Actually it looks like the TL-WN821N has the AR9287+AR7010 chip-set which is why
it doesn't have the 5Ghz range. I have ended my search though, Alfa wireless
adapters are fully compatible with BackTrack Linux 4 R2. I found this one that
has all the features I am looking for (AWUS051NH)
http://www.amazon.com/Alfa-AWUS051NH-802-11a-Wireless-9dBi/dp/B002BFO490 . I
was able to confirm its compatibility here:
http://www.backtrack-linux.org/forums/hardware-compatibility-list/39064-alfa-awus051nh-works-out-box-bt4-r2.html
. Search ended.