Most people wonder how come a police vehicle heading towards a crime scene is immediately followed by vehicles of journalist and ambulances. Transmission are eavesdropped by the journalists or anyone else with little eavesdropping device called mobile radio scanner.

A radio scanner is a security device that can be used to monitor multiple discrete signal frequencies automatically. It scans different frequencies and stops when it detects a signal on any one of them. After the transmission stops it begins to scan the other frequencies .This mobile radio scanner or a police scanner is a communication receiver originally intended for monitoring Very high frequency (VHF) ranging from 30 MHZ to 300 MHZ which is used for FM radio broadcast, television broadcast, land mobile stations for the purposes of emergency, military and business , Amateur Radio, marine communications, air traffic control communications and air navigation systems .It can also be used to monitor the frequency immediately above the VHF which is Ultra high frequency.UHF which consists of electromagnetic waves with frequencies ranging from 300 MHZ to 3 GHZ, which is used in public service agencies for two-way radio communication. A mobile radio scanner which monitors the VHF and UHF land mobile radio systems can tune into different kinds of modulation.

Modern mobile radio scanners have advanced microprocessors that can help it to scan hundreds of channels per second and store thousands of channels. Some of the recent models can follow trunked radio systems and APCO-P25 which is a set of standards for digital radio communications used by federal, state and local public safety agencies in USA to enable them to communicate between them.

A mobile radio scanner is used in emergency services like police and fire services. It also helps in the field of journalism, crime scene investigation, airline services and spying. Many people also use these devices for fun like spying on friends and neighbors to pull off pranks on them. Some of those scanners like BC350C are sold to public commercially. This scanner has facilities like preprogrammed search which can help to find interesting “action band”, search lockout which lets you to shutdown unwanted or uninteresting frequencies and so many other advanced features. Scanners have transformed from an 8 channel crystal controlled devices to handheld and desktop devices which are faster, lighter, cheaper and more advanced than the scanners of the old times.

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While many people use these  radio scanners for fun, some of them use it with evil motives like keeping track of the activities of law enforcement to benefit in their intentions. Other than being used in law enforcement agencies these radio scanners are used for high frequency communication in corporations and emergency services to keep their information private. Seeing the rising in number of intrusions taking place in their communication devices the corporations began to broadcast it digitally. This move eventually opened up the market for digital radio scanners as technology kept pace with the advances made by the corporations.

APCO Project 25 is a set of standards for digital radio communications used by public safety agencies in USA to help them communicate with each other. It is similar to European Tetra protocol though not linked with it in any level. The project was introduced after September eleven attack .According to this protocol a spectrum of digital frequencies has to be assigned for the usage of law enforcement and emergency services. But this move was jeopardized by the advent of digital radio scanners which made the transmissions available to the public as good as the old scanners did with high frequency broadcasts. These digital radio scanners are available in plenty of mortar store which sells electronic goods.

Post 9/11 the US government decided to encrypt the digital frequencies to stop the public from intruding using the digital radio scanners. This opened up a series of controversies and plenty of liberal advocates tried to defend the right of the public to intrude into the airwaves using right to information act .Their efforts were futile as government chose to encrypt their digital transmissions. Some of the people tried to buy  radio scanners which can decrypt the digital transmissions but those products were legally banned. The government also passed law making it a violation to posses decrypting digital radio scanner which can be punishable by jail time and fines. This is the present scenario in US for digital radio scanners.

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These scanners used to be slow, expensive and huge. It had very small number of users. But due to the advancement made in communication technology the VHF radio scanners are made available to public usage for much lower prices. This is mainly because of the use of microprocessors which allows the scanner to store thousands of channels and also it can monitor hundreds of channel at the same time.  The VHF radio scanner is one of those scanners used to intrude into the airwaves consisting of very high frequencies.

The range of VHF ranges from 30 MHZ to 300 MHZ which is used for FM radio broadcast, television broadcast, land mobile stations for the purposes of emergency, military and business , Amateur Radio, marine communications, air traffic control communications and air navigation systems. The characteristics of VHF propagation are perfect for short-distance terrestrial communication, with a range generally somewhat farther than line-of-sight from the transmitter. The ionosphere does not usually reflect VHF radio whereas HF is usually reflected and Thus restricting the transmission to a very small area helping short distance communications. The effects that atmospheric noise and interference from electrical equipment have on VHF are much lesser when compared to lower frequencies.

This vhf radio scanner is used in many fields which use airwave spectrum of 30 MHZ to 300 MHZ. The vhf scanners of today are equipped with many IF band passes. The VHF radio scanner uses a receiver with very narrow a bandwidth that produces pictures saturated in blacks and whites. This kind of vhf radio scanners are also used in walkie-talkie. These scanners have become portable and affordable to be used by the public which makes it more and more popular opening up a huge market especially in the countries like US.

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