From spy cameras to lost cell phones, hidden devices that transmit a radio signal can be detected with the right techniques and equipment. Simple "sweeping" devices detect targets for which they were designed and expensive engineering equipment can allow an expert to "see" the radio waves in the room. Some signals, such as "spread spectrum" transmissions, don't stay still long enough on one frequency to be detected as a single transmission, so still other techniques are required.

Sweeping a room for hidden radio signals is a little complicated because there are so many signals normally present and hidden devices often use special transmission techniques to avoid detection. A device may use analog, digital or GSM mobile phone-based communication. Advanced "bug trackers" may include detectors for all of these, catching intermittent signals as well. Miniaturization and the use of microwave frequencies means devices can be extremely small.

Cell Phone Detection

A mobile phone SIM card is at the root of many surveillance devices.   A mobile phone SIM card is at the root of many surveillance devices.

With a bug tracker device with GSM capability, the user can find his lost cell phone somewhere in the sofa; but mobile phones, their software and the SIM cards that many of them use hold the keys to the mobile phone network and are all important tools for surveillance. Small audio and video recorders use a SIM to connect to a mobile network and forward video files, even accepting commands by SMS text message.

Engineering Tools

Hunting an elusive bug -- and doing a thorough job -- calls for an engineer with an arsenal of detective tools led by a spectrum analyzer. A special kind of display device that shows radio signals across a selected range, the spectrum analyzer can give you enough information to locate low power devices, intermittent signals and signals at unexpected frequencies that might not be caught be standard sweeping.

Spread Spectrum and Other Technologies

Spread spectrum radio transmission involves changing transmission characteristics such as frequencies in a way that allows a receiver to follow. An excellent technique for avoiding interference, it also makes signals hard to detect. Spread spectrum became popular with the military but is also one of the technologies used to make modern Wi-Fi more reliable. Used by hidden devices, it is hard to detect by fixed signal but will still show radio frequency energy to skilled investigators. In addition, laser-based transmitters and other newer devices may be difficult to detect.

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