This paper covers the essential aspects of modeling surface roughness for microwave applications based on underlying physics. After a short summary of the relevant field theoretical fundamentals, surface roughness metrology and commonly used roughness parameters are described. Existing models and their limitations are discussed before the recently proposed Gradient Model is introduced. To this purpose, the modeling approach, the derivation from Maxwell’s equations, model predictions, and their experimental verification are shown. Reasonable choices for effective material parameters reflecting the electromagnetic effects of surface roughness as well as a corresponding surface impedance concept are derived. Both concepts allow for easy application of the Gradient Model with 3-D field solvers or analytical models. The obtained simulation results illustrate roughness impact on loss and phase delay in typical transmission lines. Comparison to measurement results up to 100 GHz shows that the Gradient Model accurately predicts these quantities for rough conductor surfaces. As it is not limited to transmission lines only, it significantly improves the design process for arbitrary microwave applications with 3-D field solvers for this frequency range.

This paper describes an accurate modeling approach taking into account the impact of surface roughness on both the attenuation and the phase coefficients of the propagating TE ₁₀ mode in rectangular waveguides. The proposed approach is based on a transmission line model for waveguides and a general surface roughness model, which are both derived from physical relations, i.e., Maxwell's equations. Thus, it is neither limited to explicit frequency bands nor to certain surface profiles. Based on the proposed approach, the specific influence on the propagation coefficient and the line impedance of a waveguide is quantified in the typical transmission frequency region and also around the cutoff frequency. Also, the roughness-dependent shift of the cutoff frequency is covered by the proposed model and explained analytically. Propagation characteristics predicted by the proposed approach absolutely coincide with full-wave simulation by CST Microwave Studio with the same input parameters. Furthermore, WR10 waveguide samples are fabricated and their surface profile is characterized using a confocal laser scanning microscope. Model predictions agree with electrical measurements up to 90 GHz for the attenuation coefficient within ≤10% and within 0.08% for the phase coefficient in the transmission region of the waveguides. In addition, the difference between normally distributed and arbitrarily distributed surface profiles is covered by the model and pointed out in this paper at rigorous measurements even at very smooth surfaces with root mean square-roughness of less than 300 nm. Finally, different techniques for utilization of this approach and possible application fields are presented.

In this letter, an additively manufactured 3$,$dB hybrid coupler is presented. While the geometrical structure is generated by a stereolithography (SLA) printer from UV curable photopolymer resin as one single part, conductive coating is provided by an electroless silverplating process. The utilized slotted waveguide approach improves the solvent flow during metal plating and therefore enables the fabrication of complex waveguide structures. Measured imbalance between the through and coupling ports stays below 1.5$,$dB over the entire E-band, while the respective phase difference ranges only between 87$° and 91$° despite the challenges of additive manufacturing in such frequency regions.

This work proposes a fabrication approach for 3D printed waveguide paths based on a combination of an optimized electroless silver plating process with a waveguide-specific design rule for general metal plating by introducing large scale non-radiating slots into the broad and narrow waveguide walls with a periodicity shorter than quarter of the guided wavelength. Experimental fabrication of straight sections of the proposed slotted waveguide with various gap dimensions yields an optimum gap size as result of a trade-off between silver plating quality and leakage losses through the sidewall gaps Moreover, a successful practical application of the proposed approach is presented in terms of a 3D printed multiple bend waveguide interconnect in three different space dimensions which imposes a complex task for conventional manufacturing techniques due to the need of multiple cutting planes for split block assembly. The proposed approach benefits of low cost, moderate handling effort and independence of the concrete geometry to manufacture making it therefore especially interesting for distribution and feeding networks in the context of rapid prototyping, automotive and space related applications.

This work proposes an approach to generate high performance rectangular hollow waveguides for applications in E-and W-Band by stereolithography 3D plastic printing and subsequent copper electroplating. In order to improve the metal deposition process, large and densely positioned sidewall gaps are identified as a key parameter in the design for manufacturing of such transmission lines with the proposed fabrication strategy. Measurements of fabricated prototypes achieve an overall attenuation of about 8 to 10 dB/m over wide bandwidths and thus, perform in similar ranges as compared to conventionally manufactured waveguides. Despite the large sidewall gaps, radiation leakage has been measured in an order of -50 dB, suggesting feasibility to many practical applications. The proposed approach offers fundamental contributions to the design for manufacturing of 3D printed waveguide systems and increases the applicability of conventional electro-plating processes.

This work evaluates different metal deposition approaches for a two staged additive manufacturing process for 3D horn antennas, divided into the deposition of plastic and conductive material. The demonstrated fabrication work-flow allows for antennas of low cost, low weight and therefore demonstrates the vast benefit of additive manufacturing. The comparison of a commercially available cast aluminum horn to the fabricated prototypes reveals a deviation of up to 3 dB in terms of transmission losses in co-polarization over a distance of 1.85 m. Cross-polarization measurements are presented revealing a larger variation among the different metalization approaches. The results shown in this paper demonstrate a successful application of additive manufacturing in radio frequency engineering. Moreover, one of the presented approaches makes only use of 3D printing systems and is therefore, potentially suitable for totally automated manufacturing.