does my powerbook g4 have wifi?

Hi, Noemi. Yes, Wifi = Airport; Airport is just Apple's name for its wireless networking hardware and software, which conforms to the same 802.11 standards as everyone else's.

Open System Preferences>Network and select Network Status from the lower pull-down menu in the window that appears. Is Airport shown as an option in the lower portion of the window? If so, you have an Airport card installed and can connect to wireless networks through it. Some Powerbook G4s were supplied without Airport cards, but they all have a slot to put one in.

...does powerbooks have poor antena?

Yes, in my experience. My son's Aluminum Powerbook with Airport Extreme has less than 1/3 the range of my Titanium Powerbook with original Airport card, which in turn has poorer range than most other laptops I've been around.

Yes: buy a third-party wireless card to use in the Powerbook's CardBus slot. Some third-party cards, in addition to their built-in antennas that protrude outside the Powerbook's metal case (which is beneficial), also allow the connection of an extended external antenna to improve range even more.