Checkout this cell Jammer or we can say Dirty transmitter. A Dirty transmitter transmits junk within the cellular phone bands. This model is good enough to knock out pesky cell phones for about 10 to 15 feet, just perfect for a quite cup of coffee in your favorite coffee shop.
Via : EEweb
“The 555 timer [8 pin] IC simply makes a noise. It’s coupled via C4 [electrolytic] to modulate the MRF transistor oscillator. With C1 set at roughly 1/3rd, you will be close to 900 MHz. By sweeping the C1 trimmer capacitor, you can swing the output frequency from 800 MHz to 2 GHz with the transistor and values shown.”