72 lines
3.7 KiB
Plaintext
72 lines
3.7 KiB
Plaintext
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MOBILE TRACKING EQUIPMENT or "Bumper Beepers".....by The Mad Phone-man
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You remember the little "bug" installed on the bad guys car in the James Bond
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flicks that allowed Bond to follow the car from a distance? Well this file is
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a tutorial on them.
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First, they do exist, I've built my own, but even the best commercial units
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intended for law enforcement purposes wont do what the Bond model purports
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to do, that is, give a printout on a moving map showing the route driven by
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the bug toter.
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The basics of the unit are the transmitter, which is about the size of
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a pack of cigarettes and is held on via a magnet. And a receiver, using 2
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identical antennas, coupled to a center zero meter which gives a heading
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towards the transmitter. More on these later.
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The receiver/display unit is used by pilots, amateur radio operators, and
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law enforcement and security personnel to track the movements of the
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transmitter
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usually at short ranges, the civil air patrol uses these units to find
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downed aircraft by tracking the emergency beacon, activated by the
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impact of the crash. Amateurs play "hunt the fox" to keep illegal transmitters
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out of the ham bands. Law enforcement personnel track the movement of people,
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drugs, and weapons by attaching a transmitter to the object (or suspect's
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car) to be followed.
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The transmitter is usually a small VHF or UHF battery operated package
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dangling a 19" flexible antenna (about the thickness of piano wire). The
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transmitter does not "beep" per say, but transmits a continual carrier.
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The FBI uses 167.xxx mhz for theirs and the local DA uses the intercounty
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police freq of 155.37. I have seen military models that use 149.xxx mhz
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around here (air force).
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Now the receiver:
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Two identical antennas mounted on the chase vehicle (usually magnetic mounts)
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feed a pair of PIN diodes that feed a phase detector which samples the
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receiver's IF output. When the received signal is directly in front of you,
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signals arrive at exactly the same time at each antenna. This is calibrated to
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read center 0 on the meter. (Incidentally the unit can't tell if the signal is
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in front or in back of you, so the need to make sure you follow the subject
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reasonably closely is apparent). If the bug travels say to 10 o'clock on the
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compass rose, the needle will swing to 4 o'clock on the meter. The object here
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is to always drive towards zero and you follow the bug in the most direct
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direction. With a little practice,you can follow a subject on an adjcent
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street without loosing him.
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The meter swings because the signal arrives later at one antenna than the
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other, causing a voltage change in the phase detector (an Exar Radio-teletype
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decoder chip in my model).
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Some recent units ive seen have Light emitting diodes in a 360 degree circle
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and use 4 antennas. This gives you full circle detection capabilitys as the
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phase between pairs of antennas is calculated also.
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Now, prevention:
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The easiest way to detect if you've been planted with one of these little
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transmitters, is to walk around the car or whatever with a portable frequency
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counter and check for an alien RF signal. This is also the recommended
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method to de-bug your home. A small freq counter with 1.2 ghz capabilitys
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sells for around $100. today. If you do find a transmitter, have fun with it.
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Stick it on a train heading out of town, a Greyhound bus, or a over the road
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tractor-trailer rig....my favorite is to stick it on one of their own
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vehicles and watch them chase themselves....hehehe.
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The Mad Phone-man (c) 1988
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