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From: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Newsgroups: sci.electronics
Subject: Re: Detecting Radar Detectors
Message-ID: <28522@netnews.upenn.edu>
Date: 15 Aug 90 18:58:48 GMT
References: <324@bally.Bally.COM> <2860@nems.dt.navy.mil> <1990Aug07.150400.3929@pmafire.UUCP> <229@dynasys.UUCP>
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Reply-To: depolo@eniac.seas.upenn.edu (Jeff DePolo)
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In article <229@dynasys.UUCP> jessea@dynasys.UUCP () writes:
>This is called AECM - Active Electronic Counter Measures  -in military terms.
>It is indeed illegal.  You must have a license from the FCC in order to
>transmit and you are not going to get one.  I have thought of building one,
>myself, but I've considered it a waste of time since they would eventually
>catch you.  I don't think the officers would realize what was happening
>the first couple of times.  But the word would get out and they would set
>a trap of some sort to catch that dastardly soul that would pull such a
>hideous crime.
>One other note here.  The way radars work is that waves are bounced off of
>cars and the frequency is changed depending on how fast the car is going.
>The radar measures the shift and approxiamates how fast the car was going
>according to how much of a shift occurred.  Conceivably, you could build
>a AECM unit that would detect the incoming frequency and broadcast back
>some other frequency to fool the detector.  Of course, you would still have
>the bounced frequency mixed in, so I'm not sure how well it would work.

The way I see it, you would have to 
  1.  Receive and count the incoming signal
  2.  Subtract the shift in the received signal caused by your velocity
  3.  Subtract the #2 shift again to compensate for the shift when
      transmitting your new signal
  4.  Add the approprite shift to correspond to 55 MPH
  5.  Retransmit at this new frqeuency

That would be the most effective way.  However, it would cost way too
much and require quite a bit of hardware.

The easy way is to take a CW carrier on approximately the same frequency
as the radar gun is transmitting on and AM it at a frequency 
corresponding to the frequency you want the radar gun to hear.
This AMed carrier, when mixed in the radar gun, will yield an AF signal
which the radar gun will interpret as a Doppler shift. 

This should work on the "true" Doppler radar guns that use AF frequency 
counters to determine speed, but probably would not work on PLL radar guns, 
which are quite plentiful because they cost less.  Anyone else have any ideas
on this?

								--- 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)