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