# Wideband Circularly Polarized Antenna Based on a Non-Uniform Metasurface
## Antenna Design
### Antenna with Non-Uniform MS  
Next, the MS is modified with different sizes of unit cells (\(W_{i}\
eq W_{o}\)) to achieve better performance. The simulated performances of the antenna with a uniform MS and non-uniform MS are illustrated in Figure 5. It can be seen that, by using a non-uniform MS, an additional CP band around 6.8 GHz is  
Figure 4: Simulated AR of the uniform MS-based antenna with different values of (**a**) patch size (\(W_{p}\)) and (**b**) unit cell size (\(W_{i}=W_{o}\)).  
Figure 3: (**a**) Geometry and simulated (**b**) \(|\)S\({}_{11}\)] and (**c**) axial ratio (AR) of the antenna with and without a uniform MS. The optimized dimensions for the antenna with MS are as follows: \(W_{s}\) = 40, \(H_{s}\) = 1.52, \(W_{p}\) = 12, \(d\) = 6.8, \(L_{f}\) = 8.5, \(W_{f}\) = 3, \(p\) = 10, \(W_{i}\) = \(W_{o}\) = 8.8 (unit: mm).  
produced, which contributes to significantly increasing the antenna's overall 3 dB AR BW to 33.3% (5.0-7.0 GHz). The additional CP resonance can be adjusted by tuning the size of the outer MS unit cell, \(W_{o}\). Figure 6 shows the antenna's AR characteristics for different values of \(W_{o}\). Obviously, tuning \(W_{o}\) has a significant influence on the CP band at around 6.8 GHz, which increases as \(W_{o}\) is decreased.  
Since the MS is a radiating aperture of the proposed antenna, the surface currents (\(J_{s}\)) on the MS are considered to demonstrate the CP radiation of the proposed antenna. Figure 7 shows the simulated \(J_{s}\) at the three different minimum AR points of 5.2, 6.2 and 6.8 GHz. It can be seen that, in these cases, when the phase changes from \(0^{\circ}\) to \(90^{\circ}\), the vector \(J_{s}\) is rotated in a clockwise direction. Thus, the antenna will generate left-hand CP (LHCP) radiation in the broadside direction. At 5.2 GHz, this resonance is created by the driven patch, and thus the \(J_{s}\) is highly concentrated at the four center unit cells of the MS. At 6.2 and 6.8 GHz, more currents are occupied in the outer unit cells because these resonances are generated by the surface waves propagating on the MS. However, it can be seen obviously that at 6.9 GHz, the \(J_{s}\) on the outer unit cells is very strong. This explains why an additional band can be obtained by modifying the uniform MS to a non-uniform MS.  
Finally, the impedance matching of the proposed design is considered. The parameter studies indicate that the feeding position is a critical parameter to determine the antenna's matching performance. Figure 8 shows the simulated reflection coefficients and ARs against the variation of feeding position, \(L_{f}\). Here, increasing \(L_{f}\) results in a significant improvement in the matching performance. Regarding the AR response, tuning the feeding position changes the length of the feeding  
Figure 5: Simulated (**a**) \(|\)S\({}_{11}\)1 and (**b**) AR for the antenna using a uniform MS and non-uniform MS.  
Figure 6: Simulated (**a**) \(|\)S\({}_{11}\)1 and (**b**) AR of the non-uniform based antenna with different sizes of the outer unit cell, \(W_{o}\).  
line. This variation will lead to different coupling scenarios between the driven patch and the MS. Thus, the AR characteristic, particularly in the high-frequency band, is affected.  
Figure 8: Simulated (**a**) \(|\)S\({}_{11}|\) and (**b**) AR for the non-uniform based antenna with different feeding positions, \(L_{f}\).  
Figure 7: Simulated surface current distributions at (**a**) 5.2 GHz, (**b**) 6.2 GHz and (**c**) 6.9 GHz for the antenna with a non-uniform MS.