Customers often inquire about the insertion loss of rectangular waveguides. The answer always depends on the waveguide’s construction and how it’s used. However, there are some general guidelines that can help predict or assess waveguide insertion loss.

The first guideline is to recognize that insertion loss is comprised of attenuation as well as return loss. That is, insertion loss is the sum of return loss and attenuation loss. Return loss is caused by reflections that may occur at either end of a waveguide section or at discontinuities within. Depending on the relative phase or the delay between the reflections, their combined effects may be either additive or off-setting. The return loss of a straight waveguide section is typically dominated by reflections that occur at the waveguide flanges.

Attenuation is caused by signal leakage or conductor losses in the waveguide walls. The minimum possible attenuation is determined by the conductivity of the waveguide material. Based on the conductivity of the material, the attenuation constant of a waveguide may be calculated. Expressed as the fraction of signal power dissipated per unit length, the minimum attenuation is shown in the accompanying graph for copper waveguides ranging from WR-03 to WR-15. For each waveguide size, attenuation decreases as the frequency increases. This behavior results from the smaller currents induced on the waveguide walls at higher frequencies, which more than offsets the effects of decreasing skin depth.

Extruded copper waveguides can exhibit attenuation values close to their predicted minimum values. Electroformed waveguide sections often perform nearly as well. Waveguides produced by various other means, such as machining or Wire EDM, have rougher surfaces and exhibit higher attenuation owing to the relationship between skin depth and surface roughness. Various plating materials and chemical finishes can also affect waveguide attenuation.

Waveguide tapers, bends and twists can also increase attenuation slightly due to localized higher-order propagation modes and the increased surface currents that support them. Such mechanical features can also cause minor reflections that may contribute significantly to the return loss.

For a metrology-grade straight waveguide section such as Eravant model STQ-WG-12025-FB-CF, insertion loss is often dominated by attenuation. If the maximum return loss is -30 dB, its contribution to insertion loss is less than 0.01 dB. At 75 GHz, the minimum attenuation of a WR-12 copper waveguide may be calculated as 0.019 dB/cm or about 0.12 dB for a 2.5-inch section. The accompanying data plot shows measured insertion loss of around 1.0 dB. The insertion loss is lower at higher frequencies, indicating that conductor losses are mainly responsible for the attenuation. The measured attenuation level of 0.16 dB/cm, when compared to the predicted minimum value of 0.019 dB/cm for a perfectly smooth copper waveguide, suggests that either the surface roughness or the plating material may be contributing to higher attenuation. This is a typical result for metrology-grade machined waveguide sections, where electrical and mechanical stability are prioritized over low insertion loss. To minimize insertion loss, an extruded or electroformed waveguide may be used.