Integrated optical antennas are key components for on-chip light detection and ranging technology (LIDAR). In order to achieve a highly collimated far field with reduced beam divergence, antenna lengths on the order of several millimeters are required. In the high-index contrast silicon photonics platform, achieving such long antennas typically demands weakly modulated gratings with lithographic minimum feature sizes below 10 nm. Here, we experimentally demonstrate a new, to the best of our knowledge, strategy to make long antennas in silicon waveguides using a metamaterial subwavelength grating (SWG) waveguide core loaded with a lateral periodic array of radiative elements. The mode field confinement is controlled by the SWG duty cycle, and the delocalized propagating mode overlaps with the periodic perturbations. With this arrangement, weak antenna radiation strength can be achieved while maintaining a minimum feature size as large as 80 nm. Using this strategy, we experimentally demonstrate a 2-millimeter-long, single-etched subwavelength-engineered optical antenna on a conventional 220 nm SOI platform, presenting a measured far-field beam divergence of 0.1° and a wavelength scanning sensitivity of 0.13°/nm.