Miniaturized UWB antenna probe for UAV-compatible ionic-solution sensing via near-field reflection and far-field transmission

Abstract This paper presents a compact approach for microwave sensing of ionic solutions using near-field reflection and far-field transmission measurements with a miniaturized ultra-wideband (UWB) planar antenna probe designed for unmanned aerial vehicle (UAV) platforms. The proposed probe features a low-profile structure (41 mm × 32 mm × 0.813 mm) facilitating integration with UAV platforms. By incorporating branch structures into a split-ring resonator (SRR), the probe achieves a broad measured impedance bandwidth (2.9–13.04 GHz, S₁₁ < -10 dB).The sensing capability of the proposed probe is experimentally evaluated using NaCl and KCl solutions with different concentrations through two complementary measurement modes: near-field reflection based on S11 and far-field transmission based on S21. Both solution types exhibit clear concentration-dependent variations in the measured S-parameters, demonstrating that the sensing response is not limited to a single type of ionic solution. To evaluate concentration discrimination and measurement repeatability, five repeated measurements were conducted for each NaCl concentration. For the tested concentration interval of 0.5 percentage points, changes of 2.508 dB in S11 at 7.5 GHz and 1.812 dB in S21 at 10.7 GHz were observed, exceeding the corresponding measurement variations and demonstrating distinguishable responses for the smallest tested concentration interval of 0.5 percentage points under the present laboratory conditions. Furthermore, simulations indicate that mounting the probe on a UAV carbon fibre arm causes limited changes in radiation characteristics, while measurements show inter-element coupling below −30 dB in the two-antenna configuration. The agreement between simulated and measured antenna reflection responses, together with the laboratory-based dual-mode sensing experiments, supports the potential of the proposed probe for future UAV-integrated microwave sensing and broadband wireless communication applications.