sampson@killer.UUCP (Steve Sampson) (03/03/88)
There's been some guessing recently how police radars operate and I thought I'd throw this out for consumption. Police radars are Continuous Wave (CW). They transmit one frequency and mix that with any incoming frequency. The result being a Doppler shift. Police CW does not measure range (police don't have time to sort, therefore CW-FM techniques are not required) and most are set up to display the largest Doppler shift. The officer gets a read-out in MPH of the fastest auto closing or opening. The transmitter consists of a Gunn diode followed by a diplexor consisting of either a majic-t waveguide section, or more recently, a ferrite circulator. This is coupled to an antenna. The mixer is a Schotky diode producing audio. This audio is converted from frequency to MPH based on the transmit frequency. Gunn diodes are very unstable. Sort of like a magnetron only worse. The frequency drifts all over the place. The most often used jamming technique has been Amplitude Modulation. A Gunn diode is modulated by a frequency equal to the desired Doppler shift. Since this is considered the worst possible jamming method, the more power the better. The carrier and sidebands are used in place of the radars. As with any AM system, the sideband power is >50% of the carrier. This means that power must significantly increase with range to maintain effective jamming. Road and Track did a test with 100mw jammers and they proved ineffective. The advantage of AM jamming over any other is that it is the best deception. Other techniques (more powerful) announce your presence. Such as fast swept jamming. If the officer monitors the audio, they can identify a spoofer very well, especially if your the only closing target. ------------- S. R. Sampson "Let's get Dole into a hospital, and remove that pen that's stuck in his hand", "Robertson is a two steps past insane" The GRAND old party is now the Grand OLD PARTY...