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Radar and Lidar Jammers

Microwave Jammers
From time to time traffic radar jammers appear in magazine and internet ads. Most if not all of these jammers are useless, absolutely no effect at any range under any conditions.

The legality of jammers is also in question and somewhat up to the whims of bureaucrats and politicians regarding enforcement and court interpretation of the law. Several U.S. states and some countries prohibit the use or possession of a police radar jammer.

For the United States Federal Communications Commission to consider an intentional radiator legal the field radiation must meet FCC Rules Part 15, and the device must perform some function for the public good. Police radar jammers are not considered good for the public by the FCC.

The FCC considers the use of traffic radar jammers as malicious interference and strictly prohibited by the Communications Act of 1934, as amended, as well as by FCC rules. Anyone using a jammer risks such penalties as losing FCC licenses, paying a fine, or facing criminal prosecution. (FCC DA 96-2040).

Passive jammers
Passive jammers are suppose to re-radiate the radar signal after distorting it, adding noise or rapidly shifting frequency, in such a way the true target reflection is masked. A passive jammer does not generate or amplify a signal, only channel or redirect the radar energy back toward the radar.

For this method to work the jammer antenna would need to be at least as large as the vehicle - not practical. Additionally the jammer antenna would need to be almost perfectly aligned to the radar antenna - not lightly. Passive jammers have absolutely no effect on any radar under any circumstances.

In 1997 the FCC ruled passive jammers violate federal regulations because the jammers radiate energy that, or at least is intended to, adversely affect the ability of law enforcement officials to protect public safety on the highways. Before this ruling passive jammers were not considered transmitters and not covered by FCC regulations.

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Active jammers
There are 2 types of active jammers, one continuously transmits, the second only transmits when a signal detected. Many police radars can detect jamming signals even when the radar is not transmitting, this is a good reason a jammer should only transmit when a radar signal is present.

Continuously Transmitting
May set off Interference
or Jamming Indicators.
Transmits when Signal Detected
May get speed reading
before jammer can react.

There are 2 basic jamming techniques used against police radar, noise jamming and deceptive jamming. Noise jamming transmits amplified noise to desensitize the receiver and obscure legitimate targets. Deceptive jamming transmits a signal that causes the radar to register a speed set by the jammer. Both techniques could set off the radar interference or jamming indicators.

Noise Jamming
Desensitized radar receiver
obscuring legitimate targets.
Deceptive Jamming
Transmits a deceptive fake
spee fixed or set by operator.

Deceptive jamming transmits a signal anywhere in the radar receive band. The signal is slightly amplitude modulated at the radar Doppler frequency rate of the fake speed. This technique works on the same principle as a tuning fork.

Laser Jammers
Most laser jammers are completely ineffective. A few jammers are effective, about half the time, for vehicles at ranges greater than 500 to 1000 feet. All jammers are completely ineffective at ranges less than 500 feet. The jammers have the same problem as laser detectors, the narrow lidar beam never strikes the detector aperture. Laser Jammers are useless at the ranges lidar is most effective.

Laser jammers are ineffective because the jamming signal must closely match the lidar pulse width and rate which varies with model. The jammer must detect the waveform and generate a match, then synchronized transmission to the next detected pulse.

Police Radar Information Center

Radar and Lidar Jammers
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