## 3 Experimental Results and Discussions
### Experimental Results  
The S-parameters of the fabricated array were measured by using an Agilent vector network analyzer (E8363B). Figure 12 shows the measured and simulated reflection coefficients of the proposed antenna. The measured input reflection coefficients are below  
Figure 11: Photographs of some crucial parts of the antenna prototype. (**a**) RGW-to-GGW divider. (**b**) GGW-to-RGW divider. (**c**) Capacitive slits of GGW. Figure 10: Fabricated antenna array prototype. (**a**) SLA-printed prototype. (**b**) Surface-metalized prototype. (**c**) Surface-polished prototype. (**d**) Unassembled parts. (**e**) Fully assembled antenna structure. (**f**) Measurement setup. \(-10\) dB within the bandwidths 19.91-20.72 GHz (3.9%) in K-band and 28.22-29.19 GHz (3.5%) in Ka-band. The corresponding impedance matching bandwidths acquired from simulation results are 19.76-20.77 GHz and 27.45-29.41 GHz for the K- and Ka-bands, respectively. While the measured bandwidths are smaller than the simulated values, both measurements and simulations show reasonable agreement, especially on the response of the antenna prototype in the upper region of the two frequency ranges. The discrepancy observed between the measurement and simulation results may be attributed to fabrication tolerance and imperfections that resulted from the 3D printing and metallization processes. Figure 13 shows the measured and simulated coupling levels between the two ports of the antenna structure. The measurement and simulation results show that a high level of isolation (>35 dB) between the two ports is realized over the two frequency bands. This is partly due to the stacked structure that allows the K-band and Ka-band arrays to be at different layers. The far-field radiation patterns of the antenna were measured in an anechoic chamber, as shown in Figure 10f. Figures 14 and 15 present the simulated and measured normalized radiation patterns of the array in the x-z and y-z planes at 20 and 28.5 GHz. Overall, the measurement results are in good agreement with the simulation results. In the x-z plane, the 3 dB beamwidths of the RHCP and LHCP beams are \(14^{\circ}\) and \(13^{\circ}\), respectively, and the sidelobe levels are lower than \(-13.3\) and \(-12.2\) dB, respectively. The patterns in the y-z plane are similar to those obtained for the x-z plane. Figure 16 shows the simulated and measured axial ratios (ARs) as functions of frequency over the lower and higher operating bands. The 3 dB AR bandwidths determined from the measurements are 19.89-20.52 GHz (3.1%) and 28.26-29.31 GHz (3.6%), which mostly overlap with the \(-10\) dB impedance matching bandwidths for both bands. A noticeable difference can be seen between the simulated and measured results in a small fraction of the higher band for frequencies  
Figure 12: Simulated and measured reflection coefficient versus frequency. (**a**) S11 for lower frequency band. (**b**) S22 for higher frequency band. Figure 13: Simulated and measured inter-port coupling level versus frequency. (**a**) S12 for lower frequency band. (**b**) S21 for higher frequency band. below 28.5 GHz. This can be caused by multiple factors, including fabrication error and measurement uncertainty of the far-field characterization system. The gains and simulated efficiency of the antenna array at several frequency points within its two operating bands are summarized in Table 3. The maximum measured gains are 20.1 and 20.3 dB for the lower and higher band, respectively. The antenna prototype provides efficiency of at least 88.9% over both operating frequency bands. Given the aperture size 52 mm \(\times\) 52 mm, the calculated aperture efficiencies of the array are 66.25% at 20 GHz and 32.6% at 28.5 GHz. The results indicate that the proposed dual-band, dual-CP antenna can simultaneously achieve high efficiencies, high aperture efficiencies, and high gains. Figure 16: Simulated and measured axial ratio for the (**a**) lower frequency band and (**b**) higher frequency band.