19db Omni Antenna - No Effect

Hi,

I'm a noob at this, so maybe someone can clarify the basics for me.
I've been using an EnGenius 362-EXT usb wifi card and wanted to improve my reception. I'm sharing internet in an apartment building, so I thought of getting a better antenna. I splurged on a 19dbi omni directional antenna (its massive, like 88cm) but once installed, I didn't noticed a reception difference.
Is the usb card not powerful enough to power the antenna? (does that statement even make sense?) I just don't see why they make antennas so large, when it doesn't seem to add any reception. I've decided to order a hawking signal booster, but will it make a difference? Will it improve reception, and what should I do with the antenna? Was it a waste of money?
Thanks
Eric