## 1 Introduction  
Microwave tumour ablation is a percutaneous cancer treatment. The treatment consists of a needle-like applicator being inserted through the skin and guided to a target tumour using medical imaging systems. Microwave energy irradiated from the applicator's end tip into the surrounding tumorous tissue elevates the temperature to over 100 \({}^{\circ}\)C, causing immediate cellular death [1, 2, 3]. The microwave energy is irradiated by a microwave antenna inside the applicator, which is either uncooled or actively cooled with circulated chilled saline water or CO\({}_{2}\)[4, 5, 6, 7, 8, 9, 10]. The surrounding tissue undergoes mechanical and electrical changes during ablation treatment due to the sharp rise in temperature. Since the dielectric properties of biological tissue are frequency-dependent and temperature-dependent, numerous studies have been conducted to measure dielectric properties in various tissues, mainly using techniques such as open-ended coaxial reflection and resonant cavities [11, 12, 13, 14]. However, the main disadvantages of these techniques are the small sensing volume of the open-ended coaxial probe [15] and the destruction/manipulation of the material under test by resonant cavity techniques [14]. Moreover, the heating methods employed (thermal bath, microwave oven, and supplementary ablator, amongst others) might be limiting, as they do not accurately emulate the thermal environment induced through ablation due to different heat deposition rates and water mobility [16, 17, 18]. Hence, there is increasing interest in investigating the transient effects of heat deposition rates in tissue with dual-mode applications. This approach utilises the same ablation antenna for both treatment and quasi-simultaneous measurement of the scattering parameters, which can be transformed into the dielectric properties of the surrounding tissue [19, 20, 21]. Existing conversion models, initially developed for open-ended coaxial probe measurement techniques [22, 23, 24], are typically adapted for the specific applicator used in the ablation procedure. These models facilitate the conversion of measured scattering parameters into complex permittivity values by characterising the antenna's electromagnetic behaviour using reference liquids with well-defined dielectric properties. Once calibrated, the quasi-real-time scattering parameters measured during ablation are converted into complex permittivity values, allowing for assessment of tissue properties [19, 20]. This emphasises the importance of uncooled antennas, given their direct contact with the measured medium (resulting in higher sensitivity to transients) and reduced complexity. However, structural changes caused by high temperatures during ablation adversely impact its efficacy [19, 20]. Thus, this study expands on previous research [25] by thoroughly investigating PTFE expansion in uncooled coaxial insulated monopole antennas under ablation conditions. While prior work identified deformation phenomena, this research demonstrates that pre-annealing is an effective mitigation strategy. The designed experiments characterise thermal expansion and illustrate the improvement effects of pre-annealing on the coaxial cable. Given that the uncooled insulated monopole antenna is intended for dual-mode applications, specifically quasi-simultaneous scattering measurements and thermal treatment, minimising variations caused by structural changes significantly improves the reliability of scattering measurements, enabling accurate permittivity extraction through conversion models. In prior investigations, ablation experiments were conducted on ex vivo porcine liver utilising an uncooled insulated monopole antenna. Visual examination revealed the expansion of PTFE, as illustrated in Figure 1. The quarter-wavelength monopole antenna is designated as (D), whereas the expansion observed following ablation is denoted as (F).