Octave-Bandwidth Single-Polarization Horn Antenna with Negligible Loss
The super-extended C-band presents one of the greatest challenges to traditional low-sidelobe horns, with its comparatively long wavelength requiring large, massive structures. The trifurcated horn does not require excessive mass at C-band compared to a simple horn, but its performance is less than optimal. Here, we consider a metamaterial-based rectangular horn that simultaneously maintains both the low-sidelobe and low-backlobe performance of the corrugated horn and the low weight of the trifurcated horn.
Figure 2.6 compares the aperture distributions and threedimensional radiation patterns for several rectangular horns: an unlined horn, a trifurcated horn, and a metamaterial horn. For an unlined rectangular horn, the significant sidelobes are in the Я-plane, resulting from the abrupt change in the aperture field magnitude at the Я-plane walls of the horn, shown in Fig. 2.6a. The Я-plane sidelobes are very low as a result of the smoothly tapered aperture field magnitude in the Я-plane. The trifurcated horn creates a stepped aperture distribution in the Я-plane, resulting in lower primary sidelobes, but comparable backlobes to an unlined horn, shown in Fig. 2.6b. Placing the appropriate metamaterial liner on the Я-plane walls of the horn creates a smoothly tapered aperture field distribution in the Я-plane, shown in Fig. 2.6c. With the aperture field magnitude tapered in all planes, the metamaterial horn radiates a pattern with very low sidelobes and backlobes, unlike horns with more uniform field distributions, such as the plain unlined horn or the trifurcated horn.
The horn considered here has a 2.8A x 2.6A (25 x 23 cm) aperture, and a total length of 5.7A (50 cm), where l is calculated at the low end of the band. The feed waveguide is WR177. These values lead to a flare angle of 23°. The Я-plane dimension was chosen to give approximately equal Я- and Я-plane patterns, maximizing the efficiency when illuminating a circularly symmetric reflector. The waveguide feed was chosen to be close to cutoff at the low end of the band in order to maximize the operating bandwidth. These dimensions lead to a peak gain ranging from about 16 to 24 dB across the super-extended C-band.
Figure 2.6 Aperture electric field distributions and 3D radiation patterns for the (a) unlined horn, (b) trifurcated horn, and (c) metahorn. Note that the metahorn outperforms both of the alternatives in sidelobe levels and especially in backlobe levels. This performance is comparable to that available from a corrugated horn, but at a fraction of the weight. Reprinted, with permission, from Ref. 14, Copyright 2013, IEEE.