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Analysis of Passive Radar Jamming
by BillSF

How a mass produced 'passive radar jammer' works. The American unit costs about $250.00 and lacks frequency sensing and driver setting of speed to be on radar gun. It returns a 1000Hz tone (55mph @ 12.545 GHz) and is not really passive as the radar signal itself isn't able to power it. So in the exoxy that is to 'preserve this secret' must be a 3V lithium cell!

Mass produced price: less than Hfl. 100,-- (with features USA unit lacks)

By both asorbing and reflecting in a more powerful way than a whole car, this device is able to send back false info. If a cop is listening to the Doppler signal, he'll hear the much louder 'fake' return and the real return. The 'fake' return will be at a lower frequency! (therefore lower speed) In the US, some states require that the signal be 'listened to' in order to get a conviction, effectively outlawing 'flash traps'.

An electronic 'jammer' would return a proper signal, but is illegal as it actually sends on its own. That would be the only reason i'd advise not using it. Another funny side effect is it is probably set to give the 55 mph tone for 10.545GHz. So if it is 90km/h in the X band, it would be 24 mph at 24.545GHz or 38km/h and for the flashtraps operating at 36.000GHz, it might show a 26km/h rate, unsafe (too slow) speed and grounds to get flashed! (Might need some adapting here) The best unit (not that expensive) would have some simple method to determine which range the signal is, and send back the 'proper tone' set by the driver. Hope this isn't too technical, but if you want more read on:

method to determine which range the signal is, and send back the 'proper tone' set by the driver. Hope this isn't too technical, but if you want more read on:

T = Doppler tone (Hz)
S = Speed of auto (km/s)
C = Speed of light (km/s)
F = Doppler RADAR Frequency (Hz)

T = FS/C

1.0545E10*100/2.997E8 = 3518Hz @ 100km/h

The returned tone is in proportion to speed and radar frequency. To make the arrest legal, some jurisdictions require listening to the tone. Obviously, the 24.5GHz unit is for city use and ofcourse, has a meter to show the strongest signal. Obviously at higher speeds the tone is too high to discern and
some sort of pitch change electronics are required if listening is.

The 36GHz unit is allways used for very short range 'flash traps'. All the above can be beaten with non-transmitting technology. (This may or may not be legal)

The actual box is a retroreflector that is also a corner reflector to act as the radar antenna. A simple diode can both detect and modulate a false return signal. This is an extremely clever device and the inventor has chosen to keep it and himself a secret. The improved device is clearly my invention and shall be effectively 'open source'.

Implementation of a passive radar jamming device.

While transmitting on police radar bands *may* be illegal as they represent 'malicious interferrence', the frequencies are actually in the ISM (Industrial, Scientific, Medical) bands and can be regarded as pretty much 'public air space'. There are three frequency ranges police radar can be found:

X-band = 10.545GHz +/- 100MHz US, Canada, England and Scandinavia.
= 9.410GHz D, NL?
= 9.910GHz +/- (varies by country) rest of Europe

K-band 24.150 or 24.545GHz (and the whole range +/- 100MHz)

Ka-band = 33.4 - 36.0Ghz

The X-band units have the highest power and are used on lonely long stretches of highway. It is a good range for the police as all those microwave door openers use this range too, adding confusion to detectors.

K-band units are the popular 'radarguns' used for instant-on, 'your busted' speed traps. A few hundred mW of power wouldn't be surprising and they can be detected 'miles ahead'.

Ka-band units are used for the camera traps. Legality is in dispute everywhere and they were probably first used in the USA. Power is exceedingly low and could be pulsed. Don't be surprised at <1mW of power that may set your Stinger off as you get flashed!

It is well possible different ranges may be used in different places. In general, it is these three that are 99% of the problem. For generating the 'correction tone', X = 10GHz, K = 24.25GHz and Ka = 34.67GHz. Regional variences can be adjusted for. Calculation of the tones is outlined above.

The sensitivity need not be very great and no super expensive super-het technology is required. Corner reflector or horn antennas, tuned circuits, diode detection and low noise opamps will get any radar within 'strike range'. You determine what band the signal is in and respond with the Doppler tone
that is approiate for the band in question. Basicly if the horn is 'shorted' or 'open' it will reflect 'brightly'. If it is fed into a proper termination, it will reflect no signal. An RF switch would select the conditions and do it at the 'proper' frequency corresponding to legal speeds. This is the deluxe and proper way.

A simpler method would involve three tuned circuits and pin-diode (as in 1N4448) switches. Each one would be modulated with the legal speed tone which could be adjusted for the place used and the speed laws ofcourse. The tuned circuits could be fine-tuned for the region used. This device would probably best be powered by the automobile. The first example could use a small battery and last for several years. The first method would also be able to alert you to what band you just beat! The simpler method could be constructed at home for a very modest price (less than Hfl. 25,--) assuming the constructor had access to a means to calibrate it. (read: radar units!) Precision PCB techniques may eliminate the calibration.

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