It’s true that “On Atheros-based cards, encryption (WEP, WPA, etc.) does not work when compression is enabled”.
In which page of the manual is written that…?
Thanks
It is written in wireless documentation,
http://www.mikrotik.com/testdocs/ros/3.0/interface/wireless.php
Ok. Thanks Sergejs
Where does this limitation come from?
Where does this limitation come from?
Atheros themselves.
(I believe this is because there is not enough CPU power to do both, therefore Atheros have set the hardware to only allow one or the other, not both.)
FYI: if typed “atheros compression” in the search field no entries came up. You might want to look into improving your docs site. Just as the OP I find it difficult to find stuff to the point that I’m contemplating on making local copy of your website and indexing it myself 
The ubnt radios are atheros based and they can do both WPA and compression at once.
More precisely only atheros chipsets support compression. However encryption is not suported when compression is turned on.
Above link does not mention it however I read elsewhere that compression is not supported on RB4xx at all even if chipset supports it.
Personally, I use Google to search the wiki. The built in wiki search is not very good.
http://www.google.com/search?as_q=atheros+compression&as_sitesearch=wiki.mikrotik.com