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I think he means Mobile Jammers. If you did mean Mobile/Cellphone Jamemrs, they work in many ways. One of the many ways could be to produce a sweep of different frequencies that cancel out the mobile frequencies in that area. The heart of the equipment is Voltage Controlled Oscillator. The range is dependent on how the equipmment is built. The jammer can be made to jab anywhere from 1m to 100m if i remember it right, though more range is still possible.
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recently a VVIP came to our city. before the escort vehicle there was a van with antenna. i heard that it is jammer to block all surrounding wireless communications so as to avoid any remote operated bombs. am i clear now? this jammer's principle is what i needed.
I agree with the previous explanation, The objective of a jammer is to avoid intellegible wireless radio communication, it is done by transmitting signals, (wich maybe just sine waves covering the objective band or random signals which cover the objective frequency) with a power greater than the objective transmitter power.
I am using objective frequency and objective power, because there are many. just check the distribution of the spectrum. A good jammer may cover the whole spectrum.
I think the operating principle of a jammer is self explanatory, broadcast enough noise on a given frequency to prevent other devices from being able to use it, this is easily found in a dictionary and the original poster should have spent at least a few seconds of his own time looking up the answer. Mind you with very few exceptions these devices are illegal. Even the one's a VIP may use are likley to be technically illegal.