Inside a Cell-phone Jammer
Electronically speaking, cell-phone jammers are very basic devices. The simplest just have an on/off switch and a light that indicates it's on. More complex devices have switches to activate jamming at different frequencies. Components of a jammer include:
Every jamming device has an antenna to send the signal. Some are contained within an electrical cabinet. On stronger devices, antennas are external to provide longer range and may be tuned for individual frequencies.
The main electronic components of a jammer are:
• Voltage-controlled oscillator - Generates the radio signal that will interfere with the cell phone signal
• Tuning circuit - Controls the frequency at which the jammer broadcasts its signal by sending a particular voltage to the oscillator
• Noise generator - Produces random electronic output in a specified frequency range to jam the cell-phone network signal (part of the tuning circuit)
• RF amplification (gain stage) - Boosts the power of the radio frequency output to high enough levels to jam a signal
Smaller jamming devices are battery operated. Some look like cell phone and use cell-phone batteries. Stronger devices can be plugged into a standard outlet or wired into a vehicle's electrical system.
Alternatives to Jamming
While the law clearly prohibits using a device to actively disrupt a cell-phone signal, there are no rules against passive cell-phone blocking. That means using things like wallpaper or building materials embedded with metal fragments to prevent cell-phone signals from reaching inside or outside the room. Some buildings have designs that block radio signals by accident due to thick concrete walls or a steel skeleton.
Companies are working on devices that control a cell phone but do not "jam the signal." One device sends incoming calls to voicemail and blocks outgoing calls. The argument is that the phone still works, so it is technically not being jammed. It is a legal gray area that has not been ruled on by the FCC as of April 2005.
Cell-phone alerters are available that indicate the presence of a cell-phone signal. These have been used in hospitals where cell-phone signals could interfere with sensitive medical equipment. When a signal is detected, users are asked to turn off their phones.