A car radar detector is a 12 volt DC electronic device employed by drivers, to alert them if their vehicle's speed is being acquired. Radar (short for Radio Detection And Ranging) detectors comes
from the first speed measurement technologies where radio waves around 10.5Ghz in frequency (X-band) were fired from a law enforcement officer at a moving vehicle. The radar beam would bounce off the moving car and the change in the wave's frequency (known as the Doppler effect) could be electronically be calculated to determine the vehicles speed.
Most car radar detectors are windshield mounted via a bracket with suction cups that presses onto the glass, although some stealthier packages are designed to be hidden upon installation, mounting in the front bumper, grill or other discrete positions. A separate antenna is usually mounted somewhere to the front of the vehicle, whilst the control box and/or visual alert component is mounted in the cabin, whereby the driver has easy access to control the volume and hear or see the alerts.
Many companies specialize in custom installations, mounting the radar detector kit in vehicles so as you couldn't tell they were there. Some companies can blend them in to things like rear view mirrors, clocks behind the dashboard, or concealing the entire package except for a single LED light.
The radar detector works by picking up the police frequency at a distance, and alerting the driver by visual and/or auditory signals. In theory, microwave from radar guns will travel forever. This is the case in the vacuum of space, but not the case on earth. Materials such as humidity, dust, pollution, buildings, and competing microwave signals absorb and disperse the signal, reducing the effective range of a police radar gun. It is at this distance, before the radar beam is strong enough to bounce back to the police car, that a driver equipped with a radar detector stands the greatest chance of an early warning.
Whilst some users wanted to detect radar, others wanted a radar detector jammer, which would block a Police radar gun from acquiring your speed.