The equivalent medium theory of metamaterials provides a way to obtain their effective constitutive parameters. However, because of its non-reciprocity, the complexity of the electromagnetic coupling, and a metallic bottom layer, it has been challenging to retrieve them from a metamaterial absorber. In this paper, we propose a method without any approximation to obtain them, in which the non-reciprocity and the strong electromagnetic coupling are included. Compared with the three methods such as symmetric metamaterial method, asymmetric metamaterial method and metasurface method, our method can reveal the metamaterial absorber’s electrical and magnetic resonance and show its electromagnetic coupling coefficients. To deal with a metamaterial absorber with a metallic bottom layer, four corners of the metallic bottom layer in the unit cell are removed, making it possible to retrieve the electromagnetic parameters. Surprisingly, these results show that the metamaterial absorber with a metallic bottom layer in our example operates in a negative refraction state at the half absorption frequencies, which helps further understand the absorbing mechanism of these metamaterial absorbers.

Multiple electric resonators loaded with lumped resistors are usually constructed in a unit cell to design broadband metamaterial absorbers. In this paper, we propose a simple way to achieve ultrabroadband microwave absorption. Only one split ring resonator loaded with one lumped resistor in the absorber's unit cell in our design. Its measured reflection coefficient is lower than −10 dB in the frequency range 14.20 GHz to 32.98 GHz. Its bandwidth is 18.78 GHz, and its absorption frequency covers the Ku-, K-, and Ka-bands. The absorber has a 1.524 mm dielectric layer, and the thickness of the copper foil on both sides of the dielectric layer is 0.035 mm. Therefore, the total thickness of the absorber is only 1.594 mm. The edge length of its unit cell is 1.8 mm, which is about 1/4 of the wavelength of 40 GHz, so the oblique incidence of electromagnetic waves will not excite higher-order modes below 40 GHz. In addition, we study its absorption mechanism in detail. It is discovered that besides selecting a suitable lumped resistance value, the construction of two magnetic resonances and an electric resonance between them is necessary to achieve broadband absorption.