What causes phase change over temperature in a coaxial cable assembly and how do I minimize it?
Cable Phase Stability
Phase stability over temperature is critical in systems where multiple signal paths must maintain a fixed phase relationship: phased array antennas, interferometers, and calibrated test setups. A typical PTFE-dielectric coaxial cable changes phase by approximately 600 ppm/°C, which translates to approximately 1.2° of phase change per degree per GHz per foot. For a 6-foot cable at 10 GHz over a 30°C temperature variation, this is 216° of phase change, which would completely disrupt a phased array with phase tolerance of ±5°.
| 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 is TCE?
TCE (thermal coefficient of electrical delay) is the fractional change in electrical length per degree C, expressed in ppm/°C. A cable with TCE of 100 ppm/°C changes its electrical length by 0.01% per degree. At 10 GHz, 1 foot of this cable changes phase by 0.4° per degree C.
Does cable length matter for phase stability?
Yes. Phase change is proportional to cable length. This is why minimizing cable length is critical in phase-sensitive systems. A 1-foot cable with TCE = 100 ppm/°C changes by 0.4°/°C at 10 GHz; a 10-foot cable changes by 4°/°C.
Can I compensate for temperature-induced phase changes?
Yes. Digital phase calibration measures the actual phase at known temperatures and applies a correction. Analog phase compensation uses a temperature sensor and a voltage-controlled phase shifter. Both approaches add complexity but can reduce the effective phase variation to <1° over the temperature range.