Dash mounted radar on a police cruiser Speed Measurement Labs tests products all the time and we work with them to determine what is the best technology every year.

Radar guns have been in use since for 40 plus years, since the 1960s. Most people did not know what radar guns were until the mid 1970s. Radar guns use what is referred to as "RF" energy pulses to transmit down the highway towards a vehicle. This RF (radio frequency) pulse is an electro-magnetic wave, transmitted from an RF transmitter inside the radar gun.  The Bel STi Driver and Valentine 1 radar detectors are best to detect these radar guns.

It almost sounds safe, but RF energy can be dangerous when you are to close the energy burst. It is similar to the energy in a microwave, which is quite capable of cooking chickens and steak. I hope you get the analogy.

The first useable radar guns came out with an X Band radar frequency at 10.525 Giga hertz (GHz). That is equal to 10,525,000,000 cycle of energy per second. I can't talk that fast. The RF transmitter is a relatively small, very well machined, metal device with a transmit and receive cavity, fixed with an electronic, microwave crystal oscillator that generates high frequency electrcal pulses. These pulses are sent into a cavity, which is designed to amplify the high frequency pulses and let them escape through an opening into an antenna, which is mated and matched to the transmitter.

The RF pulses must travel outward in a metal antennae, that is precisely machined for that frequency, thus little power is lost. The antenna is usually a round antenna with certain critical features not necessary to discuss here. In either case the RF magnetic pulse exits the antenna and travels in the direction the gun is aimed. As the pulse travels down the highway, the RF energy burst gets larger in width and height very quickly.

This means the original energy is quickly dissapated across a wider and wider wave of energy. If a vehicle is encountered by this RF energy wave, the RF pulse reflects off any metal of the vehicle, but passes through any fiberglas, glass or plastic. At the instant of reflection, the frequency of the X band radar gun, 10.525 GHz, changes ever so slightly. This is called the Doppler Shift. The reflected RF pulse is much smaller is size, and of course as it reflects back towards the gun, it too continues to expand in size.

The frequency change is directly proportional to the speed of the vehicle, and is a relatively low frequency change. As you can see the signal getting back to the radar gun has been reduced in energy. How much? An X band gun maight be 25 Watts of energy. That was quite a bit of power. By the time the reflected energy with the Doppler frequency shift gets back to the radar gun, the amount of energy may only be several micro volts of detected energy. The receiver cavity in the radar gun, discussed above, now has to receive the microvolts of energy coming back from the vehicle.

The radar gun receiver is very sensitive and can detect this weak signal from a typical 3/4 mile distance, maximum. It knows the outgoing frequency, detects thre incoming frequency, subtrcts the two, and the exact Dopper shift is now used to be processed by the radar gun electronics. This all happens in about 1/2 second for the older guns and about 1/5th of a second for the new guns. So, even if you have a radar detector, the radar gun is pretty fast.

However, the driver does have an advantage with radar guns. If multiple vehicles are traveling down the highway, the radar gun will simply detect the highest amplitude signal. If two vehicles are traveling at identical speeds, the radar gun will detect the speed of the larger vehicle. If somebody is speeding in a group of vehicles, the radar gun does not know who is speeding, thus the radar gun oeprator must make a decision. K band radar was introduced in the 1980s. It is 24.15GHz (24,150,000,000 herts per second. In Europe it may operate at 24,125,000,000 hertz. Almost the same.

In the late 1990s, Ka started introduction: 33.8GHz and 34.7 GHz. Then 35.5GHz wide band was introduced. Then photo radar was introduced, but that is another article. The reality of the situation is the slow migration from X band to Ka band over the years, expansion of radar guns used to acquire vehicle speed, and the increase power that is constantly being aimed at traffic, at your brain!! Think of all that RF energy every day.