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TUCoPS :: Hardware Hacks :: chipsdet.txt

Detecting California Highway Patrolmen by their walkie-talkie emissions




So...you're driving down the freeway and you want to go faster than
55mph?  Well I can tell you how to do it without worrying about the CHP.
This idea is NOT a radar detector.  And it does NOT work with local
police, only the CHP.  It is a cheap version of the CHiPs detector you
may have seen advertised in Car and Driver for upwards of $300.

First...how it works.  You may have seen local police with a two-way
radio clipped to their shirt pocket.  They like this because they can
immediately call for backup if they get into trouble.  These radios work
on the same frequency as their car radios and allow communication with
the station house (the base station).  The CHP has similar personal
mobile radios, but they work on a different system.  The reason for the
difference is that CHP officers may roam many miles away from their base
station, unlike the local police.  A portable radio would not have the
range to call back to their base.  So what the CHP personal mobile
radios do is they work on a different frequency than their car radios.
This frequency is called a mobile extender frequency.  Their portable
radios transmit to their car radios, and their car radio takes the
signal, changes it to it's own frequency, and relays it back to the base
station.  The mobile extender frequency radios that the CHP officers
carry on them only have a range of 3 miles.

So, if you have a common police scanner (perfectly legal to own and
operate), and tune it in to the mobile extender frequency, whenever you
pick up anything on that frequency you know that a CHP officer is within
three miles of you.  The stronger the signal, the closer he is.  This
idea isn't perfect.  You only can pick up the signal if the officer
leaves his portable radio on while he is in his car (they always do) and
you can only pick something up when something is being transmitted.
Fortunately, CHP officers constantly talk to their base.

So, to implement this idea in the cheapest manner, go to your local
Radio Snack store and get their cheapest scanner.  Barring any sales,
that would be their two channel portable scanner that uses crystals.
Buy it, and order a crystal for the frequency 154.905 (it must be
exactly this).  A better idea is to get a non-crystal using scanner for
about $100.  This scanner can pick up any of thousands of frequencies
without any add-ons being required. Don't be confused if it is called
eg. a "10 channel scanner".  In this case the ten channels are
equivalent to the preset buttons on you car radio.  And like your car
radio it can still pick up ALL frequencies within the specified bands.
The two channels of the crystal-using scanner really refer to only two
frequencies that it can receive.  As you can see, the more expensive
scanner is vastly more useful, though the cheaper scanner will work
fine.  Scanners more expensive than $100 usually only add bells and
whistles that you probably don't need.  The $100 scanner does have other
useful features over the $50 one.  You can plug it into the cigarette
lighter of your car and therefore you won't need to rely on batteries;
you can use your more sensitive car antennae instead of the one built
into the scanner,  and you have the option of letting the more expensive
unit scan between the mobile extender frequency and the regular CHP
frequency to get a better feel for the officer's range and what he's up
to.  As with the crystal-using scanner the frequency to tune into is
154.905.

Now you can drive at the speeds that the freeways were designed for. But
please, if you use this idea, wear your seatbelts!



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