# Millimeter-Wave High-Gain Dual-Polarized Flat Luneburg Lens Antenna with Reflection Cancellation
## 4 Implementation, Measurements and Discussions
### Measurement of FLLA  
The fabricated prototype was measured to characterize its performance following a standard procedure. A vector network analyzer (VNA), Keysight N5225B (10 MHz-50 GHz), was used to measure the \(S\)-parameter. The measured \(S\)-parameter of the FLLA is shown in Figure 13a. It can be seen that the measured common impedance bandwidth for two ports is 27.5-32.6 GHz in terms of both \(S_{11}<-10\) dB and \(S_{22}<-10\) dB, and the measured couplings between two ports are smaller than \(-16\) dB over the bandwidth. The measured curves are in accordance with the simulations in trends, and a small frequency shifting is observed on \(S_{11}/S_{22}\) curves. The far-field radiation patterns of the fabricated FLLA were measured in a far-field anechoic chamber (0.5-110 GHz) in Southeast University. The measured gain of both the FLLA and the feeding stacked patch antenna is shown in Figure 13b. The measured gain curves exhibit a reasonable agreement with the simul  
Figure 12: Photos of prototypes: (**a**) feeding stacked patch antenna; (**b**) FLL; (**c**) entire FLLA; (**d**) measuring in anechoic chamber. Figure 13: Measured \(S\)-parameter and gain of prototypes: (**a**) \(S\)-parameter for FLLA; (**b**) gain for FLLA and feeding patch antenna. a measured gain of 16.55-18.88 dBi for port-1 and 16.5-18.67 dBi for port-2 over the impedance bandwidth, comparing with the measured gain of 6.4-8.9 dBi for the feeding antenna. The slight difference between the gain of port-1 and port-2 is assumed to be affected by the used supporter. Without loss of generality, the measured radiation patterns of the FLLA excited by port-1 were compared with simulations at 27, 29, and 31 GHz, respectively, as shown in Figure 14, which show good agreements. The measured 3 dB beam-widths in the E- and H-planes were 17\({}^{\circ}\) and 16.5\({}^{\circ}\), respectively. The measured side lobe levels (SLLs) were smaller than \(-\)13 dB in the E-plane and \(-\)15 dB in the H-plane. The cross-polarization levels were \(-\)19 dB in the main lobe for 27 and 31 GHz, while that for 29 GHz degenerated to \(-\)15 dB. Meanwhile, the FTBRs were greater than 19.5 dB at all three frequencies, demonstrating that a desired small reflection was achieved. Figure 14: Simulated and measured radiation patterns of the FLLA: (**a**) E-plane at 27 GHz, (**b**) H-plane at 27 GHz, (**c**) E-plane at 29 GHz, (**d**) H-plane at 29 GHz, (**e**) E-plane at 31 GHz, (**f**) H-plane at 31 GHz. Without loss of generality, the scanned beams in the E- and H-plane when port-1 is excited were measured, as shown in Figure 15. The measured steering angles in E-plane were 0\({}^{\circ}\), \(\pm\)7\({}^{\circ}\), \(\pm\)17\({}^{\circ}\), \(\pm\)22\({}^{\circ}\), and \(\pm\)25\({}^{\circ}\), corresponding to the feeding antenna's shifts of 0, 0.35, 0.7, 1.05, 1.2 mm in the E-plane, respectively. The steering angles in the H-plane were 0\({}^{\circ}\), \(\pm\)6\({}^{\circ}\), \(\pm\)15\({}^{\circ}\), \(\pm\)19\({}^{\circ}\), and \(\pm\)24\({}^{\circ}\), corresponding to the feeding antenna's shifts of 0, 0.35, 0.7, 1.05, 1.4 mm in the E-plane, respectively. In the measured scanning angle ranges, the gain loss in either the E- or H-plane was smaller than 1.72 dB or 1.7 dB, respectively. The measured scanning beams exhibited narrow angle ranges and smaller gain loss than those of simulations. The reason is that the actual shifts in the feeding antenna cannot be accurately controlled as desired at a millimeter scale, which would involve an error to the steering angles. Meanwhile, the used supporter was backed to the feeding patch antenna, prohibiting the backward movement of the feeding antenna. But this cannot ensure that the feeding antenna moves exactly in the transvers plane. If a forward movement is happened during the transverse moving and the feeding patch antenna is closer to the FLL in turn, the gain will be increased for the oblique radiated beams.