How Traffic Radar Works
There are many kinds of radar: Military radar, weather radar, and air-traffic control radar. Traffic radar is quite different and falls into its own category.
The first category of radar is highly sophisticated equipment.
Traffic radar, by contrast, is very simple.
Traffic radar does not sweep.
Traffic radar does not use a modulated beam.
Traffic radar does not have a radar screen.
Traffic Radar: The Basic Idea
Traffic radar's constant beam acts very much like a searchlight, both in its shape and in its method of operation. That's because microwaves behave much like light waves. They travel in straight lines and they are easily reflected. Metallic objects like cars, trucks, guard rails, and overpasses make the most effective reflectors, sending glints of microwaves around in unpredictable directions just like glints of light.
Unlike light, however, you can't see the radar beam because microwaves are invisible. But they are very easily received by a radio tuned to microwave frequency. And, in fact, such a radio connected to a compact antenna forms the basis of all traffic radars.
Traffic radar works by shining its microwave searchlight down the road.
How Traffic Radar Measures Your Speed
Traffic radar calculates speed from the reflections it receives. It uses a phenomenon of physics know as the Doppler principle. We've all heard how the Doppler principle works with sound waves. The classic example is heard along railroad tracks. As the train approaches, you hear the sound at a fixed pitch. The instant the train passes and begins to move away, you hear a lower pitch. The train itself is making the same sound both coming and going, but t a stationary listener, the speed of the train adds to the pitch of its sound as it approaches, and subtracts as it departs. This change from true pitch is called the Doppler shift and the magnitude of the change depends only upon the speed of the train.
Traffic radar applies this Doppler principle to microwaves.
How Far Away Can Radar Clock You?
Going back to the searchlight analogy, we all know there's a limit to the effectiveness of any spotlight. The more powerful it is, the farther it reaches. The same applies to radar. Since power costs money, traffic radar is far less powerful than military radar.
Traffic radar's low power means that it has limited range. It's a fact of microwave life that the strength of the beam diminishes with the distance. The farther it has to travel, the less energy it'll have when it gets there. For example, the radar operator may spray your car with microwaves while you're still a mile away. But the reflected signal has to travel that same mile back to the radar before it's of any value. If it's so weak when it gets back that the radar's electronics can't read it, then no speed computations is possible. You're out of range.
Radar range depends upon two things: power of the radar and reflectivity of the
For highway vehicles, radar reflectivity is mostly a matter of size and shape. The smaller the vehicle, the smaller its reflection, and therefore the shorter the range. Some cars are out of range on some radars until they drive within 500 feet of the antenna. On the other hand, big, flat surfaces perpendicular to the beam make excellent reflectors. The same radar that may be blind to a small car 500 feet away can see a semi truck a mile and a half away. From this you should conclude that the principle of radar is quite easily understood, but the details of its operating behavior are hard to predict with accuracy.