Police Radar and How it Works

A Way to Defeat It

A legal device that defeats police speed radar detection systems is possible. This device disables all bands of police radar, and can have substantial benefit with laser speed guns, too. It even has rather low cost!

There is a continuing huge market for Radar detectors in all vehicles, even though they have been made illegal to have or use in some states. In recent years, there have been some products promoted and sold which purport to defeat or jam Police speed radar. Most of these involve some sort of radio transmitter which sends out a signal in or near the Police band being used for that type of radar. These devices are highly illegal, and even being in possession of such a device is a Federal felony, with the probability of lengthy jail time. It is a very bad idea to get or make one of these!!

In our discussion below, we will clarify many things about speed radar. We'll even explain the 65 mph trees which everyone has always laughed at and which people have assumed was a result of a faulty device. It was NOT a faulty radar, and was operating exactly as designed.

There IS a LEGAL way to defeat speed radar. We have invented three different configurations of devices which will do the job!

Speed Radar Operation

First, we need to discuss the operation of speed-detecting radar.


The radar speed gun has quite a bit of sophisticated electronics in it. First, there is a transmitter, which creates a signal (called a carrier wave) at a specific frequency (of whichever band the radar is designed for). We will use as an example 1.5 Ghz, which means that one and a half billion sine wave pulses are created every second. This signal is not modulated like a signal from a radio station would be.

Some people seem to assume that this frequency is absolutely precise and that it doesn't ever vary. In the real world, variations in component values and dependence on temperature cause continuous slow changes in the carrier frequency. (This will come up again later). To simplify our example, we will assume a frequency of EXACTLY 1,500,000,000 Hertz.

Next, a tiny fraction of this signal is taken off and kept in the radar gun, to be used later. Then, most of the signal is amplified and sent out the front of the radar gun.

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Signal Radiation

This signal is radiated out toward your car. As the signal spreads out, its strength gets weaker and weaker. It complies with the inverse-square law of physics. Going twice as far away makes the signal 1/4 as strong, and the radiation pattern is twice as wide and twice as high. Various radar guns have different spread patterns, but a block away, the pattern may be about 30 feet in diameter. This means that the signal strength is less than 1/100,000 of its original strength. (By the way, that is why there is no radiation danger to you in your car.)

This is the signal that gets to your car. If your car passes through any part of the 30-foot diameter circle (actually, cone), the signal will hit it. The radar gun does NOT have to point precisely at your vehicle.

Reflection off your vehicle

Then your car reflects it. Reflection occurs in two ways.
A tiny fraction of this energy that is reflected in all directions, happens to be reflected back in the exact direction of the originating radar gun. The total radar signal reflected exactly back toward the source radar gun is the sum of these specular and diffuse components. You can probably see from this that on the whole, the signal strength from a huge semi tractor and trailer is likely to be far stronger than that from your little compact car. (That situation is actually due almost entirely to the much larger diffuse reflection. The specular reflection can be of pretty similar strength.) Even though that reflected signal is stronger, it MAY NOT be the one that the radar gun notices. (More about this later.) In any case, the actual total signal strength reflected back FROM your vehicle to the radar gun is even far weaker than when it first got to your vehicle, so it's REALLY miniscule now!

As an experiment in 1987, we started fitting the front of a huge old 1972 Ford van with a sloping flat, mirror shiny, surface, tilted back (upwards) at about a ten degree angle. The whole front of the van was this sloped mirror surface. (It would NOT have been safe to drive since you couldn't see the road!) Such a modified vehicle was ugly as sin, but was INVISIBLE to Police speed radar, because ALL of the signal was reflected (in a specular manner) up and out into space! In order for this approach to work effectively, the mirror surface had to be absolutely clean! We discovered that if it got even a small amount of dust on it, there was enough diffuse reflection to send a signal back to the radar gun, and it was no longer invisible to the radar.

When the US Government was designing the Stealth Bomber and the other Stealth technologies, they faced dealing with both of these types of radar reflections. We heard a story that they had done such an excellent job of making the entire airplane anti- and non- reflectant, that it was totally invisible to radar sitting on the Tarmac. EXCEPT when a pilot sat in it! His glasses and helmet and face were NOT Stealth modified, and therefore reflected a signal back, and therefore made the plane's location known to the radar! (I understand that they solved this later!)

Have you ever noticed the weird angular shapes of a Stealth airplane? That's related to an attempt at reducing specular reflection back toward an enemy radar, much like our experiment with the shiny wedge on that old van. Of course, they combined that basic shape with anti-reflective coatings and other technologies. If you had several billion dollars to spend, you could apply military Stealth to a car and make it invisible to Police RADAR!

