemes, France) to prove that our designs and fabrication method is suitable for electrically small antennas (Figure 1b to d) and they indicate their suitability for next-generation compact communication systems.

References
[1] J. J. Adams et al., Advance Materials, 2011, 23, 1335–1340
[2] M. Kong et al., Journal of Electromagnetic Engineering and Science, 2017, 17, 228–232
[3] X. Chen et al., Science Advances, 2023, 9, 0357
[4] F. Liu et al., Small, 2018, 15, 1804055
[5] D. Dengiz et al., Advance Materials Technologies, 2023, 8, 2201991
[6] P. Velvaluri et al., Scientific Reports, 2021, 11, 10988