## 2 Antenna Design and Development
Figure 6: Simulated reflection coefficient of equivalent circuit of inverted L-shaped planar monopole antenna on a partial ground plane in vacuum and with FR-4 substrate. Figure 7: The evolutionary stages of the branch inverted L-shaped partial ground plane patch antenna: (**a**) Antenna I, (**b**) Antenna II, (**c**) Antenna III, (**d**) Antenna IV. Figure 8 compares the simulated reflection coefficients of Antennas I, II, III and IV and Figure 9 shows the corresponding simulated axial ratio. In Antenna I, the simulated RLBW (\(15_{11}\)\(1\leq-10\) dB) was 50.84% (1.1-1.85 GHz), achieving circular polarization (AR \(\leq\) 3 dB) with a 29.45% ARBW (1.1-1.48 GHz). The simulated RLBW and ARBW of Antenna II were 53.69% (1.09-1.89 GHz) and 64.53% (1.06-2.07 GHz). The square ring branch in Antenna II significantly enhanced the axial ratio bandwidth (64.53% vs. 29.45% for Antenna I). The simulated RLBW and ARBW of Antenna III were 54.66% (1.09-1.91 GHz) and 64.55% (1.07-2.09 GHz). In Figure 9, the solid square branch of Antenna III rendered AR close to 1 dB in the upper L-band (1.5-2.0 GHz). The low AR (\(\leq\)1 dB) was attributable to the larger surface area of the solid square branch, vis-a-vis the square ring branch (Antenna II). The larger surface area mitigated the coupling effect between the inverted L-shaped radiating patch and the adjoining branch (Figure 10). In Antenna IV, the simulated RLBW and ARBW were 53.12% (1.106-1.906 GHz) and 70.88% (1.02-2.14 GHz). The triangular corner truncation of the partial ground plane enhanced the ARBW from originally 64.55% (Antenna III) to 70.88%. The enhanced ARBW rendered Antenna IV suitable for a wide range of L-band applications. Figure 8: The simulated reflection coefficient of Antennas I, II, III and IV. Figure 9: The simulated axial ratio of Antennas I, II, III and IV. Figure 10a,b illustrate the coupling effect between the inverted L-shaped radiating patch and the adjoining branch at the center frequency of 1.5 GHz and the 0\({}^{\circ}\) phase of Antennas II and III. The coupling effect was mitigated as the surface area of the square branch increased (Antenna III). Figure 11a-d illustrate the surface current distribution of the inverted L-shaped patch antenna with a corner-truncated partial ground plane diagonally adjoined with a square branch (Antenna IV) at the center frequency (1.5 GHz) at 0\({}^{\circ}\), 90\({}^{\circ}\), 180\({}^{\circ}\), and 270\({}^{\circ}\) phase. The surface current density was very high around the feed line and relatively high near the adjoining area between the square branch and the corner-truncated partial ground plane. The perturbation induced by the diagonally adjoining square branch generated right-hand circular polarization (RHCP). At 0\({}^{\circ}\) phase, the surface current vectors traveled in the +X direction and rotated to the -Y direction at 90\({}^{\circ}\) phase. At 180\({}^{\circ}\) phase, the surface current vectors moved in the -X direction and turned to the +Y direction at 270\({}^{\circ}\) phase. The surface current rotated clockwise, achieving RHCP in the -Z direction. Figure 11: _Cont._  
Figure 10: The coupling effect between the inverted L-shaped radiating patch and the adjoining branch at the center frequency of 1.5 GHz and 0\({}^{\circ}\) phase: (**a**) Antenna II (square ring branch), (**b**) Antenna III (solid square branch).