I own two STX boards that should act as main backbone link to IGW. The problem is that I am missing 40Mhz channel option that i can see in command line interface, but could not enable.

When I set the option channel-width to 40Mhz I get an error message "incompatible band and channel-width". I would appreciate if someone could help me solving that problem.

if you want to use 40mhz channel you have to use the A standard.
Instead if you use the N standard you have to enable extension channel in the HT tab of the wireless interface.

I've tried it and it still does not show 40Mhz option. May it be that it is hidden since the device is using Mikrotik L3 licence. When I'm searching the problem in the Google, most commom solution is to change HT-extended channel option but in my case this isn't helping.

I will try to upgrade the software to newer version and see will it help.

when you select HT extension channel it will be 40MHz wide.

you can look up in this 11n card discussion why it is so:
http://forum.mikrotik.com/viewtopic.php ... hz+channel

I've tried it and it still does not show 40Mhz option. May it be that it is hidden since the device is using Mikrotik L3 licence. When I'm searching the problem in the Google, most commom solution is to change HT-extended channel option but in my case this isn't helping.

I will try to upgrade the software to newer version and see will it help.

SXT's do not support 40Mhz Channels, no matter the mode you select.

The '40Mhz' Channel mode you are thinking of is 20+20 (20Mhz Main channel + 20Mhz HT Extension Channel), this is not regarded as a 40Mhz channel as the RB only enables the extension channel when required, so majority of the time there is only noise in the main channel.

If you upgrade to 5.4, you'll see the channel width report as (20/40MHz HT Above) / or below, depending on what you selected.

But at this stage 802.11n hardware does not support 40mhz channels. (This will come, and allow for 600mbit 802.11n, which is part of the original spec, however fairly useless until we get gbit Ethernet )