Re: "Radar Scramblers"- getting longer

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

On Thu, 30 Oct 1997 15:28:31 -0500, Arryn Milne wrote:

>DeWitt Harrison wrote:
>>[ ... ] I think the car-to-doohickey-ratio
>> would be something like 2 square meters divided by 0.0005 square meters.
>> Fudge all you want, but the cop's supersensitive-califragilistic zapper will
>> see all car and no doohickey. 
>
>Isn't a non-transmitting jammer an oxymoron? [ ... ]

Bingo.  But the Phazer argument is that noise is mixed with the
reflected return. More below.

>Anyway, still have to worry about vascar, pacing, and lidar :-(

Sadly true. This means we still have to be awake while driving.
--------
And on Thu, 30 Oct 1997 16:44:37 -0500, Frederic Breitwieser wrote:

>The unit I have, the Phazer II, was in fact tested with X-band (I own a
>unit for measuring racing speeds <G>), and a friend of mine is a NY state
>trooper, and yes, it does screw up the KA and Pulsing KA (Instant on).
>
>Never tested it against Lidar.

With all due respect, Frederic, I'm still waiting for that Phazer Shootout
report in R&T / C&D.  One of these mags did look at a lidar stealth
license plate cover once. Results negative on that one. Regarding X
band, any decent radar detector can sniff it out a mile away around
a corner. It's continuous broadcast bounces off of everything.
--------
And on Thu, 30 Oct 1997 20:44:28 -0500, Kirby Smith wrote,

>The operative word is corner reflector.  I won't speak to radar, not my
bag, but
>your average bicycle reflector (made up of zillions of mini corner reflectors) 
>has an optical crosssection of roughly 100 meters squared.  And it only
occupies
>an area of 0.0025 meters squared.
>
>The above comment is not intended to support the idea of passive jamming (a 
>contradiction in terms).  Passive countermeasuring may be doable, but is
usually
>difficult for a variety of reasons.  Better to transmit a deceptive or 
>interfering signal.  [ ... ]

I did say "fudge all you want," so suggesting a corner reflector is fair. A
corner reflector having physical dimensions much larger than the signal's
wavelength has the wonderful quality of reflecting directly back to the
source without needing to be aimed accurately. So a corner reflector may
certainly return as much signal as a much larger object which is a poor
reflector. But it cannot be larger than it is. (100 square meters of what?)

At any rate, I will easily agree that my car-to-doohickey-ratio could well
be reduced from my initial 4000 to 1 estimate, at least in the case of lidar
with it's wavelength on the order of 0.000001 meter. This is because
  1) at such small wavelengths it's possible to build small reflectors (bicycle)
with narrow beam patterns and
  2) the beam pattern of the light returned from the car will much wider
because its many reflective surfaces are not organized for this purpose.

The upshot is that as the source-to-target distance becomes greater, the
bicycle reflector becomes relatively more efficient.  At some distance,
it is quite possible that the small, narrow beam reflector will return as
much light to the source as will the car, even though the total amount
of light bouncing of off the car is much greater. Of course, placing
the bicycle reflector behind a windshield will drastically reduce its
effectiveness as the light suffers large reflective losses in both directions.

Radar is a different matter.  The highest frequency band in current use
is Ka at 26.5 - 40 GHz (wavelengths in the 7.5 - 11.3 mm range).
Compared to lidar, the wavelength of Ka radar is roughly 10,000 times
greater.  The bicycle reflector now must be scaled up accordingly or
suffer from greatly increased beam width. Now the small passive reflector
and the front of the car are at parity in terms of efficiency and now
my 4000 : 1 ratio isn't farfetched.  In the case of the Phazer, you
must also consider the losses incurred in a passive mixer intended to
add noise to the reflection. (One of you radio types ought to speculate
on what this circuit would look like.)  To spare Phil any additional agony
on this topic, please email privately. I've got Jasik's Antenna Engineering
Handbook open to radar reflectors.

For putting up with all this nonsense, I make a gift to the list consisting
of my revolutionary, low cost, anti-lidar passive jammer design:
  Cover an old soccer ball (football) with small bicycle reflectors attaching
them with springs from discarded ball point pens. Mount this assembly on
top of a 10 meter mast.  As you drive along at high speed, the rush of air
will cause all the sprung reflectors to flutter wildly thereby imparting a big
doppler noise component to any lidar return signal. The mast will allow
you to baffle the lidar operator even before your car has cleared the horizon.
Failing that, the operator will be paralyzed by laughter. Everyone has my
permission to copy this design freely.

Thanks for your patience,

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