🚀 go-pugleaf

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


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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