The Signal's Reflected Trip Back

The reflected signal spreads out from your car and gets weaker again. By the time it gets back to the radar gun, it can be far less than one-billionth of its original strength.

Again in the Gun

Remembering that the gun retained a tiny amount of the radiated signal, we can now electronically compare the retained and reflected signals to learn several things. If we had wanted to, we could have timed the delay until the reflected signal got back, and found how far away the car was, like aircraft radar does. But Police DON'T CARE how far away the car is, so this processing does NOT occur in a Police speed radar gun.

There is a phenomenon called the Doppler shift, which causes the frequency of any signal radiated from an object (including a reflected signal) to be shifted by a very specific amount. The size of this frequency shift is dependent on the speed of the moving vehicle, and on almost nothing else. The equation is f(reflected)=f(source) * Sqr.Rt((c+v)/(c-v)), where c is the velocity of light, and v is the velocity of your car. The speed of light is REALLY fast! For a car going 100 mph (toward the radar gun), this only represents a frequency shift (increase) of our original signal to 1.500000150 Ghz, a VERY tiny change. It is SO small a change that it would seem impossible to even recognize it. The ONLY way to even know there was a change is through combining (ADDING) the retained signal with the reflected signal. In the process of this signal addition, several new signals appear, one of which is at the frequency of the difference! This process is called beating the signals together. So we get an resulting output (difference) signal of about 150 Hz for the 100 mph car. Lower speeds give lower (difference) frequency. Every frequency corresponds uniquely with a specific speed, virtually exactly proportional to vehicle speed.

An important effect results from this method of getting this difference of frequency. Since the signals being added were created within about one one-millionth of a second, from the same source oscillator, very little accidental frequency change could have occurred. Even though the (carrier) frequency gradually drifts due to temperature and other effects, little change can occur in such a very short time. Five minutes later, that same radar gun might have warmed up and is now oscillating at 1.500001000 (a change much larger than the final measured difference) but it would still be precise because BOTH the retained and reflected signals would have been shifted identically.

Final Analysis

Since the difference frequency is virtually directly proportional to the target vehicle speed, a simple circuit converts the 150 Hz signal into a readout of 100 mph. A 75 Hz difference signal would show as a 50 mph readout.

The radar gun has a circuit that retains the highest previous difference reading, and compares all new readings with it. A higher new difference reading replaces the previous retained/displayed value. This is important!

The speed determined in this way is the speed difference between the police car and the target vehicle. These comments have described the situation for a stationary police car, the common situation. Some radar guns meant for use in moving Police cars are also connected with the Police car's speedometer, to automatically adjust the output reading to the correct value for the target vehicle.

Still with us?

Deficiencies in Speed Radar Operation

We felt it necessary to include this fairly full description in order to address several subjects that seem to mystify everyone, such as the 65 mph trees. And to assure you that we know what we are talking about. So, even if you didn't follow all the details above, it's OK.


Now you know a LOT about speed radar, and you know the answers for some long-standing mysteries of it.

We've gone through a lot of this because we haven't seen any other easily available source for this information. Also, to get you to realize that we have comprehensive knowledge of the subject.

You also now know something about why Stealth aircraft look so odd!

For the Police!

Since many millions of drivers presently have some kind of radar detector in their cars, police are often annoyed by their equipment being defeated in this way. We have a suggestion of an improvement for them!

From the discussion above, it should be clear that there is no actual reason for a continuous radar beam to be transmitted. (That continuous transmitted signal is what radar detectors receive and identify to alert the driver.) If, instead of sending a continuous signal, a signal pulse that was only one one-millionth of a second long was sent out, everything in the design concept of Police radar as described above works properly and effectively, displaying the target vehicle's speed.

For the moment, say that the radar gun had a timer circuit such that one such pulse was sent out each second. A target vehicle's radar detector would only have one-millionth as much total received radar signal to try to analyze and interpret, making the signal far below the threshold level for triggering an alert for the driver. In addition, since all modern environments have many extraneous electronic radiations everywhere, in a millionth of a second, NO radar detector could possibly be set sensitive enough to determine that that very brief signal came from a police radar gun and not a nearby garage door opener or other source.

In reality, it would probably make sense to send such pulses several times a second, so there would be no delay in getting a speed readout. The length of signal should probably not be less than about one one-millionth of a second, since that represents about 1000 total feet of how far light can travel in that time interval (500 feet to a target and 500 feet back).

The best feature of this simple modification is that police officers would be subjected to FAR less radiation from the radar gun that is only a couple feet away from them. By being subject to only around one-millionth of the radiation (same intensity of radiation but for only one-millionth of the time), any possible health implications for the officers should be greatly reduced.

