By TEBAOFLEX Jyen. Originally published on September 28, 2026.
Phase-stable microwave cable selection requires more than a low-loss dielectric. Temperature, mechanical flexing, connectors and assembly restraint can change electrical length. A cable’s insertion loss, impedance match and phase stability are related design considerations, but one favorable measurement does not prove the others.
PTFE behavior is construction-dependent
PTFE-based dielectrics can show nonlinear temperature-dependent electrical behavior. Expanded PTFE should not be described as eliminating every phase transition or guaranteeing a flat phase curve. Gore’s technical discussion describes complex temperature effects in its ePTFE cable constructions. Characterize the actual finished assembly across the required temperature range, including heating and cooling.
Use the correct phase-delay relationship
For a small delay change Δτ at frequency f, the phase change is Δφ = 360 × f × Δτ degrees. Illustrative calculation: at 18 GHz, a 1 ps delay change corresponds to 6.48 degrees. This is a calculation, not a measured TEBAOFLEX specification.
Another hypothetical example uses a 2 m line, a constant velocity factor of 0.695 and the speed of light approximated as 3.00 × 10⁸ m/s. The nominal delay is about 9.59 ns. A 2000 ppm change in that delay is about 19.2 ps, giving approximately 124.3 degrees at 18 GHz. The assumed delay change must come from actual characterization before this calculation can predict a real assembly.
Specify the VNA measurement setup
- Record assembly length, connector configuration, frequency range and the chosen phase reference.
- Calibrate at the defined reference planes and control test-lead movement, connector cleanliness, mating torque and fixture repeatability.
- Record temperature soak, temperature measurement location, sweep direction, heating/cooling sequence and any hysteresis.
- Report phase change against a specified reference state. Use appropriate phase unwrapping; a wrapped trace can conceal large accumulated changes.
Separate thermal stability from flex stability
A thermal sweep on an undisturbed specimen does not establish phase repeatability during repeated bending. A flex test should define bend radius, angle, movement, torsion, number of repetitions and the location of restraints. Measure residual change after returning to the original position as well as change during movement if the application depends on it.
Likewise, a low VSWR does not guarantee low phase drift. Review insertion loss, return loss, phase change, group delay where relevant and connector repeatability as separate acceptance items. Connector size alone is not a universal frequency rating; the exact connector and assembly design control the usable range.
Procurement evidence
Ask for plots and raw results identifying the construction, serial number, conditions and uncertainty. Do not transfer another manufacturer’s trademarked cable-family performance to a different supplier. Aerospace or defense suitability requires the actual contractual qualification and traceability, not a phase-stable description alone.
Sources and scope
Gore: temperature-dependent insertion loss and phase
These references explain the relevant methods or manufacturer guidance. They do not certify a TEBAOFLEX product or establish results for a supplied batch.
Discuss your specification
For a project-specific cable specification, send the duty, installation drawing and required acceptance criteria to TEBAOFLEX. WhatsApp: +86 185 9591 3361. Request documented product ratings and test evidence before approval.