"Frank Alforo" wrote in message
...

"Ed Price" wrote in message
news:CAw9c.15750$Q45.10237@fed1read02...

"Frank Alforo" wrote in message
...
Anybody have any info the material used on the stealth aircraft to
absorb
radar signals?

The 297 hits on Google for "radar absorbent material" was information
overload? The 3,730 hits for "RF anechoic material" frightened you? And
the
70,100 hits for "stealth aircraft design" sent you scurrying here to see
if
we had anything for you?

At least it's nice to see you doing your homework before Sunday night.

Ed
wb6wsn

Tnx for the tip, Ed. Your point is well taken. But it seems to me that the
same can be said for the great majority of the posts seen here. Perhaps we
need a moderator to keep irksome and inane posts from cluttering up the
newsgroup and provoking the ire of the members who visit here.

Frank


Naah, an official moderator isn't needed. Besides, he'd get burned out too
fast anyway.

BTW, absorbent material is NOT 377 ohms per square. The traditional
absorbent material is decidedly low-tech, consisting of open-cell urethane
foam with carbon particles dispersed along the cell walls. The grungy way to
make this is to cast a monster billet of foam, maybe 10' long by 4' by 4'.
The billet is then squashed flat in a press, and submerged in a solution of
carbon particles (lampblack; and seems I heard of that being made by
scraping the soot of a propane flame off of a copper block), water and
ethylene glycol (for wetting). The billet is allowed to expand, soaking up
the solution, and then drips dry. It's just like a huge kitchen sponge.
Particles of carbon are then left disperesed, almost painted, on the cell
walls. Then, after some more oven drying, they cut the billet, with a hot
wire, into those fancy pyramidal shapes. Then, some latex spray paint for
that high-tech blue or white look. And then they sell them to you for $175
per 24" square!

If you think of a plane wave propagating along, it has a 377 ohm wave
impedance over a unit area of wavefront. As the wave encounters the tips of
the pyramidal cones, a bit of the wave begins to encounter a slightly lower
impedance. As the wave propagates further, the impedance over the unit area
decreases. The wave begins to lose energy to ohmic heating of the lower and
lower impedance medium. Finally, at the base of the cones, the wave is
trying to propagate in a much lower than 377 ohm medium, and it's giving up
energy rapidly to warm the carbon particles. (With enough energy lost, it
might even melt the foam or ignite it.)

The tapered shape is used so as to present a gradual change in impedance.
The thing to avoid is any sudden change in impedance, which would result in
a reflection. There is much debate on optimizing the shape; right pyramids,
truncated pyramids, rotated sections of the pyramid, cones; some have
advantages in certain applications. The particles creating the loading can
be adjusted; sometimes ferrite particles are used with, or in place of, the
carbon.

(Before urethane foam was used, early researchers did use horse-hair. Maybe
one of them had some auto upholstery experience to draw on.)

All stealth is a combination of absorbing what you can, reflecting what you
can't (in more benign directions), trapping energy in labyrinths and
avoiding resonant structures (like exposed weapons or long panel seams,
etc). Another approach is to use multi-layer 'sandwiches" to create ohase
cancellation to a wave, but this is really difficult to optimize for more
than one frequency.

Ed
wb6wsn