Radar is an acronym for radio detecting and ranging. It was developed for military use in the late 1930s and was used during World War II. After the war, radar technology was adapted to civilian applications. In 1950 state agencies and municipalities began experimenting with radar to catch speeders.

The first radar devices used by police to monitor traffic were stationary, but in the 1970s, solid-state integrated circuits allowed radical downsizing of radar devices. This gave rise to radar guns, usually used by police officers sitting in patrol cars. A new frequency band for radar transmission, the K band, vastly increased the use of radar technology by police departments.

Two technologies help drivers avoid getting caught by police radar: radar detectors, which let drivers know that they are being subjected to radar, and radar jammers, which actually block police radar from tracking a car.

A radar detector will beep, light up, display a text message or actually talk to alert a driver to the presence of a radar device.

The popular perception of radar detectors is that speeders use them to avoid entrapment by authorities. But in many cases a radar detector will keep drivers at a consistent, legal speed. For instance, some municipalities and states have developed an ingenious safety device called a "drone" that emits safety warning system (SWS) radar.

SWS drones cause a radar detector to beep, warning the motorist to slow down. They also can alert drivers to accident-prone spots, as well as construction and other road hazards. Older detectors, however, cannot differentiate between the drone warning of a road hazard and police speed monitoring devices.

Newer microwave technology takes the SWS drone concept a step further by providing drivers with specific information about high-speed emergency vehicles, stationary road hazards or trains equipped with so-called safety radar transmitters. Newer "smart" radar detectors give drivers a special audible and visual warning that describes the approaching hazard.

Radar detectors used to be illegal in many states. However, now radar detectors in cars are illegal only in Virginia and the District of Columbia. It is illegal, however, for large vehicles such as buses and trucks that weigh more than five tons to have radar detectors.

Because individual states and municipalities are equipped with different radar devices, some research is required by the consumer. Know what methods of radar are being used in your area before you settle on a radar detector type.

Police scientists still are seeking ways to make undetectable radar devices, while manufacturers come up with more advanced detectors. In 1987 some police departments began using Ka Photo, which could take pictures of the speeding vehicle. In 1993, Ka Stalker arrived, which operates on newly expanded K band frequencies above those of Ka Photo.

The latest detecting development is the laser speed gun, an extremely accurate device that doesn't use radar and therefore can't be detected by any standard radar detector. Laser guns as well as Ka Photo and Stalker are very expensive. A laser gun can run as high as $4,500, three times as much as a standard radar gun. And they operate from a stationary position only, which is the main problem with the original X band devices.

With municipal and state budgets continually squeezed, not many police forces have been able to justify the higher-priced units. As a result, only about 500 laser devices are in use throughout the country out of an estimated 200,000 traffic measurement devices nationwide.

Variations

Radar detectors use the basic component of all radio devices, the superheterodyne circuit, or superhet. Not all detectors are designed to detect all four bands -- X, K, Ka Photo and Ka Stalker -- and laser. If your locality still uses only X and K bands, then a detector that finds only these bands may suffice.

So-called "wideband" detectors can receive all four radio bands. Many full-range detectors distinguish not only between the differing bands but emit a different tone depending on which band is detected. You'll need a special detector to detect laser gun activity, unless you spring for an all-inclusive detector, which will pick up any and all speed detection types. Of course, you'll pay more for the added detection.

Most units beep when they detect a radar signal. The beeping gets louder and faster as the driver gets closer to the source. Some premium models emit a voice warning, "slow down, slow down," that speeds up as you get closer to the radar source. Most detectors employ a segmented LED meter that gives a visual reading on the radar's proximity and power.

The sensitivity of the detecting model is measured by a decibel rating. The farther away the detector is from the radar-monitoring device, the more sensitive the detector must be to pick up the signal. Most detectors have a sensitivity range from 100 dB to 115 dB, with higher numbers indicating increased sensitivity, meaning your detector could alert you to the presence of a radar device up to three miles away - plenty of time to slow down.

