sampson@attctc.Dallas.TX.US (Steve Sampson) (01/16/90)
> Simon Travaglia, University of Waikato, Hamilton, New Zealand writes: > I'm pretty baseline about how detectors work etc. Can someone explain > or mail me how the theory of it works. (I understand the basics of > reflections etc, is it Dopler-ish?) > What's to stop you syncing in with the Speed Detector and sending > back info to say that you are doing 500mph? The radar uses a Gunn diode for the transmitter which drifts alot. The frequency is unpredictable. Therefore you cannot measure it and jam at a certain Doppler offset. The most popular techniques has been the AM jammer. You use a Gunn diode and modulate the power supply with the Doppler shift you want the radar to read. This does not work with moving radar, and is very poor on stationary radar. The radar bandwidth is usually the Audio frequencies, so 500 MPH will be out of band and attenuated. Probably should stay below 100 MPH. If I was going to build a jammer it would sweep the whole frequency band using just a carrier, probably with a couple hundred Watts from a TWT :-) This would prevent any reading with a fast enough sweep rate (can't lock-on). The following is an article I wrote, it's unpublished except for once being uploaded to rec.ham-radio. It's public domain. ---------- Police Radar, Detectors, and Jammers Steve R. Sampson April 1988 Introduction The radar systems used by police as well as detectors used by the public, have matured and become more sophisticated over the years. Some radars even have cameras attached now to photograph the speeders. The cheapest radar detector on the market will outperform the detector of even five years ago. All of them work as advertised, that is, warn of radar emissions on two or three frequency bands. Where you spend your money is in increased sensitivity and optional features. The old crystal detector is fully obsolete and superheterodyne detectors have become state of the art. The Radar The police radars operate in three frequency bands. The first is X-Band with a frequency centered on 10.525 GHz (Giga- Hertz) and 13.45 GHz. The second is K-Band centered on 24.150 GHz. The third is Ka-Band and is centered on 34.31 GHz. These radars are not very sophisticated as far as frequency generation and transmission go. They generally use a Gunn Diode in a tuned cavity connected to a waveguide, circulator and antenna. ((Figure 1 )) The circulator is simply a ferrite rod inserted into the waveguide. The purpose of the circulator is to direct the transmitter power out the antenna and the receive power to the mixer. In older radars a Majic-T performed this duplexing function. The circulator performs much better (by about 6 dB) and allows much smaller antennas to be used. The mixer circuit is also quite simple, usually consisting of a diode. The reflected target frequencys are mixed with a sample of the transmitter frequency (which comes from some of the power going around the circulator) producing an audio frequency difference which is directly related to the velocity of the target. Most of the improvements to the police radar have been in the tracking circuitry. This type of radar is called Continuous Wave (CW). Both the transmitter and receiver are always on so there is no need for a Local Oscillator (LO). The disadvantage of CW radars is that they do not measure range. For police work however, the officer is only concerned with immediate traffic and they can visually acquire the target. With the radar tracking the fastest target, the officer merely goes after the fastest moving vehicle. This procedure is not complex or error prone as you might think. If an officer can't decide who the offender is, they can wait for easier pickings or provide a warning to both drivers to slow it down. The whole purpose of traffic radar is to keep the traffic down at a safer speed and penalize those who refuse. The fines come in handy when it comes time to pay for all those that have killed or injured themselves (gas for the fire trucks and ambulances, etc). Doppler Effect When a radar beam strikes a stationary target, such as the ground or buildings, the same frequency returns as was transmitted. When a radar beam strikes a moving target however, the frequency is shifted relative to the speed of the target. The faster a target is moving, the greater the frequency shift. This resulting frequency shift is called a Doppler Shift. You can hear Doppler shift in many activities. Race cars coming down the main stretch for example, have an increasing frequency as they come towards you, while decreasing in frequency as they go away. The formula for Doppler shift in Hertz is: ((Formula 1 )) The wavelength of the transmit frequency in centimeters can be computed using: ((Formula 2 )) For the police radar frequencies of 10.525 GHz and 24.150 GHz, the wavelengths are 2.85, and 1.24 centimeters respectively. To find the Doppler shift for each Mile Per Hour, simply substitute one (1) for Velocity (V). This results in 31.35 Hz per MPH in X-Band and 72.06 Hz per MPH in K-Band. A car moving at 55 MPH would cause a frequency shift of 1724.25 Hz at X-Band (55 x 31.35). This math is done electronically using a frequency counter and gate divider in the radar display. To accurately measure the speed of a target the radar should be pointed directly into the path of travel. The reason the Race car frequency changes as it goes by, is because you are offset away from the path of travel. The angle between you and the car is constantly changing. If you get down next to the track, then only one frequency will be heard as the car approaches and then recedes. This principle is important to the police officer also. They must point the radar into the traffic to get the true speed. Any difference in angle results in an incorrect speed (slower). The measured speed (Doppler shift) decreases as the angle increases, until at 90 Degrees the speed is