I am not reaching theoretical wireless throughput

Issue: The device overview shows that my devices are not connecting at full speeds. 802.11n throughput depends heavily on 40MHz channel width and additional antennas.

• iPad 1 : 802.11n certified but 1 Antenna, 20MHz only : max. theoretical throughput: 65Mbit/s
• 1 Antenna, 40MHz : 150Mbit
• Netgear WNDA3200 USB Stick: 802.11n , 2 Antenna, 40MHz : max. theoretical throughput: 300Mbit/s
• 3 Antenna , 20MHz: 195Mbit
• 3 Antenna , 40MHz: 450Mbit

The iPad 1 is in compliance with 802.11n but delivers only slightly better performance than previous 802.11g.

Root cause: 40MHz in 2.4GHz bands is often deactivated because of interference with other channels. There are only 3 20MHz channels without overlapping available in the 2.4.GHz band.

Solution: For a complete table see IEEE_802.11n-2009