802.11n force 40mhz channel

[PackElend]

@prozakas

In noisy environments, where interference degrades transfer speeds, it is sometimes better to use narrower channels (i.e. 20MHz), because that means 3dB higher Tx power (keep in mind that total Tx power is then spread into half of bandwidth meaning twice as high power amplitude) which in turn means 3dB better SINR for receiver and that means better throughput

could you explain this in more detail as I cannot find adequate documentation on this phenomena

I have just found a got explanation in IEEE 802.11n/ac Data Rates under Power Constraints:

When a station is already using the maximum allowed power, it can either use all this power over a 20 MHz channel, or spread it over the different 20 MHz sub-channels of the selected wide channel. For example, an HT station operating in the 2.4 GHz band, can either use 100 mW over a single 20 MHz channel, or divide this power over the two sub-channels belonging to a 40 MHz channel (each 20 MHz sub-channel is allowed to use 50 mW only). We note that this power is also shared between the different spatial streams. So if the station sends 4 Nss on a 40 MHz channel, the part of each stream per sub-channel is 12.5 mW only (i.e. 100/(24) = 12.5). The
same thing is true for 802.11ac stations which can either use all the 200 mW with a single stream over a 20 MHz channel, or spread the power over up to 8 streams and 8 sub-channels (i.e. 160 MHz channel). So each stream is allowed to use 3.125 mW only per sub-channel. This power spreading may limit the transmission range significantly.

and here in a nice table https://www.duckware.com/tech/wifi-in-the-us.html#channelwidthrange