Rotation and robotic multi-axis scanning free-space measurement method for electromagnetic performance evaluation of curved stealth structures

Abstract

Since the last decade, stealth technology is developing at a much higher speed with a lot of research going on to evaluate the developed structures. Radar absorbing structures (RAS) is a key part of stealth technology which absorbs certain frequencies to reduce radar detection. Vector network analyzer (VNA) plays an important role in the RAS performance evaluation of stealth structures. The conventional methods of measuring the electromagnetic properties of specimens are closed-cell measurement utilizing coaxial cable and waveguides at high measuring accuracy and free-space measurement utilizing radiating antennas to measure electromagnetic properties of larger stealth structures without being damage done to the specimen. All these conventional evaluation systems can scan a single point at a time. A scanning free-space measurement (SFM) system was a step towards automated scanning of large stealth structures. This system was capable to scan the full field of flat linear structures in the x-y plan using two linear stages. But SFM is limited to curved specimens such as leading-edge and cylindrical structures, and the return loss is highly affected by the specimen curvature. In this study, a 3-Axis scanning free-space measurement (3-Axis SFM) system is developed. The system has two linear stages and a rotational stage for raster scanning of the specimen. The system performance was checked by scanning a cylindrical RAS specimen of radius 100 mm. The results were then compared to the SFM system which showed that the Signal-to-noise ratio of 3-Axis SFM return loss is quite high compared to the SFM system. A coordinate-based scanning algorithm was also developed for 3-Axis SFM to ensure a fixed interval scan of complex geometrical shape specimen like wing leading-edge at lower incident angles. The algorithm achieved a normal incident scan for the NACA-M3 symmetrical leading-edge specimen, non-symmetrical airfoil specimen and S-shape double-curvature specimen. The results of obtained through coordinate-based scanning using 3-Axis SFM were confirmed through 6 Axis robotic SFM results. Different scan parameters and results can be visualized in GUI software developed in C++ using QT and QWT frameworks. Finally, the inspected results were curvature compensated to get the actual RAS performance of the specimen. The geometry-based PEC reflection-loss prediction algorithm accuracy is further improved by adding a stand-off-distance effect and more specimen curvatures data to the algorithm. The accuracy of the algorithm is verified by comparing predicted PEC results to the measured PEC results of the corresponding specimen.