From: Matt Blaze <mab@crypto.com>
Date: Fri, 13 Aug 93 11:52:56 PDT
To: koontzd@lrcs.loral.com (David Koontz )
Subject: Re: Beepers can also be used to track you down!
In-Reply-To: <9308131749.AA12091@nebula.lrcs.loral.com>
Message-ID: <9308131837.AA11141@crypto.com>
MIME-Version: 1.0
Content-Type: text/plain



>>Basically, they'd send pings to your beeper throughout the city they expected
>>you in, then they'd find out which cell you were in.  After that, they can
>>use a small radar-like gun to actually find your beeper, also by pings.
>
>>There's probably a way to disable the transmitter in the beeper, but I
>
>All you need is a faraday cage, an all metal enclosure with metal to metal
>contacts between all its surfaces.  A Copper brillo type scrub pad, expanded
>out to make a pocket big enough for the pager.

No, that won't work - the pager won't be able to receive the signals for
paging you.  You need a one-way faraday cage (OWFC).  You can get
these from a good electronic supply house, but they're rather expensive and
specialized so they don't usually list them in their catalogs.  You have to
call and ask.  Law enforcement and the military are the main users of OWFC's,
so they may refuse to sell you one without proper authorization, or they may
deny knowing about them at all.  Expect to pay at least $500 bucks for a pager-
sized one, much more for a room size model.  Use a fake name when buying, or
expect extra attention from the authorities.

If you have no luck finding someone willing to sell you one, you can make your
own, but it's a bit of a tedious process.  A brillo pad is a good starting
point, but you have to insert diodes at least every 1/64th wavelength to allow
the RF energy to flow into the cage but not out.  For a 450mhz pager, every
centimeter or so will do.  Surface-mount diodes are a good choice because of
their small size.  Basically, expand out the brillo pad as described in the
previous post, then mark 1cm size squares  around its entire outer surface.
At each intersection point, cut throw the mesh and solder in a diode, making
sure that the emitter side is pointing either in or out for all the diodes.
(I don't remember which side the diodes are supposed to point, but you
can easily turn the mesh inside out when you're done.  Whichever way lets
you receive pages is the right way.).    Be warned that buying a large number
of surface mount diodes is considered somewhat suspicious, so to play it safe
and order them with a fake name and in small quantities from several suppliers.

It's been over a year since I made an OWFC for my pager and the results have
been astounding - I am certain that the authorities are no longer using my
pager to track my location.

Be warned, though, that those so-called holographic images on Visa and
MasterCards can easily be used for much the same purpose, although I've been
unable to prove that they are actually using this technology to track people
on any kind of large scale.  Safest bet is to carry your credit cards along
with your pager in your OWFC.