How do I choose between waveguide and coaxial cable for millimeter wave frequencies above 40 GHz?
mmWave Interconnect Selection
The choice between waveguide and coaxial cable at millimeter wave frequencies involves tradeoffs in loss, bandwidth, power handling, flexibility, and cost. Neither is universally superior; the optimal choice depends on the specific application requirements.
| Parameter | Semi-Rigid | Conformable | Flexible |
|---|---|---|---|
| Loss (dB/m at 10 GHz) | 0.8-2.5 | 1.0-3.0 | 1.5-5.0 |
| Phase Stability | Excellent | Good | Fair |
| Bend Radius | Fixed after forming | Hand-formable | Continuous flex OK |
| Shielding (dB) | >120 | >90 | >60-90 |
| Cost (relative) | 2-5x | 1.5-3x | 1x |
Frequently Asked Questions
What about cost?
Waveguide components at mmWave frequencies are precision-machined (EDM or CNC) from brass or aluminum, making them expensive. A WR-10 straight section costs $200-500. Equivalent 1.0mm coaxial cables cost $300-800 but provide much wider bandwidth. Neither is inexpensive at mmWave frequencies.
Can I transition between them?
Yes. Waveguide-to-coax transitions (adapters) are available for all standard waveguide bands. They introduce 0.5-1.5 dB loss and limit the usable bandwidth to the waveguide band. The transition uses a coaxial probe inserted into the waveguide, coupling the coaxial TEM mode to the waveguide TE10 mode.
What about substrate-integrated waveguide (SIW)?
SIW creates waveguide-like structures within a PCB using rows of vias as the sidewalls and copper planes as the top and bottom walls. SIW provides the low radiation and good isolation of waveguide with the manufacturing simplicity of PCB technology. Loss is higher than metal waveguide but much lower than microstrip at the same frequency.