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Test and Measurement Equipment Calibration and Uncertainty Informational

What is the effect of cable flexure on measurement repeatability at millimeter wave frequencies?

What is the effect of cable flexure on measurement repeatability at millimeter wave frequencies? Cable flexure (bending, movement) causes phase and amplitude changes in the transmitted signal, and at mmWave frequencies this effect is significantly amplified compared to lower frequencies: (1) Why cable flexure matters at mmWave: phase change: a cable bend changes the electrical length by a small amount (ΔL). The phase change: Δφ = 360 × ΔL / λ. At 5 GHz (λ = 60 mm): ΔL of 0.1 mm → Δφ = 0.6°. At 40 GHz (λ = 7.5 mm): ΔL of 0.1 mm → Δφ = 4.8° (8× worse). At 110 GHz (λ = 2.7 mm): ΔL of 0.1 mm → Δφ = 13.3° (22× worse). These phase changes directly affect S-parameter measurements (S21 phase, S11 on a Smith chart). Amplitude change: cable bending also causes amplitude variation due to: internal conductor displacement (changes the characteristic impedance locally), dielectric compression (changes the propagation velocity), and outer conductor deformation (changes the shielding). Typical amplitude stability: standard cable: ±0.5-2.0 dB per bend at 40 GHz. Phase-stable cable: ±0.05-0.2 dB per bend at 40 GHz. (2) Phase-stable cables: specially designed cables minimize flexure-induced phase and amplitude change. Construction: corrugated outer conductor, PTFE or air dielectric, and strain-relief at connectors. Examples: Gore Phaseflex (< 0.1 dB amplitude, < 5° phase per flex at 40 GHz), Junkosha MWX (< 4° phase per flex at 40 GHz), Huber+Suhner Sucoflex (< 3° phase at 26.5 GHz). Cost: $200-1,000 per cable (10-50× more than standard cables). Length: keep as short as possible (shorter cable = less flexure effect and less loss). (3) Impact on VNA measurements: if the cable moves during calibration or measurement: the calibration is invalidated, because the cable was part of the calibrated path. Best practice: secure cables to the bench with cable clamps and do not touch or move them after calibration. Use phase-stable cables for any measurement above 20 GHz. Recalibrate if the cable is accidentally moved.
Category: Test and Measurement Equipment
Updated: April 2026
Product Tie-In: Calibration Kits, Standards, Cables

Cable Flexure at mmWave

Cable phase stability is one of the most overlooked but impactful factors in mmWave measurement quality. Many "mysterious" measurement problems are traced back to cable movement.

  1. Performance verification: confirm specifications against the application requirements before finalizing the design
  2. Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
  3. Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
  4. Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Common Questions

Frequently Asked Questions

How do I test cable phase stability?

Connect the cable between two ports of a VNA. Perform a full 2-port calibration. Record S21 magnitude and phase. Flex the cable by bending it 90° and releasing. Observe the change in S21 magnitude and phase. Repeat for multiple flex cycles. The maximum change per flex cycle is the cable phase/amplitude stability specification. A good phase-stable cable shows < 5° phase and < 0.1 dB amplitude change per flex at 40 GHz.

Can I use semi-rigid cable at mmWave?

Semi-rigid cable (solid outer conductor) has the best phase stability (it does not flex). However: semi-rigid cables cannot be bent after installation. The cable path must be planned precisely during assembly. If the cable needs to be repositioned: it must be re-bent (which can damage the cable). Semi-rigid is ideal for: fixed installations (inside instruments, fixed test fixtures). For lab benchtop measurements: flexible phase-stable cables are preferred (they can be repositioned within their stability specification).

How does cable loss affect measurements at mmWave?

Cable loss at mmWave is significant: at 28 GHz, a standard SMA cable has 1-2 dB/m loss. At 67 GHz: 2-4 dB/m. At 110 GHz: 4-8 dB/m. This loss reduces: the effective dynamic range of the measurement, the power delivered to the DUT (for TX testing), and the signal level at the receiver (for sensitivity testing). Mitigation: use the shortest possible cables, use low-loss cable types (air-dielectric, larger diameter), and subtract the measured cable loss from the measurement result.

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Glossary

Related Terms

Noise Figure → LNA → Noise Floor → S-Parameters → Insertion Loss → Return Loss →
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