Additive manufactured (AM) metals are a subject of much interest for their performance in passive microwave applications. However, limitations could arise due to artefacts such as surface texture and/or roughness resulting from the manufacturing process. We have therefore adapted a parallel plate microwave resonator for the accurate measurement of the surface resistance of flat metal plates, allowing for microwave current flow in two orthogonal directions by simply exciting a different resonant mode (at 5.3 and 6.4 GHz), without the need to remove and re-fix the sample. The systematic and random errors associated with the measurement of surface resistance are very small, less than 1% and 0.1%, respectively. The technique is demonstrated with measurements on a range of samples of the alloys AlSi10Mg and Ti6Al4V, manufactured by laser powder bed fusion, in addition to traditionally machined samples of bulk metal alloys of aluminium and brass. For AM samples of AlSi10Mg we have studied the effect on the surface resistance of directional roughness features, generated by the laser raster paths, in directions transverse or parallel to microwave current flow. Importantly for passive microwave device applications, we demonstrate that these samples exhibit no systematic anisotropy of surface resistance associated with such surface features.

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A new method for accurate measurement of the microwave surface resistance of flat metal plates is proposed and verified experimentally, based on a sapphire dielectric resonator. System losses are accounted for by having continuous and controlled variation of the distance between the dielectric and the sample under test.

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