🚀 go-pugleaf

In article <1991Jun24.231722.12091@kodak.kodak.com> ornitz@kodak.kodak.com (Barry Ornitz) writes:
>In article <1991Jun24.055534.24442@cs.mcgill.ca> mingmar@cs.mcgill.ca 
>(Ming MAR) writes:
>>Can anyone here explain how radar absorbing materials work?
>
>In principle, these materials present a resistive impedance to the illuminating
>wavefront.  The radar waves are converted to heat.  In practice, no radar
>absorbing material is perfect.  Tricks with the geometry are often done to
>enhance the absorption at particular frequency bands (like making the thickness
>of the absorber an electrical half wavelength so the reflected wave from the
                               ^^^^
>underlying metal surface is out of phase with the wave reflected from the
>surface of the absorber - by adjusting the absorption you can get enhanced
>performance with the destructive interference between the two waves).  Spiked
>geometry is often used to minimize the effects of impedance matching the
>absorber.
>
Minor quibble.  The thickness of the absorber is made to be 1/4 wavelength
so the reflected wave from the underlying metal survace is out of phase.
Think about it for a minute, and you'll see what I mean.

>>Are you saying that the windshield glass reflects radar back to
>>the gun?
>
>Of course it does.  Glass has a dielectric constant higher than air.  It is
>not as good a reflector as metal, of course, but radar waves do reflect off
>glass.  The same can be said for any material, of course.  It is merely a
>matter of radar cross section as to whether a police radar gun has acceptable
>range to the officer. Diminish the radar's range enough and it will be useless.
> 
Hmm.  On most of the cars I have seen, the windshield is tilted back at
a good angle.  45 degrees might be average.  I'm no aerodynamicist, but
I'd guess that the reason for this is to reduce the coefficient of drag.
Anyway, whatever radar is reflected off windshield glass is likely to
dissipate into the atmostphere above the car where it will be reflected.

RCS (Radar Cross Section) is dependent on the orientation of the object
in question from the radar source.  On a car, it is likely that the radiator,
bumper, and possibly headlights and part of the hood are the biggest
contributors to the RCS as viewed from a policeman's radar.  Most of the other
reflective surfaces are angled such as to reflect the radar anywhere but back
to the source.  

Placing RAM in front of the major contributors to the RCS of a car will
reduce the effective range of the policeman's radar gun.  According to
The International Countermeasures Handbook, a reduction in RCS of 20 dB
will result in a reduction in effective range of the radar of 68%.  Given
that typical dielectric RAM produces a loss in the 20 dB range, one could
significantly reduce the effective range of a policeman's radar by strat-
egically applying it to one's car.  Magnetic RAM is even more efficient,
often achieving losses upwards of 30 dB.  Even if one only covers part
of the reflectors in one's car with RAM, significant reductions in the
range of police radar can be achieved.

Below, I've included an excerpt from some mail I sent out a while ago to
others that expressed an interest in the subject to me via email.  It
briefly explains the operation of the two major kinds of RAM.

How RAM works:

1) Magnetic Radar Absorbing Material:

A thin material, effective 1/4 wave length of the microwave radiation
incident upon the absorber, will produce a reflected and transmitted
wave.  The transmitted wave, upon reflection from the conductive back-
ing, will emerge 180 degress out of phase with the reflected wave.  The
two will destructively interfere with each other and cancellation
results.  For magnetic absorbers of this type, reflectivity reductions
of 20-25 dB are typical up to around 10 GHZ, and 25-30 dB above 10 GHZ.

2) Dielectric Radar Absorbing Material:

These absorbers operate by effectively altering the dielectric properties
through the material.  The front face of the absorber has a surface imp-
edance closely matching that of free space, thereby producing little front
face reflection.  The loss factor increases through the material, causing
dissipation of the electromagnetic energy.

Advantages of magnetic RAM include thinness, greater attenuation than
dielectric RAM, and tuneability.  Tuneability is also a disadvantage
if broadband absorption is desired.  A Typical thicknesse for a 10GHZ
(X-band) magnetic RAM is .065 inches.  Magnetic RAM must be backed by
a conductive surface.  Some manufacturers will supply their Magnetic
RAM with a conductive coating on one surface for application to non-
metallic surfaces.  Magnetic RAM must also be painted or otherwise
sealed to prevent the metallic particles it contains from oxidizing.

Advantages of dielectric RAM lie mostly in it's broadband attenuation.
It is generally thicker than magnetic RAM, with a typical thickness of
5/16 of an inch or more for coverage from 10GHZ through 100GHZ.  Also,
dielectric RAM can be applied to non-metallic surfaces.  Care must be
used when applying dielectric RAM as it has a front and a back and will
only work when properly oriented.

Magnetic RAM typically costs around $55 per square foot.  Dielectric
RAM typically costs around $20-25 per square foot.

- Bill Dorsey   (sdorsey@eng.umd.edu)