schema jammer
hi, just thought I'd butt in here,
I'll for-go the 'moral' issues raised by this, and the warnings of legality, since it would be breaking the law. Most countries come down very heavy on such actions, not just 'radio' emissions, but specifically phone interference. Safe to say, I strongly disagree, on many levels.
You're all talking about GSM, off the top of my head, second generation mobiles (most common) work on 900 and 1800Mhz. Uplink and downlink are separated by about 50Mhz, band width is 25Mhz for 900, and 50Mhz for 1800. CDMA phones are very tolerant to interferers, but they can be jammed, just like you 802.11b LAN can be jammed, although the spreading sequence in CDMA and code are significantly longer. All these systems have hopping capability, and will hop anyway from time to time to spread network traffic (no-one has the same channel for long). If you are near a base station that has both 900 and 1800Mhz capability, and you want to jamm all mobiles in a specific area, you would need two separate transmitters. OR - A big, fat, dirty transmitter that spits out harmonics like their going out of fashion
Although all mobile GSM systems are really good at coping with this, you wouldn't have to damage the radio links too much for mobiles to stop working. Most phone companies would rather cut you off than give you a 'bad line'. peoples phones would still show they have coverage, but they would be unable to make or recieve a call.
My advice would be, keep it simple. You want a wideband transitter to nail either the uplink, or downlink. (I would go with downlink since the signal from the phone is strong, and the signal from the BS is weak, and you would be nearer to the phones). Also, let there be harmonics, specifically, 2nd order (double the centre frequency), that way you can get 900Mhz and 1800 at the same time. So just build a FM transmitter, feed it with noise to randomise and spread the band, and don't bother with the usual hassle of reducing emissions. Sharp edges in your PCB, no shielding, and lots of power (3-5watts?). No point in complicating it, unless you want to 'eaves-drop' in which case, it becomes an electronics nightmare.
Still disapproving,
BuriedCode