What are Police Radar Jammers?
The Ins and Outs of Police Radar Jammers
Are radar jammers legal?
We are often asked whether it is legal to build radar jammers that disable the police speed measurement radar systems. Is such a device even possible? Radar jammers have been used in the military ever since radar came into wide use. The purpose was to protect aircraft from enemy targeting systems. Radar used in the warfare industry was very accurate and could be used on cannons to target and shoot down enemy aircraft. If one could disable these systems, friendly aircraft could be protected. In later years radar was used to accurately track aircraft around airports to improve air travel safety.
Speed Limits
As more and more automobiles started using the nation’s highways, the concern for driver safety promulgated the need driving rules and limits. To enforce speed limits Doppler radar systems were developed and sold to police departments. Naturally, drivers were not happy about being forced to obey arbitrary (in their opinion) speed limits. When the municipalities started assessing fines for the speeders the ‘war’ began. Many drivers felt that the use of radar to get a vehicle speed was a violation of privacy and there were even a few court battles over that view – the courts usually sided with the cities.
Radar Detectors
Meanwhile, cities quickly learned that radar could be used to fund their police departments and help them balance their budgets. During the 1960’s radar detectors were developed to give the driver advanced warning when police were operating speed detection radars. As detectors became more effective the industry developed more radar bands, then more technology-driven ways to avoid detection and maintain the income stream.
The detector industry responded with ever increasingly capable detectors to combat this technology explosion. With the use of ‘instant-on’ radar, however, detectors were relegated to a backup position. After all, they could not react to a signal that was not there. When the officer triggered the instant-on pulse, the car was already too close to take corrective action. Therefore the need for radar jammers arose.
Police Radar Jammers
Basically, radar jammers are devices designed to block or interfere with the proper operation of a radar transmitter. Car radar jammers specifically target the Doppler radar systems used by police officers to measure the automobile’s speed. Police radar jammers are usually transmitters very similar to the radar system they are designed to block. They generate a radio frequency signal very close to the police radar frequency and try to overpower or confuse the radar. The radar jammer then emits that signal in the direction of the police radar to try to keep it from getting an accurate speed reading.
The two most popular techniques for police radar jammers are amplitude modulation and frequency modulation.
Amplitude Modulation
In the AM system, the car radar jammer generates a signal at or close to the police frequency, then turns that signal on and off at a speed equal to the Doppler shift it wishes to create. Adjusting the switch frequency varies the simulated Doppler shift and hence, the speed displayed on the police radar. The police radar has a mixer diode the combines the radar’s transmitted frequency with the signal reflected from your car. (When a radar signal hits a moving object the frequency is shifted proportionally to the speed of the car, i.e., the Doppler Shift) If one could get the mixer to detect the amplitude shift caused by the car radar jammer turning On/Off, then it would show the lower frequency Doppler shift instead of the car’s higher speed.
This works great on the bench and in parking lots. But, when the car’s speed is greater than the simulated Doppler shift, there is a problem. The police radar jammer actually picks up its own Doppler shift from the car moving and transmits that speed to the radar gun. This can actually increase the distance the radar can pick up the true speed – an outcome truly not desired! This problem can easily be solved by adding an isolator to the car radar jammer to prevent its own Doppler shift but these are very expensive. Also, one would need a car radar jammer for each frequency band allocated to the police radar.
Frequency Modulation
In the FM system, the police radar jammer still generates a radio frequency similar to the radar and emits it in the direction of the gun. In this case, however, the frequency is changed (or swept) across the radar operating frequency to use the same mixing mechanism in the radar gun. By sweeping the frequency across the radar band police radar jammers create multiple Doppler shifts that are constantly changing at the sweep rate. Ever since the 1960s, when a tree was clocked in Florida at 75 miles per hour, the courts have ruled that police radar must get 8 identical readings on a target before it is allowed to display a speed.
