radar jamming is illegal. that was my disclaimer. it all comes down to continuously jamming any signal with a radar signal of your own. this will require you to make your own low power radar transmitter (its pretty easy). first your gunna need a Gunn Diode (a small semiconductor). the semiconductor will create/generate microwaves when supplied with 5v. you will also need to enclose the Gunn Diode in a resonater. you could also use a terminal regulator to convert your cars 12v to the 5v you need. the toughtest part of this project is the fine tuning. this thing is sloppy if you cannot tune it so you will need the use of microwave measurement equipment.
now, as you most likely already know, cops use a few radar bands. i will list the most common.
- K band = 22 ghz
- X band = 10.525 ghz
- Ka band = 33.4 - 36.0 ghz
for more on the frequency spectrum - link

there are three main ways of jamming the radar signal. the first is

Active,
Continuous Noise Jamming.

Continuously broadcasts white noise of high amplitude, causing radar guns to read random numbers, preventing a reading of the actual speed of the car. The radar gun takes about 8 measurements of speed, and only outputs a speed if the 8 measurements agree.

* Advantages
Always works except sometimes at VERY close range
* Very easy to build and operate, since it is always on

* Disadvantages
Very Illegal: Jamming or attempting to jam a police radar gun is a federal felony punishable by fines up to $75,000 and one year in jail.
* Very easy to detect, since you are broadcasting a loud signal to everyone around. Most modern radar guns have detectors for these signals, alerting the cop when he is being jammed in this manner.
* Radar detectors will not work, since your own jammer is constantly broadcasting radar anyhow.
* The cops can shoot an anti-speeder missile which homes in on your jamming signal.

Active,
Selective Noise Jamming.

This is a variation on how the Active Noise Jammer is used. This is not continually on, as the implementation above is. Instead, a radar detector detects radar, and then triggers the active jammer described above for several seconds while the driver slows down to a legal speed. The jammer then shuts off.
Note: this requires that the jammer operate faster than the radar gun, so that the jamming signal is outputted before the reading is complete.

* Advantages
Works most of the time.
* Less likely to be detected by jammer detectors, since it is on for a shorter period of time, and since it will be on only when the cop’s radar gun is transmitting. Many radar guns’ jammer detectors do not work when the gun is transmitting, because it reads the radar being transmitted by the gun itself.
* The anti-speeder missile described above will not work.

* Disadvantages
Just as illegal as continuous active jammers, with the same penalties
* If the radar gun is well built and faster than the jammer device, it can get a reading of a car’s speed before the jammer turns on
* Some more modern DSP radar guns can detect radar jamming even when the gun is transmitting
And the second method is

Active, Deception Jamming.

Which the the only method that will keep you from getting arrested. A radar detector detects radar, and then transmits a frequency corresponding to a given (legal) speed. This frequency is read by the radar detector, which mistakes it for the Doppler shifted echo off of the car, and gives it a fake reading of whatever speed the jammer prefers. This method requires that the jammer knows the EXACT frequency that the cops are transmitting.

* Advantages
It is not detected by the radar gun as jamming.
* Works very well if one knows the frequency which the cops are using (which is a given for the X and K bands). Radar detectors operating in the Ka band can use any number of different frequencies. Some deception jammers are actually sophisticated enough to read a (fixed) Ka frequency and then transmit the appropriate jamming frequency; however, due to constant FCC crackdowns on jammer manufacturers, these are very difficult to find.

* Disadvantages
Still just as illegal as all active jammers, with the same penalties
* Once again, if the radar gun is well built and faster than the jammer device, it can get a reading of a car’s speed before the jammer turns on.
* Many modern day Ka band radar guns switch rapidly between many different frequencies, foiling all deception jammers since they cannot determine which frequency to transmit

The last method are

Passive Jammers.

they are legal jammers. these devices re-radiate the radar signal after distorting it (adding noise and/or shifting frequency) in such a way the true target reflection is masked by the distorted signal. A passive jammer does not generate or amplify a signal, only channel or redirect the radar signal (after distorting) back toward the radar.

* Advantages
Legal in most states.

* Disadvantages
Certain states (including California, Utah, Minnesota, Ohio, Indiana and Oklahoma) have laws prohibiting any jamming device
* The large antennas required to absorb the incident radar beams can actually increase your car’s visibility to radar detectors, allowing them to get a reading from a larger distance than otherwise possible.
* For this method to work the jammer (distorted signal) power must be as large as or greater than target reflected power -- the jammer antenna would need to capture well over half of all the radar energy striking the target (a very large jammer antenna), and be aligned to the radar antenna. To date all known passive jammers have had no effect on any radar under any circumstances.
thanks