This paper reports on the detailed design and experimental demonstration of a metamaterial-enabled low-sidelobe horn antenna (metahorn) based on principles similar to those of earlier soft horn antennas. The target application is a linearly polarized feed horn in the super-extended C-band (3.4-6.725 GHz) for communication satellite reflector antennas. The paper describes the detailed design and manufacturing of the -plane metamaterial liner (metaliner) based on a freestanding wire grid without the need for a dielectric substrate material. The measured copolarized and cross-polarized antenna patterns from the feed horn demonstrate over an octave pattern bandwidth with negligible loss. The results show similar bandwidth with lower sidelobes and backlobes than those of the trifurcated horn that is currently used as the standard C-band feed for single linear polarization. This demonstration shows promise for lightweight metamaterial horns to replace heavy and expensive C-band corrugated horns.