why wont my iphone 4 connect to wifi without putting phone over modem?

i turn my wifi on and it does nothing so turn it off and back on again and its searching but still can not find a broadband to connect to? but if i put my phone over the broadband it connects but i have to stay in the same room as the broadband it disconnects! i have heard so many different reasons and solutions and i dont know which one is right!!!

anyone experienced this or know of what to do to fix it? i have ios 6.1.3 and have heard that there has been problems with connecting to wifi with ios 6!

also been told it could be the wifi card in the phone but it wouldnt even find a broadband if it was the wifi card, would it? please help!!!