# Efficient Implementation of Polymer Microwave Fiber Links Employing Non-Galvanic Coupling Mechanism
## 2 PMF Overview
### PMF Electrical Performance Simulation Results  
By utilizing one of the aforementioned polymer materials, the attenuation of an appropriately designed PMF is less dependent on its structure selection, as the inherently low dielectric tangent of such materials guarantees low-loss propagation. However, the dispersion and field confinement of the PMF are strongly influenced by the PMF's geometry. To demonstrate this, we present an investigation of the performance of various PMF structures. The four examples that we choose to present are based on common dielectric waveguide geometries, such as the cylindrical dielectric rod [4] and the rectangular cross-section dielectric [27]. More specifically, the following geometries are considered: (1) a hollow-core cylindrical PMF; (2) a hollow-core rectangular PMF; (3) a solid-core rectangular PMF; and (4) a ribbon PMF. These four PMF geometries are depicted in Figure 4.  
Figure 3: E-field distribution of a rectangular cross-section core PMF: (**a**) 2 mm \(\times\) 1 mm and (**b**) 2 mm \(\times\) 0.5 mm. All four PMF structures consist of a polyethylene core (\(\epsilon_{r}=2.35\) and tan\(\delta=8\times 10^{-5}\)) and foam polyethylene cladding (\(\epsilon_{r}=1.36\) and tan\(\delta=5\times 10^{-5}\)) and are designed for single-mode operation in the 110-170 GHz band. In Table 1, the PMFs' core dimensions are shown, while each PMF has cladding with a 6.8 mm diameter. All simulations were executed in the ANSYS HFSS 3D EM simulator. In Figure 5, the attenuation and the group delay of the four structures are shown, in TE11 mode for the three rectangular cross-sections and HE11 mode for the circular cross-section. In the single-mode region of operation, weaker field confinement, as shown for the hollow-core rectangular PMF, leads to lower attenuation and group velocity dispersion (GVD), as most of the guided wave's power is propagating in free space. However, a strongly confined field offers much better coupling in terms of insertion loss, which overpowers the small increase in attenuation and dispersion. It is important to note that the rectangular cross-section PMF supports two families of modes. For this analysis, only the TE-like mode family was investigated, for two main reasons: (1) the TM-like modes are suppressed due to the coupling mechanism (as will be shown later in this paper) and (2) the two mode families are orthogonal to each other, meaning that the TM-like modes do not affect the performance of the TE-like modes, and vice versa, in terms of dispersion. This is based on [PERSON]'s assumption [27], which states that the Ex component of the TE-like modes is greater than the Ey component, which is approximately zero, and vice versa for the TM-like modes. This causes the orthogonality of the two mode families and, as a result, allows for full duplex links in rectangular PMFs [15]. \begin{table}
\begin{tabular}{c c} \hline
**PMF Structure** & **Dimensions** \\ \hline Hollow-core cylindrical PMF & \(Rin=0.5\) mm \\  & \(Rout=1\) mm \\ \hline Hollow-core rectangular PMF & \(h=0.6\) mm \\  & \(w=2.4\) mm \\  & _wall thickness_\(=150\) μm \\ \hline Solid-core rectangular PMF & \(h=0.5\) mm \\  & \(w=2\) mm \\ \hline Ribbon PMF & \(w=3.2\) mm \\  & \(h=0.32\) mm \\ \hline \end{tabular}
\end{table} 
Table 1: Dimensions of the PMF geometries. Figure 4: PMF cores evaluated. All four PMF structures exhibit similar attenuation, but the field confinement of each structure introduces a tradeoff between the group delay variation and coupling robustness. As we will see later in this paper, the solid-core PMF indeed presents lower coupling losses, and this makes it the most suitable solution for short-to-medium-distance links. Note that the constant coupling losses of the link have a greater impact than the group delay variation and attenuation. To facilitate feasible communication connections, the PMF should possess the capability to create bends with a minimal curvature radius. The nonlinearity in how the propagation constant changes with the frequency leads to waveguide dispersion.