sdorsey@eng.umd.edu (Bill Dorsey) (06/26/91)
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)
frank@snowy.rice.edu (Franklin Tae-Sung Kang) (06/26/91)
In article <1991Jun25.174433.25693@eng.umd.edu> sdorsey@eng.umd.edu (Bill Dorsey) writes: >In article <1991Jun24.231722.12091@kodak.kodak.com> ornitz@kodak.kodak.com (Barry Ornitz) writes: >>>[Question about radar reflecting off glass] >>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 > [stuff about radar be reflected due to angle of windshield] In the Stealth Aircraft Handbook, it states that the cockpit was a great source of reflected radar waves. The radar would penetrate through the cockpit glass and reflect back through all the 90 degree angles(which are the best shape to return radar energy). To fix this, they sprayed the glass with some type of metal such that the gaps bewteen the metal droplets were too small to allow the radar waves to penetrate. I suspect the penetration of glass is also dependent on the wavelength used, but it seems like the greatest advantage would be to make the windshield as reflective as possible so the reflections would go away from 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 > [misc stuff deleted] >of the reflectors in one's car with RAM, significant reductions in the >range of police radar can be achieved. I had thought police radar could get readings from a mile or two away. If that range was reduced to a 1/2 mile, that is still well within the range most police use to clock cars. As much as I would like it, I really don't think RCS could be reduced such that it would be usefull against getting clocked by radar. Perhaps a more feasible method would be to get fake returns as someone suggested. I could swear that some mentioned in rec.autos that the cooling fan returned something in the 40mph range. One could make the fan very reflective while minimizing the RCS of the car... Why don't we forget this idea and work on turning the car into a black body. Then we'll lick the laser speed detecting devices at the same time. The down side is that it could get pretty hot during the summer. Hmmm, in fact your car would look like a featureless black blob. There goes getting a candy apple red car. :-) Frank