Signal shielding device prevents remote-controlled detonation of bombs
In this year's Youth Olympic Security, Nanjing Metro Police specially introduced two new security tools, which are signal shielding devices and mobile X-ray machines.
The signal shielding device is shaped like a suitcase, with two long poles standing on its black main body.
According to the police, once the shielding device is turned on, all mobile phone signals and wireless signals within 50 meters will be blocked. "This is to prevent criminals from using remote control to detonate. Once the instrument is turned on, there is no signal, and criminals can be prevented from remotely detonating the bomb," the police said.
Nowadays, all kinds of jammers are common in the market, and everyone's cognition of jammers is uneven. What kind of understanding should we have on the shield?
The first point: everyone thinks that the higher the power of the cell phone signal jammer, the better the shielding effect. In fact, this is not completely correct. Many manufacturers take advantage of the loopholes that inexperienced users cannot detect the actual power, and will deliberately exaggerate the openness of the shielding power, resulting in the nominal power index of the equipment and affecting its reference value. For large-scale shielding systems, shielding skills are an important factor affecting the shielding effect. Therefore, the advantages brought by advanced shielding skills are largely greater than the output power factor of the shielding equipment.
Second point: It is also incorrect to think that directional antennas can be used to reduce external disturbance. Because the directional antenna covers the main lobe, side lobe and back lobe. The main lobe coverage angle is defined as the coverage angle of the mobile phone signal shielding when the external power of the main lobe drops by 3dB. This means that the radiation intensity on the side and back of the directional antenna is only much lower than the main lobe. Therefore, if the space is very close to the antenna, it is still relatively shielded, and reflections from shielded buildings can still cause external disturbances. In addition, if used inappropriately, it will also lead to disturbances upstream, which will increase the scale of disturbances. Therefore, whether the equipment technology can meet the conditions, the use of antennas and the debugging of the system are far more meaningful than whether to use directional antennas.
The third point: Everyone thinks that the mobile phone signal blocking interval is a hard indicator. In fact, the interval is not an absolute value. It is related to the strength and quality of the signal in space. That is to say, the interval is meaningless. The same shielding power, if the interval is weaker at 100m - 70dB, then only 12.5m at the strong -52dB. The shield spacing is related to the antenna gain. And the same power shielding host applies different gain antennas, and its interval will be very different. For example, antennas with a gain of 16 dB are separated by 100 meters, and antennas with a gain of 4DB are separated by only 25 meters.