Path: utzoo!mnetor!uunet!amdahl!nsc!decwrl!reid
From: reid@decwrl.dec.com (Brian Reid)
Newsgroups: sci.electronics
Subject: Re: radar countermeasures
Message-ID: <305@bacchus.DEC.COM>
Date: 1 Mar 88 20:11:24 GMT
References: <4596@pucc.Princeton.EDU> <20271@bu-cs.BU.EDU> <912@spar.SPAR.SLB.COM> <1123@pasteur.Berkeley.Edu>
Reply-To: reid@decwrl.UUCP (Brian Reid)
Organization: DEC Western Research
Lines: 34


In the 1960's I worked as a programmer in a university laboratory whose
focus was on low-noise electronics and interplanetary microwave communication.
(moon-to-earth telemetry, for example).  Our lab had quite a number of
hot-shot microwave people in it.

One of the guys built what I consider to be the ultimate radar jammer. It was
passive. His car had a plastic grille. Behind the grille he put a 12-inch
audio loudspeaker. On the surface of this loudspeaker he had glued several
thousand copper dipoles, sprinkled at random, that were carefully cut to be
half-wave (or was it quarter-wave; I can't remember) at police radar
frequencies. The dipoles were individually dipped in varnish so that when the
crossed each other as they were sprinkled on the speaker grille, they would
not make electrical contact.

This speaker was connected to an audio oscillator in his car, whose frequency
he could control with a knob on the dashboard. Because the copper dipoles
were the correct length, they reflected (absorbed and reradiated, probably)
vastly more of the radar signal than the body of the car. By carefully
choosing the waveform that went into the loudspeaker, he could set the
frequency of the doppler-shifted signal received by the police radar.

One of my most joyous memories of my college years was the Sunday morning
that I helped him calibrate the thing with a radar gun borrowed from the
university police (some friend of a friend was a policeman who was amused by
the concept, and he let us use their radar for the morning). We were able to
pretty much dial the speed that the cops would see, anywhere from the true
speed of the car up to about 200 mph. It didn't seem to work to set a speed
lower than the car was going, but if the car was traveling 60mph and the dial
was set right, the radar readout would show 200mph, which was so obviously
false that (in theory, anyhow) the cop would have no data.

This guy drove like Buck Rogers, and during the time I knew him he never got
a radar speeding ticket, so I guess it worked.