It has a 30-day evaluation trial and works perfectly on Windows 7. Passmark WirelessMon is what you're looking for. Is Windows choosing the stronger signal? If so, why isn't that working for you? However, my advice to you would be to focus on what's causing your issue rather than trying to patch around it. Windows will choose the 20MHz signal if it's stronger even if it yields a lower transfer rate.

I have this issue myself with one access point that uses a 20MHz bandwidth and one that uses a 40MHz bandwidth.

One common irritation is that the strongest signal may not yield the fastest transfers. (You can turn it down if frequent AP changes are disrupting connectivity, and you can turn it up if you're getting stuck on a poor AP.) This essentially decides how much better a signal has to be for Windows to switch access points with the same SSID. On some WiFi cards, you can tune the 'roaming agressiveness' from the device manager. Windows will generally select the strongest signal among access points with the same SSID in the same band. The manufacturer of your WiFi card might offer a custom connection manager that offers this functionality, but I've never seen it. At least, not with any standard solution.

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