Transmission Line Loss
Understanding Transmission Line Loss
Transmission line loss is one of the most important practical considerations in RF system design. Every dB of cable or trace loss directly reduces system sensitivity and must be accounted for in the link budget.
Loss by Transmission Line Type
| Line Type | Loss at 1 GHz | Loss at 10 GHz |
|---|---|---|
| RG-58 coax | 0.6 dB/m | 2+ dB/m |
| LMR-400 coax | 0.08 dB/m | 0.3 dB/m |
| Semi-rigid 0.141 | 0.3 dB/m | 1.0 dB/m |
| Microstrip (FR4) | 0.3 dB/cm | 1.5 dB/cm |
| Waveguide (WR-90) | N/A | 0.01 dB/m |
Frequently Asked Questions
What causes transmission line loss?
Conductor loss (skin effect, increases with sqrt(f)), dielectric loss (loss tangent, increases linearly with f), and radiation loss (from discontinuities). Conductor loss dominates at low frequencies; dielectric loss dominates at high frequencies.
How do I choose the right cable?
Balance cost, flexibility, loss, and power handling. Short lab connections: RG-316 or semi-rigid. Outdoor runs < 30m: LMR-400. Long runs: 7/8 inch hardline. Above 18 GHz: consider waveguide. Always calculate total loss against your budget.
When should I switch to waveguide?
When coaxial loss becomes excessive: above 18 GHz for most applications, above 40 GHz for all. Waveguide loss is 10-100x lower than coax. The crossover point depends on run length and acceptable loss budget.