What's wrong with the Wi-Fi on my iPhone 3GS?

I bought a second hand iPhone 3GS, so naturally, I wasn't expecting anything perfect. But one problem I found was that my phone can hardly connect to the Wi-Fi in my house, or anywhere else for that matter. When I'm sitting in the living room, where the Wi-Fi box is, it connect but only with 2 bars, when I go literally anywhere else in the house, it can't find the Wi-Fi, even in the next rooms. I know it isn't a problem with the Wi-Fi because my sister has an iphone 5 which works everywhere in the house, my dad has my sisters old iphone 3gs which works everywhere in the house, and I have an old 2nd Generation ipod touch which also works everywhere.
My only guess is that the internal antenna for the Wi-Fi in the phone is broken or very weak!
Any help or suggestions?

You need to replace your iphone's wirless antenna