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If you want a USB Wireless adapter that works out of the box with Backtrack 4 purchase the Long Range Alfa 500mW USB Adapter: AWUS050NH

The card can be put in monitor mode as well as packet injection.

I purchased mine from ebay: Here

Specs:
* Support IEEE 802.11n draft 3.0 standard
* Support IEEE 802.11 a/b/g standards
* Support 2.4GHz and 5.8GHz frequency band, MIMO (Multiple Input Multiple Output)
* High speed transfer TX data rate up to 150 Mbps
* High speed transfer RX data rate up to 300 Mbps
* High Transmitter Power
* Brand NEW

Specifications :

* Standards : IEEE 802.11a/b/g/n
* Interface: USB 2.0 mini USB
* Data Rate:
o IEEE 802.11b: 1, 2, 5.5, 11Mbps
o IEEE 802.11g: 6, 9, 12, 18, 24, 36, 48, 54Mbps
o IEEE 802.11N: 20MHz up to 144Mbps, 40MHz up to 300Mbps
* Frequency Band:
o IEEE 802.11 b/g : 2.412GHz – 2.484GHz
o IEEE 802.11 a: 5150 MHz – 5250 MHz (Japan) ; 5150 MHz – 5350 MHz and 5470 MHz – 5725 MHz (Europe) ; 5150 MHz – 5350 MHz and 5725 MHz – 5850 MHz (USA)
* Output Power:
o 27dBm@ 11 b/g
o 21dbm@11 b/g/n HT20
o 20dbm@11 b/g/n HT40
o 18dbm@ 11a -
o 13dbm@11a HT20-(14dbm)
o 14dbm@11a HT40-(13dbm)
* Sensitivity for IEEE 802.11 b/g:
o Legacy Mode:
1M : -91
11M : -91
6M : -93
54M : -77
o HT20 Mode:
MCS15 : -74
MCS8 : -92
MCS7 : -75
MCS0 : -92
o HT40 Mode:
MCS15 : -70
MCS8 : -89
MCS7 : -73
MCS0 : -88
o (Typically @PER < 8% packet size 1024 and @25°C ± 5°C)
* Sensitivity for IEEE 802.11 a:
o Legacy Mode:
6M : -93@5180MHz ; -92@5320MHz ; -93@5500MHz ; -93@5825MHz
54M : -78@5180MHz ; -78@5320MHz ; -78@5500MHz ; -78@5825MHz
o HT20 Mode:
MCS15 : -74@5180MHz ; -75@5320MHz ; -75@5500MHz ; -74@5825MHz
MCS8 : -92@5180MHz ; -93@5320MHz ; -92@5500MHz ; -88@5825MHz
MCS7 : -74@5180MHz ; -75@5320MHz ; -75@5500MHz ; -74@5825MHz
MCS0 : -92@5180MHz ; -93@5320MHz ; -92@5500MHz ; -88@5825MHz
o HT40 Mode:
MCS15 : -71@5190MHz ; -71@5310MHz ; -70@5590MHz : -68@5795MHz
MCS8 : -90@5190MHz ; -89@5310MHz ; -89@5590MHz : -86@5795MHz
MCS7 : -73@5190MHz ; -73@5310MHz ; -72@5590MHz : -69@5795MHz
MCS0 : -90@5190MHz ; -88@5310MHz ; -87@5590MHz : -87@5795MHz
o (typically @PER < 10% packet size 1024 and @25°C ± 5°C)
* Data Modulation Type: BPSK, QPSK, CCK and OFDM
* Security: 64/128bit WEP, 802.1X support, Wi-Fi Protected Access WPA, WPA-PSK, WPA2
* Supporting O.S.:
o Windows 2000 / XP / Vista
o Mac (10.4.x/10.5.x)
o Linux (2.4.x/2.6.x)