Comparison of Metahorns with Homogeneous and Inhomogeneous Metasurfaces
Lining a cylindrical horn antenna with a homogeneous metasurface yielded the geometry and simulated performance shown in Figs. 2.19a-c. The horn's bandwidth was limited by the cutoff frequency at the low end and by the excitation of higher-order modes and thus undesired cross-polarization at the upper end of the band, both depending on the exact metasurface patch dimensions. Larger patches lowered the cutoff frequency, but also lowered the frequency at which the higher-order modes appeared and rose above the tolerated level of -30 dB.
Figure 2.19 (a) Rendering of horn with a homogeneous metasurface liner and
simulated 5n (b) and cross-polarization (c) for various metasurface dimensions. (d) Rendering of horn with an inhomogeneous metasurface liner. (e) Exponential curves defining the metasurface patch dimensions' dependence on position in the horn. Simulated 5n (f) and cross-polarization (g) for the inhomogeneous metahorn. Reprinted, with permission, from Ref. 16, Copyright 2013, IEEE.
Creating a horn with an inhomogeneous metasurface, as shown in Fig. 2.19d, allowed larger patches in the waveguide and horn throat to lower the waveguide cutoff frequency below 10 GHz, while smaller patches further out on the horn walls prevented higher-order modes from being excited significantly below 20 GHz, as visible in Figs. 2.19f-g. Rather than having a discrete transition between patch sizes, the exponential curves in Fig. 2.19e smoothly tapered the patch sizes to minimize any higher-order modes that would be excited by a metasurface transition.
For comparison, the cross-polarization of a comparable corrugated horn is also shown in Fig. 2.19g. Although the corrugated horn has the best cross-polarization level over a narrow frequency range at the low end of the ^u-band, the metahorn exhibits comparable or better cross-polarization over a much broader frequency range, including the entirety of the ^u-band.
Similar to corrugated horns, other important considerations for the design of metasurface-based hybrid-mode horns include the horn flare angle and the tapering profile of the metasurface unit cells. Numerical studies of these parameters and their effects on the horn performance showed that, like the corrugated horn, the flare angle has a significant effect on the horn's reflection and radiation properties, as shown in Figs. 2.20a,b. The tapered profile of the inhomogeneous metasurface patch dimensions also has a significant effect. Rapid tapering from the large patches to the small patches creates a stronger discontinuity that can excite higher-order modes and thus increase cross-polarization levels, while tapering that is too slow fails to suppress higher-order modes properly. In spite of these effects, it is readily apparent that the proper choice of flare angles and tapering profiles can lead to a metasurface-lined horn that performs comparably or better than the corrugated horn across a very broad bandwidth. As these metasurfaces operate on the tails of resonances and exhibit minimal intrinsic losses, the metahorn antennas have negligible added losses due to the metasurface liners.
Figure 2.20 Input reflection coefficient (a) and relative cross-polarization (b) for metahorns with various semi-flare angles. Metasurface patch tapering profiles (c) and their corresponding effects on the horn's cross-polarization (d). Reprinted, with permission, from Ref. 16, Copyright 2013, IEEE.