Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!hao!oddjob!gargoyle!ihnp4!alberta!ncc!lyndon
From: lyndon@ncc.UUCP (Lyndon Nerenberg)
Newsgroups: sci.electronics
Subject: Re: radar countermeasures
Message-ID: <10076@ncc.UUCP>
Date: 29 Feb 88 22:10:50 GMT
References: <4596@pucc.Princeton.EDU>
Organization: Nexus Computing Inc.
Lines: 34
Summary: It's been done...

In article <4596@pucc.Princeton.EDU>, MJSCHMEL@pucc.Princeton.EDU (Michael J. Schmelzer) writes:
> Think about it: A little black box in your (innocuous looking) Oldsmobuick
> that would, upon receiving a threat signal from smokey, rather than just
> sit there and beep like an idiot telling you to slow down even though its
> already too late, would send back a signal to your friendly Highway
> Patrolman on his X or K or whatever that would register a perfectly
> legal double nickels on the Law's satanic little radar gun's readout.
> Speed with impunity! Drive those Interstates at the speed God made them for!

Yes, it can be done. I built one of these with a friend several years
back using a 10GHz Gunnplexor with the short horn antenna. What we
based the idea on was that the radar gun transmitted just above the
top of the 10GHz amateur band and measured the doppler offset of the
reflected signal to determine the speed of the object. As it turns out,
the receivers in the radar guns aren't to picky about which reflection
they track (high side of pilot freq. vs. low side) - they lock on to
the strongest signal that even looks close. As a result, it was possible
for us to conduct "propogation tests" just below 10.5 GHz (the top of
tha amateur band) that were guaranteed to screw up the readout on the
radar gun. Because the Gunnplexor is rather temperature sensative, it
wasn't possible to "dial in the speed" you wanted the gun to read (although
we did build a voltage divider controlled by a set of thumbwheel switches
that tried to vary the frequency to match the expected doppler offset
for a given speed). There were also problems with calculating the fudge
factor to compensate for the (our) vehicle speed.

The end result was we spent a couple of weeks driving around town
watching a *lot* of policemen poke their heads out from behind the
cars with very *confused* expressions on their faces :-)

I also recall some companies in the U.S. selling these boxes for a few
months before the FCC (?) stopped the practice (sometime around 1978?)

--lyndon  VE6BBM (maybe not for much longer :-)
{alberta,ddsw1,utzoo}!ncc!lyndon