Dual-Polarization Ku-Band Metamaterial Horn
Many satellite communication systems employ dual-polarized Ku- band antennas, including corrugated horns as reflector feeds. Ku- band corrugated horns, while providing excellent performance over bandwidths approaching an octave, are relatively expensive to manufacture with sufficient tolerances, and they have a high weight. Metasurfaces based on PCB manufacturing, however, promise a less expensive and lightweight option while providing comparable sidelobe levels and cross-polarization to corrugated horns.
Modeling and Simulation
Figure 2.12 shows the geometry of the metahorn. The horn is fed by a WR62 waveguide, which connects via a tapered adapter to a 15.8 mm square waveguide, followed by a straight section of square waveguide before connecting to the throat of the metahorn itself. The chosen horn dimensions lead to a gain in the vicinity of 20 dB across the operating band. The metamaterial liner is the printed- patch balanced hybrid metasurface presented earlier.
Figure 2.12 Square metahorn geometry, including details of the dielectric mode-matching plug located in the horn throat.
Unlike the wire-grid horn, the metasurface in the dual-polarized square horn must extend all the way to the throat of the antenna to avoid increased cross-polarization levels from the higher-order modes that would be excited. Similar to corrugated horns, the square metahorn then required a mode converter at the horn throat to convert from the TE10 fundamental waveguide mode to the desired hybrid mode. Corrugated horns achieve mode conversion through a sequence of grooves (corrugations) of varying depth in the waveguide and horn throat. As corrugations would negate the potential cost and weight savings of the metasurfaces, an alternative approach had to be found. Analogous to dielectric core horns, a dielectric plug was created, but terminated at the horn throat, as shown in Fig. 2.12. Typical dimensions of the plug were I = 0.6A, d = 0.4A, Ia = 0.6A, and Ib = 0.16A. As evidenced by later measurements, this dielectric plug proved to provide an effective conversion mechanism for the waveguide modes, leading to low cross-polarization across the entire ^,-band.
Because of the dielectric materials involved, a high-frequency structure simulator (HFSS) proved to be the suitable choice for simulating the horn’s performance. As with the wire-grid horn, symmetry allowed the size of the structure to be reduced to a quarter of the full horn. The quarter-metahorn was simulated, lined with the metamaterial structure, as shown in Fig. 2.12. The metasurface liner consisted of four trapezoidal panels fitting the walls of the horn. Partial unit cells were removed from the edges for manufacturability; their absence did not significantly affect the horn’s performance because the electromagnetic fields are minimal in the corners of a soft horn.