Path: utzoo!mnetor!uunet!nbires!hao!ames!ptsfa!dmt
From: dmt@ptsfa.UUCP (Dave Turner)
Newsgroups: sci.electronics
Subject: Re: radar countermeasures
Message-ID: <4155@ptsfa.UUCP>
Date: 1 Mar 88 20:57:16 GMT
References: <4596@pucc.Princeton.EDU>
Reply-To: dmt@ptsfa.UUCP (Dave Turner)
Organization: Pacific * Bell, San Ramon, CA
Lines: 38

In article <4596@pucc.Princeton.EDU> MJSCHMEL@pucc.Princeton.EDU writes:
>speaking, of course, hypothetically) to build a radar TRANSPONDER?


I had the same idea 25 years ago.

Most speed radars measure the doppler shift of the received signal.
For a given transmitted signal one can calculate the return frequency to give
any desired doppler shift (desired indicated mph). If the transmitted signal
always had a constant known frequency all that is necessary is to generate
a fixed frequency that will provide the desired readout.

A more elaborate transponder would subtract the speed of the moving car
from its received frequency to compensate for the doppler shift of the
moving car before transmitting a suitable higher frequency to give a legal
speed on the police radar.

To combat hand-held radar that are turned on and off to try to defeat
radar detectors the transponder should not transmit except when a signal
is received.

The police radar guns that I've read about will lock onto the strongest signal
so the transponder signal should dominate the radar gun readout.

I never built such a device but Radio Electronics magazine for August, 1986
(p. 39) has a construction article that will work. They call it a
"Radar Speed-Gun Calibrator" for X and K bands. Illegal? You bet.


The same idea should work for laser speed detectors that use doppler shift
but the implementation would probably be more difficult.
But if sufficient power is available just blast the speed detector, the police
squad car, etc. It would also be useful to blast cars that run red lights,
cut you off without signaling, jaywalking pedestrians, etc. :-)


-- 
Dave Turner	415/542-1299	{ihnp4,lll-crg,qantel,pyramid}!ptsfa!dmt