Can a signal jammer really shield all signals?

Although the time for the college entrance examination this year was affected by the epidemic and was postponed to July 7-8, in order to ensure the information security of the examination room, the corresponding measures have not been reduced at all. Operators have issued announcements that wireless signal jammers will be turned on during the examination period in accordance with the requirements of the Education Bureau, which will affect the communication quality of mobile phone users around the school. Communication will not return to normal until the end of the examination. What is the effect of turning on the signal jammer during the college entrance examination? The invigilator at the entrance of the examination room is waving a metal detector. Is it really useful?

To know what is the effect of shielding mobile phone signals, let's first understand the working principle of mobile communication. As a mobile communication device, a mobile phone needs to establish a connection with nearby base stations when communicating, and then access the core network to find the communication destination. In the above process, all information is carried and transmitted through electromagnetic waves. The electromagnetic wave frequency band used by the mobile phone is related to the system used by the mobile phone and the operator. But in general, the electromagnetic wave frequency band used by the three operators is distributed in the range of 850-4900MHz. If the college entrance examination organizer wants to shield the mobile phone signal in the examination room, it is impossible to make the 2G/3G/4G/5G base station near the examination room not work, otherwise all the communication equipment in this area will not be able to communicate normally. Then the only way is to shield the mobile phone signal in the examination room.

When it comes to shielding mobile phone signals, the first thing that comes to mind is metal shielding, that is, placing the mobile phone in a closed metal box to block the propagation of electromagnetic waves through metal. However, it is obviously impractical to make all classrooms in the examination room into metal-enclosed classrooms. then what should we do? It's very simple. You only need a small cell phone signal jammer to block all cell phone signals in the examination room.

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The cell phone jammer scans from the low frequency band of the forward channel (the channel from the base station to the mobile phone) to the high frequency band at a certain speed during the working process, and forms interference when the mobile phone receives the message signal, so that the mobile phone cannot detect the slave base station The normal data sent can not establish a connection with the base station, showing no signal and no service status. For example, when the mobile phone communicates with the base station, the signal jammer is like a messenger, deliberately making noise around you. The noise signal is so large that the mobile phone can't understand what the base station is saying, and the mobile phone can't hear what the base station is saying. In fact, this is not to shield the mobile phone signal, but to obliterate the signal sent by the mobile phone in a large number of noise signals, thereby achieving the purpose of interference.

At present, the mobile phone signal jammers used in the examination room are all full frequency bands, which can interfere with all frequency bands of the three operators 2G/3G/4G/5G, and WIFI signals and Bluetooth signals can also be shielded. The above is the principle of mobile phone signal jammer. However, the frequency range of electromagnetic waves ranges from a few Hz to several hundreds of GHz, and each segment may be used as a communication signal. Will there be some special frequency bands that are used in the examination room? This is true for other radio communication tools, and radio communication tools with dedicated frequency bands "came into being" in this context. The working frequency band of these devices is selected outside the frequency range covered by the mobile phone jammer, and the mobile phone signal jammer does not work on them.

For this special radio communication tool, the examination room is specially equipped with a monitoring vehicle equipped with a radio monitor for real-time monitoring of radio wave signals of unknown origin. So how does the radio detector work? Because electromagnetic waves have the characteristics of "near big and far small", by constantly moving the detection vehicle, the monitoring equipment can locate the signal transmitter according to the distance and the strength of the signal, and capture the offenders using these radio communication tools on the spot. Nowadays, drones have also appeared in the college entrance examination examination room, which can perform signal positioning faster. Therefore, neither mobile phones nor these special radio equipment can play a role in the college entrance examination room.