By the way, this micro-pulse version of Police radar is NOT the same as the "instant on" radar that is now used by some Police departments. That approach is initially off and the transmitter is just turned on (manually) by the Officer to detect a specific vehicle. Since it is initially off, no radar detector could sense the presence of the radar gun, until it is actually turned on. The new micro-pulse approach described above is continuously functional but could never be sensed by any existing or future radar detector.

Our Invention

Any of the THREE configurations of our LEGAL device consistently and dependably defeats the radar process from functioning. These devices work against ALL bands of radar, and even laser speed guns (but there are certain limitations there). (It would even work against the proposed improvement just suggested for police radar!)

Some people hopefully realize (from the above discussions) that I know what I am talking about. The three configurations of the invention are all fairly inexpensive. One configuration could probably be mass produced for around $5 in material!. The other two have certain additional options and flexibility and involve about $50 in materials in volume production, maybe less.

We filed paperwork toward a patent for our device.

I have absolutely NO interest in spending hours of my time with people who want to build a single unit for themselves. In 1998 and the early part of 1999, I honored enormous numbers of e-mail inquiries regarding this device. In many cases, people would send 20 or 30 e-mails to me, trying to pry out details of how it operates.

For a while, I even offered rather low cost plans for the invention (with contractual restrictions about never manufacturing or selling devices based on my invention). It got ridiculous where many people would essentially harass me in attempts to get me to divulge the information, rather than springing for $10 to buy those plans.

I have better things to do that waste my time with such people. I had presented this page and my invention as an effort at public service.

In any event, about the middle of 1999, I stopped accepting requests for this information. I have left this page up on the Internet because it presents a lot of information about the functioning of speed radar that doesn't seem to be available anywhere else.

Therefore, you're out of luck to get the information on how to make my device. It's too bad, too, because it works great! By the way, it is very justifiable to be cautious or skeptical of the various garbage that is offered by companies trying to scam the public. And there are plenty of companies that are using the Internet to do that.

So, if you're just a motorist that is upset at getting a ticket or too many tickets, and hope that I will tell you how to make one, sorry, but you're out of luck. I have stopped even responding to such e-mails. I know that's somewhat disrespectful, but there have been many thousand such e-mails, each of which probably think they're the only one contacting me. I have no intention of hiring a dozen people to respond to e-mails from people who want something for free.


As bonus information, here's some discussion on VASCAR. Police actually have a speed detection method that has many advantages over radar. It is not affected by rain or wind or fog. It can be accurate from any direction, including from airplanes! It is called VASCAR. Prior to the general use of speed radar, VASCAR was the common method Police used (in many states) to arrest speeders. It is seldom used any more because of how common radar has become, but if a lot of people got devices like the ones proposed here, which completely defeat speed radar, they could easily go back to VASCAR, with its several advantages. (My device, or ANY device cannot defeat VASCAR).

VASCAR's operation is extremely simple. It involves very simple algebra. An example that you could do will demonstrate the basic idea of VASCAR. Say you're driving cross-country across the Midwest, and there are crossroads that pass over the highway from time to time. On a sunny day, you can tell very accurately when a vehicle goes under the bridge because it is not in the sunlight and suddenly appears dark. For argument sake, say two bridges are one mile apart (this is unnecessary for the concept but makes for simple numbers in my example). You notice that your vehicle takes exactly 60 seconds to go from under one bridge to under the next. This actually confirms to you the fact that your speedometer is showing your 60 mph (or one mile per minute). As you're driving along, you happen to see a vehicle behind you that seems to REALLY be going. You count the seconds that vehicle needs to go from one bridge to the next, and you find it to be 40 seconds. In a VASCAR-like way, you can tell how fast that car is going! In 40 seconds, it went as far as you do in 60 seconds. This means that it is going 60/40 or 3/2 your speed. Since you know you are going 60 mph, that car is going 90 mph.

You actually don't need to know how far apart the bridges are. All you actually need are the number of seconds that YOU take to travel some distance and the number of seconds that the other vehicle takes to travel the same distance, and your vehicle speed. You do not need ANY other information to know extremely accurately how fast he is going! That's essentially VASCAR.

If bridges aren't available, you could use billboards or signposts or utility poles or any other identifiable points for the start and end points of your comparison segment. That's generally what Police did when they used VASCAR. There are actually several ways that VASCAR could be used. The most common involved an Officer choosing any two roadmarks as the end points for his speed checking interval. He would generally drive his own car through the distance at a particular set speed and time it. This gets him two out of the three pieces of information he needs for VASCAR. The only remaining piece of information is how long your car takes to go that distance. (There are variations in how he can establish the first two, but they all result in the same basic information).

