image https://cdn2.macworld.co.uk/cmsdata/features/3605043/wireless_diagnostics_performance_thumb.jpg

WIFI Diagnostic
Forget the network

You may find that forcing your device to forget the network can help. Turn off Airport, go to System Preferences > Network, then select Wi-Fi in the list on the left and then click Advanced. Select the network you wish to forget, press the (-) and agree to Remove.

Once you’ve done this your Mac and other devices using your iCloud Keychain will not join that network.

How to fix Wi-Fi not working problems on Mac: Forget network
Use Apple’s Wireless Diagnostics

You can also get an idea of whether other devices are causing your signal to drop by using macOS’s built-in Wireless Diagnostics utility.

To generate a graph like the one shown above, follow these steps:

Open Wireless Diagnostics. Either search for it using Spotlight (Cmd + spacebar), or hold Option/Alt and click on the Wi-Fi icon at the top right of the screen, then select Open Wireless Diagnostics.
Before you click Continue to run a report, go to the menu and click on Window > Performance (or hit Alt + Cmd + 5)

This will generate three graphs that will tell you about the transmission rate, signal quality, and signal and noise levels. If you monitor this for a few hours you may be able to identify whether there is a problem.

The top graph displays the data rate of your wireless network in Mbps. The level of the graph will be dictated by your router and other equipment you have connected to it. The important thing, in troubleshooting terms, is that the rate is reasonably consistent. If you are seeing dips in the data rate, or a complete drop-off, it indicates that there is a problem.

The middle graph, labelled Quality, displays the ratio of signal to noise over time. Ideally, it should be a straight-ish line with small spikes. If you notice frequent dips in the line, it’s likely that something is interfering with your Wi-Fi signal.

The bottom graph, labelled Signal, displays both the signal strength and measured noise. Both are shown as dBM, or Decibel-milliwatts, a commonly used unit of the absolute power of radio signals. A reliable signal should have a signal strength of somewhere between -60 and -10dBm and a noise level below -75dBm. The narrower the gap between the two lines on the graph, the more unreliable the signal is likely to be.

If you notice sudden increases in noise, try to identify when and why they occur. Does it happen, for example, when a wireless phone handset is in use, or when a microwave is switched on? If you identify that a particular appliance is interfering with the signal, move the router away from the appliance that’s causing the problem.

You could also try varying the height of the router as well as its horizontal position to see if that has any effect on the reception when the interfering gadget is in use.