# Partial Discharge Wideband Full-Band High-Gain Resonant Cavity UHF Sensor Research
## 3 Resonant Cavity UHF Antenna Sensor Design
### Microstrip Monopole Patch Antenna Body Design  
#### 3.1.1 Antenna Design Based on Curved Flow Technology  
The dielectric substrate of the microstrip monopole antenna designed in this paper is rectangular rigid material FR-4 epoxy resin, the relative dielectric constant of the substrate \(\varepsilon_{r}=4.4\), the dielectric loss tangent angle tan\(\delta=0.03\), and the thickness of the dielectric substrate \(h=2.8\) mm. The microstrip monopole antenna designed in this paper uses SMA coaxial feed with a characteristic impedance of \(50\)\(\Omega\).  
Since the energy of the PD signal is concentrated in the range of \(500\) MHz-\(1.5\) GHz, and the common monopole patch antenna is often used in the frequency band above \(3\) GHz in the communication field, if we want to make it detect partial discharge, we need to extend its working band.  
In this paper, the patch of monopole patch antenna in Figure 1 is slotted, and a stepped structure is added by using the curved flow technique, and finally the antenna is constructed by using HFSS software as shown in Figure 3.  
Figure 3: Diagram of monopole antenna based on curved flow technique: (a) antenna front; (b) antenna back.  
Curved flow technology is a technique to make the surface current flow along a specific structure on the metal patch of microstrip antenna by making slots or adding slits, introducing multi-step structure, etc. Such a design can free the equivalent length of the antenna from the physical length limitation, thus effectively increasing the equivalent length of the microstrip antenna and improving the bandwidth of the antenna without increasing its size [15; 22; 23; 24]. Each parameter of the monopole patch antenna based on the curved flow technique is shown in Table 1.  
HFSS software is used to simulate and analyze the antenna with the above parameters and structure, and the data of VSWR is shown in Figure 4. The VSWR of the monopole antenna improved by the curved flow technology is less than 5 in 380 MHz-3 GHz, and the working band of the antenna basically covers the frequency band of the UHF signal, which meets the design requirements of the PD UHF sensor for power equipment and can be optimized subsequently.  
Figure 5 shows the simulated gain direction of the antenna at the center frequency point of 1 GHz, the highest gain is only 1.27 dBi, and the directionality is poor, necessitating subsequent optimization to improve the directionality of the antenna gain.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value (mm)** \\ \hline Substrate length \(L_{\text{S}}\) & 205.2 \\ Substrate width \(W_{\text{S}}\) & 241.2 \\ Patch length \(L_{\text{P}}\) & 134.0 \\ Patch width \(W_{\text{P}}\) & 120.6 \\ Rectangular slot length \(L_{\text{MI}}\) & 42.9 \\ Rectangular slot width \(W_{\text{MI}}\) & 60.9 \\ Step structure length \(L_{\text{MZ}}\) & 6.7 \\ Step structure width \(W_{\text{MZ}}\) & 44.2 \\ Microstrip line length \(L_{\text{W}}\) & 53.6 \\ Microstrip line width \(W_{\text{W}}\) & 5.1 \\ Dielectric substrate thickness h & 2.8 \\ Floor length \(L_{\text{G}}\) & 48.9 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Geometric parameters of monopole antenna based on curved flow technique.  
Figure 4: VSWR diagram of curved current monopole antenna.