A new waveguide attenuator element utilizing corrugated metallic surface combined with resistance card

This paper is concerned with a new type of attenuator element for use in waveguide circuits, The element consists of a resistance card on a corrugated metallic plate in a rectangular waveguide excited in the mode. The principle of operation is based essentially on the fact that since the guide having the corrugated metallic plate acts as a slow-wave structure and since an axial component of the electric field is produced on the surface of the corrugation, the resistance card laid on the corrugation causes a power loss. Hence by choosing appropriate values of the corrugation depth and of the resistance per unit area of the resistance card, a fairly large attenuation of the electromagnetic waves passing down through the guide results. The change of the propagation constant as a function of the resistance of the card is experimentally observed at around 4,000 mc. The value of the phase constant is in good agreement with the theory, but the experimental value of the attentuation constant is larger than the theoretcial prediction. For example, a waveguide, having the cross section of 2.5 cm 5.00 cm, the corrugation depth of 1.17 mm and the corrugation pitch of 0.4 mm, is theoretically expected to yield a propagation constant of 0.15 neper/cm for the card resistivity of 400 ohms/cm², while the experimental value measured under the same condition is 0.26 neper/cm.