Meaning
High-frequency loss computations determine the power loss of electromagnetic waves travelling through a hollow metallic transmission line. When designing sub-millimetre systems, a waveguide attenuation calculation must account for both the bulk conductivity of the metal wall and the additional losses caused by surface roughness. This calculation combines the theoretical smooth-wall attenuation with empirical or numerical correction factors.
The result gives the total loss in decibels per unit length. This mathematical analysis is critical for determining the viability of substituting lightweight molded plastics for heavy metal blocks.
Loss Calculation
Mathematical models combine the physical dimensions of the waveguide with the operating frequency to find the theoretical conduction loss. In a typical waveguide attenuation calculation, the formula uses the metal conductivity and skin depth to determine the surface resistance. This is then multiplied by the current distribution across the waveguide walls to yield the smooth-wall loss value.
Roughness Multiplier
Surface roughness increases the effective path length of the currents and raises the conduction loss. When the waveguide attenuation calculation incorporates correction factors, the computed loss values rise to match experimental measurements. The choice of correction factor depends on the ratio of surface roughness to the electromagnetic skin depth.
Substrate Contribution
Plastic waveguides are molded in two halves, plated with metal, and then joined together to form the hollow channel. If the molded plastic has high surface roughness, the sputtered metal coating will replicate it, leading to a high waveguide attenuation calculation. This calculation allows engineers to specify the maximum allowable roughness for the molded substrate to meet the system loss budget.
