Why are there multiple antennas with 802.11n devices? Are the small USB 802.11n adapters with no external antennas just as good as the internal NIC with three big antennas?

One of the big benefits to 802.11n is its use of multiple simultaneous communication channels, which breaks the data stream into multiple parts and transmits each part across a different antenna to corresponding antennas on the receiving end. The technique is called MIMO.

All types of 802.11n NIC devices use multiple antennas. Just because you don’t see multiple antennas doesn’t mean they’re not
there. For example, the SMC SMCWUSBS-N 802.11n wireless USB adapter uses two internal antennas. The 802.11n standard should
allow up to four antennas, but most devices use two or three. This reduces the effectiveness of MIMO slightly, but USB and PC Card
NICs should provide just as much throughput as PCI NICs with full-sized antennas at close range.

Tags: Mobile Broadband, Bluetooth, Wi-Fi