The only sources of error in VASCAR are the Officer's reaction time in pressing the start/stop timing button and his judgment on precisely when a vehicle was passing a landmark. In the case if the bridge shadows, the second error possibility is almost zero, but for an Officer in a moving Police car, determining the precise second when you, a quarter-mile ahead, pass some landmark, could be off by a number of feet. If an Officer would use a baseline of 60 seconds or so, a one second error would only represent 1.5% error. But that's an awfully long baseline. Essentially, VASCAR determines your AVERAGE speed for the measured distance. (RADAR determines your instantaneous speed). In a mile long stretch of highway, most drivers would see the Police car and severely slow down. Since he is only determining the AVERAGE speed for that distance, that average would be substantially less than your beginning speed. Therefore, most Officers tended to use the shortest possible baselines for VASCAR, to get a speed average before you might see him and slow down. If he used a 10-second baseline (around a block and a half), a half-second error in clicking the beginning time and a half-second error in clicking the ending time could total a one second error, which represents a 10% error in speed determination (either up or down). (That type of poor Officer understanding of VASCAR is a main reason it got replaced by radar. Some Officers would try to use even shorter, five-second baselines to get a speeding value even faster before being seen. Such Officers really gave VASCAR a bad name because they would sometimes get VASCAR results that were 20% off. A vehicle actually traveling a legal 65mph would be arrested for going 78mph, because of the incorrect use of too short a baseline. A lot of people got improperly arrested because of that. Eventually, some important people got improperly arrested in that way, and RADAR came along anyway, so VASCAR almost disappeared from Police use.)

I mentioned airplanes. If you have driven long distances, you must have occasionally noticed that, out in the middle of nowhere, a white stripe was painted across all the lanes (like at a stop sign) and then another, maybe 15 seconds later. These lines are so Police aircraft can easily use VASCAR. If they marked those lines 1/4 mile apart, then an Officer in an airplane could just count the seconds you take to go from one to the other. If you take 15 seconds, simple algebra (or his little chart) says you're going 60 mph. If you took 10 seconds, you would be going 90 mph, and so forth. The airborne Officer has a handy chart that shows speed versus seconds and can instantly know the average speed any vehicle was going in that 1/4 mile. He just calls a Police car and has the car arrested, with a VERY accurate speed.

VASCAR has only that one big deficiency. It only determines the AVERAGE speed a vehicle is traveling over the baseline distance. If a violator would see the Police car and greatly slow down during the timing section, the measured AVERAGE speed would be far less than the instantaneous speed he began at. Back when VASCAR was really popular, many motorists were improperly arrested (due to police incompetence resulting in inaccurately high VASCAR speeds) but the person seldom knew enough of the above logic to defend his true innocence in court. This it the other reason why VASCAR got replaced by radar, the fact that a lot of people got tickets they didn't deserve with VASCAR, not because of any flaw in VASCAR but in lack of understanding of the Officer using it. If an accused would have known enough to challenge the results based on a too short baseline, he probably would have been properly adjudged innocent.

First fully invented and Developed, Jun 1991,
First Published on the Web: Sept. 6, 1998

This page - - - - is at
This subject presentation was last updated on - -

In the summer of 2012, a strange experience happened to me. I was driving on a 30 mph main road in Homewood, Illinois. I was driving east, at possibly around 35 mph, as nearly all traffic generally drove at around 5 mph above that posted speed. A rather large SUV was about a block behind me, and it was clearly going a lot faster than I was. Going the other direction, a Police car was going along, but he made a U-turn and quickly pulled ME over! He even showed me that his Radar showed 44 mph which he was convinced was due to me! I could not get the Officer to understand that he had gotten a radar echo from the LARGER vehicle which was behind me and he definitely had decided that I had been speeding, several blocks away from him and traveling in the opposite direction. He was a very nice Officer and I think he noted that I am a Minister in a Christian Church and he decided to just give me a verbal Warning. I had not been able to convince him that his Radar gun had actually gotten a radar echo from a much larger vehicle behind me which his Radar gun had recognized. Police Radar guns ONLY display the FASTEST speed they get an echo for.

The reason I am mentioning this here is that even the Cops do not realize that a Radar beam has some width, which, in this case, happened to also include a larger vehicle behind me, where the Cop did not recognize the implications of that.

Finally, some visitors to this page realize that I am very knowledgeable on this subject and so they decide to impose on me to help them fight some speeding ticket they have already received. Please do not annoy me with such things. Once you have been given a ticket, very little can be done unless you want to hire a lawyer for thousands of dollars. I cannot help you there.

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C Johnson, Theoretical Physicist, Physics Degree from Univ. of Chicago