(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: