MacOS has a nifty wifi signal strength diagnostics tool hidden under the wifi icon in the menu. Option-click the icon and choose Open Wireless Diagnostics:

wifi diagnostics

Ignore the Wireless Diagnostics window that opens, and from the Wireless Diagnostics Window menu, click Performance. You’ll be able to see your wifi Rate, Quality and Signal.

Your signal to noise ratio should be above 20, which is presumably why the MacOS wifi diagnostics tool has 20 as the first label on its y Quality axis:

Screenshot 2021-01-26 at 10.56.40

Here, I’ve got an RSSI of -65dBm and a Noise value of -94dBm, which means the signal-to-noise value, aka Quality is -65 - -94 = 29.

There ought to be a direct way to open the Performance tool from terminal, perhaps passing an option to:

/System/Library/CoreServices/Applications/Wireless\\ Diagnostics

but I haven’t discovered it yet. If you do the above, you still need to open the Performance tool from the Wireless Diagnostics Window menu.

You can also poll the wifi data from the commandline with:

/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | egrep -E 'agrCtlRSSI|agrCtlNoise|lastTxRate'

which will show you your signal, noise and rate (in Mbps).

     agrCtlRSSI: -70
    agrCtlNoise: -88
     lastTxRate: 88

If you want to poll it regulary combine this with watch:

watch -n1 "/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | egrep -E 'agrCtlRSSI|agrCtlNoise|lastTxRate'"

which runs the command every second (-n 1), and you can find those wifi dead zones.