I have an old Airport Extreme that I had put away because the modem supplied with my internet plan is wireless. After reading your post I pulled out my AE and hooked it up. It gave me a glimmer of hope when it connected from the distant room, but then it disconnected and gave me an ! on my wifi icon. So unfortunately not even the Airport Extreme will let me use wireless from a distance (2 rooms away with a hall with no walls leading straight to it). Maybe beause it's an old one (??) 802.11n. I've only ever used Mac products and know nothing else. This is so dissapointing from Apple.
I have this problem to. To be clear I think that there are two issues being discussed:
1. Poor wifi that caused by router settings. In this case you should still be able to 'see; local wifi networks, connection and speed issues may exist or
2. Poor wifi that is caused by a hardware fault, this limits what the laptop can 'see', so when I'm comparing my MBP with another device, the MBP can't 'see' that same number of networks and looses sight of my router way before the other device. - This is what's happening to me.
So far my PD has consisted of trying a previous OS - no change
Disconnecting the 3 antenna connections and trying them 1 at a time. Connections J0 and J2 showed the same behaviour as when all 3 connected, by J1 showed nothing.
Next steps are to try connecting J0 (and/or J2) antenna to the cards J1 connection, this will tell me if the issue is the card or the antenna.
Frankly given the amount of discussion on this issue I surprised that this hasn't be raised as a recall issue, c'mon Apple show this issue the kind of support that you're known for. Great customer service and support is part of the reason customers like me choose Apple products.
Bug report filed
So the results are in of the single antenna tests:
Antenna Board Result
J2 J2 Normal
J2 J1 V poor
J1 J1 V poor
J1 J0 Normal
I could have done more testing but the antenna connectors are quite fragile. The results look fairly conclusive, in that Board J1 always produced a weak or no connection.
So next I'll get a new board (unless Apple see sense and authorise an out of warranty repair?) and see what happens....