Interesting. There is an optical technique called the "crab's eye lens"
which uses a deep parabola with a detector (non imaging) at the top of
the curve. It works by glancing reflections of the light entering the
open end making it to the detector without being able to make the
roundtrip back out before first hitting the detector. If the walls were
lossy and the curve approximated by straight lines, one would have the
inverse of the usual pyramids used in anechoic chambers. I wonder if
they would work as well?
Tomm
m.ford wrote:
>----- Original Message -----
>From: "Peter Chadwick" <g3rzp@g3rzp.wanadoo.co.uk>
>To: "Larry Carman" <lncarman@swbell.net>; "'Brass Pounder'" <wf3j@comcast.net>
>Cc: <amps@contesting.com>
>Sent: Saturday, February 04, 2006 12:39 PM
>Subject: Re: [Amps] FW: Â RFI Question
>
>
>
>
>>Larry asked:
>>
>>
>>>Exactly what is the composition of radar
>>>
>>>
>>absorbent material and how is it applied to the metal surfaces? How does rf
>>absorbent material react to different ranges of wavelengths?<
>>RAM (Radar Absorbent Material) varies, depending on the application. For
>>anechoic chambers, it's usually in the form
>>of pyramids of some foamed plastic, heavily loaded with graphite, and is
>>pretty filthy stuff to be touching as you're
>>setting up in the chamber. This works reasonably well dow to around 400MHz,
>>depending on the size of the pyramids.
>>For lower frequencies, powdered ferrite is added to the mix, and even ferrite
>>slabs are used, placed under the
>>pyramids. It has been known for enough energy to be absorbed from a microwave
>>transmitter/antenna in an anechoic
>>chamber for the pyramids to be heated to the point where they catch fire!
>>Very nasty, with lots of toxic gases and
>>fumes, and carbon dirt EVERYWHERE!
>>Stealth aircraft rely heavily on the reflection back to a monostatic radar
>>being low, because the signal goes off in
>>another direction - this caused by the geometry of the various surfaces: I
>>suspect the RAM used there is a resistive
>>material, possibly with ferrite loading again.
>>73
>>Peter G3RZP
>>_______________________________________________
>>
>>
>
>
>As I understand it, the surface of the pyramids reflect the wave
>deeper and deeper into the panel where they cannot escape by
>reflection and are thus attennuated.
>
>Mike k1ern
>
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