Prototype and Measurements
Figure 2.13 shows photographs of the fabricated prototype. The horn and metasurfaces were constructed separately, and then the four trapezoidal sections of metasurfaces were placed inside the horn. The metasurfaces were fabricated using standard PCB techniques, including vias. The dielectric mode-matching plug was mounted in the section of straight waveguide behind the horn throat. Alternatively, a metahorn could be constructed almost solely of the metasurface PCBs, as the ground plane layer of the PCBs could serve as the horn walls. The only potential issues are the mounting flange and the connections between the four PCB walls, depending on the requirements of the specific application. This approach would further reduce the weight and cost of the metahorn antenna.
Figure 2.14 shows the measured radiation patterns for the metahorn at 12, 14, and 17 GHz, compared with the patterns for an unlined horn. The low sidelobes and negligible backlobes indicate that the metasurface indeed tailors the horn’s field distributions appropriately for a soft surface, in contrast with the high sidelobes and backlobes exhibited by the unlined horn. Moreover, the main beams from the metahorn are nearly rotationally symmetric, i.e., the metahorn exhibits (approximately) a polarization-independent pattern—an ideal characteristic for dual-polarized communication systems with circularly symmetric reflectors.
Figure 2.13 Photographs of the fabricated square metahorn prototype, together with a close-up of the front and back of its metasurface.
Figure 2.15 shows detailed comparisons between the measured and simulated metahorn radiation patterns. Again we see the nearly identical main beams regardless of the f-plane. The patterns show some slight asymmetries, but these are likely artifacts of the measurement range rather than due to higher-order modes arising from imperfections in the horn. Cross-polarization is only apparent for the f = 45° plane, as expected. At 12 and 14 GHz, the crosspolarization remains below approximately -30 dB, but it creeps up around -25 dB at 17 GHz. This increase results from the excitation of undesired higher-order modes in the waveguide and/or horn. An improved mode converter would likely lower these levels.
Figure 2.14 Three-dimensional normalized radiation patterns for the measured metahorn (top) and simulated unlined horn (bottom).
Figure 2.15 Simulated and measured metahorn radiation patterns in principal planes at 12, 14, and 17 GHz. Note that the main beam patterns are approximately identical for all planes at a given frequency, producing radiation patterns that are nearly independent of polarization.
Figure 2.16 shows simulated and measured metahorn performance across the ^u-band. Measurements and simulations agree reasonably well. The metasurface exhibits a resonance just below 11.5 GHz, which leads to a corresponding peak in the curves. This resonance frequency is most likely the cutoff frequency in the horn throat, since the low-index metamaterial increases the cutoff frequency compared to that of an empty horn. Above that point and throughout the ^,-band, the metasurface operates on the tail of its resonance, which leads to better return losses and minimal absorption loss.
Figure 2.16 Summary simulated and measured performance of the square metahorn: directivity (a), sidelobe level (b), input reflection coefficient without (c) and with (e) the dielectric plug, cross-polarization measurements versus simulations (d), and simulated dielectric plug comparisons (f).
It is apparent from Fig. 2.16 that the dielectric mode converter is necessary to reduce cross-polarization levels. Moreover, modifying the plug design from its initial geometry to improve its mode conversion operation significantly improved cross-polarization levels at the higher frequencies, visible by the black dot-dashed curve in Fig. 2.16f, where the cross-polarization remains on the order of -30 dB across the ^,-band. Conveniently, the region around 15-16 GHz where the cross-polarization is highest lies outside the typical transmit and receive bands used for satellite applications.