Does Wireless Diagnostics "Best 2.4 Ghz" even work?

If you can switch to 5GHz it would be better.

My experience is that channel 6 tends to be the default channel most routers choose.

Best 2.4GHz is just measuring signal strength and signal noise levels at the location where you Mac is sitting, and suggests the channel that has least interference from routers.

1, 6, and 11 are the 3 channels that do not overlap with each other (basically each channel uses a total of 5 channels worth of bandwidth in a bell curve). If your neighbors are not using one of those channels or WIreless Diagnostics says their signal strength is very weak (a large negative number is a weak signal), and if the noise on that channel is low (a high negative number such as in the 90's is a low amount of noise), then use that channel, but if the neighborhood has lots of people moving in and out (apartments, condos, row homes, etc...), then keep checking to see if someone hasn't added a new router to the mix.

And if possible switch to 5GHz which has more channels and each channel does NOT overlap the others and the range is shorter so more routers can fit into the same area.

It excludes the channel you are using from the options ( or, it includes it as potential interference). Try running it with your WiFi access point (router) off. That will allow it to pick your current channel if that is the best.

I Think 6 is most common. 1 is default for Apple.

I just tried using it to determine if I could boost the speed from my Airport Time Capsule. Using an app on my iPhone to monitor wifi signals in real time, I used Wireless Diagnostics to run a scan. The results indicated that the best channels to use for 2.4 GHz were 2 or 11 and 36 or 40 for 5 GHz. Based on the channels in use as viewed in my iPhone app, that made sense.

Here's the weird part: after I switched to those channels, my signal strength for both 2.4 and 5 dropped. For 2.4, the strength dropped from around a consistent average of 75% to about 60%. On the 5 GHz, it dropped from 59% to 23%. I can't explain why the signals dropped so much when using the recommended channels. All I know is that I reproduced the results multiple times from various locations in my house, always with the same result.

So while the Diagnostics app can help you find what theoretically should be the best channels based on the channels used by the networks around you, it doesn't necessarily mean that those channels will in practice result in a stronger, more consistent signal and/or faster speeds.

Did you turn off your own WiFi base station when you did your test? See Barney-15E's reply above.

I personally do not use the test. I look at signal and noise strengths from the WiFi scan and Choose a channel that has the lowest competing signal strength and low noise.

Mostly I only do this when traveling, as there is good separation between my house and the neighbors, so it is not as much of an issue at home.

The Wireless Diagnostics can generally be started by Option-Click on WiFi menu bar icon, and selecting Open Wireless Diagnostics..., or you can use

Finder -> Go -> Go to folder -> /System/Library/CoreServices/Applications -> Wireless Diagnostics, and from the Wireless Diagnostics menus select 'Scan'