How do I transition from rectangular waveguide to circular waveguide?
Waveguide Cross-Section Transition Design
The TE10 mode in rectangular waveguide and the TE11 mode in circular waveguide have similar field distributions (a single half-wave variation across the broad dimension) and can be coupled efficiently through a gradual cross-section change. The transition design must maintain field continuity while avoiding excitation of the TM01 mode (which has a fundamentally different field distribution) or higher-order TE modes.
| 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
What dimensions should the circular end have?
The circular waveguide diameter should be chosen so that its TE11 cutoff frequency matches or is slightly below the TE10 cutoff of the rectangular waveguide. For WR-90 (cutoff 6.56 GHz), use a circular waveguide with diameter ≈ 26.9mm (TE11 cutoff ≈ 6.54 GHz). This ensures the same operating frequency range.
Can I handle both polarizations through the transition?
The transition converts the rectangular TE10 mode to one polarization of the circular TE11 mode. The orthogonal TE11 polarization is not excited by the transition (it would correspond to a TE01 rectangular mode, which is not present). To handle both polarizations, you need two transitions or an OMT on the rectangular side.
Is the transition reciprocal?
Yes. The same transition works identically in both directions. A signal entering the circular end in the TE11 mode (correct polarization) exits the rectangular end as TE10 with the same loss and reflection characteristics.