Ever since the late '80s to early '90s, cell phones have been an integral part of our lives. They enable quick communication and connection to the Internet. Cell phones can prove to be a nuisance in many situations, such as at the movies or at the theater. Even worse, they can be a security risk, in which case a signal jammer can be used to stop them in certain situations.

Two Kinds of Signal Blockers

There are two types of signal jammers that can be used to block a cell phone signal. The first kind is a passive jammer. These do not require power, but do require more robust installation. The other kind is an active jammer. These are easier to use, but require an active power source. They are also portable, a distinct advantage over passive systems.

Passive Signal Blockers

Cell phones use electromagnetic waves to communicate between the handheld unit and the cellular towers that form the cellular network. These signals can be blocked by using what is known as a faraday cage. A faraday cage is essentially a metal box. This is why cell phones will cut off in the elevator. The signals cannot pass by the conductive walls. Passive signal blockers usually come as transparent films with wires embedded in them for windows or as solid films for walls. They can also come as paints with conductive particles. Installation requires covering the peripheral surfaces of a room, or in some cases, a building with the films or paint.

Active Signal Blockers

Active signal jamming uses a device that emits a signal to prevent cellular signals from working. There are a number of ways this is achieved. For instance, the signals may cancel each other or the jamming signal may drown out the cellular signal. The signal jammer must work in the frequency of the cellular signal. Different cellular bands require different jammers, although there are some jammers which work over a number of different bands.

Legality of Signal Blockers

The biggest issue surrounding the use of a signal blocker is the legal ramification. In the U.S. federal agents may use signal jammers in various cases to prevent or control the communication of targets of investigations in certain situations. Private use, such as within a company to prevent the transmission of trade secrets, may also be allowed with a permit. Outside of the U.S. the laws vary. It is wise to find out the local laws for possession and use of a signal jammer before attempting to use one.(from eHow)