What is the maximum usable frequency of a coaxial connector and what determines that limit?
Connector Frequency Limits
In the fundamental TEM mode, the electric field is radial and the magnetic field is circumferential, with no field variation around the circumference. The TE11 mode has one full cycle of field variation around the circumference and requires a minimum frequency (cutoff frequency) to propagate. Below cutoff, only the TEM mode exists, and the connector behaves predictably.
| 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 |
- 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
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
Can I use SMA above 26.5 GHz?
Measurements show SMA connectors typically perform acceptably to 27-28 GHz in practice, but with degraded repeatability and higher VSWR. Some SMA connectors specified for 'DC-27 GHz' use tighter tolerances. Above 28 GHz, performance degrades rapidly and 3.5mm or 2.92mm connectors should be used.
Are 3.5mm and SMA compatible?
Mechanically, yes. 3.5mm connectors use the same thread and coupling nut as SMA. A 3.5mm male mates with an SMA female and vice versa. However, the precision of a 3.5mm-to-SMA interface is limited by the SMA's looser tolerances, giving SMA-grade performance, not 3.5mm-grade.
What about the dielectric in the connector?
Most coaxial connectors use air dielectric with PTFE support beads at intervals. The PTFE beads slightly lower the cutoff frequency due to εr > 1 and create small reflections at each bead. Precision connectors minimize bead effects through careful geometry. Air-line connectors (no beads) provide the best performance but require more careful handling.