Microwave physics of flame-sprayed Kanthal and other circuit loss coatings

A phenomenological study of flame-sprayed Kanthal and other popular rf lossy-conductor coatings was prompted by the extensive reliance on them -- for stability and other control purposes -- in numerous microwave tube types and their at times insufficient effectiveness or reproducibility. A dependable, convenient and versatile X-band "circular electric" test cavity greatly facilitated the investigation which included identifying the X-band-effective permeability (and its response, if any, to annealing) for several magnetic alloys in smooth, solid form. Thus the observed surface resistivity of a plasma-sprayed Kanthal, iron-powder or other fragmented coating could be expressed as the product of three factors arising from resistivity (via its square root), permeability (likewise) and physical structure. In the sprayed-Kanthal case it was possible to extend this systematic probing: by lapping away the surface roughness the third factor could be further factored into "superficial roughness" and "internal structure" components. Such breakdown by factors not only reveals their actual relative importance (prevalent preconceptions not withstanding) but is instructive regarding the interplay of loss achievable and associated reproducibility for both present and future coating approaches.