# Thermal Impact of 5G Antenna Systems in Sandwich Walls
## 4 Conclusions  
In this work, the 3D finite element method was used to perform steady-state thermal analysis primarily on three types of walls: the sandwich wall, the signal-transmissive wall using stainless steel as connector material (the current design), and the signal-transmissive wall using copper as connector material (the previous design). Compared to the sandwich wall (with a U-value of 0.1496 W/m\({}^{2}\)K), the current design slightly increased the U-value to 0.156 W/m\({}^{2}\)K, which is below the maximum recommended U-value of 0.17 W/m\({}^{2}\)K for  
Figure 14: Temperature profile for the cut plane, which is 20 mm away from the exterior surface: (a) Signal-transmissive wall (current design); (b) 3 \(\times\) 3 signal-transmissive wall (current design). Figure 15: Temperature profile for the cut plane, which is 22 mm away from the exterior surfaces: (a) Signal-transmissive wall (previous design); (b) 3 \(\times\) 3 signal-transmissive wall (previous design). external walls specified by the National Building Code of Finland. The building energy simulation showed that in Helsinki, an increase in the U-value by 0.0064 W/m\({}^{2}\)K (from 0.1496 to 0.156), would result in only an additional external wall heat loss of 0.545 KWh/m\({}^{2}\) per year (equivalent to approximately 0.062 euro/m\({}^{2}\) per year if district heating is utilized). However, the previous design raised the U-value to 0.3 W/m\({}^{2}\)K, which greatly exceeded the U-value limit specified by the National Building Code of Finland, leading to an additional heat loss of 12.8 KWh/m\({}^{2}\) per year (23.5 times that of the current design). The 3D numerical models (Figures 11 and 12) demonstrate a significant reduction in the thermal bridging effect from the previous design to the current design. Additionally, if a highly thermally conductive material, such as copper, is used for the center connectors in the signal-transmissive wall, heat flow may not always occur in a direction perpendicular to the building element planes. As a result, heat transfer paths could vary from layer to layer, leading to poor performance of the parallel path method. Furthermore, the results of comparing the U-values of signal-transmissive walls calculated using the parallel path, isothermal planes, and ISO 6946 combined methods showed that the isothermal planes method provided more accurate results. In addition to the three wall models investigated in this work, thermal analysis was also conducted for a configuration of 3 \(\times\) 3 signal-transmissive walls, consisting of nine signal-transmissive walls arranged in a 3 \(\times\) 3 grid, using both numerical and theoretical methods. The calculated U-values for the 3 \(\times\) 3 configuration were found to be the same as those for the standalone signal-transmissive walls. Additionally, it was observed that the areas affected by thermal bridging were generally smaller than the design dimensions of a signal-transmissive wall, which are 150 mm \(\times\) 150 mm. In the future, we plan to conduct a 3D transient heat transfer simulation for an apartment with partially signal-transmissive external walls. Currently, performing such a simulation is difficult due to the high computational demand, and it may require the use of supercomputing in the future. Material choices affect both the thermal and electromagnetic properties of the signal-transmissive wall. The use of material with higher electrical conductivity would improve the electromagnetic transmission of the system. In the future, we need to study how materials, such as copper could be used in coaxial cable assemblies without compromising thermal insulation. Furthermore, the use of foldable coaxial cable assemblies should be studied. By folding the coaxial cable, the heat transfer should be smaller, which makes it possible to use other materials than stainless steel in the antenna system.