It depends on your physical location and the TV signal coverage of the local broadcasting stations in your area. You can find information on the available channels on websites such as the FCC DTV Reception Maps page. Other websites may also be available to help you discover available stations in your area.
Actual signal strength may vary depending on a variety of factors including building structure, neighboring buildings and trees, weather, and the type of antenna used. Signal strength may be weak when using an indoor antenna or in hilly areas.