Location Attempt of Multiple Heated Spots in a Polymer-insulated Coaxial Cable by Frequency Domain Reflectometry

We already reported that a combination of frequency domain reflectometry (FDR) and inverse fast-Fourier transform (IFFT) is a powerful tool for locating the position where an abnormality or degradation occurs in a cable up to a length of around 1,000 m. In our previous research, we always set a single spot of abnormality. This paper shows that this method is also applicable if abnormalities occur simultaneously at more than one spot. A coaxial cable of 31.5 m, insulated with low-density polyethylene, was heated at two different positions using two heating chambers. Each heated portion was about 47 cm long. The portion closer to the measurement terminal was heated to various temperatures up to 100°C, while the other portion was heated to 100 °C. As a result, it was confirmed that the two portions can be located if the temperature of the heated spot is 10°C higher than the surroundings when the highest measurement frequency is above 800 MHz.