I had the same problem with my iPhone 4 and could not figure out why. Then I started messing with it and I realized the problem. I thought this would have nothing to do with the wifi connection but it does. When you first take the back off your phone there is a panel you have to take off that is by the camera. I believe like 5 screws including an abnormally long one. Well there is a tiny metal fitting that fits around the screw when you thread it in. Well mine fell off the first time I took my phone apart but I figured it meant nothing so I left it off. Then put my phone back together and wifi was weak. I thought for a few days and messed with it and I put that metal piece back on and I had 3 strong wifi bars. I thought that couldn't be it so I took it off again and it was weak again.