Is 40MHz channel width on 2.4GHz band still frowned upon?

I have a TP-Link Archer C7 v2 running OpenWRT 23.05 in the United States. I'm continually tweaking my config in OpenWRT in order to get the best performance out of this old device. In addition to a selection of 20MHz or 40MHz channel width on the 2.4GHz radio, OpenWRT gives me the option to "force 40MHz mode" in the Wireless Network > Advanced Settings tab.

Based on a thread I found from a few years ago (40MHz 802.11N setup saved but is 20MHz wide) and on advice I've seen elsewhere online, it seems at best inconsiderate and at worse possibly illegal to run a 40MHz width channel on 2.4GHz.

On the other hand, I've also noticed that a number of Eero wifi mesh points in my neighborhood run on 40MHz channel widths in the 2.4GHz band. In addition, a post in a thread from earlier this year regarding the Archer C7 v2 recommended "change to 40mhz channel width for 2.4Ghz, and since every new AP around you forces 40Mhz without scanning these days, you may have to force 40Mhz too in the advanced tab section".

My question, then: Is the 'force 40MHz width' option only intended to be used for completely isolated WiFi installations where there's no chance of channel overlap, or is this option becoming somehow more acceptable?

Forcing 40 MHz is only supposed to be used in the U.S. when it doesn't cause overlapping interference with other AP's and clients don't advertise they are 20 MHz width only, but there are no penalties for selling hardware that fails to drop back to 20 MHz width when it should. You've already figured out where that road goes.

The 2.4 GHz spectrum in my neighborhood is an absolute mess too. It's full of mesh systems using 40 MHz wide 2.4 GHz channels - probably for no better reason than justifying the "big number on the box" - overlapping not only with each other, but also with a mess of other 20 MHz channels. Many of the 20 MHz channels are not even spaced on 1, 6 and 11 and overlap with each other as well.

I run 20 MHz wide 2.4 GHz SSIDs on the least crowded of 1, 6, and 11 for 2.4 GHz only IOT devices and to extend range outside the home for occasional use on the property, but I don't expect much from it.

Which channel width gives you better throughput on 2.4 GHz? I expect you'll find 40 MHz doesn't help much (interference), in which case you might as well try to be a good neighbor and stick with 20 MHz. You could even stick with 1, 6 and/or 11 to set a good example for your neighbors. It won't help, but you'll sleep better at night :wink:

Also might be good to remind users of the note that appears - at least on the LuCI web GUI:

Using this option does not comply with IEEE 802.11n-2009!

The legal questions aside, wider channel widths are more susceptible to interference (and range will be lower, as the same fixed transmit power will have to span a wider width). If you have multiple overlapping BSSIDs, 20 MHz is likely to provide better performance and range than insisting on 40 MHz, the 2.4 GHz band just doesn't have enough space for that.

My main goal has been to coax some more throughput out of the 2.4GHz band for devices that don't perform well on 5GHz when they're not close to the AP.* I don't want to be a bad network citizen or disregard 802.11n-2019. It does seem like some APs disregard it by design, however. I suppose IEEE standards are more guidelines than laws, so there probably aren't consequences for this other than people being grumpy with you, as opposed to wireless transmission power levels which I think have some legal weight at least in the US.

*My 5GHz issues may warrant their own thread - I'm not sure if it's due to our old house's construction, the capabilities or configuration of the 5GHz radio on this device, capabilities of the client devices, or a combination, but the 2.4GHz band has always been more solid in terms of latency, packet loss, and speed of authentication/association. 5GHz definitely beats 2.4GHz in throughput, as long as you're not too far from the AP and your device supports 802.11ac.

FWIW, here are wireless scan results run from the AP itself... if anyone has advice on choosing channels, channel width, etc, I'd appreciate it.

iwinfo phy0-ap0 scan (5GHz)
iwinfo phy1-ap0 scan (2.4GHz)

The problem is not guidelines, the problem is altering operation of the radio (if [the chip] it wasn't originally designed to do so). Nonetheless, as I linked from an older thread, merely enabling that setting won't do anything unless the band is in fact clear.

The 5 GHz issue could possibly be attributed to things like the density of older walls, etc. I experienced an issue in the same room with a device being parallel to a wall. Rotating the device to be perpendicular to the wall solved the issue.