Or it could be that there aren't many more AP's around within your antenna's range... which is always a possibility.

No, the antenna should function fine on your adapter. Your really not going to find anything much more powerful than the adapter you already have.

The only way that you might find more AP's is if you get above all the obstacles in your area (homes, foliage, etc...), and scan for some. So ultimately in order for you to pick up more AP's....you'll have to get up high somewhere, roof for example. I can guarantee you'll get more up there.

The walls of your home are one of the signal killers. So no matter how powerful your antenna or device is, you'll still loose a lot of its power when it has to go through your home's walls.

Most adapters you'll see on store shelves, regular G cards at least, will have a transmit power of 70 or 80 mW, which is very weak compared to your adapter's. 500 mW is about as good as your going to get. Your antenna is a 15 dBi. I don't understand dBi as well as I should, but i believe its the amount of beamwidth the antenna has. The higher it is, the more narrower the beamwidth gets (so higher is not always better). For example, a 5 to 10 dBi omni would be the perfect antenna for a home or office looking to cover many computers. The downside to an omni is that it doesn't cover devices or computers directly below or above the antenna, due to its vertical polarization...meaning that devices directly above or below the antenna may get poor signal quality.

You may try a high gain directional antenna. Focusing all 500 mW in one direction may pick up quite a lot of AP's.

Borgslayer