How do I design a waveguide twist section to rotate the polarization by 90 degrees?
Waveguide Twist Design
Waveguide twists are used when two components need to be connected but have different polarization orientations. This occurs frequently in antenna feed assemblies where the transmitter output is horizontally polarized but the antenna feed requires vertical polarization, or vice versa. The twist physically rotates the waveguide and the propagating mode along its length.
| Parameter | Standard Rect. | Ridged | Circular |
|---|---|---|---|
| Single-Mode BW | 40% (1.25-1.9 fc) | 50-150% | 26% (1.31:1 ratio) |
| Attenuation | Low | Moderate (3-5x) | Low to very low |
| Power Handling | High (kW-class) | Moderate | High |
| Polarization | Single | Single | Dual (TE11) |
| Cost | Low (commodity) | Medium | High (specialty) |
Frequently Asked Questions
Can I use a 90-degree twist at any frequency within the waveguide band?
Yes, a well-designed gradual twist works across the full single-mode bandwidth of the waveguide. The VSWR may vary slightly across the band, with the best performance near the center frequency. The twist length should be designed using the guided wavelength at the lowest operating frequency for worst-case performance.
What about a 45-degree twist?
Any rotation angle is possible. For a 45-degree twist, the same design rules apply: the total length should be at least 2λg multiplied by the ratio of the twist angle to 90 degrees. A 45-degree twist over 1λg produces similar VSWR to a 90-degree twist over 2λg.
Is there an alternative to a physical twist?
A septum polarizer or a 90-degree waveguide junction with an internal polarization-rotating iris structure can achieve the same effect in a shorter physical length. These are more compact but more complex to manufacture and typically have narrower bandwidth than a gradual twist.