How do I calculate the attenuation per unit length of a transmission line at a given frequency?
Transmission Line Loss Calculation
The total attenuation of a transmission line determines how much signal is lost per unit length. For short interconnects (< 1 inch), even moderately lossy substrates may be acceptable. For long runs (antenna feeds, distribution networks), the loss budget constrains the choice of transmission line and substrate.
| 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 |
Cable Selection Criteria
The three loss mechanisms have different frequency dependencies: conductor loss αc ∝ √f (due to skin depth decreasing), dielectric loss αd ∝ f (linear with frequency), and radiation loss αr ∝ f² (quadratic). At low frequencies, conductor loss dominates. At high frequencies, either dielectric loss (for lossy substrates) or radiation loss (for thick substrates at mmWave) dominates.
- 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
Loss and Phase Stability
For system design, the required attenuation per unit length is determined by the link budget. If the transmitter-to-antenna path includes 6 feet of transmission line and the loss allocation is 3 dB, the required attenuation is < 0.5 dB/ft. This eliminates lossy options and constrains the selection to low-loss cables or waveguide.
Frequently Asked Questions
How do I minimize total loss?
Choose the lowest-loss transmission line type that meets your mechanical constraints: waveguide for fixed installations above 10 GHz, semi-rigid coax for flexible routing, and low-loss PCB substrates for board-level interconnect. Keep all transmission line runs as short as physically possible.
Do connectors add loss?
Yes. Each connector pair adds 0.05-0.3 dB depending on frequency and quality. SMA at 10 GHz: ~0.1 dB. 2.4mm at 40 GHz: ~0.15 dB. In a system with 6 connectors, the total connector loss can exceed 1 dB, which may be significant.
What about temperature effects on loss?
Conductor loss increases with temperature because resistivity increases (copper: +0.4%/°C). Dielectric loss also generally increases. Total loss may increase 10-30% over a 100°C temperature range. Specify maximum loss at the highest operating temperature.