# Partial Discharge Wideband Full-Band High-Gain Resonant Cavity UHF Sensor Research
## 4 Antenna Performance Simulation Analysis  
### Voltage Standing Wave Ratio  
Voltage Standing Wave Ratio (VSWR) is a parameter used to describe the ratio of reflected signal strength and forward signal strength at the input port of the antenna, measuring the impedance matching ability between the input and output circuits of the antenna. When the signal enters the transmission line and reaches the antenna sensor, part of the signal will be reflected back to the transmission line, while the other part will be transmitted into the air; these reflected waves will be superimposed with the original waveform, creating the so-called \"standing wave\", which has a negative impact on the transmitted energy and signal. VSWR can be calculated by the following equation:  
\[\mathrm{VSWR}=\frac{|U|_{\mathrm{max}}}{|U|_{\mathrm{min}}}=\frac{1+|\Gamma|}{ 1+|\Gamma|} \tag{12}\]  
where \(\Gamma\) is the reflection coefficient of the antenna input, defined as the ratio of reflected power to incident power. The lower the voltage VSWR, i.e., the closer to 1, the better the impedance matching effect of the input port, that is, more electromagnetic waves are transmitted to the antenna. On the contrary, the higher the voltage VSWR, the more impedance mismatch, indicating that more signals are reflected back, which will degrade the antenna performance, leading to signal loss, increased bit error rate, and other problems, and may even cause damage to the circuit.  
In addition, since the UHF antenna sensor operates in a UHF environment, the signal attenuates quickly, so it is necessary to ensure that as much signal as possible is transmitted to the antenna to ensure the sensitivity and accuracy of the antenna. Therefore, a lower VSWR value is critical for the quality and stability of the transmission circuit in this case, so a lower VSWR is a fundamental requirement for the design and use of high-performance UHF antenna sensors. When VSWR \(\leq\) 2 the antenna sensor receives the best electromagnetic energy, but in fact VSWR \(\leq\) 5 can meet the engineering needs, so this paper will use VSWR \(\leq\) 5 as the PD detection bandwidth of power equipment [28; 29; 30; 31; 32; 33; 34].  
The Fabry-Perot resonant cavity monopole antenna model in Section 3 above is constructed using HFSS simulation software, and the VSWR curves are obtained from its simulation in the frequency range of 300 MHz-3 GHz as shown in Figure 12. The simulation results show that the VSWR of the UHF antenna designed in this paper is less than 5 in the frequency range of 0.41 GHz-1.69 GHz and 1.75 GHz-3 GHz, which meets the design requirements.  
Figure 11: Resonant cavity monopole antenna structure diagram: (**a**) front of the feed antenna; (**b**) back of the feed antenna; (**c**) partially reflective cladding; (**d**) resonant cavity monopole antenna.  
The VSWR of the physical antenna was measured using an E5063A vector network analyzer manufactured by Agilent with the same sweep range of 300 MHz-3 GHz. As shown in Figure 13, in the 0.37 GHz-3 GHz VSWR \(\leq\) 5, VSWR performance is good. Comparing the measured and simulated results, the overall trend of the measured VSWR results is basically the same as the simulated results, and the measured results are basically better than the simulated ones in the whole measurement band, but the oscillation is more obvious and meets the requirements of engineering design.