## 2 Double-Layer Tapered Slot Antenna Design  
A dissemble view of the proposed EM absorber applied DLTSA is shown in Figure 1a. Note that the EM absorber is better placed in both sides of the tapered radiating slot for the monostatic RCS reduction (i.e., for the waves incoming from +\(\varepsilon\) direction). The feed line of a conventional TSA should contact the lossy absorber directly and cannot avoid the antenna gain loss [17]. The beam should also be tilted with such asymmetric configurations. Therefore, in this work, the DLTSA is designed to have asymmetric tapered slots with the EM absorber layer at the outermost part, as shown in the figure. Such a configuration not only makes the antenna design symmetric, but also allows the feedline to be in between the slots, not in direct contact with the lossy absorbers as shown in Figure 1a. The side views of the DLTSA without and with the EM absorber are shown in Figure 1b,c, respectively. Among the commercial EM absorbers in the market, we selected Laird (London, UK) Eccosorb FGM-125 designed for RCS reduction in the X-band [18]. As can be seen from Figure 1a,c, we place the EM absorbers on the tapered slot ground plane, such that they are not contacted with the feed line. The metal is made of copper, and Taconic TLY-5 substrate is used. The design parameters are given in Table 1. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(L_{1}\) & 19.6 & \(L_{4}\) & 5 \\ \(L_{2}\) & 9.7 & \(r\) & 0.4 \\ \(L_{3}\) & 15.6 & \(W\) & 7.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Design parameters. The units are in millimeters. The simulated results of the impedance and radiation gain performance of the DLTSA in Figure 1b,c are compared in Figure 2. In Figure 2a, where the reflection coefficients of the DLTSA according to the presence of the FGM-125 absorbers are plotted, we can see that those of both antennas show a very small difference, while well-matched in the Ka-band. What can be found in Figure 2b, where the \(\theta\)-sweep radiation pattern of antenna gains are plotted, is that the radiation patterns show little difference, whereas both show directive beams at 35 GHz. Note that the gains are 7.1 dBi for the proposed design with FGM-125 absorbers and 7.0 dBi without the absorbers. The radiation efficiency is about 88%. The little difference of 0.1 dB is caused by the lossy EM absorber. Note that the improved front-to-back ratio of 4.1 dB of the case with the FGM-125 is due to the lower backward radiation with the presence of the EM absorber layer. To further validate the proposed design in Figure 1c, it is compared with a conventional TSA that is composed of a single tapered slot layer with the feedline being exposed to the EM absorber. The size and position of the EM absorbers are kept the same from the conventional TSA but they are now in direct contact with the feed line like in Figure 3a. Figure 3a illustrates the surface current distribution at 35 GHz for a conventional TSA with the FGM-125 in place. It is clearly observed that the current distribution is not confined toward the radiating slot but is scattered to the absorbers. The associated reflection coefficients and the radiation patterns are deviated from the one without the absorber as can be observed from Figure 3b,c. On the other hand, the DLTSA with the absorber facilitates a  
Figure 1: Proposed double-layer TSA (DLTSA). (**a**) Dissemble view. (**b**) Side view of the assembled DLTSA without EM absorbers. (**c**) Side view of the assembled DLTSA with EM absorbers. smooth current flow transition to the slotted radiator despite the presence of the absorber as presented in Figure 3d. As a result, a conventional single-layer TSA hardly avoids the performance deterioration from the lossy absorbers. Figure 4 compares the simulated reflection coefficients and radiation patterns for the antennas in Figure 3a,d, namely conventional TSA and the proposed DLTSA with absorbers.