# Wideband UHF Antenna for Partial Discharge Detection
## 4 Fabrication and Measured Results  
The proposed UHF antenna was printed on a FR4 substrate with the dimensions of \(89.5\,\mathrm{mm}\times 138\,\mathrm{mm}\times 1.6\,\mathrm{mm}\), and the photograph of the fabricated antenna is shown in Figure 10. The S\({}_{11}\) of the antenna was measured in an anechoic chamber (W = 3 m, L = 6 m, and H = 3 m) using a vector network analyzer (Agilent Technologies E5071C). Figure 11 depicts the simulated and measured S\({}_{11}\) parameters of the proposed antenna. It is shown that S\({}_{11}\) was less than \(-10\) dB from 740 to 1500 MHz. The measurement result agreed well with the simulation result. Slight discrepancies between the simulation and measurement could be caused by fabrication and measurement errors.  
In order to estimate the PD detection ability, we set up a test system in our laboratory, as shown in Figure 12. We used a RG316 coaxial cable to imitate the conducting ground wire of the HV power equipment. Toward this purpose, the protective dielectric layer and the metal net in the middle of the coaxial line were peeled off so that the EM waves could be emitted to the outside. At the end of the coaxial cable, we connected a 50 ohm termination to imitate the wire that is grounded to the earth. The other end of the coaxial cable was connected to port 1 of the vector network analyzer. The fabricated  
Figure 11: Simulation and measurement results (S\({}_{11}\)) of the proposed UHF antenna.  
Figure 10: The prototype of the proposed UHF antenna: (**a**) top view and (**b**) bottom view.  
antenna was placed below the center of the coaxial cable, same as Figure 3. The SMA connector of the antenna was connected to port 2 of the vector network analyzer. The PD detection ability could be inferred by the S\({}_{21}\) parameter. Note the test system employed in this work was not exactly same as the previous simulation in Section 2 because there were discontinuities of the peeled coaxial cable. Nonetheless, the test setup in our work could obtain the trends of the coupling performance since the S\({}_{21}\) parameter was proportional to the coupling coefficient of Section 2.  
We investigated the S\({}_{21}\) parameters for the different distances (10 mm, 20 mm, and 30 mm) between the peeled coaxial cable and the antenna. Figure 13 shows the actual measurement picture for the distance of 10 mm. As shown in Figure 13, the 10 mm-thick Styrofoam was placed under the ends of the coaxial cable and fixed with tape, which made the coaxial cable to be firmly placed 10 mm above the antenna. Note that more Styrofoam was stacked for the tests with longer distances. As shown in Figure 14, the S\({}_{21}\) parameters lay between \(-10\) and \(-40\) dB in both simulation and measurement results. Discrepancies between the measurement results and simulation results might be caused by fabrication and measurement errors including antenna misalignment, the non-perfectly straight peeled coaxial cable, etc. It should be noted that it would be useful to provide a exact methodology to extract the coupling coefficient of Section 2 using the available measurement equipment. In future work, this extraction methodology should be addressed and studied.  
Figure 12: PD experimental set-up for the antenna test.