How Radar Detectors Work

The BEL 975R Vector Remote radar detector: Speeders can reprogram this detector's sensitivity to match changing police technology.

Photo courtesy

Picking Up Signals

In the previous sections, we saw how police use traditional radar as well as new laser technology to catch drivers speeding. As it turns out, conventional radar is relatively easy to detect. The simplest radar detector is just a basic radio receiver, something like the one you use to pick up FM and AM radio stations.

The air is full of radio signals -- they're used for everything from television broadcasts to garage door openers -- so for a receiver to be at all useful, it must pick up only signals in a certain range. The receiver in a radio is designed to pick up signals in the AM and FM frequency spectrum, whereas the receiver in a radar detector is tuned to the frequency range used by police radar guns. Periodically, the frequency range used by the police is expanded, and speedsters everywhere have to invest in new detection equipment.

A basic radar detector won't do you much good if the police officer drives up behind you and turns on the radar gun. The detector will alert you, but by that time, the officer already has all the information he or she needs. In many cases, however, detectors pick up the signal before the speeding car can be tracked. Police often leave their radar guns turned on for a long period of time, instead of activating them after sneaking up behind a car.

Radar guns have a cone- or dish-shaped antenna that concentrates the radio signal, but the electromagnetic wave quickly spreads out over a wide area. The radar gun is configured so that it only monitors the speed of a particular target, not everything in the vicinity, so chances are a detector will pick up the radio signal well before the radar gun recognizes the car.

Of course, with this sort of detector, you're relying mostly on the luck of the draw -- if the police officer decides to target you before any other car, you're caught. Modern detectors offer much more extensive protection for speeders, as we'll see in the next section.