The distance a scanner is able to receive varies based on the type of scanner, the location of the radio, and many other factors. A portable scanner inside a home may only receive 10 miles or so. A base station scanner with a back-of the-set antenna slightly more. A scanner connected to an outdoor antenna often will receive double or triple these ranges. If you have a portable scanner inside a car, the metal roof of the car will attenuate the signal you're trying to receive. A mobile scanner
antenna mounted outside of the car with a magnetic or other type of mount will significantly increase the distance you can hear and the clarity of most stations.
The distance a scanner will receive depends on many factors including your elevation; the frequency, elevation, transmitter power, and tower height of the transmitting station; the terrain and buildings in your area; whether cellular, paging and other transmitters in your area; whether the system you're monitoring is a wide-area network, and more. Every area and every situation differs. Go to scannermaster.com for antennas, pre-amplifiers and other hardware that will help you increase the
range you are able to receive.