I stumbled on article http://wiki.mikrotik.com/wiki/Manual:Wireless_Advanced_Channels on mikrotik manual.
Strange, I cannot get to this option neither on RouterBoard not PC.
/interface wireless channels
simply does not exist.
How we can access it?
I just found out the same. Such menu option as explained in the manual does not exist.
Hope MT will read this and give us an answer…
clarification added to article
This must be the first time an ROS feature is explained before its even available!
Usually we have to wait months so not years for a good explanation on the available options… 
Did I dream I saw a document with details of which channels could be used for 40MHz on 802.11n ??
I can’t seem to find it now.
Anyone else remember seeing it ?
Nick.
They are the same as on normal a/b/g. Just 20Mhz with one extension channel of 20 makes 40Mhz. As long as the combination stays within the regulatory domain’s band limits you have a 40Mhz channel.
Any news with advanced channel feature ? I wrote about this future to support, but i never received ticket number 
if you never received a ticket number, you didn’t write to support. anyway, this feature should be in v5.10 or later.
Ok. sorry!
What a great feature. I was really waiting for this (7, 14 and 28Mhz BW).
I hope this comes out in the next version.
Normis, is this feature available yet in any implementation? If not, any ideas when it may be released for testing?
Thanks!