# Bandwidth-Enhanced Circularly Polarized Crescent-Shaped Slot Antenna via Circular-Patch Loading
## 3 Experiment Results  
To validate the simulation results, a prototype of the proposed antenna with the optimized parameters, as tabulated in Table 1, was fabricated for measurement. Figure 6 displays the fabricated antenna, which has overall dimensions of 50.8 \(\times\) 50.8 \(\times\) 0.5 mm\({}^{3}\). The reflection coefficient was tested using an Agilent 8510C RF network analyzer. A comparison of the reflection coefficients is illustrated in Figure 7, in which good agreement was apparent between simulation and measurement results. The measured and simulated frequency bandwidths for \(|S_{11}|<-10\) dB were 92.33% (1.79-4.86 GHz) and 91.07% (1.80-4.81 GHz), respectively.  
Figure 5: Simulated electric-field distributions in the slot with period \(T\) at: (**a**) 2.1 GHz; (**b**) 3.5 GHz.  
Figure 8 presents the simulated and measured ARs and LHCP gains versus the frequency in \(\theta\) = 0\({}^{\circ}\) (broadside direction). The radiation characteristics of the fabricated antenna were measured in an RF anechoic chamber, which was 5.5 \(\times\) 5.5 \(\times\) 5.0 m. A dual-polarized rectangular horn antenna was utilized to acquire the AR and LHCP/RHCP waves. The 3 dB AR bandwidths were 86.08% (1.80-4.52 GHz) and 84.40% (1.89-4.65 GHz) in the simulation and measurement, respectively. It was evident that the 3 dB AR bandwidth was entirely enclosed by the \(-\)10 dB reflection bandwidth. In addition, the measured LHCP gains varied from 0.39 to 4.09 dBic within the measured 3 dB AR bandwidth. It was also observed that the antenna gains decreased as frequency increased. This occurred because the main beam was tilted from the broadside direction owing to the asymmetrical configuration of the proposed antenna. The simulated and measured normalized radiation patterns on the two principal planes (the \(x-z\) (\(\phi\) = 0\({}^{\circ}\)) and \(y-z\) (\(\phi\) = 90\({}^{\circ}\)) planes) for 2.1 and 3.7 GHz are displayed in Figure 9. The proposed antenna radiates LHCP in the \(+z\)-direction and RHCP in the \(-z\)-direction. In addition, the radiation patterns at 3.7 GHz are asymmetric and tilted from the broadside direction. Reasonable agreement between simulation and measurement was found. Discrepancy could be attributed to inaccuracies during the fabrication process and experimental tolerances.  
Figure 6: Fabricated antenna.  
Figure 7: Simulated and measured reflection coefficients of the proposed antenna.  
Figure 10 plots the simulated and measured ARs versus the observation angle of the proposed antenna at two frequencies, 2.1 and 3.7 GHz. As observed, due to the higher cross-polar levels that  
Figure 8: Simulated and measured ARs, and left-handed circular polarization (LHCP) gains versus frequency.  
Figure 9: Simulated and measured normalized radiation patterns of the proposed antenna: (**a**) 2.1 GHz; (**b**) 3.7 GHz.  
could be found in the radiation patterns at higher frequencies, the 3 dB AR beamwidth became slightly narrower, with an increase in frequency. The measured 3 dB AR beamwidths at 2.1 GHz were 69\({}^{\circ}\) and 78\({}^{\circ}\) on the \(x-z\) plane and \(y-z\) plane, respectively. At 3.7 GHz, the measured 3 dB AR beamwidths were 55\({}^{\circ}\) on the \(x-z\) plane and 76\({}^{\circ}\) on the \(y-z\) plane. Furthermore, reasonable agreement was found between simulation and measurement results.  
To evaluate the performance of the antenna, Table 2 summarizes a comparison of the key performance characteristics of this work and the previously published CP slot antennas [8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Note that \(\lambda_{c}\) represents the wavelength at center frequency \(f_{c}\) of the 3 dB AR bandwidth. As can be observed in Table 2, the performance of the proposed antenna is competitive, exhibiting broader CP bandwidth as compared to the previous antennas. The proposed antenna also has an acceptable gain for a common slot antenna. Moreover, the antenna size of 0.55 \(\times\) 0.55 \(\lambda_{c}^{2}\) is more compact than in earlier works [9; 11; 12; 13; 15; 16].