## 4 Experimental Setup  
Performance testing consists of two key measurements: communication distance and sensitivity. A standard commercial dipole with the length of 55 mm operating at 2.3-2.5 GHz is used as the interrogator antenna. For communication distance measurement, the sensor antenna remains fixed while the interrogator antenna is positioned using a fixture pole to alter its height in a vertical direction. A ruler is employed to measure the distance. Besides that, some absorbers are employed to prevent unwanted interference from the surroundings. As the distance between the antennas increases, the backscattered signal is observed until the S\({}_{11}\) peak diminishes to the level of background noise, indicating the communication limit. The experimental setup for communication distance measurement is shown in Figure 3a.  
Figure 2: Schematic diagram of backscattering-based wireless temperature sensing system.  
Sensitivity is measured by monitoring the resonant frequency shift as the temperature increases from 25 to 50 \({}^{\circ}\)C and then in 50 \({}^{\circ}\)C increments up to 500 \({}^{\circ}\)C. The antenna distance is fixed at 5 cm, which satisfies the far-field condition for both antennas. The interrogator antenna is connected to port 1 of the VNA and positioned directly in front of the sensor antenna to ensure proper polarization alignment and is located outside the furnace, as illustrated in Figure 2. A thermal insulator cover made of alumina foam isolates the heating environment inside the furnace. Alumina foam is widely used as a high-temperature thermal insulator in various industries, including industrial manufacturing, aerospace, and power generation applications. A K-type thermocouple measures the actual temperature inside the furnace and verifies the accuracy of the furnace's temperature controller.  
Both measurements utilize the Time-Domain Gating (TDG) method applied in the VNA to eliminate environmental interference and isolate the backscattered signal received by the interrogator antenna [20]. The inset in Figure 2 illustrates the Time-Domain Gating (TDG) process, where an S\({}_{11}\) null in the original frequency domain is transformed into a peak after gating. The FFT (Fast Fourier Transform) converts S\({}_{11}\) to the time domain, allowing unwanted reflections to be removed. After applying IFFT (Inverse Fast Fourier Transform), the gated signal is converted back to the frequency domain, isolating the antenna's resonant response. As a result, the original S\({}_{11}\) null shifts to a peak, representing the dominant resonance frequency of the antenna. The experimental setup for sensitivity measurement is shown in Figure 3b.  
To verify heat containment, Figure 3c shows the external surface temperature of the thermal insulator cover, measured using a non-contact infrared thermometer, which reads 54.6 \({}^{\circ}\)C while the internal furnace temperature exceeds 400 \({}^{\circ}\)C. This confirms the alumina foam as a thermal insulator that effectively minimizes heat leakage, ensuring a safe environment for the interrogator antenna and measurement instruments while maintaining measurement accuracy and system stability. To further enhance heat isolation, it is recommended to use a thermal cover made of a low-thermal-conductivity material.