If ath9k is installed but not automatically loaded at boot, i.e. if "lsmod | grep ath9k" returns nothing, first try to load it manually with "modprobe ath9k". If your wifi adapter works after that, just do as root:
echo "/sbin/modprobe ath9k" > /etc/modprobe.d/wifi.conf
so that the module be loaded at boot time.
If that doesn't work, maybe your adapter is not supported by the module. To check type:
lspci -nnk|grep -A3 Net
This will confirm you the driver in use (if any) as well as the kernel module(s) usable with your adapter. In addition this will give you both the vender and device IDs in the form [VVVV
DDD] at the end of the first line. For instance here:
bash-4.1$ lspci -nnk|grep -A3 Net
00:19.0 Ethernet controller : Intel Corporation 82566MM Gigabit Network Connection [8086:1049] (rev 03)
Subsystem: Lenovo ThinkPad T61 [17aa:20b9]
Kernel driver in use: e1000e
Kernel modules: e1000e
03:00.0 Network controller : Intel Corporation PRO/Wireless 4965 AG or AGN [Kedron] Network Connection [8086:4230] (rev 61)
Subsystem: Intel Corporation Device [8086:1011]
Kernel driver in use: iwlagn
Kernel modules: iwlagn
So my Wireless controller has vendor #8086 and device #4230.
modinfo <module name>|grep -i <device #>
should tell you if this device is supported or not. For instance here:
bash-4.1$ modinfo iwlagn|grep -i "4230"
returns one line:
which shows that my adapter is supported.
EDIT post edited to include the "-i" or "ignore case" option of grep command as the device #, being written in hexadecimal characters, can include letters. Thanks to dr.s for noticing.