Police scanners
By: Carl Broady, Current Not yet Rated

Radio scanners are radio receivers which are capable of scanning through a number of user programmable channels or frequencies. Radio scanners can also be user programmed to search or scan between two given frequencies in predetermined steps.
These Radio scanners are available at different price levels with different features and capabilities. The more expensive scanners can cover a much wider range of frequencies, have more memory channels and can scan through the channels very much faster than the lower priced models.
Radio scanners must be programmed with frequencies into some or all of its memory channels. The Scanner is then set to "scan memory" mode. It will then listen momentarily to the frequency in each of its programmed memory channels. The scanners memory channels are numbered and the scanner will run through them in ascending order.
A radio scanner has a squelch control which is a circuit, usually controlled by a knob that can be set to suppress the audio output of the receiver and cause the scanner to skip or ignore a channel when the signal received is too weak. This control is set to attenuate or tune out any natural or man made background radio noise.
A scanner will run through its user programmed frequencies and it will stop on a channel which has a signal strong enough to break the squelch, that is higher than the minimum squelch setting. There is also a delay feature available on most scanners which can be set so that the scanner will stay on a channel until the transmission ends or it can be programmed to stay on the frequency for a user determined amount of time, say 15 seconds or so before it resumes scanning.
Until a few years ago and before Radio Scanners were developed, to achieve the same results as a 50 Channel radio scanner you would have to have 50 radios, each one tuned to a different frequency, then each of these radios would have to be turned on and, if nothing was heard off-again. The next radio in line would then have to turned on then off again and so on down the line of radios until a signal was received on one of the radios. Each of these radios would have to be turned on and off again sequentially in under two seconds.
A number of the latest scanning radio receivers radio scanners have a very wide frequency range Many manufacturers offer wideband scanners which can tune from 152 kHz to 1310 MHz.
The typical household analog transistor FM radio receiver usually has a frequency range from about 85 MHz to 105 MHz and the tuning display or dial is approximately 2 ½ inches long. If the tuning range of the typical household transistor radio was the same as a wideband scanner and it displayed the tuning frequency in the same way it would have to have a display or dial over 25 feet long.
A radio scanner can receive most analog FM and AM radio transmissions and are mainly used to monitor voice radio communications. Most two-way radio uses narrow FM. Most FM broadcast stations utilize wide FM. Some of the higher end scanners are capable of receiving both narrow and wide FM signals and can be used to listen to broadcast stations as well. On a scanner that receives only narrow FM, broadcast stations will sound a little pinched or tinny. A few high-end scanners are even capable of receiving digital radio transmissions.
Radio scanners are used mainly by radio hobbyists to monitor the two-way radio traffic from police departments, fire departments, ambulances, marine and ship to shore radio, the Coast Guard, aircraft communications from the tower to the aircraft or from one aircraft to another, amateur radio, CB radio, business radio, which might include towing companies, railroads, taxis etc, family radio walkie-talkies and even race car drivers talking to their pit crews during a race.
Radio scanners can also be used to listen in on telephone conversations on some of the older wireless or cordless telephones.
The more modern cordless telephones now use spread Spectrum technology to prevent telephone conversations from being overheard using radio scanners.
Laws were passed in United States making it illegal to sell scanners which are capable of or easily modifiable to receive the frequencies that are used by the cell phone companies. For the most part the cell phone companies have also started to use spread Spectrum technology and encryption to prevent any eavesdropping and keep cell calls private.
Most radio scanner enthusiasts use their scanners to listen in on the radio communications of their local police, fire and ambulance departments.
Because all radio transmitters in United States have to be registered with the FCC, all the information about the transmitters, their transmit frequency and their location is readily available to the public. in the USA it is very easy to look up all the frequencies in your city or your and your surrounding townships for the services that are of interest to you and program these frequencies into your radio scanner.
When the owner of a radio scanner hears a siren in their neighborhood then they can switch their scanner on and usually within a few seconds they will be listening to the radio traffic between the responders and the dispatchers and find out exactly what the emergency is and which departments are responding to it.
Radio scanners are available as a base models for home use or hand-held units. A basic entry-level scanner will cost under $100. A higher end scanner with all of the bells and whistles will cost several hundred dollars.
Even the low-cost entry-level Radio scanners provide many hours of very informative entertainment but a mid range scanner costing a couple of hundred dollars is still an extremely good value and a really good investment.

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