Low Frequency Gain Enhancement of Cavity-Backed Electrically-Small Hybrid Spiral Antenna for High-Voltage Partial Discharge Sensing

In this study, we propose a sensor for partial discharge (PD) sensing based on an electrically small hybrid spiral antenna, measuring approximately 0.087 wavelengths in diameter at 200 MHz. The sensor utilizes a hybrid design that integrates equiangular and Archimedes spiral antennas. Spiral antennas encounter significant gain loss when the wavelength exceeds their circumference, limiting their low-frequency performance. This can impede the early detection of PD, as the initial electromagnetic (EM) PD signature is concentrated at the low-frequency end of the ultrahigh frequency (UHF) band. In addition, PD sensors are often enclosed within a metallic cavity to shield them from surrounding noise. However, this further exacerbates the challenges of low-frequency performance, with the band of interest potentially falling well below the cavity cutoff frequency. In this study, we alleviate this limitation by integrating a capacitively loaded ring resonator with the spiral antenna, thereby enhancing its low-frequency gain without increasing the antenna size. Experimental results from a prototype sensor with an aperture diameter of 130 mm demonstrate that the proposed ring resonator increases the realized gain in a relatively wide bandwidth from 220 MHz to 650 MHz by as much as 18.3 dB, with minimal impact observed at higher frequencies. Moreover, in situ PD detection test results demonstrate an improvement of 0.6 dB in the mean received power, along with a spectral efficiency improvement in the frequency range of 290–525 MHz, with a peak improvement of 21.2 dB in the received power spectral density (PSD) at 410 MHz.