Only 3 non-overlapping channels in 2.4g wifi: 1, 6, and 11. Each uses the next two channels above and below the center frequency.
One thing to note about using any type of range extending technique to access distant wifi - if you cannot hear the other mobile stations accessing the access point, you will generate/experience a performance degradation.
Technical reason: 802.11 uses carrier sense, multiple access with collision avoidance (CSMA/CA), somewhat akin to the CSMA/collision detection of wired Ethernet. However, on Ethernet, every station can hear all others, and it's full duplex. With wifi, you don't know that you can hear each station, and it is half duplex to boot. Therefore, if there are other stations that you can't hear, you can expect an increase in collisions and therefore retries, with the problem becoming exacerbated exponentially by the number of nodes who cannot hear each other and the amount of data each tries to transmit.
It is always better for the wifi network and each station on it if each station can hear all others. This 20+ mile shot is almost certainly a point to point shot between two stations only. I didn't watch the video.