# Efficient Implementation of Polymer Microwave Fiber Links Employing Non-Galvanic Coupling Mechanism
## 3 PMF-AiP Coupling
### Coupler Design
As illustrated, the antenna directs the electromagnetic field in the desired lateral direction. Recall that the evaluation of the antenna's near-field characteristics without the presence of the PMF is not sufficient. The verification of the correct operation of the structure needs to be considered. For this purpose, the near-field characteristics in the presence of the PMF are re-evaluated, as well as the correct behavior of the electromagnetic wave, meaning that a propagating wave must be visible. The PMF used for the evaluation of the antenna's near-field characteristics is the solid-core rectangular PMF that was discussed in the previous section. In Figure 14, the  
Figure 11: Simulation results for the Poynting vector magnitude pattern at 140 GHz. Figure 12: Simulation results in terms of the E-field 3D distribution without (**left**) and with (**right**) a magnifier at 140 GHz. Figure 13: Simulation results in terms of the normalized H- and E-field patterns and the Poynting vector 3D graph without a PMF at 140 GHz (**left** to **right**). normalized E- and H-field patterns at 140 GHz are shown. It is clear that the PMF affects the antenna's performance. Although the field patterns of the coupler structure are valuable, the most important performance metric of the coupler is the total coupling loss to the PMF. In Figure 15, the coupling losses for the four different PMF geometries are illustrated. As was expected from the analysis of the PMF characteristics, the solid-core rectangular PMF presents the lowest coupling loss. It is important to mention that the simulation results represent only the coupling losses of the structure. The antenna's input network insertion loss is not included and the PMF's propagation loss is de-embedded. As mentioned earlier in this paper, an advantage of the Vivaldi antenna is that the input signal creates an electromagnetic field with a profile similar to the TE-like modes of the rectangular PMF. This effect is very beneficial for the overall performance, as it ensures single-mode operation. In other words, the Vivaldi antenna effectively suppresses the TM-like modes. This suppression is illustrated in Figure 16, where the TM-like modes are below \(-60\) dB and the TE21 mode is approximately at \(-18\) dB. To achieve this suppression, the proposed Vivaldi antenna requires only the differential mode of the dual coplanar strip  
Figure 14: Simulation results in terms of the normalized H- and E-field patterns and the Poynting vector 3D graph with a PMF at 140 GHz (**left to right**). Figure 15: Coupling \(|\)S21\(|\) for the four PMF cases. line at the input network. The common mode not only does not suppress additional modes but also reduces the directivity of the antenna, as illustrated in Figure 17.  
A very important aspect of a wireless coupling structure for a PMF is the robustness of the coupling mechanism, i.e., the displacement tolerance of the PMF in front of the antenna. Due to the relatively small size of the package and the PMF, the precise placement of the PMF in front of the antenna is almost impossible. The proposed structure offers high tolerance regarding the PMF displacement of approximately 0.02 dB per 10 \(\upmu\)m of displacement, as shown in Figure 18.  
Figure 16: Coupling losses of the fundamental and higher-order modes of the solid rectangular PMF. Figure 17: Poynting vector magnitude for (**a**) common mode and (**b**) differential mode of the differential coplanar strip line at 140 GHz. Figure 11:0 (a) Horizontal, (b) lateral and (c) vertical PMF displacement at 140 GHz.