How do I transition from coaxial cable to rectangular waveguide with minimum loss and reflection?
Waveguide Transition Design
The coax-to-waveguide transition must convert the TEM mode in the coaxial cable to the TE10 mode in the waveguide. The fundamental challenge is that these modes have very different field distributions and impedances (50 Ω vs ~500 Ω), requiring an impedance transformation and field pattern transformation simultaneously.
| 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) |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
What bandwidth can a simple probe achieve?
A single-probe transition with fixed back-short achieves 15-25% bandwidth for return loss > 20 dB. Bandwidth can be extended to 30-40% by adding a tuning screw or iris near the probe. Ridged transitions achieve the full waveguide bandwidth (40%).
Where does the loss come from?
The primary loss in a coax-to-waveguide transition is the mismatch loss from imperfect impedance matching. A well-designed transition with 25 dB return loss has only 0.01 dB mismatch loss. The remaining loss (0.1-0.3 dB) comes from conductor loss in the probe, gasket loss at the flange, and radiation at the coaxial-to-probe junction.
Can I design transitions at mmWave?
Yes, but the small dimensions make fabrication challenging. At W-band (75-110 GHz), the WR-10 waveguide is 2.54 × 1.27 mm, and the probe diameter may be 0.1-0.2 mm. CNC machining or MEMS fabrication techniques are used. Alternatively, substrate-integrated transitions use a PCB probe to couple between a 1.0mm coaxial connector and the waveguide aperture.