We explore the design of a high capacity multi-radio wireless network using commercial 802.11n hardware. We first use extensive real-life experiments to evaluate the performance of closely located 802.11n radios. We discover that even when tuned to orthogonal channels, co-located 802.11n radios interfere with each other and achieve significantly less throughput than expected. Our analysis reveals that the throughput degradation is caused by three link-layer effects: (i) triggering of carrier sensing, (ii) out of band collisions and (iii) unintended frequency adaptation. Using physical layer statistics, we observe that these effects are caused by fundamental limitations of co-located radios in achieving signal isolation. We then consider the use of beamforming antennas, shielding and antenna separation distance to achieve better signal isolation and to mitigate these problems. Our work profiles the gains of different physical isolation approaches and provides insights to network designers to realize high-performance wireless networks without requiring synchronization or protocol modifications.