Re: "Radar Scramblers"

[DeWitt Harrison <de@aztek-eng.com>]

On Mon, 27 Oct 1997 19:54:00 -0600, Will Gadd wrote:

>I recently saw an ad for a device that "reflected" radar and lidar signals,
>supposedly wrecking the return signal by adding an FM signal etc. Given
>that I've had three tickets in the last year (despite having virtually none
>in the years before), I'm looking for better anti-revenue technology than
>my current radar detector. Do any of the Tech Gurus or just plain
>opinionated have an opinion about these devices? [ ... ]

Well, I suppose I fall somewhere between techie and just plain opinionated.

It's really pretty simple. All passive, reflecting "jammers" are a hoax and
fail the common sense test as well. Imagine how your vehicle appears
to a radar or lidar receiver's antenna. It's analogous to your pointing
a flashlight beam into the night and observing objects by the reflected
light, an oncoming car for example. Your ability to see this car depends
on the total amount of car-reflected light returned to your eyeballs
compared to the amount from other reflective objects. These amounts
depend on physical and apparent sizes (closeness) and the reflective
properties of the front of the car, etc.. Now try to imagine how it would
be possible for that car to become invisible to you by sticking some
little reflective doohickey behind the windshield.  We'll even let it be a
funny, twinkling little doohickey. Common sense tells you that the car
will still be perfectly visible to you, because the returned light that you
see comes virtually entirely from the car itself and not from the "jammer"
doohickey. In techie jargon, the signal-to-noise ratio (quality) of the
reflected signal at the receiver cannot be degraded in the slightest by a
relatively tiny passive reflector. "Mr. Zulu, lock the Phasers to scam!"

Passive schemes that could work are not generally practical. The
entire target could be covered with non-reflective, absorbing materials.
The front of the car could be shaped like a big mirror which reflects
everything away from their points of origin.  Then your car would look
like the stealth bomber.

Active schemes amount to overpowering the reflected signal with
a transmitter in the target object. Suppose that when you directed
your flashlight beam at that oncoming car, it responded by switching
on a trillion candle power searchlight into your face (must be Phil!).
You wouldn't be seeing car, or anything else for a while after that
and you would have an extremely poor signal-to-noise-ratio (SNR).
So far, active jammer technology has proven to be too expensive and
unreliable for widespread use in automobiles. But folks are trying.
There was a web page called www.mr2.com:80/TEXT/radar_faq.txt
which described some efforts. It might still be out there.

The old caution applies; if it's too good to be true, guess what ...
Meanwhile, a top quality radar/lidar detector increases your odds
of timely warning significantly provided you use it in concert
with all your other senses.

HTH

DeWitt Harrison    de@aztek-eng.com
Boulder, CO
88 5kcstq