Coaxial Cable Loss
Understanding Coaxial Cable Loss
Cable loss is often the largest and most overlooked loss in an RF system. A 50-foot run of RG-58 at 2.4 GHz loses over 15 dB, which is more signal than most amplifiers provide. Selecting the right cable for each application is essential for system performance.
Loss Mechanisms
- Conductor loss: Ohmic resistance of the center conductor and shield. Increases as sqrt(f) due to skin effect. Dominant at low frequencies.
- Dielectric loss: Absorption in the dielectric material. Increases linearly with frequency. Dominant at high frequencies. PTFE (Teflon) has lowest dielectric loss.
- Radiation: Shield leakage. Minimized with high-coverage braided or solid shields.
Common Cable Types
| Cable | OD (in) | Loss at 1 GHz | Loss at 10 GHz |
|---|---|---|---|
| RG-174 | 0.100 | 0.98 dB/ft | 3.3 dB/ft |
| RG-316 | 0.098 | 0.46 dB/ft | 1.7 dB/ft |
| RG-58 | 0.195 | 0.35 dB/ft | N/A |
| LMR-240 | 0.240 | 0.12 dB/ft | N/A |
| LMR-400 | 0.405 | 0.07 dB/ft | 0.22 dB/ft |
Frequently Asked Questions
How much loss does coaxial cable have?
Cable loss depends on cable type, length, and frequency. At 1 GHz, a quality low-loss cable (LMR-400) has 0.07 dB/ft while a small flexible cable (RG-316) has 0.46 dB/ft. Loss increases with frequency and length.
How do I choose between cable types?
For antenna feeds (long runs), use the lowest-loss cable that fits: LMR-400 or equivalent. For test bench connections (short runs), use flexible cables like RG-316. For microwave (>18 GHz), consider semi-rigid or phase-stable cables. Balance loss, size, flexibility, and cost.
Does cable loss change with temperature?
Yes. Cable loss increases slightly with temperature due to increased conductor resistivity and dielectric loss. The change is typically 0.1-0.3% per degree C. Phase also changes with temperature, important for phase-critical applications like phased arrays.