Radio Scanner node for the CRT.
A Radio Scanner is a radio reciever capable of recieving and filtering many frequencies, and frequency ranges. Some scanners can even track trunked frequencies.
Most scanners recieve the following frequencies:
10 meter- 25 cm HAM band.
Low and Hi VHF bands.
Aircraft bands.
137Mhz-144mhz unencrypted mobile military land bands.
UHF bands
The public service 800Mhz band. All scanners are required to block the 800Mhz cellular telephone band.
Citizen's Band frequencies.
Some recieve AM and FM frequencies as well.
Most radio scanners have a plethora of features. The following are a list of features common to most scanners made in the last 5 years:
Trunked communications tracking. Most scanners can follow trunked communications. The two main standards are Motorola and Ericsson.
Range scanning. This is the ability to scan one or several ranges only, such as the public service band, or military land band. It is more efficient then scanning all the possible ranges of the scanner.
Individual frequency scanning. Most scanners hold between 25 and 500 individual channels. This enables a user to scan only for channels they want to monitor. For instance, instead of scanning the entire public service band, which takes a long time, one could scan only the police frequencies.
All scanners have a lock function, by which one can keep the scanner on a specific frequency. This is useful when listening to a conversation with gaps in it.
Automatic hold. Many scanners will hold for a second or two when they come across a frequency with active communications. This is related to the 'lock' function.
Most modern scanners can follow standard Ericsson, GE and Motorola trunking. Trunking is a type of frequency management. Trunked communications switch frequencies within a given band, so that more communications can take place at the same time.
Methodology
There is a certain methodology to efficient scanning. Basically, one should limit the number of frequencies scanned, in order to be more efficient. For instance, if you want to listen to incoming flights, police to dispatcher communications, and Voice of America on shortwave, there is no point to scanning every range the scanner has access to. Also, if the scanner is currently working its way through the Aircraft band, there is no way it will get to the public service band in time to receive "we got a 10-55 on I-95." Limiting frequencies can be a good way to skip what isn't interesting.
Limiting one's scanning can be done in several ways. First, if one is only interested in listening to police and other public services, they can limit the range to the 800 Mhz band. If the user knows the specific frequencies that they want to listen to, they can program those in. They will probably have to if listening to trunked communications, as very few scanners can automatically recognize and track a trunked channel. One can also use a combination of the above described methods.
Function
Scanners have microchip based electronics that control how they scan. A scanner is basically a mini-computer. As it scans each frequency, it measures signal strength, if it is stronger then the background noise, then the scanner pauses on that frequency. This is controlled by the squelch knob found on all scanners. Scanners also have electronics that limit access to the cell phone frequencies and certain government frequencies.
The following information was gathered mostly from my dusty closet of a brain. Some of the info on frequencies and features was found in the manual for a Pro-94 Radio Shack scanner. | 
