Samsung galaxy s3 won t connect to wifi unless near router?

My Samsung galaxy s3 won t connect to WiFi or will have a bad signal unless it is near the router. I have had this phone for a while now, and I have always been able to connect to the WiFi from my room, but just recently I have been able to because I would have a bad connection or because it wouldn t connect at all. Whenever I get near the router I see the bars go up, and my internet is faster. I m pretty sure this is my phone. I have tried it with two different places, and different routers, and it kept happening. Please tell me something I will be able to understand, also please don t tell me to take it in to the shop.

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