ornitz@kodak.kodak.com (Barry Ornitz) (06/25/91)
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. >Radar cross-section is reduced but not eliminated. Generally >speaking, I'm not disagreeing -- merely quibbling. > >> it takes >> only a windshield or a head-lamp lens to get an accurate enough >> reading of the vehicle speed. > >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. >I'm not exactly sure what you mean by "stealth aerodynamics." I >am an aerodynamicist. I received my Master of Engineering from >the University of Toronto Institute for Aerospace Studies in >1977. You tell me what you mean by "stealth aerodynamics" and >I'll tell you if I know anything about it. That part of the >discussion can be conducted in sci.aeronautics. >ideas, but I'd like to know for sure.) As an aerodynamicist, you should know all aircraft produce a bow wave of slightly compressed air in front of the aircraft (this is getting beyond my expertise here!). This bow wave, being composed of air of slightly greater density than the surrounding air, will have a slightly higher dielectric constant. Once again, radar can pick up this dielectric discontinuity - even if the plane is made of perfectly absorbing materials. This effect will be small, but it will be there. >I have never implicitely or explicitely contend that RAM reduces >radar cross-section to zero. If that is the _only_ point you >wish to make, then fine, we are not in disagreement. Barry ----------------- | ___ ________ | Dr. Barry L. Ornitz WA4VZQ | | / / | | Eastman Kodak Company | | / / | | Eastman Chemical Company Research Laboratories | |< < K O D A K| | Process Instrumentation Research Laboratory | | \ \ | | P. O. Box 1972, Building 167B | |__\ \________| | Kingsport, TN 37662 615/229-4904 | | INTERNET: ornitz@kodak.com -----------------
mingmar@cs.mcgill.ca (Ming MAR) (06/30/91)
In article <11351@idunno.Princeton.EDU> stratton@phoenix.Princeton.EDU (D. Alexander Stratton) writes: >In article enno@technix.oz.au (Enno Davids) writes: >> >>On reading this it struck me that Stealth should be quite visible on the >>Doppler radar systems used at some airports to look for 'wind shear' and >>otherwise track moving air. Comments? >> > To detect wind shear with doppler radar, >the moving air mass must possess enough moisture or aerosol content to >provide enough return signal to the radar. But I'd like to hear what some >radar experts think. > I've cross-posted this because there doesn't seem to be any radar expertise in the sci.aeronautics newsgroup. One question that comes to mind is: what kind of range does a wind shear detection system have anyway?