Car radar jammers that sweep across the band create many ‘Doppler shifts’ that are identical to the reflection from the car. In this case, the speed of the car is irrelevant as any accumulated Doppler shift actually helps with the confusion. Obviously, with all these signals coming into the radar, the gun can never get the 8 readings it needs to display a speed. Only Consumer Microwave Company in Dallas was able to successfully market this type police radar jammer in the 1970’s.
Unfortunately, these car radar jammers also suffered from the expense of needing a separate transmitter for each of the three radar bands the police are authorized to use.
Passive Radar Jammers
About 30 years ago Rocky Mountain Radar introduced a new type of police radar jammer. It uses the same principle as the FM radar jammer described above, but with one significant difference; it did not contain a radio frequency generator. This is important because the FCC determined that car radar jammers are illegal if they contain an RF generator and emit that energy.
This new technology has neither! The company discovered that it could build highly effective police radar jammers by using a very high gain antenna and merely mixing the incoming police frequency with a series of Doppler shifts that it generated internally; then it reflected this modified frequency back to the police radar gun. When the radar gun received the modified signal it looked identical to that reflected from your car, except there were many of them. It could not get the requisite 8 identical speed readings and stayed blank. Car radar jammers that use this technology are effectively protected from radar speed readings and are effectively ‘invisible’ to police radar.
Of course, nothing is perfect, Since police radar jammers that are passive are limited by the amount of energy collected from the police radar, there is a limit on how close one can get before losing the advantage – punch through. On the positive side, because these Passive Radar Jammers do not generate any signals but use those generated by the police radar, they are not limited by the number of bands the police are allowed to use.
Laser Radar Jammers
Laser radar jammers (Lidar Jammers) are very similar to police radar jammers with one notable exception; they are not governed by the FCC. The main legal limitation for a laser radar jammer is the power they are allowed to transmit as governed by the FDA. Once again there are two types of laser radar jammers that have been produced.
The first and most popular laser radar jammer is the brute force broadcast type. In this design, a series of infrared LEDs are switched on when laser radar is detected. The theory is that the sheer amount of power generated by the array will overcome the Lidar. Of course, this approach suffers from two main problems. First, since the raw power is just turned on and left on for a period of time, the detection circuit in the police laser radar gun can simply use a cheap capacitor to filter it out. Yes, it does see the increase in ambient infrared noise, but it has good filtering. The second problem is that laser radar is very fast. The laser radar jammer must detect the laser first, then turn it on which means that the laser radar MAY get a reading before the system even kicks in!
Rocky Mountain Radar uses a technique driven design in its laser radar jammer. With this design, the transmitter LEDs are always turned on but they are switched ON/OFF 8 million times per second. By switching the diodes on and off at this rate, the laser radar jammer creates pulses identical in shape to those generated by the laser gun. Since it runs continuously, the pulses are never synchronized with the laser radar pulses and the whole system becomes nearsighted and confused.
Are They Legal?
All the police radar jammers discussed above are NOT legal by FCC Ruled except for the passive radar jammer. This is principally due to the fact that they contain an RF generator and they emit energy; the passive radar jammer does not have either. There are no Federal laws, other than the power restriction by the FDA, restricting the use of laser radar jammers.
This is not the whole story, however. To this date, about 11 states have banned the use of ANY police radar jammer or laser radar jammer in those states. Virginia and Washington, DC ban all detection and jamming systems. The other states, fearing the effectiveness of Passive Radar Jammers, have changed their laws to include these in their statutes. Only one company produces passive radar jammers and they have included a convenient switch to disable the police radar jammer while keeping the radar/laser detector in operation. Go here for a list of states that now ban these products.
So, Where can I get One?
Police radar jammers are no longer available for sale in the US. Passive radar jammers are available from RockyMountainRadar.com. Laser radar jammers are sold by several radar detector companies, principally by Escort radar and Rocky Mountain Radar. Feel free to contact us with any questions about radar technology or our signature radar scrambler, The Judge 2.0.
MB Churchman, President
Rocky Mountain Radar