Low-Profile and Low-Dispersion Artificial Impedance Surface in the UHF Band Based on Non-Foster Circuit Loading

Conventional artificial impedance surfaces are too thick for practical use at ultrahigh frequencies (UHFs). Thin structures with passive capacitive loading are possible in the UHF band, but only with narrow bandwidth. This paper presents a non-Foster loaded artificial impedance surface to achieve lowprofile and wide bandwidth simultaneously in the UHF band. The high resonant frequency associated with a thin structure is reduced by the synthesized non-Foster-type impedance load. For a demonstration, we designed a 5-mm-thick actively loaded surface for obtaining a high-impedance condition supporting slow transverse magnetic (TM) surface waves with a propagation index of 1.5, which corresponds to a 420jΩ impedance surface. The non-Foster impedance is synthesized to be equivalent to -2.8 pF || -3.6 nH. Both the non-Foster impedance and the surface are fabricated and tested with a near-field measurement. The retrieved refractive index shows a constant TM propagation index of 1.5 over a bandwidth of 450-800 MHz, a 56% fractional bandwidth. By varying the biasing voltage of the non-Foster circuit, the impedance can be tuned from 250 to 420 jΩ, while maintaining low dispersion over the same bandwidth.