Bad WiFi reception when using external screen I know it doesn't make any sense (at least, it doesn't to me), but I have the strangest situation over here: 

Upstairs:
I use a MacBook Pro (mid 2010) with a i7 2.66 GHz processor and 4GB of DDR3. The notebook is on my desk on my right side. I've connected a 23" ACD as external screen. When the screen is connected, the lid is closed.  

Downstairs:
AirPort Extreme (4th generation) which sends a signal (n-standard) to my Mac (and all other devices in our house).

For some reason, when the external ACD is connected I got a very bad signal. Signal strength even goes down to 1 sometimes. But when the ACD is disconnected and the lid is opened, the signal strength increases to normal proportions (40-80).

I can't figure out why the signal strength is influenced by connecting the ACD or not.
Can you?
I may have an answer for you.

The AirPort card in this model is located at about where the play/pause and volume buttons  are and the antennas are built into the display. When you close the lid, the antennas are being pointed sideways instead of up. 

Not only that, but now you've added an extra layer of glass and aluminum to block the radio waves being transmitted from your AirPort card to your AirPort Extreme. This is probably what's causing the issue.

There's not really a fix for this, as it's how the MacBook Pro was designed. 

A workaround, however, would be to leave your MacBook Pro lid open when connected to your display. If you prefer your main screen to be the external display, you can change which display is primary. There are some pretty good instructions out there that explain how to do this.