A Miniaturized Anechoic Chamber: Omnidirectional Impedance Matching Based on Truncated Spatial Kramers-Kronig Medium

Spatial Kramers-Kronig (KK) medium provides a new platform to achieve omnidirectionally reflectionless absorption without using any gain elements. However, impedance-matched spatial KK media realized in previous experiments generally have flat geometries with large transverse dimensions. Realizing finite spatial KK absorbers in free space background is important for many practical applications but has not been demonstrated experimentally thus far. Here, the concept of spatial KK relation to cylindrical coordinates is extended and a double-resonance space-frequency KK permittivity profile is implemented in a circular geometry. Omnidirectional impedance matching to free space is achieved by truncating the spatial KK absorber at the boundary where the relative permittivity is unity. For the experimental verification, a two-dimensional 6.6-wavelength-diameter doughnut-shaped anechoic chamber is implemented using the designed spatial KK medium. The measured electric field distributions and experimental imaging confirm the omnidirectionally reflectionless property, demonstrating that this miniaturized anechoic chamber is suitable for electrically small scatterer measurements and imaging. The proposed finite spatial KK absorber and fabricated miniaturized anechoic chamber show great promises for electromagnetic shielding and compatibility.