## 2 Design and Implementation of On-Line PD Monitoring System
### Partial Discharge Detectors
#### 2.2.2 Ultra-High Frequency Antenna
The use of chip-resistor of higher resistance allows to widen the bandwidth even more (e.g., for \(R=7.2\)\(\Omega\), the \(-6\) dB bandwidth grew to 90 MHz). Unfortunately, it is at the expense of VSWR growth and a slight change of resonant frequency \(f_{r}\), which is shifted to the lower frequencies. In turn, change in distance \(H\) between the patch and ground plane  
Figure 5: Schematic diagram of the meandered PIFA. influences the resonant frequency to a greater extent than the bandwidth. The results of the simulation proved that for \(H\) = 3 mm the resonant frequency was 319 MHz, and for \(H\) = 6 mm it increased to 357 MHz, wherein \(-6\) dB bandwidth was, respectively, equal to 47 and 56 MHz. Based on the simulation results, the prototype construction of meandered PIFA of dimensions presented in Table 3 was manufactured. The antenna was equipped in cylindrical protective housing made from polytetrafluoroethylene (PTFE) and matched to the cavity of the active dielectric window. The antenna with the active dielectric window, inspection hole cover, and the module for conditioning and digital processing of PD pulses constitutes a complete PD sensor labeled as ISO-002-ADW/UHF (Figure 8). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Indication** & **Value (mm)** \\ \hline Width of the patch & \(L_{1}\) & 62.5 \\ Length of the patch & \(L_{2}\) & 100 \\ Length of the slit & \(S_{1}\) & 50 \\ Width of the slit & \(S_{2}\) & 2 \\ Distance between patch and ground plane & \(H\) & 5 \\ Diameter of the circular ground plane & \(D\) & 14 \\ Distance between feeding point and resistor & \(d\) & 26.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Dimensions of the prototype meandered PIFA. Figure 6: Simulated VSWR for meandered PIFA with various loading chip resistors and constant \(H\) = 5.0 mm. Figure 7: Simulated VSWR for prototype meandered PIFA with various distances \(H\) between patch and ground plane and with the same loading chip resistor with resistance \(R\) = 5.4 \(\Omega\). Having installed the antenna in the active dielectric window, the VSWR characteristic was measured using a vector network analyzer type KC901S+ (Deepace, Guangdong, China). The measurements showed that compared to the simulation results, the VSWR curve is slightly shifted towards lower frequencies (Figure 9). It is probably an effect of the antenna surroundings influence, i.e., metal transformer tank and inspection hole cover, which were not taken into account in the simulations. The measured resonant frequency of the antenna was equal to 329 MHz (VSWR = 1.13; S\({}_{11}\) = \(-\)24.3 dB), and \(-\)6 dB bandwidth equal to 73 MHz (from 280 to 353 MHz). In order to check the performance of meandered PIFA, comparative research was conducted, in which a commercial UHF disk sensor with a diameter of 15 cm and measured resonant frequency \(f_{r}\) = 475 MHz was used as a reference antenna. Both sensors were installed in an oil-filled transformer tank of dimensions \(1200\times 800\times 750\) mm, equipped in two ceramic dielectric windows and an electrode system for generating surface discharges on the pressboard sample. The distance between the PD source and the central point of the antenna, in the case of both sensors, was the same and equal to about 45 cm. The pulses were registered with a four-channel oscilloscope type Tektronix MDO 3104 with a sampling frequency of 5 GS/s. The level of the apparent charge of the generated partial discharges was monitored by the use of the standard measurement set compliant with the IEC 60270 standard (Figure 10). Figure 8: Components of the ISO-002-ADW/UHF sensor: 1 = meandered PIFA, 2 = N-type connector, 3 = cylindrical PTFE-housing, 4 = active dielectric window, 5 = enclosure for partial discharge (PD) pulses conditioning module and analog-to-digital processing module, 6 = fiber optic cable gland, 7 = cable gland for power supply, 8 = shielding tube for signal cables, 9 = inspection hole cover. Figure 9: Simulated and measured VSWR of the prototype meandered PIFA.