On 2007-08-07, R. Mark Clayton <nospamclayton@btinternet.com> wrote:
><paranoid bob <paranoid.bob@example.com>> wrote in message
> news:157da111e89a3cdd310496a490a4524b@msgid.frell. theremailer.net...
>> "R. Mark Clayton" <nospamclayton@btinternet.com> wrote:
>>
>>> > Anyone know where I could find a short range (10 feet max), short
>>> > burst (just long enough to disconnect a call), directional mobile
>>> > phone jammer? Just to disconnect loudmouths on the train, directional
>>> > so it doesn't affect someone quietly texting in different direction?
>>> >
>>> > thanks
>>> >
>>>
>>> Given that mobile phones still work under pantograph wires and under
>>> bridges
>>> etc. (but not tunnels) you will need a fairly long burst, and pretty high
>>> energy to defeat a frequency agile GSM. In any event you would only mess
>>> up
>>> the receive segment. The phone would hand off to another base station in
>>> another direction fairly soon too. With CDMA or UMTS you would have even
>>> less luck.
>>
>> A friend who knows or thinks he knows (!) a bit about electronics said
>> that the way to do this would be to generate white noise and modulate
>> it to a band around the main frequency (900 or 1800 megahertz, I think
>> he said).
>>
>> ?
>
> What do you think the spark on the power pick up does - not only that it is
> connected to a bloody good aerial (the wire).
>
> Without knowledge of the precise times and frequencies that the particular
> phone is using it would require a lot of energy to effectively jam it (and
> then probably only one way). Sat with this in your lap, Lenny would soon be
> using you as proof of his theories about EMF causing injury (enough will).
> Not only that but you would get nicked.
I don't think it is that bad. You only need to block one transmission
direction to disconnect the call; if you are closer to the phone than the
tower the tower->phone direction would be the one to pick. I think
GSM
phones require at least a 6 dB signal-to-noise ratio to operate, so if you
can put a signal on the phone's receive channel at about the same power
level as the the tower you'll block it. I think the tower is limited to
less than 2 Watts (32 dBmW) per
GSM channel. If the tower is 1 km away
from the phone and you are 10 m away you'll need to get 1/10,000th of the
tower's output power on the channel to match it, or about 200 uW. There
are 124 channels in the 900 MHz band and 374 channels at 1800 MHz, so to
cover all channels at once at 200 uW would take about 25 mW for the
900 MHz band and 75 mW for 1800 MHz, 100 mW total. If I haven't made
a mistake it seems the jammer would need a lower power output than mobile
handsets typically produce to block all the channels in both bands at once
to a distance of 10 meters. And the transmission isn't pulsed, so Lenny
might be happy.
As for getting caught, I've read anecdotes about what it took to locate
low-level, bandband sources of unintentional interference to GPS receivers
and this was not all that easy even when the interference was broadcast
continuously. I'm not sure how they'd find a broadband 100 mW source
that was turned on for only a minute or two every once in a while.
I think a jammer that blocks all
GSM channels would be fairly straight
forward to build. I hope this doesn't become popular.
Dennis Ferguson
Linear Mode
