What causes higher order mode propagation in a coaxial cable and how do I avoid it?
Coaxial Cable Mode Analysis
A coaxial cable with perfect symmetry only supports the TEM mode, regardless of frequency. In practice, any asymmetry (off-center inner conductor, connector misalignment, bend) can excite higher-order modes once the frequency exceeds the TE11 cutoff. The TE11 mode has fields that vary sinusoidally around the circumference, and it requires the average circumference to be approximately one wavelength.
| 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 |
Cable Selection Criteria
Below the TE11 cutoff, any TE11 energy excited at a discontinuity decays exponentially (evanescent mode) and does not propagate. Above cutoff, the TE11 mode propagates with its own phase velocity and wavelength, interfering with the TEM mode and causing frequency-dependent amplitude and phase variations.
Loss and Phase Stability
Smaller coaxial cables have higher cutoff frequencies because the average conductor circumferences are smaller. This is why mmWave measurements use small-diameter cables (0.047-inch and 0.034-inch) with cutoff frequencies above 60-100 GHz. The tradeoff is higher loss per unit length due to the smaller conductor dimensions.
- 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
Connector Interface
When evaluating what causes higher order mode propagation in a coaxial cable and how do i avoid it?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
Frequently Asked Questions
Can bends excite higher-order modes?
Yes. A sharp bend breaks the axial symmetry and can excite the TE11 mode even below its free-propagation cutoff. The excited mode is evanescent (decaying) below cutoff but can cause significant reflection at the bend. The effect is proportional to the bend sharpness and inversely proportional to the margin below cutoff. Use gentle bends (radius > 5× cable diameter) to minimize mode excitation.
What happens at the connector interface?
Any misalignment between the cable and connector centers (eccentricity) excites the TE11 mode. Precision connectors minimize this through tight dimensional tolerances. The mode coupling is proportional to the eccentricity normalized to the cable diameter and increases with frequency. This is why precision connectors cost more; they require tighter manufacturing tolerances.
Is there a mode between TEM and TE11?
No. The TE11 is the first higher-order mode in coaxial cable. However, for large diameter cables (waveguide-like dimensions), many TE and TM modes can propagate. The standard WR-series waveguide has a frequency range chosen to support only the TE10 mode, with the TE20 as the next mode defining the upper frequency limit.