## 3 Results and Discussion
### PTFE Thermal Expansion  
The mean (\(\overline{\mathrm{X}}_{\Delta L}\)) and standard deviation (\(\sigma_{\Delta L}\)) of the change in PTFE length, measured using a toolmaker's microscope, were calculated and are represented by the purple plots in Figure 6 at an ambient temperature of 30 \({}^{\circ}\)C. The non-annealed antennas exhibited PTFE expansion with an \(\overline{\mathrm{X}}_{\Delta L}\) of 0.35 mm and a \(\sigma_{\Delta L}\) of 0.11 mm, while the annealed antennas exhibited PTFE shrinkage with an \(\overline{\mathrm{X}}_{\Delta L}\) of \(-0.18\) mm and a \(\sigma_{\Delta L}\) of 0.22 mm. In Figure 6, data measured during both heating and cooling cycles using the camera setup are displayed with red or blue plots. There was agreement between the camera-based measurements obtained at the end of the cooling cycles (at 30 \({}^{\circ}\)C) and those measured with a toolmaker's microscope. This reinforces the validity of the camera setup. At the end of the cooling cycle, the non-annealed antennas exhibited an \(\overline{\mathrm{X}}_{\Delta L}\) of 0.34 mm and a \(\sigma_{\Delta L}\) of 0.09 mm, while the annealed antennas exhibited an \(\overline{\mathrm{X}}_{\Delta L}\) of \(-0.13\) mm and \(\sigma_{\Delta L}\) of 0.12 mm. These results reaffirm previous findings, indicating PTFE expansion for non-annealed antennas and permanent shrinkage for annealed ones. The greater standard deviation seen in both sample groups from the toolmaker's microscope measurements is attributed to unavoidable minor variations in antenna placement before and after the experimental measurements. Conversely, the developed camera system maintained consistent antenna positioning throughout the entire experimental process. The yellow line in Figure 6 represents theoretical unrestricted PTFE thermal expansion, \(\Delta L\), as calculated by Equation (1). The mean initial length (\(\overline{\mathrm{X}}_{L0}\) = 12.95 mm) measured using a toolmaker's microscope was inputted as the initial length \(L_{0}\) in Equation (1). The change in temperature, \(\Delta T\), was calculated from the initial temperature \(T_{0}\) of 30 \({}^{\circ}\)C and  
Figure 6: PTFE thermal expansion in mm as a function of temperature. the temperature of the peanut oil's target steady state temperature, \(T_{\rm ss}\). Two values were considered for the linear coefficient of thermal expansion of PTFE, \(\alpha_{1}\), 124 \(\times\) 10\({}^{-6}\)\({}^{\circ}\)C\({}^{-1}\) and 135 \(\times\) 10\({}^{-6}\)\({}^{\circ}\)C\({}^{-1}\), reflecting the variability range of this parameter found in the literature [28]. Disregarding the shrinkage observed in the experimental plots produced by the annealed antennas (light red/blue), there is notable agreement with the theoretical plot (yellow), as depicted in Figure 6.  
Copper has a high thermal conductivity of 391 Wm\({}^{-1}\) K\({}^{-1}\)[40]. Consequently, there is a negligible thermal difference between the exposed PTFE section and the PTFE covered by the outer conductor. Additionally, copper has a low \(\alpha_{1}\), 17.6 \(\times\) 10\({}^{-6}\)\({}^{\circ}\)C\({}^{-1}\), especially relative to PTFE [41]. Therefore, during annealing, the covered PTFE could be considered restricted PTFE in terms of thermal expansion. Due to its very high viscosity, it is assumed that, during coaxial cable manufacturing, the PTFE was extruded using a high-pressure paste or ram extruder. This results in high residual compression stress in the PTFE tube, which causes non-linear expansion due to stress relaxation when it is heated. As can be seen in Figure 6, the highest elongation occurred around its glass transition temperature, \(T_{\rm g}\), at 126 \({}^{\circ}\)C, especially during the cooling cycle, that is, from a hotter to colder stage. Persistent PTFE residual polymer stresses following the extrusion process in coaxial cable manufacturing will contribute to the accentuated non-linear expansion achieved by non-annealed antennas.