It is possible to build an AMPS cellular phone system jammer using commonly available parts.  This will only work with the older, analog 800 MHz cellular phone system and is for reference only.  The theory is to generate a signal that is in the same frequency band as the phone is operating in, but at a much higher power than the normal received phone signal.  Sweeping that signal across the entire cellular phone band will jam any phone in the immediate area.

The most important part of the jammer is the initial RF signal source.  Generating the RF signal is very easy today with an IC from Maxim IC.  The MAX2622 is a monolithic voltage-controlled oscillator that covers the frequency range of 855 MHz to 881 MHz.  Review the datasheet (151k PDF) then order a few MAX2622's from Maxim directly.  The device is in a tiny little 8-pin µMAX package, so you may want to mount it on a SurfBoard from Digi-Key, part number 33108CA-ND, to make it more easy to work with.  Note that you'll need to glue a copper ground plane to the bottom of the SurfBoard using a scrap piece of copper clad PC board material.  Drill the ground vias as needed.

The next part of the jammer is the sweep generator circuit, this can surprisingly be built using (mostly) parts from Radio Shack.  It's just a twin-tee tone generator buffered/amped by a LM741 op-amp.  This outputs the sweeping voltage that connects to the MAX2622's voltage tune pin.

You'll also need to aquire an old bag-style cellular phone (Nokia/Tandy/Uniden are best) for it's power amplifier module, usually a PF0030.  This will be the intermediate power amplifier and will drive the final power amplifier.  There are a few possibilities in this stage and they will be described later.

Additional Notes

Can also be used to jam 800 MHz trunking and SMR radios (i.e. racist cops) and will do a little damage to the 900 MHz ISM band.

The third harmonic of a sweeping 800 - 828 MHz signal will jam devices operating in the 2.4 GHz ISM band.  Great for taking out overzealous ISPs.

Select a picture for larger image.

Overview of the jammer with the RF power amplifier, battery and the directional antenna.

Internal view.  The case is from an old Uniden cellular phone.

Closeup of the PF0030 and the associated controls.  You can see the MAX2622 and its SurfBoard mounted directly on the PF0030's RF input line.

Spectrum analyzer plot.  The spikes are actually rolling through the band.

Closeup of the Motorola Mostar 30 Watt RF power amplifier.

Tap point inside the Mostar radio.  Remove the 10-ohm surface mount resistor in series with the RF input into the power amplifier, and insert your RF drive there.  Don't exceed 250 mW of drive power.

Stupid homebrew 800 MHz directional antenna.