Many ultrasensitive detectors trigger false alarms by picking up stray radio frequencies in the X- and K-band frequencies. Many garage door openers, for instance, operate on frequencies that a detector reads on the X-band meter. Sensitive detectors usually include anti-falsing circuitry that rejects spurious signals and other interference.

Tech talk

Police radar measures changes in frequency in a focused microwave radio signal reflected by a moving object, in this case, your car. The principle, called Doppler shift, is simple: When you listen to a train whistle, the whistle sounds higher in pitch when the train approaches than when it moves away. Also, the faster the train goes, the greater the change of pitch of the whistle in both directions.

For cars, microwave signals from traffic radar equipment are emitted at a frequency and then bounce back from the moving vehicle at a different frequency. When the police radar device is stationary, the reflected signal from an approaching vehicle will rise in frequency according to the vehicle's speed. The higher the speed, the greater the increase in frequency. When a vehicle moves away from the radar beam, the converse occurs. The frequency of the reflected radio signal will drop according to the speed of the vehicle. Traffic radar units measure only the difference in frequency between the original signal and its reflection.

A radar detector is similar to an FM radio receiver. Employing super heterodyne technology, a radar detector tunes to the frequencies of traffic radar devices, reads them and relates the information to the driver in audible beeps or LEDs.

The authorities first used thin X band, a 10.525 GHz frequency. Standard K band operates in 24 GHz and works faster over shorter distances. Ka Photo occupies a thin range of the Ka frequencies between 34.2 and 34.4 GHz. For Ka Photo, the radar unit takes continuous traffic readings. A camera triggers when a threshold speed registers. If, for example, the threshold level is 40 miles per hour, any car going faster than that speed is photographed. If the photograph shows the license plate, the owner of the vehicle is issued a citation. Because Ka Photo is automatic, it doesn't need a human operator.

Ka Stalker has the widest range of all radar devices, from 34.4 to 35.2 GHz. The entire Ka band operates at very low power levels to make the radar beam more difficult to detect. Options allow the police user to shift the frequency at will over the entire Ka band.

Most radar devices provide accurate readings but trained personnel must interpret them. Among the technical drawbacks to police radar is the inability of monitoring equipment to provide reliable vehicle identification, except for the still rare Ka Photo guns.

The laser speed gun bounces a laser beam, at intervals, off a moving vehicle. It gauges the distance at each interval and computes the speed by measuring the time it took to get from one position to the next. Individual vehicles can be pinpointed at distances of several hundred yards, even in dense city traffic.

Purchase

The range of the radar detector should be at least three times the effective range of current radar speed measurement devices. Most high-quality detectors claim a range of a mile on level ground. Both visual and audible alarms should indicate signal strength, enabling the driver to determine the vehicle's distance from the radar installation.

Some salespeople may tout a particular detector by claiming high sensitivity. High sensitivity means that the police signal can be read around curves and over hills, favorite patrol car hiding places. Sensitivity is important, but it should be balanced with selectivity, the ability to filter out unwanted radio waves. A radar detector that is too sensitive may cause false readings. If you decide on an ultra or high sensitive detector, get one with a filter that can eliminate these. Remember that these sensitive detectors must be able to pick up the full range of Stalker radar.

Another factor to be aware of before buying: An insurance company may not insure your detector unless it is permanently installed.

Connections

Most radar detectors include several mounting options, including a windshield mount - usually a suction cup, a hook and loop fastener for removable dashboard or sun visor mounting, and a coiled power cord, which gets plugged into your car's cigarette lighter. You can mount a detector permanently, but most models are designed to be plugged and unplugged and removed.

The size of the detector determines whether installation is required. Since radar detectors often appeal to thieves, most users want them out of sight. Many new models weigh less than a half-pound and are small enough to fit into a coat pocket. Most detectors come with accessory kits for installation.