I have the Alfa 500mW USB Wireless B/G Adapter. It comes with a 2 dbi antenna.

I have purchased a 15 dBi Omni 2.4Ghz WiFi WLAN Omni Antenna.

This antenna, which includes an adapter cable, has not improved the reception of wireless networks whatsoever.

I have tested it with Windows Xp, and with Linux using Kismet and Airodump-ng.
Because the results are identical with the standard 2dbi antenna or the new 15dbi, does the alfa wireless usb adapter have a limiter or something.
I live in Metro Vancouver and with the increased range I should be picking up a lot more networks.