Impedance characterization of Laser-Induced Graphene (LIG) at X and Ku bands for low-profile and flexible microwave structures

Laser-induced graphene (LIG) is emerging as a sustainable and cost-effective alternative to conventional metallic conductors for microwave applications. However, the understanding of its electrical behavior at high frequencies remains limited. This work presents a comprehensive analysis of the surface impedance of LIG traces in the X (8–12 GHz) and Ku (12–18 GHz) frequency bands, by considering three different sets of laser parameters and correlating the measured electrical properties with the morphological features of the conductor. Results show that, for all the cases considered, the LIG can be modeled as a purely resistive sheet up to 18 GHz, and thus, the surface resistance remains close to its dc value when beam defocusing is applied. Conversely, for other manufacturing options, such as single-pass and multipass scribings, the surface resistance increases by 30%–40% due to sample defects. The extracted impedance is validated both numerically and experimentally using two representative microwave structures: an ultrawideband (UWB) monopole antenna (1–18 GHz) and two flexible 4 x 2 arrays of resonant scatterers working at 9 and 9.5 GHz. In both cases, good agreement is observed between simulation and measurement, with the monopole antenna showing less than 3-dB difference in the realized gain. In the case of the flexible arrays, instead, frequency shifts up to 150 MHz are reported as a result of thermally induced bending of the precursor during laser processing. These findings demonstrate the reliability of the extracted impedance parameters and underscore the importance of incorporating substrate deformation into numerical simulations for more accurate predictions.