Design of a Tunable Scaled Absorber Using Magnetorheological Fluid Under a Magnetic Field

In order to achieve wideband scaled measurement of low observable object scattering characteristics, the scaling design of the related absorbing material has been within the scope of researchers over recent years. In this work, a tunable material was designed to meet the wideband and high-precision requirements for scaling measurements. The material was produced using magnetorheological (MR) fluid, composed of silicon rubber and flaky carbonyl iron particles (CIPs), and an electric magnet. According to the results, applying the magnetic field made the CIPs in the MR fluid aligned with respect to different parallelism depth. Furthermore, the larger the field strength, the lower the permittivity and the higher the permeability. Based on the Maxwell–Garnett mixing rule, two parameters could be interpolated within a frequency range of 18–40 GHz. Changing the magnetic field resulted in the tunable absorbing property of the composite at frequencies of 2–40 GHz and the adjustable reflection loss in the region of −22 dB to –3 dB. While the thickness of the full-size absorbing coating was 0.7–0.9 mm at 10 GHz and a scale factor of 4, the designed material was as thick as 0.8 mm, and the average radar cross-section (RCS) error of coating plates was less than 0.5 dB. In addition, the scaled frequency band could be tunable within a range of 36–40 GHz as the absorber was as thin as 2 mm, and the mean RCS error was found to be less than 0.6 dB. Therefore, the MR fluid was shown to be a promising candidate for scaled measurements.