If you've got a standard wireless setup it is probably a neighbor very close to you. To pinpoint it we would need to get into the details of Radio Direction Finding, which would be more expensive and time consuming than it's worth.
If you'd really like to catch them and offer to share the cost of your internet, secure your wireless network with a good password and then see which of your neigbors starts driving to Starbucks a lot, or has the phone or cable company van in their driveway installing their own internet. You could also put some foil over your antenna and see if you spot any neighbors wandering around their backyards with a laptop in their hands searching for a stronger signal!
|