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Napier
10-10-2006, 09:47 AM
How do I make a plane or sheet surface that absorbs radio waves but doesn't reflect or transmit them?

I'm thinking a resistive sheet that has so many ohms per square, and whose structure is smaller than maybe 1/2 or 1/4 wavelength. But how many ohms?

Or doesn't this work for some reason?

ZenBeam
10-10-2006, 12:12 PM
You can't make a single resistive sheet that has the properties you want. You'd have to either have two sheets with space between them (look up Jauman absorber), or a sheet that supported both electric and magnetic currents (well, interacts with the magnetic field in a way that can be modeled with magnetic currents).

As for why a planar sheet with only electric currents can't do what you want, the currents you set up on the sheet will radiate just as much on both sides of the sheet, by symmetry. To stop transmission, they have to radiate enough to cancel the incident field, but to not reflect, they can't radiate at all.

ZenBeam
10-10-2006, 12:16 PM
OK, "Jaumann absorber" will get you more hits than with one "n".

David Simmons
10-10-2006, 06:00 PM
If the wave enters the sheet from space, or essentially from the air, and you want to avoid reflection, the ratio of electric to magnetic field intensities in the sheet should be 377, which is the impedance of free space. To avoild transmission through the sheet or reradiation from the sheet, the material must absorb the wave and convert it to heat as soon as it enters the sheet.

I doubt that this is possible in a flat sheet that's perpendicular to the incident wave. The absorbers that I'm familiar with depend upon shape to provide multiple reflections with some of the wave being absorbed at each reflection. Such as an array of pyramids made of lossy material with the points tacing the wave. The wave refects many times off the sides of the pyramids with some being absorbed with each reflection, and the wave traveling into the array finally reaching the base of the pyramids at very low amplitude. However these arrays are quite thick with good absorbing arrays for microwave being a foot thick, or more.

The stealth fighter (F-117) didn't absorb the wave but reflected most of it off in a direction away from the source of the wave. And that only a certain frequencies. With a low enough frequency of wave, the object becomes a scatterer and stealth is gone.

The techniques used to get low profile on the F-22 are classified and the only ones who know about them are our designers and enemy physicists and engineers who have studied the subject.

GusNSpot
10-11-2006, 01:09 AM
The techniques used to get low profile on the F-22 are classified and the only ones who know about them are our designers and enemy physicists and engineers who have studied the subject.
Bawahahahaha, Oh My !!!! You are a man of infinate wisdom concerning the real world...... Sing it brother.......

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