billaustinthey call me Mr. BillPremium,MVM
North Las Vegas, NV
Does the USB wifi adapter have a connector for an external antenna? If not connected properly, you will lose all the gain in cable and/or connector losses.
If you are trying to do long distance, you are better off with a directional antenna. Also, the higher the gain, the flatter the beam, making it more difficult to aim. If you are going to use an omni, 12dbi would probably be better to work with.