Approaching the lowest operating frequency thickness limits with complex surface impedance of ultrathin absorbers

In this paper, the physical model of electrically thin weakly conductive film with intrinsic surface impedance is established, indicating that the imaginary part of high surface impedance is non-negligible at microwave frequencies. In the design of lossy frequency selective surface absorbers, we introduce the imaginary part of intrinsic surface impedance for the first time. With the experimentally established relationship between the complex surface impedance and the DC square resistance, this complex surface impedance allows us to accurately predict the electromagnetic response of high surface impedance film at microwave frequencies and provides an advantage in reducing the thickness of absorber. The proposed ultra-thin absorber can provide -10 dB reduction over the frequency range of 4.5-13.3 GHz. Total thickness of microwave absorber is only 0.06λ at lowest operating frequency, which is close to the theoretical limitation. The measurement is provided to verify the validity of the equivalent relationship and the reliability of the full-wave model. This study provides a new way to reduce the thickness of absorber, exhibiting promising potential for stealth technique.