Waveguide Attenuation and its Correlation with Surface Roughness

Measurements of the attenuation produced by air-filled drawn waveguide tubing have been made at a frequency of 9375 Mc/s. The theoretical and experimental performances of various waveguide surfaces are compared and discussed. Detailed studies of the roughness of the internal surfaces of certain standard commercial waveguides have been made, to determine the orders of magnitude of the irregularities and to try to set up a correlation between measured and calculated attenuation values. It is concluded that the discrepancies between these values are due solely to surface roughness, and it is doubtful whether further improvements in surface finish over that of present precision-drawn tubes are required for any applications at frequencies near or below 24 000 Mc/s. The effects of small changes in waveguide dimensions on attenuation, together with the problems involved in measuring both the attenuation and d.c. resistivity of a waveguide sample, are considered.