RF Cable S-Parameter Overview
A coaxial cable is a two-port network. Ideal S-parameters: S11=S22=−∞ (perfect match), S21=S12=−IL dB (flat loss). Real cables show frequency-dependent insertion loss, slight port mismatch, and phase that increases linearly with frequency.
Key Cable S-Parameter Metrics
| Parameter | Ideal | Real Cable (1m RG316, to 6 GHz) |
|---|---|---|
| S21 at 1 GHz | 0 dB | −1.0 to −1.5 dB |
| S21 at 3 GHz | 0 dB | −2.0 to −3.0 dB |
| S21 at 6 GHz | 0 dB | −3.0 to −5.0 dB |
| S11 (port match) | −∞ | <−25 dB below rated frequency |
| S21 phase slope | Linear | Linear (−τ·2πf) |
| Group delay | Constant | ~5 ns/m (RG58), ~4 ns/m (RG316) |
Cable Loss Measurement Procedure
- Calibrate VNA with SOLT at cable ends (include cable in the reference path)
- Connect cable-under-test → measure S21 vs frequency
- Frequency range: 100 MHz to rated max frequency
- S21 should show √f roll-off characteristic (conductor loss dominated)
- Any abrupt changes indicate damage, kinks, or connector problems
Identifying Cable Defects from S-Parameters
| Defect | S21 Signature | S11 Signature |
|---|---|---|
| Normal operation | Smooth, √f roll-off | <−30 dB, flat |
| Damaged outer jacket | Ripple begins at specific freq | Resonance at damage length |
| Bad connector center pin | Reduced S21, resonance | S11 spike at specific freq |
| Corroded shield | Extra loss at all frequencies | S11 increases |
| Cable crimped/bent | Ripple proportional to crimp severity | Ripple at λ/2 of crimp |
Cable Electrical Length Measurement
From S21 phase slope:
τ_g = −Δφ/(360°·Δf) [group delay = propagation delay]
Cable length = τ_g × v_p × c
For RG316 (vp = 0.69c): length = τ_g × 0.69 × 299.8 mm/ns
Example: τ_g = 5.1 ns → length = 5.1 × 0.69 × 299.8 = 1056 mm = 1.056 m
RF View Cable Analysis: Load cable .s2p file → S21 shows loss vs frequency, S11 shows connector match, Group Delay shows propagation delay. Compare cable types or check cable quality. Free on Android.