Listening to the "action bands"—that is, those frequencies above 30 MHz—on a scanner radio may well be the most popular of all hobby radio activities. And it's a relatively new activity, as the first scanner radios weren't introduced until about 1970.

A scanner radio is one that automatically tunes through a set of frequencies, usually called "channels," at a predetermined rate. When the scanner finds a signal on a channel, it pauses there to let you hear the communications. When the signals end on a channel, the scanner resumes tuning through its channels until it finds another "active" channel.

Scanners were a real boon to listeners because most transmissions above 30 MHz are brief, and operating frequencies are quiet for long periods between transmissions. Older radios that tuned above 30 MHz had to be manually retuned to change frequencies. If you were tuned to the frequency used by your local police department, for example, you would miss a call on the frequency used by your local fire department. Scanners made it possible to keep track of several different channels simultaneously.

The first programmable scanners were introduced in the late 1970s, and this really boosted the popularity of scanner listening. The first scanners required to you to install new frequency-controlling crystals each time you want to receive a new frequency. Not only was this expensive, there was often a delay of weeks before new crystals "cut" to the desired frequencies arrived. Programmable scanners make changing frequencies as easy as tuning a new station on an AM or FM radio. Most scanners today also have a search function that lets you seek out active frequencies that you're not aware of.

Here is a sampling of what can be heard on a typical scanner:

Police, fire, and emergency services. Few things are as gripping as listening to the police in pursuit of criminals, firefighters attempting a rescue inside a burning building, or an ambulance rushing to the hospital!

Aviation. Civilian aircraft and airports can be heard from 108 to 136 MHz, while military aircraft are found from 225 to 400 MHz.

Marine communications. 156.80 MHz is the ship calling and emergency channel, with several other channels near it. This frequency is used on rivers, lakes, etc., in addition to the oceans.

Government. Federal, state, and local governments are heavy users of the bands above 30 MHz. Listening can range from law enforcement agencies to your local sanitation and road maintenance services. This is a great way to keep track of how your tax dollars are being spent!

Ham radio. Ham radio operators are found at 50 to 54 MHz, 144 to 148 MHz, and several other bands.

Private businesses. You can hear the activities of businesses ranging from taxicab companies to motion picture crews on your scanner.

Miscellaneous. Wireless microphones, weather bulletins, and even garage door openers can be received on most scanners.

Most communications heard above 30 MHz will be in FM, with the exception of AM on the aeronautical bands. Propagation on the bands above 30 MHz is usually restricted to "line of sight." This is defined as the optical horizon as viewed from the receiving antenna plus about another 15% due to radio signal "bending" caused by the Earth's curvature. While receiving range can be increased by using an outdoor antenna mounted high in the air, most signals heard above 30 MHz will be within 100 miles or less. However, under unusual propagation conditions, stations on the 30 to 50 MHz band can be heard from hundreds or even thousands of miles away.

To improve coverage, many users of the frequencies above 30 MHz employ repeater stations. A repeater station is located on top of a mountain or tall building, and automatically re-transmits a signal received on one frequency (the input frequency) on a second frequency (the output frequency). Some favorably located repeater stations can be reliably heard at distances of over 150 miles.

For listening to radio services within a radius of about 30 miles or so, an indoor scanner antenna, like the telescoping "whip" built into many scanners, is adequate. However, reception range and signal strength will be greatly improved if you use an external outdoor antenna.

While you're generally free to listen to anything tuned by a scanner (except cellular telephone calls, wireless intercoms and cordless phones), Section 705 of the federal Communications Act prohibits divulging or using the contents of any message you hear not intended for the general public. In practice, this is widely ignored; many wrecker and towing services have scanners in their offices to keep up with traffic accidents where their services might be needed, for example. Nonetheless, the law is on the books and could be enforced. It is prudent to not divulge the contents of anything you hear on a scanner.

In 1992, Congress went further and outlawed the sale and manufacture of scanners that can tune the cellular telephone bands. There have been attempts since then at the federal level to restrict scanning listening, but all have been unsuccessful.

Various states and localities have tried to restrict the use of portable or mobile scanners in an attempt to cut down on "ambulance chasing" and similar activities. States like New York and New Jersey have criminalized the use of a scanner to hamper police, fire, and emergency services or in the commission of a crime. A local scanner dealer can give you details about any restrictions in your area.

Interest in scanning listening continues to grow.