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PICList Thread
'How to stop cellular phones?'
1998\11\19@223646 by Donald L Burdette

picon face
Has it occurred to all you jammer nuts that the idea was to ELIMINATE RF
TRANSMISSIONS?  Jammers are typically the most powerful radiators on the
band.  The only reasonable solutions are to keep people from ever turning
on the (expletive deleted) things, or to protect critical equipment from
their effects.

Sorry, that was my military training talking.  It was more or less
correct until some clever Frenchmen decided to make low power units that
would mimic the cell site and effectively lead the cell phone to a dead
end.  Eventually the user gives up and turns it off.   Of course this
does not eliminate the RF tx, but only reduces its duration.

Perhaps we should start putting metal foil on the walls of hospital
rooms.  But then I guess they'd accuse us of being in league with the
aliens!

1998\11\20@064043 by Peter L. Peres

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On Thu, 19 Nov 1998, Donald L Burdette wrote:

> Has it occurred to all you jammer nuts that the idea was to ELIMINATE RF
> TRANSMISSIONS?  Jammers are typically the most powerful radiators on the
> band.  The only reasonable solutions are to keep people from ever turning
> on the (expletive deleted) things, or to protect critical equipment from
> their effects.

This is mostly true, except GSM digital cellular is a switched packet
network with limited ECC and no active counter-measures.

This means, that if you jam 5-6 bits out of every packet (they come at
about 20msec), none will arrive whole.

The power required to do this, is the power of the cell transmitter as
sensed at the site to be protected, plus 3 dB or so, divided by (5 bits /
length of packet), where the length is of several hundred bits. You will
probably end up with something in the 0.05W league to disable any
connection chance in a 3 x 3 x 3 meter area. Such a device can be easily
attached to the critical medical equipment proper and won't bother it at
all. In fact, if the maker is concerned he might deliberately build an
algorythm into one of his processors to do just that on the side. To Tx
50mW in single bit pulses only a switching transistor and a 900 MHz
transmission line are required ;) Just drive it with a properly fast flank
(ECL former)  with the proper timing and you may have what you want. I
guess a PIC is powerful enough to generate a pulse sequence required to
drive this kind of amortized wave transmitter.

This job is for brains, not muscles ;)

Peter

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