zero. An officer pointing the radar at the side of your car would read zero even if you were doing 200 MPH. ((Figure 2)) Engineers refer to this resulting speed as Range Rate. The symbol for Range Rate is R (Dot Product, R with a dot over it), and the formula to compute it is: ((Formula 3 )) What this means is that the measured Range Rate is equal to the ground speed at the speedometer; reduced by the radar angle. For example a radar angle of 90 Degrees would result in zero Range Rate (cosine of 90 degrees is zero). A radar angle of 180 Degrees, or pointed directly into the oncoming traffic would be minus one (-1) times the cars speed, or the true speed (cosine of 180 Degrees is -1). The sign information of course is lost with the Zero Hz IF (no such thing as a negative frequency). ((Figure 2 )) The Detector Since the police radar is transmitting a signal, any receiver tuned to the same frequency can detect it. In the early days crystal diodes were used with an oscillator. The diodes were placed in a waveguide cavity and caused the circuit to oscillate when the radar frequency was detected. I bought one and the sensitivity was poor, with range less than 1/4 of a mile. False alarms were also a major problem. Various circuits and diodes were used and sensitivity slowly increased over the years. It wasn't until the advent of the superheterodyne detector that realistic long ranges became available. The comparison in performance is not unlike a crystal radio to a transistor radio. With the superheterodyne circuit, a mixing signal is used to produce a high frequency IF (Intermediate Frequency). This IF is then filtered and amplified resulting in much better sensitivity and selectivity, increasing range and reducing false alarms. ((Figure 3)) The circuit complexity and microwave devices are still quite expensive however, and that is the reason you pay about $150.00 for this type of detector. One problem with the detectors is there are many X-Band transmitters out there. This frequency is very popular with intrusion detectors and automatic door openers which will also set off the alarm. For this reason many detectors provide a 'city' and 'highway' sensitivity. What this does is reduce the X-Band sensitivity while leaving the K-Band alone. In my experience I've found K-Band alarms were 99% associated with police radar, while X-Band has many false alarms in the city. Every grocery store has installed the new automatic sliding doors and the detectors can pick them out over a block away. By using the 'city' position you can knock many of these false alarms out. There is no way for the detector to determine whether it's a police officer or a door; both emit a CW signal that look identical. These sources of false alarms do not go unnoticed by the police officers, and make a perfect spot to nab someone with a radar detector. The detectors have good sensitivity from behind also. Sooner or later your detector will go off, and like everyone you'll start looking for the source. Check that rear view mirror every now and then! The Jammer In recent years many radar jammers have hit the streets in an attempt to create an Electronic Counter-Measures (ECM) war. These are totally ineffective without a high power level. The most common jammer found is called an Amplitude Modulated (AM) jammer. It usually consists of a Gunn diode modulated by a selectable frequency (Doppler shift). The purpose of this type of jammer is so a police officer would not suspect anything while you reduced your speed. It's very easy to see why this type of jammer provides limited offensive capability however. Just like an AM CB radio, 50% of the power is in the carrier. The sidebands with the jamming information contain only a fraction of the transmitter power. That's why most radio operators use Single Sideband (SSB) radios now. On top of that, the newer police radars lock on to the highest speed when confronted with two signals of approximately the same amplitude. The effective jamming power will probably be about the same amplitude as your echo. The radar ignores the jammer and locks on to the vehicle. These low power (100 milliwatt) jammers only become effective at very close range. The fines for radar jamming can run a small city for the rest of the year however, and basically there is just no reasonable excuse for jamming tactics. Conclusion As we have seen, the CW police radar is quite simple in design and has a significant ECM advantage due to its short operating range. It performs its function easily with few errors or limitations. The radar provides the police officer with an easy method of measuring speed. The alternative is to put marks on the highway, and using a stop-watch; manually calculate each vehicles speed. This method takes time and gets quite boring after a few minutes. Buying yourself a radar detector is easy insurance that you won't be caught funding your state, county, or cities motor pool. It's surprising that many habitual speeders do not invest in detectors. Jammers are expensive and provide no advantage over plain detection. The result is that jammer manufacturers are now trying to sell their contraptions as radar calibrators for lack of any other market. A two dollar tuning fork will perform that function just as well. ---------- Figure 1 A Block Diagram of a CW radar showing the basics Figure 2 A picture of aiming the radar and the Range Rate (R Dot) error Figure 3 A Block diagram of a superheterodyne detector Formula 1 89.36 V Fd = -------- L V = Velocity in MPH L = Wavelength of Transmitter in centimeters Fd = Doppler Frequency in Hertz Formula 2 c L = --- F c = Speed of light (29.9792458 m/sec) F = Transmitter Frequency in GHz L = Wavelength in centimeters Formula 3 . R = cos(theta) * V . R = Range Rate (Negative for closing, positive for opening) V = Speedometer reading of target (true ground speed) theta = Radar angle off the targets heading --------- EOF