We report a first-of-its-kind realization of directional transmission for smartphone-like mobile devices using multiple passive directional antennas, supported by only one RF chain. The key is a multi-antenna system (MiDAS) and its antenna selection methods that judiciously select the right antenna for transmission. It is grounded by two measurementdriven studies regarding 1) how smartphones rotate during wireless usage in the field and 2) how orientation and rotation impact the performance of directional antennas under various propagation environments. We implement MiDAS using the WARP platform, and evaluate it usmg a computerized motor to rotate the prototype according to traces collected from smartphone users in the field. Our evaluation shows MiDAS achieves median of 3dB increase in link gain. Combined with rate adaptation and power control, MiDAS also improves goodput and power saving. MiDAS does not require any changes to the network infrastructure, and is therefore suitable for immediate deployment.