Miata Mailing List: March 1995, Message #9
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   From: pond@chrisco.nrl.navy.mil (Jeffrey M. Pond)
Subject: Re: Radar and Jammers
   Date: Wed, 1 Mar 1995 09:43:48 -0500

Mac, thanks for sending your message on the status of commercial "jammer"
technology.  Matt sent a message Monday which schmatically shows the flow
through approach, I replied privately but now that there seems to be
interest I'll speculate on the deficiencies of that approach:

Matt's message:
>I remember reading about one of the 'passive' radar jammers in one
>of the free technical journals about a year ago.  The way it was supposed
>to work was recieve the radar, add it to a noise source, and send it
>out a separate antenna.  Their reasoning was "no amplifier--passive"
>here's my crude drawing:
>
>                                            /
>                                           /
>                                         --
>                           ------------- |     <----
>                           |             --
>                           |               \
>                         /---\              \
>                Noise ---| + |
>                Source   \---/              /
>                           |               /
>                           |             --
>                           --------------|    ---->
>                                         --
>                                           \
>                                            \
>you gotta have power, so the passivity of this is in question.  There is
>some logic behind this, however.  Supposedly a radar run must recieve
>10 or so pulses within a specified tolerance for the reading to be
>'accurate'  The noise source makes this more difficult.  I think one
>of the major car mags tested these devices and found them to be largely
>worthless.  (It's amazing, these radar jammer questions popped up
>at the same time on this list and the BMW motorcycle list)
>
>Matt & Annoying Little Red Car
>*Army* Research Lab
>Microwave Sensors Branch
>Adelphi, MD

My personal reply to Matt:

Presuming the antenna is small compared to the car then the received signal
represents a tiny fraction of the total incident power.  Unless you add
power at the carrier (amplify - hence "active" in my lingo) you aren't
likely to overpower the signal scattered by the rest of the car unless you
have very high antenna gain.  If you did, then the noisy beam would be so
narrow that you would have to hope the police transceiver was boresighted
by your antenna.  If he/she were off to the side by a few degrees your
antenna gain would be way down (same arguement on the recieiving side of
this too - maybe that's why it doesn't really work).

Taking info from Matt's article:

18 dB antenna gain seems reasonable.  The exact process that "18-20 dB of
noise is added to the signal" is not articulated.  It sounds like they are
just degrading the signal to noise ratio by 18 to 20 dB.  It still comes
down to the fact that unless this noisy received signal swamps the signal
reflected from the car it won't be effective.  It is hard to see how that
would happen.  There might be a chance if the car has a relatively low RCS
and the antenna is pointed right at the police unit (where the antenna gain
would work to maximum advantage).  Usually the police are not that
cooperative.  I am curious, Mac, did you happen to check the operation of
these flow through devices as a function of angle from the antenna
boresight?

In general, I think there are a lot of people who exceed the speed limit
(break the law) but have rationalized that as not really being bad but for
whom the use of an active jamming device is not a realistic option when it
comes to "breaking the rules".  This is in no way passing judgement on
anyone who exceeds the speed limit, uses detectors (illegal some places),
or uses active jamming, merely my perception of human nature.

With regards to the "whirling" beenie/propeller comments.  If such a scheme
could work (in principle it would but the size may be totally impractical)
it would be enclosed in an oblate spheroid (beach ball low on air) made of
a microwave transparent material.  The size would be the concern, no one
wants to drive a Miata that looks like it has a big AWACS antenna sitting
on it (might not have to worry about adding a rollbar too)

                              Jeffrey M. Pond
                              Code 6851
                              Microwave Technology Branch
                              Naval Research Laboratory
                              Washington, DC  20375

                              e-mail: pond@chrisco.nrl.navy.mil