# High-Gain Waveguide-Fed Circularly Polarized Spidron Fractal Aperture Antenna
## 3 Experimental Results and Discussion  
The proposed antenna was implemented, and the results are demonstrated. Photographs of the fabricated sample of the Spidron fractal aperture and an assembled model are shown in Figure 7. The antenna was fed directly by the WR-90 waveguide-to-coax adapter (model number: 16094-SF40). An Agilent 8510C RF network analyzer was employed to test the antenna reflection. Figure 8 shows the measured and simulated reflection coefficients. It was observed that the measured frequency bandwidth with \(-10\) dB reflection was 9.89-11.58 GHz (15.74%). Good agreement was observed between the measurement and the simulation.  
Figure 6: Effect of the superstrate on antenna performance: (**a**) reflection coefficients; (**b**) axial ratios (ARs); (**c**) RHCP gains (RHCP: right-handed circular polarization).  
The AR value and realized RHCP gain of the implemented antenna in \(\theta\) = 0\({}^{\circ}\) are shown in Figure 9. The measured 3 dB AR bandwidth is 10.68-11.00 GHz (2.95%), with a minimum AR value of 0.66 dB at 10.85 GHz. The measured 3 dB AR bandwidth is slightly shifted toward a lower frequency range compared with that in the simulated results. Within the operating AR frequency bandwidth, the measured RHCP gain varies from 8.73 to 9.59 dBic. The discrepancy between the measurement and the simulation can be attributed to fabrication tolerance and the misalignment between the Spidron fractal aperture and the waveguide aperture. Figure 10 presents the simulated radiation efficiency of the proposed antenna versus the frequency. The antenna efficiency within the measured 3 dB AR bandwidth is approximately 91%. Figure 11 plots the measured and simulated radiation patterns of the antenna on the _xz-_ (\(\phi\) = 0\({}^{\circ}\)) and _yz-_planes (\(\phi\) = 90\({}^{\circ}\)) at 10.85 GHz. It can be seen that the antenna radiates RHCP waves and that the radiation patterns are directional, radiating toward the broadside direction. Furthermore, the measured RHCP gains are 21.5 dB higher than the left-handed circular polarization (LHCP) gain in the broadside direction on both planes. The simulated and measured ARs versus the observation angle of the proposed antenna at 10.85 GHz are illustrated in Figure 12. As observed, the measured 3 dB AR beamwidths are 46\({}^{\circ}\) and 43\({}^{\circ}\) on the xz- and yz-planes, respectively. Reasonable agreement between the simulation and the measured results is achieved.  
Figure 8: Measured and simulated reflection coefficients of the proposed antenna.  
Figure 7: (**a**) Photograph of the fabricated sample of the Spidron fractal aperture; (**b**) assembled antenna.  
Figure 10: Simulated radiation efficiency of the proposed antenna versus the frequency.  
Figure 9: Measured and simulated axial ratios (ARs) and RHCP gains versus the frequency of the proposed antenna.  
Comparisons between the proposed waveguide-fed aperture antenna and similar counterparts reported in the literature are summarized in Table 2. Note that all of these antennas are fed by a WR-90 open-ended waveguide. As observed, all of the antennas in one study [4] are linearly polarized (LP) antennas, while the proposed antenna exhibits CP operation. Compared with another waveguide-fed CP antenna [11], which also includes a superstrate to achieve gain enhancement, the proposed antenna has a wider impedance bandwidth and a higher gain.  
Figure 11: Measured and simulated radiation patterns at 10.85 GHz: (**a**) \(xz\)-plane; (**b**) \(yz\)-plane. LHCP: left-handed circular polarization.  
Figure 12: Measured and simulated axial ratios (ARs) of the proposed antenna versus the observation angle at 10.85 GHz.