Coaxial Cable Reference
Coaxial cable is the primary transmission line for RF signals above a few MHz. Choosing the wrong cable — even for a short run — can add significant insertion loss, degrade shielding, or limit power handling. This page is a quick reference for the most common types.
Cable Comparison Table
| Type | OD (mm) | Z₀ | Vel. factor | Atten. @ 1 GHz | Max power | Best use |
|---|---|---|---|---|---|---|
| RG-174 | 2.8 | 50 Ω | 0.66 | 1.5 dB/m | 25 W | Pigtails, U.FL connections, very flexible low-power |
| RG-316 | 2.6 | 50 Ω | 0.69 | 1.3 dB/m | 30 W | PTFE insulation; flexible, low loss vs RG-174 |
| RG-58 (C/U) | 5.0 | 50 Ω | 0.66 | 0.55 dB/m | 200 W | General RF cable, lab use, instrumentation up to ~500 MHz |
| RG-59 | 6.1 | 75 Ω | 0.66 | 0.6 dB/m | 200 W | Video, CATV, 75 Ω systems |
| RG-213 (U) | 10.3 | 50 Ω | 0.66 | 0.28 dB/m | 1 kW | Base station feedlines, amateur radio HF/VHF |
| LMR-195 | 4.95 | 50 Ω | 0.80 | 0.43 dB/m | 250 W | Low-loss flexible for short runs, jumpers |
| LMR-400 | 10.3 | 50 Ω | 0.85 | 0.135 dB/m | 1.1 kW | Low-loss runs to 1 GHz; antenna feedlines |
| LMR-600 | 15.9 | 50 Ω | 0.87 | 0.087 dB/m | 2.5 kW | Long runs, base station, cellular infrastructure |
| Semi-rigid UT-085 | 2.2 | 50 Ω | 0.695 | 0.84 dB/m | 100 W | PCB-to-PCB, MMIC, excellent shielding, non-flexible |
| Semi-rigid UT-141 | 3.6 | 50 Ω | 0.695 | 0.44 dB/m | 300 W | Microwave benching, test fixtures |
Attenuation Scaling with Frequency
Coaxial cable attenuation increases approximately as √f (due to skin effect in the conductors) plus a smaller f-dependent term from dielectric loss:
At 100 MHz, LMR-400 has ~0.044 dB/m. At 2.4 GHz it's ~0.22 dB/m. A 10-metre run at 2.4 GHz loses 2.2 dB — worth using LMR-600 or a tower-mounted LNA for long runs.
Velocity Factor and Electrical Length
The velocity factor (VF) is the ratio of signal propagation speed in the cable to the speed of light. VF depends on the dielectric: solid PE ≈ 0.66, foam PE ≈ 0.78–0.88, PTFE ≈ 0.69. Electrical length = physical length / VF × 360° per wavelength. A 1-metre RG-58 cable at 300 MHz is one electrical wavelength long (0.66 × 1 m / 1 m = 0.66λ — 237°).
Power Handling Limits
Two limits apply: thermal (conductor heating → insulation damage, typically rated at 40°C ambient) and voltage breakdown (at peak voltage, the dielectric arcs over). For most cables the thermal limit is lower. Power derating at high temperatures: reduce rated power by ~1% per °C above 40°C. At altitude (lower air pressure), the breakdown voltage decreases — critical for airborne equipment.
Practical Selection Guide
- 0–30 MHz, any run length: RG-213 or LMR-400. Loss is not critical.
- VHF/UHF, < 3 m: RG-58 or LMR-195. Adequate loss, easy to handle.
- VHF/UHF, > 10 m: LMR-400 or LMR-600 to keep feedline loss below 1–2 dB.
- Microwave, > 2 GHz, critical loss budget: LMR-400 or semi-rigid.
- Pigtails and internal connections: RG-316 or UT-085 semi-rigid.
- High power (> 500 W): LMR-600 or larger hardline; ensure good connector torque.