Wi-Fi antenna. Signal problems.

Two months ago I started to notice that mi iPhone 4 doesn't recognize all wi-fi networks available in one area. I asked a friend to bring his iPod touch once, and inside a shopping, his ipod showed 6 wi fi networks (my iphone none). My home wi fi network appears only when I'm ridiculously close to the modem.

I'm strarting to get really annoyed, I called MacStation Argentina (kind of a official distribuitor here) and they told me that even if my international warranty hasn't expired, they provide technical service for all products except (guess....) iPhones.

I tried restoring it from iTunes, restoring network setting (or something like that), and I am still having the same problems.

("Ask to join networks" is on...)

I'm qute handy, and I think I can repair it on my own (if I know WHAT to do). Some guy in an unofficial center for these kind of repairments, told me it would cost me 170 dolars. Its absurd...

I found an answer, something about Clem 1 and 2, but the link is broken...

I appreciate ANY help, thanks a lot,

Juan