I ask this question in relation to radar cross section returns, Are the EM reflection properties of carbon epoxy composites similar to aluminum? Also how well are EM shielding characteristics of CF composites?
Structural carbon fiber reinforced plastic, as used in aircraft fuselages and wings, is a radar reflector.
CFRP's RCS properties are similar to aluminum, except for unidirectional fiber, if its fibers are oriented parallel to the incoming signal's polarization. In the graph below, CF 1-2 are twill weave and CF 3-4 are unidirectional.
7-12 GHz corresponds to the radar X-band, in which most military radars operate. A difference of a few dB for a narrow range of signals doesn't provide much concealment, so CFRP aircraft aren't inherently stealthy.
There are non-structural radar-absorbent materials that use carbon fiber chopped to specific length. They aren't strong enough to build high-performance airplanes out of, but can be applied on top of a structural CFRP layup, or embedded into a fiberglass layup.
In general, CF must be combined with other materials, such as iron oxide, to provide comprehensive reduction.
A comprehensive review of carbon-based RAM shows that -10 dB reduction across the board is achievable with one material, but pushing it to -20 dB requires a mixture of different materials, in a layer about 2 mm thick. This has to be on top of the reflective structural skin.
Electromagnetic shielding uses the same conductive properties, but unidirectional fiber isn't a good fit for shielding, since it lets some signal through. Generally CFRP needs to be supplanted with metal mesh or a mat of chopped fiber to provide metal-like shielding.