What is the attenuation per unit length of a rectangular waveguide in the dominant mode?
Waveguide Wall Loss
The attenuation of the TE10 mode arises from ohmic loss in the waveguide walls. Current flows on the inner surfaces of all four walls, and the finite conductivity of the metal converts some of this current to heat. The current distribution varies across the waveguide walls: maximum on the broad walls (top and bottom) at the center, and on the narrow walls (sides) at the top and bottom edges.
| 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
How does waveguide loss compare to coax?
At X-band (10 GHz): WR-90 waveguide ≈ 0.03 dB/ft vs 0.141-inch semi-rigid coax ≈ 0.35 dB/ft. The waveguide advantage is about 12× at X-band and increases at higher frequencies.
Does the wall material matter much?
The loss ratio between materials equals the square root of their resistivity ratio. Copper vs aluminum: ratio = √(2.65/1.68) = 1.26, so aluminum has 26% higher loss. Copper vs silver: ratio = √(1.68/1.59) = 1.03, so silver saves only 3%. The improvement from silver is small but matters for high-Q resonators and long transmission lines.
What about waveguide surface finish?
Standard machined waveguide surface finish (Ra = 0.8-1.6 μm) adds approximately 5-15% to the theoretical smooth-wall loss at X-band. At W-band (75-110 GHz), where the skin depth is 0.2-0.3 μm, surface finish becomes critical and can increase loss by 30-50%. Electroformed or electropolished waveguide provides the smoothest surface and lowest loss.