(b) (c)
Fig. 3 (a) Equivalent circuit model of the proposed PIUWA. (b) Simulated reflection coefficient under HFSS and ADS of the proposed PIUWA under normal incidence. (c) Simulated susceptance of YL and Yt.
Configuration and Results: The theoretic model of the proposed PIUWA composed of 3-D lossy layer and planer backplate is depicted in Fig.2(a). The 3D structure of the PIUWA is designed based on a hollow cube made of FR4 substrate with a thickness t of 0.4mm and relative dielectric constant of 4.4. On the two adjacent faces of the cube, two sets of dipoles loading with lumped resistors are printed for obtain full-wave polarizations. As shown in Fig.2(c), the top meander dipole commands the low-frequency absorption and the bottom shorter dipole mainly acts on the high-frequency incident EM waves. Here we useDL and DH to represent the top and bottom dipole, RL andRH on behalf of the resistors welded on them. By employing the two dipoles, multiple resonances are generated to expand the absorption bandwidth.
The manufactured prototype of the proposed PIUWA is shown in Fig.2(b). The upper 3D layer is processed into two types of long strips with cutting slots in opposite direction for easy assembly, as shown in Fig.2(d). The width of the cutting slots is equal to the thickness of the FR4 substrate so that the strips with different gaps can be inserted and fixed. During our numerical simulations using (High Frequency Structure Simulator) HFSS, the simulated absorbing rate under normal incidence at TE polarization is presented in Fig.2(f). As observed, the proposed PIUWA exhibits a wide absorption band of 144% from 4.0-24.53GHz with a good performance of absorptance over 90% within a wide working band. For validation, the proposed PIUWA consisting 40×40 units has been tested in the microwave anechoic chamber, as shown in Fig.2(e). The detailed measurement procedures can be referred to [21]. Fig.2(f) shows the comparison between the simulated and tested results. As we can see, except for some frequency offset caused by machining errors and experimental environment, the tested result is reasonable compared to the simulation.
Equivalent Circuit and Mechanism Analysis: For the purpose of understanding the physical mechanism of the wideband absorption performance with greater depth, an illustration based on the equivalent circuit theory of the proposed PIUWA is demonstrated in this section. As shown in Fig.1(b), when the incident wave is perpendicular to the PIUWA, the electric field component is parallel to the dipoles, which is the same as CAA absorbers. Therefore, the proposed hybrid structure can be analysed using the equivalent circuit model. Considering the symmetry of the structure, we choose TE polarization for illustration. The proposed PIUWA can be expressed as two series RLC circuits connected with two sections of transmission lines as shown in Fig.3(a). We useRLL1C1 to represent DL andRHL2C2 forDH . Between DL andDH is a cascaded transmission line with the length of h2 representing the vertical substrate between the two dipoles. In addition, the short-circuited transmission line section with the length of h1 represents the conductor-backed substrate below the short dipole.ZL and ZH are the approximate impedances of the top and the bottom dipole.Zt represents the total impedance of the rest part below the top dipole. Based on the above illustration the transmission line matrix of the proposed PIUWA can be expressed as follows: