Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!dsinc!netnews.upenn.edu!eniac.seas.upenn.edu!depolo From: depolo@eniac.seas.upenn.edu (Jeff DePolo) Newsgroups: sci.electronics Subject: Re: Fooling Radar Detectors Message-ID: <28863@netnews.upenn.edu> Date: 27 Aug 90 19:13:27 GMT References: <1990Aug27.030614.16421@xavax.com> <1990Aug27.054252.12099@uicbert.eecs.uic.edu> Sender: news@netnews.upenn.edu Reply-To: depolo@eniac.seas.upenn.edu (Jeff DePolo) Organization: University of Pennsylvania Lines: 57 In article <1990Aug27.054252.12099@uicbert.eecs.uic.edu> sandin@uicbert.eecs.uic.edu writes: >I have seen for sale at a place in Chicago (Warshawski's "everything >automotive"), a radar gun. This looks to be the sort one sees on TV when >there is a demonstration of a pitcher's fastball or some such thing. >However, if such a thing were to use the same frequency as police radar >(which it probably does not, but bear with be) It probably does. X and K are the two most popular bands. The major leagues use X band guns made by Jugs (whose guns are or were actually made by CMI) and Decatur. Kustom used to make a K band pitching gun, but it isn't very popular. S band used to be used by police, but that was a long time ago and I don't think that anybody ever bothered to make a handheld S band gun - it would be pretty bulky anyway. >What would happen if one were to mount this on the hood of ones car? >I mean, as I understand it, the police sensor is expecting to recieve a >signal shifted by amount X, from getting bounced 180 degrees. Wouldn't >the same device on my car register X/2 : in other words, if I was doing >100 MPH, it would read 50, wouldn't it? Sorry, but the answer is no. The big problem with trying to transmit a CW carrier back at the police's radar gun is that radar guns aren't very frequency-stable. They tend to drift quite a bit. The way they determine speed is by transmitting a CW carrier and mixing the transmitted signal with the returning signal to get AF baseband which is fed into a counter (simplified version). Since the frequency shift is only a few hundred Hertz for a car moving at highway speeds at microwave frequencies, the chances of you getting your carrier right on frequency to cause a "better" reading are pretty slim. You can AM your carrier and superimpose a shift that the AF counter will interpret as a Doppler shift, however. This is the most common type of jammer and works to some degree, but is definately not foolproof. Many radar guns now have "jammer detectors" built in, so the police may catch on rather easily. >There are a few subtleties, like the fact that a radar gun presumably >emits pulses and you would have to match timing. All radar guns are Doppler or psuedo-Doppler (PLL) devices. They don't use pulse timing as does the new laser-based speed device. >So, is my theory full of shit, or what? It's a good idea, and has been kicked around sci.electronics, rec.ham-radio, and rec.autos a number of times. Unfortunately, we always come to the same conclusion - jammers are bad news. > Stephan Meyers c/o sandin@uicbert.cc.uic.edu --- Jeff -- -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Jeff DePolo N3HBZ Twisted Pair: (215) 386-7199 depolo@eniac.seas.upenn.edu RF: 146.685- 442.70+ 144.455s (Philadelphia) University of Pennsylvania Carrier Pigeon: 420 S. 42nd St. Phila PA 19104