Have you tried the signal boost function on your fone?
*3370# to turn on
#3370# to turn off
Shld work for ur fone, worked for mine and my friends, it boosts your signal but gives u less battery time, i think its 10% less battery time.
When you do it ur fone shld restart or ask you about voive codec, just say yes
It'll do the same thing as what an antenna booster is supposed to do but you wont need to buy one