I found lately Sept 11,2013 latest ath9k has finally the spectral feature like the proprietary driver, it was there a while ago but only lately I resync with ath9k code. I found this link http://blog.altermundi.net/article/playing-with-ath9k-spectral-scan/
and the graphical display of FFT samples
https://github.com/simonwunderlich/FFT_eval (ath9k and ath10k)
https://github.com/kazikcz/ath9k-spectral-scan
Some cool discussion at: http://comments.gmane.org/gmane.linux.drivers.ath9k.devel/8409
In 2015 found some more info via youtube https://www.bastibl.net/ath9k-spectrum-scanning/
This could be very cool to convert cheap laptop to cheap wifi RF spectrum analyzer to troubleshoot RF problem in the field, not sure how accurate or useful it can be, but it is worth a try.
I have tried FFT_eval with my XB114/DNXA-H1 AR9390 card and it is working fine, I’m still fooling around with that.
I have the luxury of “owning” real spectrum analyzer, the Agilent Fieldfox and the PSA (deluxe version), just to make some people jealous 😎
Update Feb 2015
It occurred to me that Broadcom also have spectral FFT stuff in the driver, I happened to work on the 802.11ac radio and found some code about FFT, I mean the close-proprietary driver not the open source version, however the lack of sample/test program make it difficult for me to try out their FFT stuff.
Broadcom acquired Bandspeed so they really want you to “buy” Bandspeed solution for spectral analysis. I asked many time Broadcom to have a simple test program…
Update Mar 2015
ath10k also has FFT like previous ath9k, as we know ath10k is for the new QCA98xx 802.11ac chipset, where as ath9k is for AR92xx AR93xx (Merlin type).
I unfortunately has only the CUS223 single band radio 5Ghz, I pretty much want to try using QCA98xx chipset in 2.4Ghz to FFT the whole 80Mhz as the radio is capable of VTH80. The reason being to see the big 80Mhz without switching channel.
vswr1 