On 4/4/12 6:56 AM, Robert Berg wrote:
You can get inexpensive conductive foam from Amazon.
Not all conductive foam works as a decent RF absorber. If the conductivity isn't well matched to 377 ohms, then the RF reflects right off of it. The black foam that ICs used to come in is a good example.
The sheet RF absorber (as opposed to the pyramidal kind) typically has multiple layers of conductive sheet separated by a fairly lossless foam, with the conductivities and spacing of sheets chosen to optimize the absorption for a particular frequency range and angle of incidence. As with any RF load, the important thing is the "match".
Pyramidal absorbers (like you see in an anechoic chamber) make use of "cones" (so there's not a real sharp "transition" in impedance), and for higher frequencies, the reflections head down deeper into the valleys between the peaks. (at least for angles of incidence close to normal).
All of the absorbers have very different properties at grazing angles than they do at normal incidence.
And, what you might be seeing is actually a magnetic absorber to suppress creeping waves along the surface. It's a ferrite loaded elastomer. We use a lot of it at work, for instance, around the outside of a corrugated horn to suppress back/side lobes.
There's a new "choke ring" style antenna (patented, of course, and they deserve it) which uses spikes instead of solid rings. And, they wrap the choke over a hemispherical surface as opposed to on a plane.
Much tougher to design and fabricate (no buying sets of cake pans any more), but if you want to differentiate yourself from the horde of Ashtech/D&M style chokes, you need something.
At JPL, we also use what's called a "helibowl" for ground testing. It's a quad helix or other element inside a bowl. Doesn't have much pattern close to the horizon. I suspect you can google and find more details, or if people are interested, I can ask around about design information.
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