Waveguide Mechanical and Machining Considerations

Waveguides are predominantly constructed entirely with conductive metals, with the exception of pressure windows, some gaskets  and jacketing materials. Generally, this means that waveguides are constructed in machining facilities by technicians knowledgeable in waveguide construction. Some of this knowledge is based in physics, such as the proper bend radius and lengths of bends, and some depends upon experience gained through trial, error and troubleshooting.

For instance, generic flexible waveguides tend to have a poorer RF performance in terms of insertion loss and VSWR, even though they may solve routing, misalignment  and vibration challenges. Though, if a flexible waveguide is made with the appropriate sized sections for a specific frequency, the insertion loss and VSWR can nearly match a solid waveguide for a very narrow bandwidth. Additionally, some vendors will perform a fully assembled flexible waveguide to reduce the occurrence of mechanical stress on a flexible waveguide segment that often causes the decoupling of the flex segments.

For waveguide twists and bends, there are very simple physics based rules to follow to ensure that the RF performance is optimized. Specifically with the rectangular waveguide, bends can be done in the width, known as an E-bend for distorting the electric field, or, a bend can be done on the height wall, known as an H-bend for distorting the magnetic field. The radius required to have an optimal performing bend is greater than 2 wavelengths of the lowest frequency of interest. For a 45 degree twist, or sharp bend, there is a simple rule of having the outer wall length at ¼ the wavelength of interest. However, with sharp bends, the phase of the output signal will be inverted compared to the input signal and the frequency bandwidth capability will be limited compared to other routing options. With twists, a 90 degree bend requires at least two wavelengths, where a 180 degree twist requires four wavelengths for a full inversion twist. The polarization of the RF energy is changed during a twist, and may need to be corrected for depending upon the system.