Re: Radar jamming

[Ben Baldridge <bbaldrid@ael.com>]

At 03:08 PM 4/28/97 EDT, you wrote:
>>From a purely practical point of view, it is not too difficult to build a
dirty
>noise jammer for police radar.  Dirty here meaning you are not only going to
>target the police, but anything operating in the spectrum in & around the
radar.
>This could cause all sorts of problems with all of things that set radar
>detectors off:  alarm systems, automatic doors, etc.
>
>>From a legal point of view I think transmitting in that portion of the
spectrum
>(depending on radar type and locale) is normally prohibited without a license.
>Before someone jumps in and says "but a low power transmitter is exempt under
>FCC reg xyz"  I would submit that transmitting for the express purpose of
>preventing an officer of the court from performing his legal duties probably
>doesn't come under that exemption.  You're also looking at a FEDERAL reg, not
>the local police;  you've upped the ante and the potential fine considerably.
>
>Joe Yakubik
>
>

Some remarks on radar spoofing in general, and spectrum allocation in
particular: 

1. Frequencies for police use are: 10.525 GHz (X band), 24.15 GHz (K band)
and a band centered at roughly 
34 GHz (Ka band). These frequencies are reserved by the FCC for police
radar. Anyone else broadcasting on those
frequencies is subject to prosecution by the FCC. Do not expect them to be
reasonable or lenient if they catch you jamming police radars. 

2. It should be possible to jam radars of this type by using a low power
transmitter tuned to the frequency of the radar. The transmitted signal from
the jammer may mask the doppler shift of the return signal. I am told that
police radars have very broad front ends, so a signal on a frequency
somewhere near the radar's may mask the return successfully. By low power I
would estimate something of the order of 10 - 100 milliwatts. However,
generating that much power at X, K or Ka band is not something you do with
radio shack parts. I would be very skeptical of build-it-yourself jammer
plans, or kits. 

3. Now a good way to do it would be to receive the signal from the radar,
amplify it, then mix it with a low frequency signal to mimic a doppler
shift. If you pick the right low frequency signal (I haven't done the
calculation, but I seem to remember 2 KHz or thereabouts), you can in theory
return a signal modulated to show 55 MPH, no matter how fast you are going.
Your bogus return would have to be considerably stronger than the return off
of your vehicle. This system could be keyed to operate off of your radar
detector, using the detector to trigger the rest of it. However, point 1
above still applies. And you better have a very good anti-falsing capability
in your triggering system, or you will be amplifying and rebroadcasting all
the trash signals in the spectrum. 

4. Stretching a spring across the grill of a car is, at best, an
unpredictable method. It is sort of like the rhythm method of birth control.
Spoofing radars has been done using chaff, reflective strips or fibers of
the proper length to act as 1/4 wavelength radiators. However, an effective
chaff countermeasure for police radars would probably get you arrested for
littering if you used it. 

Those are some thoughts on the subject. I will see if I can find any
information, and if so, I'll post it. 

Ben Baldridge
Antenna Laboratory
Tracor Aerospace Electronic Systems, Inc.
 
P.S. How about the bag of donuts trick? Recently someone on this net
suggested throwing donuts out the window to distract the police in pursuit.
It's certainly cheaper than developing a jammer system. Someone, somewhere
out there must have tried it. I'd love to hear more. 
Benjamin M. Baldridge