## 2 Design and Implementation of On-Line PD Monitoring System
### Partial Discharge Detectors
#### 2.2.2 Ultra-High Frequency Antenna
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Component** & **Parameter** & **Symbol** & **Value** & **Unit** \\ \hline \multirow{6}{*}{Matching layer 1  } & Thickness & \(d\) & 5.05 & mm \\  & Density & \(\rho\) & 2079 & kg/m3 \\  & Longitudinal velocity & \(V_{L}\) & 3029 & m/s \\  & Shear velocity & \(V_{S}\) & 1640 & m/s \\  & Bulk modulus & \(K\) & 11.62 & GPa \\  & Shear modulus & \(G\) & 5.59 & GPa \\  & Acoustic impedance & \(Z_{M}\) & 6.3 & MRayl \\ \hline \multirow{6}{*}{Piezoelectric ceramics (PZT-5A disc)} & Thickness & \(d\) & 6.2 & mm \\  & Radius & \(r\) & 6.35 & mm \\ \cline{1-1}  & Area & \(A\) & 126.61 & mm2 \\ \cline{1-1}  & Acoustic impedance & \(Z_{0}\) & 31.0 & MRayl \\ \hline \multirow{6}{*}{Backing layer 2  } & Thickness & \(d\) & 8.8 & mm \\  & Density & \(\rho\) & 2546 & kg/m3 \\ \cline{1-1}  & Longitudinal velocity & \(V_{L}\) & 1790 & m/s \\ \cline{1-1}  & Shear velocity & \(V_{S}\) & 845 & m/s \\ \cline{1-1}  & Acoustic impedance & \(Z_{B}\) & 4.56 & MRayl \\ \cline{1-1}  & Bulk modulus & \(K\) & 5.74 & GPa \\ \cline{1-1}  & Shear modulus & \(G\) & 1.82 & GPa \\ \hline \hline \end{tabular}  
* 1 epoxy/alumina composite with 0.3825 volume fraction of alumina; 2 epoxy/tungsten composite with 0.08 volume fraction of tungsten.  
\end{table} 
Table 2: Parameters of piezoelectric transducer components. In the work of [21] the complex review of antennas applied for PD detection in the VHF/UHF band was presented. Among the commercial solutions, circular plate antenna (also called UHF disk sensor) prevails, since it has a simple construction and because it may be easily adjusted to the dimensions of the dielectric window, which usually is round-shaped. A disadvantage of this construction is a narrow bandwidth and the possibility to regulate the resonant frequency practically only by changing the diameter \(d\) of the circular plate (\(d=\lambda/4\) where \(\lambda\) is the wavelength). Therefore, in many research centers, the works on the elaboration of the antenna not only of compact dimensions but also of high gain and wide bandwidth, which covers the frequency band of the EM signals generated by PD in oil-paper insulation system are carried out. Among the most common wideband antenna constructions proposed by researchers, dedicated to PD detection are meander-line antenna [43], logarithmic spiral antenna [44], different types of Archimedean spiral antennas [45; 46], Vivaldi antenna [47], microstrip patch antennas [48; 49; 50], bio-inspired antennas [51; 52], and fractal antennas such as Hilbert curve fractal antenna [53; 54], H-fractal antenna [55], Peano fractal antenna [56], or Minkowski fractal antenna [57]. From the viewpoint of efficient PD detection in a power transformer, the key antenna parameter is properly adjusted bandwidth. The research results presented in [58] showed that PDs in oil-immersed power transformers generate EM pulses of frequencies from 150 to 700 MHz, wherein most of their energy was transmitted in a band of lower frequencies (up to 400 MHz). In turn, in [59] the authors registered EM signals simultaneously with the use of three ultra-wideband antennas, which jointly covered a wide measurement band from 20 MHz up to 18 GHz. The research results showed that the main PD types in paper-oil insulation (PD in oil, PD in gas bubbles in oil, inter-turn discharges, surface discharges and creeping sparks on pressboard) generate EM pulses, for which the majority of the energy is transmitted in the band from 250 to 390 MHz. For that reason, it was decided that the designed antenna would be optimized to operate in this band. In search of the most efficient and scalable antenna design, the authors tested different constructions of wideband antennas, among others logarithmic spiral antenna, circular-cross antenna, H-fractal antenna, Hilbert curve fractal antenna, and broadband planar inverted-F antenna (PIFA) [53; 55].