Connections between waveguide components are generally accomplished with flanges. A typical waveguide flange is aligned using pins and mating pin-holes. Because of the practical machining tolerances for pins and holes, flange misalignment is always possible. When one of the waveguides has an error in its aperture dimensions, or has a misalignment error relative to the other guide, the electrical equivalent is a frequency-dependent shunt susceptance located at the plane of the waveguide junction [1, 2]. Impedance mismatch effects have been studied by considering three types of misalignment separately (Fig. 1).
Fig. 1 – Waveguide misalignment possibilities include E-Plane, H-plane, and Rotational offsets
Waveguide flange misalignment can be as large as 100 µm with standard machining tolerances and 65 µm with precision fabrication methods [3]. In Figures 2 and 3, the expected reflection coefficients for a pair of rectangular waveguides with misalignment in the H-plane and E-plane are shown [4]. The expected reflection coefficient from rotational misalignment is shown in Figure 4.
Fig. 2 – Expected reflection coefficient vs. waveguide misalignment in the H-plane
Fig. 3 – Expected reflection coefficient vs. waveguide misalignment in the E-plane
Fig. 4 – Expected reflection coefficient vs. rotational misalignment
Many different types of waveguide flange have been developed over the years. At mm-wave frequencies, standard UG-383 and UG-387 flanges are often used. Mechanical tolerances for these flanges can result in alignment offsets are large as +/- 0.006 inches. Anti-cocking versions of these flanges, which include a raised outer ring to prevent waveguide cocking, are typically made with tighter tolerances and can achieve alignment precision closer to +/- 0.003 inches. Recently, newer waveguide flanges have been proposed to achieve much greater alignment precision for use at 110 GHz and above [5].
In many applications, waveguide misalignment does not result in significantly degraded system performance. When it occurs within instrumentation systems, waveguide misalignment can limit measurement precision by introducing unwanted reflections and increasing measurement uncertainty. Additionally, the performance of some components may be degraded by waveguide misalignment.
A common example is a directional coupler with high directivity. Achieving high levels of directivity requires that all ports are terminated with good impedance matching. If waveguide misalignment occurs at any port, the resulting reflections will degrade the coupler’s directivity. Other waveguide components, such as isolators and hybrid junctions, also require good impedance matching to achieve high levels of performance.
Circular waveguide connections are also susceptible to misalignment. For example, misalignment in the connection between a dual-polarized horn antenna and an ortho-mode transducer can degrade the antenna’s cross-polarization response.
To avoid problems caused by waveguide misalignment, some of the biggest challenges include understanding a variety of flange specifications, judging how much misalignment can occur, and predicting possible effects on system performance. Eravant has extensive experience solving waveguide misalignment problems, and uses that knowledge to achieve reliable and consistent results.
References:
[1] D. J. Bannister, E. J. Griffin, and T. E. Hodgetts, "On the Dimensional Tolerances of Rectangular Waveguide for Reflectometry at Millimetric Wavelengths," NPL Report DES 95, September 1989.
[2] R. F. Harrington, Time-Harmonic Electromagnetic Fields.
[3] Huilin Li, “Waveguide Flange Design and Characterization of Misalignment at Submillimeter Wavelengths”, PhD Dissertation, School of Engineering and Applied Science, University of Virginia, 2013
[4] A. R. Kerr, "Mismatch Caused by Waveguide Tolerances, Corner Radii, and Flange Misalignment," National Radio Astronomy Observatory, Charlottesville, Electronics Division Technical Note NO. 215 2010.
[5] IEEE Std 1785.2-2016, IEEE Standard for Rectangular Metallic Waveguides and Their Interfaces for Frequencies of 110 GHz and Above—Part 2: Waveguide Interfaces